simple things, u need to know,setting float levels, etc.

[grumpyvette]

yes theres lots of links, thats because theres a great deal of useful info in those links, IF YOULL TAKE THE EFFORT TO READ THRU THEM YOULL LEARN A GREAT DEAL.

Do you hate tune ups or always seem to find you spend hours and money and still never get the engine running correctly, well it takes experience and you get that only over time,if you hate tuning engines,your not alone! I really like working out the tuning intricacy's and I see so many of my friends that seem to either be totally clueless or those that would much rather drop off the car with a bottle of brandy or stop by with a case of beer and a few sub sandwiches and ask me to tweak the tune for them, which I generally will do for the cost of the jets and power valves and accelerator pumps etc required, which I generally send them to the local speed shop to pick up, that way they can get shocked at the prices and not have some silly unwarranted idea that IM some how retired early on the obscene profits, in fact a recent tune up, cost almost $240 in parts that I had the owner pick up and he saw me install those parts (plugs, rotor,distributor cap,distributor weights, ignition wire and a new coil, carb jets, and a power valve) he was very pleased with the results, stated the car felt like it picked up 50 more hp, but said if he had not personally paid for the parts and seen them installed he would have assumed that 2/3rds of that must have been labor costs.
you won,t know the answer to how well any combo runs until you do some testing and tuning with that combo, but its very likely that it should function well with minor adjustments, if the parts you select match and are properly installed.
for some reason most guys think that the constant addition of new parts solves most issues, and the truth is that the VAST majority of issues, are due to either mis-matched or improperly installed parts,or lack of tuning experience.
damn few guys ever get their car engines and drive trains functioning at near peak potential performance.
GET A FUEL PRESSURE GAUGE AND MEASURE DON,T GUESS
how can you possibly set up your fuel system unless you know the pressure and flow rates required and what currently exist's

READ LINKS

http://garage.grumpysperformance.com/index.php?threads/setting-up-your-fuel-system.211/

http://garage.grumpysperformance.com/index.php?threads/fuel-pressure-regulators.635/

http://garage.grumpysperformance.com/index.php?threads/how-big-a-fuel-pump-do-you-need.1939/

http://garage.grumpysperformance.co...ss-fuel-pressure-regulators.12776/#post-65998
http://garage.grumpysperformance.com/index.php?threads/locating-vacume-leaks.882/#post-45944

http://www.prpracingproducts.com/products-page/fuel-system-accessories/fuel-pressure-regulators/

it makes no logical sense to run 1/2" or AN#8 lines to and from a pressure regulator , or fuel pump or fuel filter,with restrictive AN#6 or 3/8" lines so use a regulator and lines and fittings that all match and allow max flow with little restriction
http://www.stockcarracing.com/techartic ... index.html

http://www.stockcarracing.com/techartic ... /fuel.html

http://www.holley.com/data/TechService/ ... uretor.pdf

viewtopic.php?f=44&t=5615&p=17136#p17136

viewtopic.php?f=55&t=109

http://www.chevyhiperformance.com/howto/45798/

http://www.carcraft.com/techarticles/cc ... r_rebuild/

http://www.vintagemusclecarparts.com/pa ... seat1.html

http://www.thecarburetorshop.com/Troubleshooting.htm

http://www.chevyhiperformance.com/howto ... index.html

viewtopic.php?f=44&t=5575&p=16927#p16927

http://www.carcraft.com/techarticles/cc ... ewall.html

viewtopic.php?f=52&t=196&p=1324&hilit=+adjusting#p1324

viewtopic.php?f=80&t=728&p=1915&hilit=+sensor#p1915

http://www.corvettefever.com/techarticl ... index.html

http://www.chevyhiperformance.com/howto ... index.html

http://members.shaw.ca/corvette86/FuelS ... gnosis.pdf

viewtopic.php?f=55&t=1442

viewtopic.php?f=55&t=211

viewtopic.php?f=55&t=11030

viewtopic.php?f=55&t=264

http://econtent.autozone.com:24991/znet ... 08fdc8.gif

http://www.carcraft.com/techfaq/116_070 ... index.html

http://www.centuryperformance.com/tunin ... g-148.html

http://www.thecarburetorshop.com/Troubl ... rdstarthot

http://www.holley.com/types/Accelerator ... 0Parts.asp

http://www.stockcarracing.com/techartic ... index.html

http://www.valvoline.com/carcare/articl ... 20000718hm

http://www.shockley.net/holley-jets.asp

http://www.mortec.com/carbtip1.htm

http://www.bgsoflex.com/holley.html

http://www.summitracing.com/streetandst ... 3B3CEE8%7D

http://store.summitracing.com/partdetai ... toview=sku

http://store.summitracing.com/partdetai ... toview=sku

http://store.summitracing.com/egnsearch ... &x=49&y=13

http://www.centuryperformance.com/fueli ... g-140.html

http://www.bob2000.com/carb.htm

viewtopic.php?f=55&t=211
WHEN TESTING<be sure the fuel pressure gauge reads correctly by comparing it to a second test gauge, these fuel pressure gauges are frequently defective
read thru these, and check out the links to parts and info, I really can,t do much better in explaining this process, and duplicating some of the posted info above is rather a waste, but if your willing to read a bit youll gain a great deal of useful knowledge

 

[grumpyvette]

Re: simple things, u need to know,setting float levels, etc

KEEP IN MIND IF you've got a carburetor equipped Chevy engine combo that produces under 450hp at the flywheel, on that set-up a mechanical pump and return style regulator is fully adequate, and probably less potential problems than an electrical fuel pump,

http://store.summitracing.com/partdetai ... toview=sku

viewtopic.php?f=44&t=777&p=2257#p2257

http://store.summitracing.com/partdetai ... toview=sku

http://www.carburetion.com/quadrajet.asp
these guys frequently have the Q-jet parts your looking for
read these

viewtopic.php?f=55&t=109

http://www.mortec.com/carbtip1.htm

viewtopic.php?f=55&t=1442

http://www.bob2000.com/carb.htm

viewtopic.php?f=55&t=1639

http://www.centuryperformance.com/tunin ... g-148.html

viewtopic.php?f=44&t=777

"ALSO for the 110gph version it said no regulator is required so what is the gain for adding one?"

thats a good question that deserves some info posted,chevy
mechanical fuel pumps throw a supply of fuel in pulsed volume, if you've ever started your engine with a loose fuel line connection to the carburetor, or tested fuel flow rates, by allowing the pump to throw fuel in a container,you've seen the fuel squirt out in pulses, a fuel pressure regulator tends to smooth out those pressure spikes the carbs needle/seat sees and provide a much steadier pressure and supply.
NOW theres a pressure overload circuit in the pump that keeps the pressure lower than a set maximum, but its not nearly as effective as running a return style fuel pressure regulator in conjunction with the pump. while its true the pump and carburetor will function without the regulator you'll generally find more consistent fuel control, and fuel levels in the carburetor if you use the regulator even with an electric fuel pump.

http://www.holley.com/12-454-25.asp

Id strongly advise buying a few books on rebuilding and trouble shooting a HOLLEY CARB

http://www.amazon.com/Holley-Rebuilding ... lley+carbs

http://www.amazon.com/Vizards-Modify-Ho ... S7J5CBAM9W

http://www.amazon.com/Holly-Carburetor- ... S7J5CBAM9W

keep in mind theres lots of small passages that can get clogged and theres several different fuel bowl gaskets, these vary by manufacturer and carb model, so be sure you use the correct gasket on the meter block and fuel bowl, if you leave debris in the fuel bowl or use the wrong gaskets they can block passages and the carb won,t function

AS ALWAYS YOUR GOING TO MISS A TON OF INFO IF YOU SKIP THE SUB LINKS

I think youll find that these related threads will provide a good deal of info you might find rather useful

http://garage.grumpysperformance.com/index.php?threads/infrared-temp-guns.11444/

http://garage.grumpysperformance.com/index.php?threads/spark-plug-info.202/

http://garage.grumpysperformance.com/index.php?threads/timing-tabs-and-indicators.1015/

http://garage.grumpysperformance.com/index.php?threads/verifying-your-real-advance-curve.4683/

 

[grumpyvette]

got a quadrajet, this info should help


http://www.hioutput.com/tech/qjetrod.html

http://www.corvetteclub.org.uk/files/do ... a_qjet.pdf

http://www.stratagaz.com/Quadrajet/Quad ... 029202.htm

http://www.florida4x4.com/tech/quadrajet/index.php

http://arapaho.nsuok.edu/~leeaq/docs/QJ ... _Paper.doc

http://www.73-87.com/7387garage/drivetrain/myqjet.htm

http://www.highperformancepontiac.com/t ... _hose.html

http://www.carcraft.com/howto/57178/index.html

http://www.highperformancepontiac.com/t ... to_01.html

http://highperformancepontiac.automotiv ... -hose.html

http://www.buickpartsdirectory.com/carbs.htm

http://www.73-87.com/7387garage/drivetrain/qjetidle.htm

http://www.geocities.com/gtopercy/Pictu ... tTech.html

http://www.mako.com.au/buick/html/qjet_tips.htm

the fuel/air ratio is only one factor, the intake runner placement in relation to the carbs venturies and the vacuum reading in the plenum and the rate of change in that vacuum during any transition from the primaries alone to all four venturies will effect the results, as will the accelerator pump shot volume and duration.
quadra-jets , generally have decent economy on the primaries, and good low vacuum reading in the plenum at full open throttle
but they are designed mostly for economy, and while they provide the required air flow for w.o.t. operation to provide good peak hp the much larger rear venturies generally DON,T provide the same precise metering that CAN, be achieve with a properly tuned square bore carb
Ive tuned both carb designs for many years and while both designs can provide both decent economy and peak hp I,d be surprised if you find many people that can tune a q-jet to smoothly transition at mid throttle under all conditions as smoothly as a good Holley or demon,its just easier to tune 4 nearly identical size venturies ,that transition smoothly as to the air pressure changes in the plenum that are roughly centered over a plenum,than two sets of vastly different cross sectional venturie areas, that rapidly reduce the vacuum in the plenum and use a large volume of fuel from the accellerator pump circuit to compensate.now obviously a slow transition from just the primaries to w.o.t. allows both designs to compensate reasonably smoothly, but go from cruise to w.o.t. by stomping on the gas pedal and theres a difference in the metering quality of both designs and how they maintain the plenum vacuum and f/a ratio.
keep in mind fuel/air ratio, will be the same if the amount of fuel and air is the same, but if one carbs throwing a fine mist and the others, provided a gulp of air mixed with a stream of fuel the power results will vary

 

[grumpyvette]

Re: simple things, u need to know,setting float levels, etc

if your cars been sitting neglected for a few years and youve drained the old fuel, added new fuel and find the cars hard to start, youll naturally need to go thru some basic checks to see if your getting spark at the plugs, ignition timing needs to be checked, the battery fully charged, the air filters are clean and your getting fresh pressurized fuel to the carb?
If You didn't blow out the fuel lines when you drained the tank, probably want to pull your tank to pump line and see if there is fuel there. If there is, I'd replace the line there , take the carb line loose (this takes two people) crank the engine with a container of some sort under the open end of the line and see if you have fuel there. If you have fuel from the tank to the pump, its a pump problem; if you have fuel from the pump to the carb, it's a carb problem,

only real thing to add is when you disconnect the carbs fuel line you can try blowing air back to the tank but the mechanical fuel pump will most likely prevent that, youll probably need to disconnect the feed side of the pump to blow air thru the line to clear that line
Id sure add a couple more gallons of fuel and a couple cans of injector cleaner to the fuel to help dissolve crud and personally ID temporarily install one of those cheap clear , throw away fuel filters ,

between the carb and mechanical pumps carb feed line, just to see what crud was being pumped to the carb for the first few hours your running the engine

Still in the thinking stage on what carb Tommy and I are going to use on his 383, I am leaning towards a Demon because I think they are a cool looking carb.

the MORE EXPENSIVE HOLLEY and DEMON CARBS also have the ability to be tuned to match the intended air/fuel ratio ,over a slightly wider range of conditions than a standard HOLLEY or EDELBROCK carburetors in my experience., so spending a bit more up front is not always a bad idea.
keep in mind a great deal of how well a carb will run has to do with how consistently the fuel system supplies fuel and which components like jets or power valves or vacuum secondary springs are used.

related info

viewtopic.php?f=44&t=585&p=24111&hilit=demon#p24111

viewtopic.php?f=55&t=211

viewtopic.php?f=55&t=6157&p=19137&hilit=demon#p19137

viewtopic.php?f=44&t=585&p=758&hilit=demon#p758

viewtopic.php?f=45&t=396&p=29211#p29211

viewtopic.php?f=55&t=1790

viewtopic.php?f=55&t=8126

viewtopic.php?f=55&t=5510
these come in handy

NOT ALL HOLLEY AND GENERIC BOWL GASKETS INTERCHANGE

http://store.summitracing.com/partdetai ... toview=sku

http://store.summitracing.com/partdetai ... toview=sku
http://store.summitracing.com/partdetai ... toview=sku

 

[grumpyvette]

Re: simple things, u need to know,setting float levels, etc

Drag Tuning and similar info FROM http://www.smicarburetor.com


A: 1) Be prepared! Make a checklist before hand, and then bring everything else too, because you?ll need it.
2) Make a log! Record the details of every run and any changes you make.

3) Be consistent! Remember details of your run, the burnout, how you staged, how you launch, what RPM you shift. If you can?t repeat this data you?re wasting time and collecting inconsistent data.

Starting with traction: make sure you can get your car to hook consistently and make sure you can put down 3 consistent 60 ft. times before any other tuning! If the car will not hook repeatedly your data will be inconsistent.

Begin your tuning with timing: Generally a motor will like the same timing no matter how the carb is tuned. Start low, say 28degrees total and go up in 2degree increments. If it doesn?t pick up after two increases, Stop. We?ll come back to this.

Timing curve: Generally a motor will like the total advance to come in as quickly as possible as long as it doesn?t cause detonation or pre-ignition. Generally this will be 2200-2800 RPM, depending on fuel.

60 Foot Times: Now that youve got traction
let?s tune the accelerator pumps. Start by adjusting. There should be no slack and also no preload between the pump lever and the pump arm. As soon as you move the throttle linkage fuel should move in a hard ?shot? from the pump nozzle. Cars with hi-stall converters may find that they have to adjust the pump arms to the point that they begin to have tension at the point where the throttle is set when the car is on the line ready to launch. This way it actually gets the complete pump shot when they release the brake and hammer the throttle.

Now you can tune the accelerator pump nozzles: Go up in size until 60ft. times increase. If they increase immediately, go down in size until the 60ft. times don?t continue to decrease.

Now it?s time to Jet: MPH is an excellent way to judge jetting. If a car is lean as you Jet-up you will see MPH increase. When MPH seems to stay the same but E.T. decreases, you?re too rich.
SIDE NOTE- If you Jet-up and get better E.T. and MPH, but your 60ft. times increase, start retuning your Acc. Pump circuit.

Remember Timing? Time to start retesting this too. Go back to 28degrees. Did this hurt or help? If it hurt go to 34 degrees. If that hurt go to 31degrees. Back to your best time? Try 38degrees. No help? Put it at 32degreees and leave it.

4) Don?t get discouraged!

5) Keep Trying New Stuff!

in many or most cases engine dieseling after the ignition is turned off is caused by a combination of a bit too rich of a fuel/air mix being drawn in and a bit too much heat in the combustion chambers this is common when a holley carbs transfer slot is badly adjusted to expose too much slot at idle or carbs with the float level set too high, or a fuel pressure regulator pressure set too high or a blocked return line on the fuel pressure regulator, or at times indicates a vacuum leak

Tuning Tips for Holley Carburetors
Q: Holley 2300/4150 Tuning Tips
A: Float Level ? The fuel level in the bowls should be set at the bottom of the sight hole so that you have to jostle the car to get the fuel to come out the sight hole when running. We set the floats in the shop to a particular setting; but fuel pressure dictates fuel level. Therefore you must set this when you first install the carburetor. Finally, after setting the floats, always give the carburetor time to burn off enough fuel to reopen the needle/seat so that your new setting is really what you are seeing. Many times people lower the floats too much because they haven?t waited a sufficient amount of time for the fuel to burn off and the new setting to take.
Throttle Blade Adjustment ? The throttle blades should be set at an rpm that is as low as possible to keep the transfer slots covered at closed throttle position. Keep the primary and secondary throttle blades adjusted equally. If you should lose your setting just back it completely off, put 1 round in the primary and 1 round in the secondary and start there.
both front and rear float levels on holley carbs with site plugs should be adjusted to the level that just allows the lower edge of the site plug threads to get damp with fuel at idle

Idle Mixture Screws ? Always adjust the 4 idle mixture screws evenly. The settings on all 4 should be the same. Our original setting is 1 1/2 turns out. If for some reason your engine absolutely needs a different setting on one side or corner you can just about bet something is wrong with the engine causing it to have a different signal/vacuum on that side/corner.

Fuel Pressure ? 6.0 to 6.5 pounds. You should absolutely know what your fuel pressure is! Many racers have no clue what their fuel pressure is, and they chase engine problems for weeks when a simple fuel pressure gauge would have indicated the problem straight away.

Vent Tube Clearance ? You must have at least ?? clearance above the vent tubes. It is ok to lower the vent tubes if absolutely necessary, but you will begin to run the risk of fuel spilling over in the turns if you are not careful.

Pump Circuit Tuning - Because of the differences in track conditions and driving styles you may need to adjust the pump circuit, i.e. pump cam/pump arm, to correct "off corner" stumbling issues. If an engine stumbles two or three times after the driver steps into the throttle this usually indicates too much fuel and can be corrected by adjusting slack in the pump arm or installing a smaller pump cam. If an engine has a "dead hesitation" and then picks right up and goes, this usually indicates not enough fuel on the pump circuit. First check that the pump arm has no slack in the adjustment then proceed to increase pump shot with either a larger pump cam or a larger pump nozzle.


Fuel Pressure and Volume Explained
Q: Check Your Fuel Pressure!!!
A: Carburetors love fuel volume, but hate pressure. Pressure creates inconsistent aeration of the fuel in the float bowl, which causes inconsistent metering. Picture in your mind a water nozzle spraying into a bucket; the more pressure used, the more froth and air bubbles are created. If you have proper volume, the optimal fuel pressure is 4PSI for modern 2 and 4 BBL carbs. High horsepower drag care may not be able to run this low because G force works against fuel attempting to travel from the fuel cell in rear to carburetors up front.
So now we have established that volume is critical. Let?s examine factors that effect low volume, and there are many.

1st the obvious:
Fuel Pump capacity
Fuel Line size
Fuel Pump style
Fuel Pump placement

Capacity: Bigger is better. But? rating methods vary; some are rated with no output pressure or restriction. Others are measured at specified output pressure (A.K.A. 110GPH at 7PSI) (These ratings DO NOT specify the size, length, etc. of inlet of outlet size used.)

Line Size: Because manufacturer ratings fail to specify inlet or outlet size, we must assume the maximum size possible was used. Therefore, any reduction in size, no matter where it occurs, will reduce volume.

Beyond size, angles in fuel lines cause restrictions that reduce volume. Every 90degree bend in the system reduces volume by 12%.

Example: Initial = 100GPH
1st 90degrees = 88GPH
Initial = 88GPH
2nd 90degrees = 77.44 GPH

Look at the fuel systems on most all vehicles, race, street, off road, etc. Almost without fail you will find at least 1 90degree bend, plus 45degrees, 30degrees etc. More common you will find 180degrees, 135degrees, 90degrees, 45degrees, one after the other.

So your 100GPH, by the time it gets to the carburetor, could be as low as 20-30 GPH. This is why fuel pump manufacturers rate GPH vs HP at what seems a completely excessive ratio. They realize that they must consider a ?worst case scenario: to safeguard against the most unacceptable installation.

To look at this situation from a completely different standpoint, view the following; Cost of engine: $10,000 for 600HP, Cost of Minimum Required Fuel Pump: $200, Cost of Maximum Required Fuel Pump: $600, Differential = $400.

Possible cost of repairing damage to engine to lack of fuel volume is a minimum of $500 and a maximum of $5,000. By using the best possible component when designing your fuel system, you protect a much larger investment, give yourself peace of mind and invest in something that you can trust to grow with you

High Flow Power Valves are intended for Alcohol applications while Standard are intended for Gas applications and a Gas application will not benefit from the use of High Flow Power Valves and in fact would run worse not better. So for gas applications stay with the Standard Flow Power Valve.


http://www.summitracing.com/parts/PHP-15001/

http://www.summitracing.com/search/Bran ... toview=SKU

http://www.percyshp.com/adjustajet.html

heres a neat little option for the HOLLEY carbs, its not really In expensive but it does allow you to effectively change the fuel flow rates far faster than you could changing JETS, its basically a needle & seat valve that adjusts like the float levels do on a Holley carb, it comes with detailed instructions , and if you use a fuel ratio meter or an IR temp gun and read spark plugs it will allow you to adjust the fuel air ratio far faster and easier than jet changes will

QUESTION
How do I adjust the fuel level on my carburetor?
ANSWER Setting the fuel level should be the first thing you do before attempting to make any further adjustments.The float level should put the fuel level just below the bottom of sight plug hole. You will make the adjustment with the vehicle on a level surface and the engine idling. You will first remove the sight plug, then to make your adjustment you will need to loosen the lock screw on the needle and seat. This will allow you to turn the adjusting nut to raise or lower the float level. Each hex flat on the nut will change the float level approximately 1/32". When you have the fuel level just below sight plug hole you will then tighten the lock screw and reinstall the sight hole plug. Make sure you have a shop towel handy in case you have any fuel leaks from the Sight plug or needle and seat adjusting nut.

http://www.vintagemusclecarparts.com/pa ... lace3.html
these brass inlet filters need to be replaced occasionally as they tend to restrict fuel flow

 

[grumpyvette]

Re: simple things, u need to know,setting float levels, etc

http://www.onedirt.com/tech-stories/eng ... for-cheap/
read thru the link above

 

[Randy_W]

Re: simple things, u need to know,setting float levels, etc

Just an fyi that I picked up in the early seventies, if you need to set the float level on a Q-jet and you don't know the exact factory setting and if you don't want lean bog or black smoke, 11/32" will work on virtually every one of them.

 

[grumpyvette]

Re: simple things, u need to know,setting float levels, etc

"GRUMPY? I have a 355 SBC with a Comp Cams 280H, double hump heads, flat tops, and two AFBs. I bought an advance curve kit and installed the lightest springs. I also bought a vacuum advance diaphram that works off low vacuum. I set initial timing at 18 degrees, hooked up the vacuum advance and had to reset the idle and the timing jumped way up. At 2500-3000 RPM the timing was about 50 degrees or so. I backed the timing off to about four, hooked the vacuum advance back up and it was better. I think I should be running a vacuum advance however it sure is confusing me. I'm wondering about the distributor. Maybe the springs are too light. Also, the throttle response is bad at low speed and it pops through the carburetors. I also know that the carburetors I'm running are too big but I think this is an ignition problem. I'm confused. Any help appreciated. Thanks in advance"



ok one step at a time ,verify TDC and get a timing tape or marked damper cover or what you need to verify timing exactly. go remove the light springs and put in the heavy springs, as a start point, you want a controlled advance, every sub system in the ignition and the carburetor, and fuel supply can be tweaked or checked and tuned to match your needs if you understand the concepts and sub systems

Idle- up too about 2500 rpm try for 14.7:1-15:1 f/a ratio
from about 2500 rpm- too about 4500 rpm try to smoothly and predictably transition the fuel/air ratio mix richer to about 13.5:1
from about 4500 rpm- too about 6500 rpm and higher ,try to smoothly and predictably transition the fuel/air ratio mix richer to about 12.5:1
this is only a starting point on the tune but it generally gets you in the ball park and tends to reduce the chances of the engine reaching detonation conditions.
the ignition advance curve needs to be checked,

get a timing light and ideally a TACH that can be used to monitor engine rpm, and don,t assume tdc AND THE DAMPER OR TIMING TABS ARE CORRECT UNTIL YOU CHECK THEM.
and look thru the links and sub links, they hold an amazing wealth of info but youll need to take some time and read thru them



once you get that set the initial timing at 6-8 degrees at idle and have the springs and vacuum allow the advance to go to about 32-34 degrees total , all in at 3000rpm-3100rpm

the threads below will help, yes I know you would rather pluck you eyes out with a red hot fork,than read links and sub links , but force your self youll learn a great deal, your going to be far ahead of guys who just guess if you know what your looking at and how to tune an engine

viewtopic.php?f=70&t=1015

viewtopic.php?f=70&t=202

viewtopic.php?f=55&t=109

viewtopic.php?f=55&t=8126

viewtopic.php?f=70&t=4683

viewtopic.php?f=70&t=2798

viewtopic.php?f=70&t=3034

viewtopic.php?f=70&t=1809

viewtopic.php?f=70&t=3438

 

[grumpyvette]

Re: simple things, u need to know,setting float levels, etc

Hey guys just a quick tip for anybody considering doing this or has already done it.

About a year ago...maybe longer, I upgraded a 650 double pumper with a genuine holley HP 750 main body.

I am very happy with the results, but now its even better.

I happened to stumble on some info on the net which prompted me to make some changes. A true HP holley carb has a 4 corner idle circuit where as the traditional double pumpers are just two corner idle circuit carbs.

Because of the fact that the HP main body would go in a 4 corner carb, the secondary idle air bleeds are much larger than a traditional double pumper.

The HP main body is supplied with #75 idle air bleeds(in the secondary) whereas a stock 750 would have a #28 idle air bleed in the secondary.

Some folks commented that their engine acted as if it had a vacuum leak after making the upgrade. After finding this out, and being that the traditional dp is a 2 corner idle circuit, they swapped idle air bleeds in the secondary to a #28 or #36 and saw much improved performance.

Even though mine was not acting weird at all, I figured what the heck....I will give it a try.

Given how well the carb was working, I was surprised but I stuffed in some #28's and definitely see better idle quality and throttle response now.

She starts even easier yet too. As I said in my case the engine did not overtly display signs of a vac leak or being too lean on the idle circuit, but with the change.....its definitely improved better than expected.

So for anybody who is doing or has done this swap...change the air bleeds in the secondaries. Heck I had to buy a kit of 10 air bleeds to quickly get what I needed. I got 8 leftover. I'll mail you a pair if ya want em haha.

 

[grumpyvette]

Re: simple things, u need to know,setting float levels, etc

http://www.quickfueltechnology.com/tech ... -operation
Carb Class: Basic Principals of Carburetor Operation

Carburetors, contrary to popular opinion, are a very basic device similar in relative design since Henry Ford and his Model T. The same basic things make them tick no matter who designed it. People over the years have heard horror stories about carburetors or maybe even had a bad experience firsthand (who hasn’t been at the race track and witnessed a carburetor fire in their lifetime?) but that should not keep you from learning how one works. So without further ado, it’s time to dispel the myths, lies, and black magic.

The first thing required for a carburetor to function properly is atmospheric pressure. Pressure is the most important variable tied to a carburetor’s performance, and without it one simply will not run! Most carburetors will have a vent tube that acts as a “port” to the fuel bowl; this “port” provides the carburetor with pressure from it’s ambient surroundings and forces the fuel to move through the metering passages as required based on engine demand. Manipulation or changing the length of the fuel bowl port can have dramatic affects on the fuel curve of a carburetor and should only be done with the assistance of a professional engine dyno. Many people think that fuel pressure is what moves fuel through a carburetor and they are which is incorrect. Fuel pressure simply pushes the fuel to the carburetor, atmospheric pressure takes over from there.

Another phenomenon a carburetor requires is something we refer to as draw. Draw is essentially what the engine wants from the carburetor in terms of air and fuel. When an engine starts going through the RPM range, draw will increase (naturally as engine speed increases air and fuel demand will as well). As draw increases a carburetor must react to properly mix the air and fuel together. Air and fuel mixture is very important and varies depending on the type of fuel you use as well the elevation you are racing at. The word carburetor gurus use for the process of mixing air with fuel is “atomization”. Atomization is where things get tricky and some black magic comes to play; carburetor manufacturers, modifiers and other fuel system related companies are always searching for more efficient ways to atomize fuel and air.

So how does the atomized fuel get to where it needs to be? This is a question that many might know the answer to - the Venturi effect - named after the Italian physicist Giovanni Venturi, the Venturi effect is a phenomenon where pressure is reduced after air flows through a constricted area. The constricted area in questions is easy to spot as it is the skinniest part of the “barrel” on a carburetor. To explain a little further, air rushes past the area with the smallest circumference causing it to speed up and form an area of low pressure right below the venturi, this low pressure will in turn pull (remember our term draw) atomized fuel from the booster venturi and send it along to the intake runners. One thing nearly all carburetors have in common no matter how many barrels is the venturi.

To sum it up; a carburetor will not run without atmospheric pressure, something to mix the fuel and a means of getting the fuel to combustion chamber - these are the basics and the details are where performance is found. Carburetor development has come a long way since Henry Ford’s Model T and each new season of racing brings some innovation giving racers an edge. Still foggy on the principals or want to know more? Give us a call, we love talking about these contraptions and helping people make the most of their race car.