demon carb trouble shooting, and bits relate to Holley also

grumpyvette · Oct 25, 2008

Tuning Tips and Trouble Shooting

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

http://www.barrygrant.com/fromBarryGran ... ual-08.pdf

http://www.barrygrant.com/demon/default.aspx?page=16

http://www.barrygrant.com/demon/default.aspx?page=5

http://www.barrygrant.com/demon/default.aspx?page=28

http://www.barrygrant.com/demon/default.aspx?page=29

http://www.barrygrant.com/demon/default.aspx?page=38
Chapter

Symptom Remedy
Fuel comes out of vent tube or runs out A) Lower float level
B) Decrease fuel pressure
C) Remove, clean/replace needle-&-seat
D) Inspect float to ensure it's moving freely

Backfires or Pops through carburetor A) Open idle-mixture screws
B) Increase squirter size
C) Increase jet size
D) Raise float level

Backfires or Pops through exhaust A) Lower float level
B) Decrease fuel pressure
C) Decrease jet size
D) Readjust butterfly position, primary & secondary

Engine won't start A) Prime carburetor with fuel
B) Pump squirters to get fuel flowing
C) Check ignition timing

Fuel leaks from throttle shafts A) Lower float level
B) Decrease fuel pressure
C) Readjust butterfly position, primary & secondary

Is rich at idle A) Increase initial timing
B) Go in on mixture screws
C) Readjust butterfly positions
D) Lower float level
E) Decrease fuel pressure

Stumbles under light acceleration A) Open mixture screws
B) Readjust butterfly positions
C) Raise float level

Stumbles under hard acceleration A) Readjust butterfly position
B) Increase squirter size

Won't return to idle A) Increase initial timing
B) Readjust butterflies

Surges at part throttle A) Open idle-mixture screws
B) Open primary butterflies & close secondary
C) Raise float level
D) Increase jet size

Emits black smoke under hard acceleration A) Lower float level
B) Decrease fuel pressure
C) Decrease jet size

READ THRU THESE THREADS AND SUB LINKED INFO

https://www.youtube.com/watch?v=HoR0uZL ... AABE37CC30

viewtopic.php?f=55&t=211

viewtopic.php?f=55&t=635

viewtopic.php?f=44&t=621

viewtopic.php?f=55&t=109&p=6685&hilit=+tuning#p6685

viewtopic.php?f=55&t=1790

viewtopic.php?f=44&t=777

grumpyvette · Oct 26, 2008

Re: demon carb trouble shooting

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HOLLEY and DEMON carbs are generally much easier to tune, trouble shoot and adjust than most of the others, the demon carbs have a small advantage in that they offer more adjustments and finer metering,in some cases and a significant number of small parts are interchangeable between SOME demon and HOLLEY CARBS now that's a two edge sword, if your a klutz it means your options allow more areas to screw up, but once you've got the basics down , the HOLLEY and DEMON carbs are generally much easier to tune correctly

let me say this, if you can follow directions and read plugs and do basic carb adjustments the HOLLEY and DEMON carbs make tuning the engine correctly far easier than something like a carter, quadrajet or edelbrock carb that, has less flexibility ID ALSO POINT OUT
that many "carb" problems are really related too or caused by ignition or cam selection or intake manifold selection choices, or matching those parts to the wrong compression ratio, rear gearing, or stall speed

max powers usually at about a 12.6:1-13:1 a/f ratio but for max mileage your looking at about 14.7:1-14.9:1 because you want the car to get decent mileage and keep the plugs clean under cruise conditions, ID try to get WOT a bit richer to reduce detonation worries,and below 4000rpm keep the F/A ratio a bit leaner, I usually try to stay at about 14.2:1 up to about 4000rpm then gradually go richer as rpms increase as it tends to reduce detonation tendency's and a larger accelerator pump and cam and power valve can cover the required f/a needs under hard acceleration vs cruising

its hardly the carbs fault if you select a great Holley or demon carb in the 700cfm-850 range and stick it on an engine combo that's basically a collection of mismatched components and then find its difficult to get it running correctly

as an example,
I had a neighbor who constantly played with his classic mustang 390 ford engine, claiming his carb was nearly un-adjustable, I quickly found the problem, his fuel presure was 3 psi , his fuel filter was mostly clogged,and his ignition didn,t advance smoothly, as the rpms increased yet he cussed that carb for months, be fore calling me over to look at his car after swearing to everyone who could hear , his carb sucked!

If a symptom comes and goes, rather at random, ID start looking at the plugs for indications of whats going on in the cylinders. ID do a compression/leak down test to verify its not mechanical in nature, Id verify the fuel pressure was consistent, Id open the carb and check internally for crud or broken parts, Id check the float levels and set the idle adjustments and if I had a F/A meter ID be using it, to look for indications, and Id use a timing light to verify the timing curve advance runs smoothly, a VOM to do a few checks on voltage and resistance on ignition components , like plug wires and check the battery voltage blunder load,and and Id use an IR temp gun to verify the exhaust temp is reasonably consistent between cylinders as the first checks, you can,t cure the problem until its isolated and clearly defined
I recently worked on a big block corvette, its got a 750 cfm vacuum secondary demon carburetor , on the stock 4 barrel intake , and it was running like crap, I checked his timing and fuel pressure then noticed the accelerator pumps and squirters were not functional, so I adjusted the nut on the spring loaded lever that controls the plunger stroke and got zero change,so I removed the 4 idle adjustment screws each one at a time and used the straw on the spray nozzle to squirt cleaner into each needle seat under pressure then re set the needle air valves at out one turn, then squirted about 1/4 of the can into each float bowl thru the side sight port after temporarily removing the glass site plugs, then I removed the front squirter and sprayed carb clearer into that opening then replaced that squirted and reset the carbs front and rear float levels
then I power squirted the 4 front and four rear air bleed openings to remove varnish and crud.from the 4 boosters, the result was the car started running significantly better and the accelerator pump and boosters started flowing fuel correctly

if youve ever wondered why Ive suggested no one use the cheap glass fuel filters, its because they are prone to leak, with less than safe results.