low vacuum reading advice requested

[pluslt]

Hey there Grumpy,

So following your advice and trying to do some of my own troubleshooting. What is spurring this on is that I just don't feel like the engine in my L48 '74 is putting out the kind of power/performance that it should. It was rebuilt approximately 3 years ago and has had about 17K miles put on it since then (at least this is what I've been told by the PO).

So I had it in the shop a month or so ago and they adjusted my timing and installed a new carb gasket. Definitly impoved the running of the car but still lacking something I feel.

So I bought a vacuum gauge and hooked it up today. What I'm getting is a needle that hovers(flutters is actually more like it)right around 8-9 inches. According to this website:

read thru these links
http://www.secondchancegarage.com/public/186.cfm

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

That means either a leaky intake manifold or late timing. To check for an instake leak it was recommended to spray starting fluid or something similar around the intake gasket. I used carb cleaner (was told by a guy at Autozone it would tell me the same thing) and sprayed it around the outside. Got no increase in RPMs or smoothing out of the engine. So.....am I left to believe that my valve timing is off? What other test should I do to verify that is the case before I get into having someone fix the valve timing (which as i understand it is a whole other can of worms.

I took my readings off the hose that goes to the PCV. Right off the PCV is a T joint that I used. I took two pictures to show you what I did where I did it and what reading I got. Just so it could be verified that I did it right.

Thanks in advance for your thoughts.

Steve

 

[grumpyvette]

If your local to west palm beach bring it over and Ill help you isolate the cause

vacuum gauges must be connected to manifold vacuum

viewtopic.php?f=50&t=821&p=1212#p1212

http://www.secondchancegarage.com/public/186.cfm

viewtopic.php?f=62&t=882&p=1390&hilit=+propane#p1390

viewtopic.php?f=50&t=723

http://www.corvette-101.com/vacuum.htm

first youll want to verify theres no vacuum leaks,youll use a un-lit propane torch not any sprays as its far more precise, you can stick the torch tip exactly where you suspect leaks,and it less messy and leaves no residual crap on the engine.
ITS important to understand how and why things work,IF you don,t have a timing light, generally you use a vacuum gauge on a STOCK engine to set timing by hooking it up to plenum vacuum , set the idle to normal idle speed (usually 670rpm-850rpm)and adjust timing to max vacuum reading, this DOESN'T,T ALWAYS result in the best possible timing but it will generally be fairly close on a DEAD STOCK ENGINE

next, Ive got to ask if you've
done a compression check,
set the ignition timing with a timing light to factory specs, if you've verified the damper and timing tab truly do show TDC or are you just assuming everything correct, and have you adjusted your valves while its idling?
its very common for guys to adjust valves incorrectly on a cold non-running engine
if YOU didn,t degree in the cam theres a good chance its installed incorrectly and if youve never pulled the valve covers and closely inspected the valve train theres a good chance the cam might have worn lobes

 

[pluslt]

If your local to west palm beach bring it over and Ill help you isolate the cause

viewtopic.php?f=50&t=821&p=1212#p1212

viewtopic.php?f=50&t=723


How to tune a carb with a vacuum gauge.
http://www.centuryperformance.com/tunin ... g-148.html


How to interpret a vacuum gauge.
http://www.secondchancegarage.com/public/186.cfm


first use a un-lit propane torch not any sprays as its far more precise, you can stick the torch tip exactly where you suspect leaks,and it less messy and leaves no residual crap on the engine

next, Ive got to ask if you've
done a compression check,
set the ignition timing with a timing light to factory specs, if you've verified the damper and timing tab truly do show TDC or are you just assuming everything correct, and have you adjusted your valves while its idling?
its very common for guys to adjust valves incorrectly on a cold non-running engine
if YOU didn,t degree in the cam theres a good chance its installed incorrectly and if youve never pulled the valve covers and closely inspected the valve train theres a good chance the cam might have worn lobes

Thanks. I have done none of those things. I'll go back and try the test with the propane. I just bought this car a couple months ago and have done no work to it myself so I don't know if all of those things were done or done correctly. In all honesty I know little about engines so to do the rest of the items on your list ( including setting the timing) I'll have to have someone else do it. As I have zero working knowledge on how to do any of it. Suppose I could read a book but I am way more of a show me person than someone who can read a how to then put it into action.

Unfortunately I do not live in or near Florida. I'm up in Maryland. Thank you for the offer though.

 

[grumpyvette]

you'll always be at the mercy of repair shops unless you get a few basic tools and some basic understanding of how each system on the car functions
a decent timing light won,t cost more than about $40-$50 and you can commonly buy them at yard sales for $10
adjusting valves is really simple once you understand the process, Id strongly suggest getting a timing light and a few basic tools and a CHEVY SHOP MANUAL FOR YOUR YEAR CORVETTE!

READ THRU THESE LINKS

viewtopic.php?f=70&t=1411

viewtopic.php?f=52&t=196

viewtopic.php?f=50&t=383

viewtopic.php?f=50&t=821&p=1212#p1212

http://www.helminc.com/helm/product2.as ... JCAQPL3BB5
this also helps

http://www.delmarlearning.com/browse_pr ... cat2ID=CHM




A rough idle is sometimes a result of a vacuum leak.

There are typically tthree vacuum sources (on earlier muscle car engines) that are in addition to the more common
intake and carb, sources that can cause a vacuum leak, these are the vacuum booster for the power brakes and the intake vacuum line running down to the vacuum modulator on the transmission, and in a few cases vacume lines running to fuel presure regulators that compensate for vacume readings.
You will need to have plugged each line individually to test each potential source, or vacuum line, you should disconnect and plug the vacuum modulator line, to the transmission, the power brake booster line, and on some later cars theres vacuum lines connected to the emission controls or auto cruise controls and see if each change helps the idle.

A vacuum modulator on a turbo -350 and turbo 400 style trans can start to leak both vacuum and suck transmission fluid, up into the intake. If you have been losing trans fluid, or the exhaust smokes under high vacuum readings it could be transmission fluid being sucked through a defective modulator up into the intake - this will DEFINITELY cause a rough running motor during idle, and smoke from the exhaust and spark plugs to be discolored on runners near the vacuum line connection to the intake manifold.

naturally youll want to use the unlit propane torch, to locate intake leaks, on the carb or intake gasket, but remember that improperly machined head or a badly machined intake , or improperly installed intake gasket can suck oil from the lifter gallery
Don't anything is good or bad till its isolated and tested good or bad. never assume anything without testing.

Your vacuum reading will depend on the displacement, compression ratio and cam timing-