Fun with vacuum leaks
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From: Mokelumne Hill
Fun with vacuum leaks
Been working on a 1976 Camper Special with 390 and Holley 4bbl. carburetor. Began with 13 inches of vacuum and slowly have found leaks in vacuum lines and increased the vacuum signal from 13 inches and running crappy to 16 inches and idling fairly good and fair throttle response with the timing at 8 degrees advanced with the vacuum line disconnected and plugged to the distributor canister. Still idles 100 RPMs over factory spec. O miles on fresh motor and rebuild on the carb. California emissions truck unfortunately. I still feel there is a vacuum leak as idle speed screw all the way out and throttle blades closed on primaries and secondaries. Oddly enough turning the idle mix screw all the way in on the passenger side has zero effect on idle quality. left side mix screw definately does however. Is it possible to have an internal vacuum leak in Holley carbs? I have worked on many and never ran into this before so unsure. Also do not know where to start looking. I have heard of warped metering blocks but do not see how those can warp. I straight edged everything and it is flat including the carb body gasket surface. Seems to me the generous gasket thickness would take up any imperfections. Hopefully some Holley guru can get me on the right path. Also I have ran the motor with all vacuum lines capped off.
Thanks,
Thanks,
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
This is a fresh rebuild engine with 0 miles on it correct?
You need to drive it, get it under load right away so compression can come up - piston ring seat. The idea is to avoid idling and messing around with carb adjustments, ignition timing &c because, the cylinder walls will glaze over.
You need to drive it, get it under load right away so compression can come up - piston ring seat. The idea is to avoid idling and messing around with carb adjustments, ignition timing &c because, the cylinder walls will glaze over.
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From: Mokelumne Hill
Used cast rings-they are seated fine. Compression is even 150 psi in each cylinder. I can add a few more degrees of initial timing and the vacuum signal goes to 18 inches and needle is smooth as it is running at 16 inches at present. Just concerned about what appears to be a vacuum leak internal to that Holley Carb as the passenger side idle mix screw is not changing the idle quality. New carb is over 500 bucks so trying to avoid that. So back to the original question are there any Holly gurus here that have dealt with a non operative idle mix screw and what did you do to get it to work ? Will not pass Kali smog test like this.
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
No such thing as an internal vacuum leak on a carburetor as such, though if the idle mixture screws do not have an effect it points to a loose or defective power valve or wrong gasket maybe.
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
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From: Mokelumne Hill
Correct and that is why I am suspecting an internal vacuum leak with all throttle blades closed and idle 100rpms over spec. No head surfaces have been milled nor the block surface for that matter. So the intake to head intake port is fine and to make sure I shot starting ether into the lifter valley with zero change in idle quality. Beyond that all factory gasket surfaces were straight edged and are flat including the throttle base plate and base of the carb itself. I do not think I have overlooked anything knowingly-it is the unknowingly I am after and have hit a dead end. So off to Holley tech to see what I can learn over there. Seems there has to be a known fix for an idle mix screw that doesn't affect idle quality considering the millions of Holleys sold and screwed with.
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Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
What I mean by that, is there's no such thing as an "internal" vacuum leak, but there is such a thing as plain ol' vacuum leak. The standard test for a defective power valve is to see if the idle mixture screws will stall the engine. Also check to see if idle transfer slots are over-exposed:
Why is my engine running rich and my mixture screws do not have effect?
"The first thing you need to check is your float level. No fuel should run from the sight hole unless you shake the vehicle. Next check the engine vacuum at idle. (in gear if A/T) if it is 12" or more a 6.5" power valve will usually be fine. Anything below 12" divide in half. For example 9" vacuum will use a 4.5" power valve. Another possible cause associated with performance camshafts are exposed transfer slots. You should not see more than .025"-.030" of the transfer slot exposed past the throttle plate at idle."
https://www.holley.com/support/faq/
Why is my engine running rich and my mixture screws do not have effect?
"The first thing you need to check is your float level. No fuel should run from the sight hole unless you shake the vehicle. Next check the engine vacuum at idle. (in gear if A/T) if it is 12" or more a 6.5" power valve will usually be fine. Anything below 12" divide in half. For example 9" vacuum will use a 4.5" power valve. Another possible cause associated with performance camshafts are exposed transfer slots. You should not see more than .025"-.030" of the transfer slot exposed past the throttle plate at idle."
https://www.holley.com/support/faq/
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From: Mokelumne Hill
Transfer slots are fine, stock camshaft in this FE, so no big overlap. Fuel level has been checked and rechecked. Has a new 6.5 power valve that came in the kit. Said valve has been checked with a MiteyVac and is good. Still trying to figure how to get that passenger side idle mix screw operational as I feel that will lead to a fix. Engine not running fat so I feel is close to the air/fuel ratio.
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
16" of idle vacuum is a little low... A healthy engine with stock cam should pull around 20" ... at Sea level. If you live in Denver, you're doing fine. What's your elevation?
Just for grins assuming you've a flatland location try advancing distributor timing for max steady vacuum. Should be close to 20". See if that helps with the idle. Idle circuit depends entirely on healthy manifold vacuum for a crisp signal through it because hardly any air flows through carb at idle. Retarded or late ignition timing reduces vacuum signal through carb and carb won't respond as well. Just spitballin' here but easy to try. If damper outer ring has slipped over the years for example timing marks will be innaccurate, leading to ignition timing errors.
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From: Mokelumne Hill
Elevation is 2700 feet. As I mentioned if I crank in some more advance the vacuum goes to 18 inches with a smoother idle (No surprise). But no smog shop will pass it like that in Commiefornia. With respect to a slipped outer ring-checked that on teardown and it was spot on. Factory pointer as well. I guess I will try advancing the initial timing up to 18 inches then see if the idle mix screws will kill it all for grins and not expecting much.
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
Right, optimal timing (best efficiency) spikes NOX emissions relative to CO and HC. Bigtime Smog producer. That sure is weird though. Have a different carburetor you can try? Sounds a little like there is a restriction in one of the idle circuits.
