TAC Tech => Mechanical => Topic started by: Gene-73 on March 04, 2021, 10:49:24 PM
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At the risk of losing this thread and any responses in the next site crash (sigh), can anyone help troubleshoot this vacuum reading? Getting a bad needle flutter at idle which smooths out at increased RPM.
https://rumble.com/ved8ir-pontiac-vacuum.html (https://rumble.com/ved8ir-pontiac-vacuum.html)
Engine and heads were fully rebuilt in summer of 2018 and installed and running by spring of 2019. Aside from the break-in on the engine stand at the machine shop, it has only been running for a few minutes at a time to move the car around the driveway and garage. Can probably count on one hand the number of times it's been up to operating temperature. Now that the body work is nearing completion I'm turning attention back to the engine. Went to tune the carb and ran into this issue.
The troubleshooting guides I've found almost all point to worn valve guides but that doesn't make sense with a fresh rebuild.
I found one YouTube video from Holley that says this can be an ignition problem. An interesting detail is that I was getting flutter like this, although not quite as bad, before the rebuild. At the time I assumed it was in the valve train, but I am re-using some of the same ignition components as before. The parts that carried over include a Mallory HyFire 6A, Comp Cams XR-i electronic points module, and the spark plug wire set. Parts replaced include distributor cap, rotor, and plugs. Anyone have any ideas on what kind of ignition problem this could be?
I don't think I have any vacuum leaks. I do believe the carburetor has some issues and is probably in need of a rebuild but would like to get this sorted out first. Also, the flutter isn't caused by the gauge sitting on the body. I get the same reading with it handheld.
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looks like sticky valves
https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge#:~:text=If%20the%20vacuum%20reading%20fluctuates,cylinders)%20is%20a%20likely%20culprit.&text=If%20vacuum%20drops%20intermittently%20at,be%20sticking%20open%20or%20dragging.
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I am not sure what kind of "ignition problem" you could have that would be bad enough to affect manifold vacuum but not so obvious that you would not have already been looking for it. Obviously a dead miss would have an effect on vacuum. I don't think the effect would be all that huge even from a dead miss.
If you've got a new cap, rotor, and wires, it's pretty difficult for any ignition events to go to the wrong place. That can be a serious problem at times, but I would say usually you know there's a problem without a vacuum gauge.
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looks like sticky valves
https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge#:~:text=If%20the%20vacuum%20reading%20fluctuates,cylinders)%20is%20a%20likely%20culprit.&text=If%20vacuum%20drops%20intermittently%20at,be%20sticking%20open%20or%20dragging.
Yes, I'm leaning toward this also since it's been sitting for a while. Maybe I just need to get it on the road and drive it to clear it out. But the article you linked also lists "an uneven air/fuel mix, erratic ignition timing, a misfire, misadjusted valves or a manifold leak near one or two cylinders also are possible causes". That's a pretty wide range of potential problems. :-?
I'll probably try a compression test this weekend to see if that provides any further clues.
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Are you running a hot lumpy cam - if yes that will cause the needle to bounce everywhere at idle