Engine miss with vac advance conneted
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Joined: Sep 2012
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Engine miss with vac advance conneted
Hello everyone, I have a 1974 L48 4 speed and I am having trouble with a slight miss when the vacuum advance is connected. I have the timing set to 8 degrees with the vacuum advance disconnected. The idle speed is 650RPM and the engine is smooth. I plug in the vac advance the idle speed goes up to 850 the timing up to 24 degrees and the engine has a slight miss. I have checked the vac advance canister with a vacuum pump and it does not leak and it starts to move at 9" and is fully engaged at ~14" of vacuum. I have even run the car with the vacuum pump operating the vac canister instead of it plugged into the vac port on the carb and have the same results. The car runs great other than at idle no surges or flat spots anywhere in the RPM range. Anyone have any ideas? Thanks
Ps the vac canister has these number stamped 437 15 which from what I have read means 15 degrees advance. This seems to agree with what my timing light says.
Ps the vac canister has these number stamped 437 15 which from what I have read means 15 degrees advance. This seems to agree with what my timing light says.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
24 degrees idle timing with the vacuum advance connected is just about perfect, and the engine should run very nicely right there. If you're picking up a slight miss with the 24 degree timing, chances are good that it's also missing at the 8-degree setting, but you just can't detect it. Use an IR heat gun to detect the cylider with the mniss or start pulling plug wires to see which one is causing you the problem: With 24 degrees timing, it's not the vacuum advance that's causing the miss - it's only amplifying a miss caused by some other issue.
Lars
Lars
Le Mans Master
Joined: Apr 2011
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From: Hermosa
This may or may not apply in your situation but I had a similar problem with my 77 L-48. turns out that the mechanical advance was sticky/ gooey from some old lubricant and not allowing the timing to retard once the rpms dropped. It was random and worse it seemed when warmer but like yours caused a miss at idle. So I could see how maybe without the vacuum plugged in the idle may be fine but once the vacuum advance is applied plus excess mechanical advance it would cause a miss at idle.
Mine also ran fine at all other rpms. Chased it around for a while cleaning rotor and cap checking plugs before I just happened on it when I pulled the rotor and messed with the mechanical and noticed that it was slow to return and sometimes did not return at all via spring pressures. Just cleaned it up with carb cleaner and the miss went away.
Mine also ran fine at all other rpms. Chased it around for a while cleaning rotor and cap checking plugs before I just happened on it when I pulled the rotor and messed with the mechanical and noticed that it was slow to return and sometimes did not return at all via spring pressures. Just cleaned it up with carb cleaner and the miss went away.
