kevdog442

69 400. Having an issue at 2-2500 rpm, engine starts to misfire. Runs nice and smooth with vacuum advance disconnected but once I connect it, it misses. Timing is advancing fine, will go up to 50 btdc or so. It’s just rough. Running pertronix now, but it did it with points and a different distributor. I’ve tried different coils plugs wires etc. but like I said it runs great with just mechanical advance, just doesn’t advance as far.

oldcutlass

Which Pertronix are you using? What is your mechanical timing set too, both total an initial? What you probably need to do is to limit the amount of vacuum advance the canister is providing to 10*.

kevdog442

this is the lobe sensor type, mechanical is around 10* initial I’ll have to check how high it goes with just mechanical

kevdog442

If it matters, it’s getting vacuum from the front port of the quadrajet

Vintage Chief

Depends on which Q-Jet carburetor you have and to which inlet the vacuum hose is connected. Above the throttle plates is ported vacuum, below the throttle plates is manifold vacuum. My preference is full manifold vacuum.

kevdog442

it’s ported, original 69 carb

BangScreech4-4-2

Try manifold vacuum. What do you have to lose?

danktx

On my car 50* advance was too much. I had a misfire so I made a limiter strap out of a piece of hacksaw blade that covered about half the slot in the in the vacuum advance can. I ended up with 11* vacuum advance and no misfire.

oldcutlass

Once you're out of the idle circuit there is no difference from manifold to ported vacuum.

matt69olds

Put the advance on manifold vacuum. be prepared to have to reset the idle speed.

if it misfires only with advance functional, and it’s really going to 50 degrees, you most likely need to limit the amount of vacuum advance. You can easily fabricate a limited tab, crane cams use to provide one with their recurve kits

I bet if you limit the total timing to around 42-45 degrees it will run much better

kevdog442

that’s my thinking. It’s a new engine, new cam…

Fun71

This is the adjustable stop that I made for my 24 degree vacuum advance canister.

kevdog442

I like that. I actually tried a different distributor that had less advance movement but still no better. May have to go adjustable. This cam is not that radical, didn’t expect these issues
Erson .477/ 290 dur. single pattern

Lonnies Performance

My car had a part throttle misfire, which happened with timing over 45deg.
Limit your vac advance as I did & problem will go likely away.

matt69olds

If you put the limiter on the other end of the canister arm, it will accomplish the same thing but also increase the amount of vacuum it takes to get the advance to begin to move. That’s sometimes an advantage of the canister provides a lot of advance, and with a weak spring.

You just need to remember to reset the timing. Putting the limiter on the other side will increase the initial timing because the starting point of the advance has moved.



Check this out.

kevdog442

Ok so it looks like I can fix this issue, but curious as to why this is happening. Is it the cam?

olds 307 and 403

It could be, did you increase compression as well? The stock carb could be lean with the more aggressive new cam.

kevdog442

yes newly built engine 10:1 pistons