Falling on face/missing/backfiring (through intake)?

[dmjlambert]

I am curious what makes you suspect but not know vacuum advance diaphragm is blown? If you have the engine idling and you connect and disconnect the vacuum advance from a manifold vacuum source, you should notice quite a difference in idle speed. Put your finger over the vacuum port on the carburetor when you don't have the hose connected, so you don't have a vacuum leak. I didn't have that difference in idle speed on mine and that's how I knew the vacuum advance was bad, and I replaced it.

[Ride The Snake!]

Thank you guys very much. Food for thought. And thank you for that vacuum advance spec sheet. I think I stumbled across that same .pdf a couple nights ago and have it somewhere in one of my browser tabs.

I had previously been running the truck with the ported vac port plugged on the carb, but I reconnected it to the canister before my test drive yesterday after playing with the idle mixture and timing. I will plug it again for now and see how it goes.

True enough about the timing tape but for 5 bucks I figured why not. I was contemplating measuring and marking the balancer at 36 degrees for now to check my total timing.

Yes, my mechanical advance weights are moving freely. I opened the dizz and checked em, and during operation I can see the mark on the balancer advance when revving the motor. I do not know the specs of my mech advance kit, but iirc I once looked at them and found numbers stamped into the weights. Is there a spec sheet somewhere for these weights? I guess there's no way to know exactly which springs I have to know the shape of the curve without measuring its behavior somehow on my motor.

I might have measured my balancer incorrectly, but using a tailor's measuring tape to get the circumference and calculating for diameter, my balancer came out to 6.17" diameter; that can't be right. Can anyone confirm if these GM balancers are 6.25" or are some of them 6.375"? I would hope I wouldn't be so inaccurate as to be off by a whole 3/10" (in the case of a 6.375" diameter balancer), which corresponds to a circumference discrepancy of ~.64" between that and which I measured.

[Ride The Snake!]

Originally Posted by dmjlambert

I am curious what makes you suspect but not know vacuum advance diaphragm is blown? If you have the engine idling and you connect and disconnect the vacuum advance from a manifold vacuum source, you should notice quite a difference in idle speed. Put your finger over the vacuum port on the carburetor when you don't have the hose connected, so you don't have a vacuum leak. I didn't have that difference in idle speed on mine and that's how I knew the vacuum advance was bad, and I replaced it.

I have only had the vac canister connected to the ported vac source on the carb, which wouldn't have any effect at idle anyway. My suspicion that it was blown was that I didn't see any discernible difference in advance movement while revving the motor and shining my timing light, plugged in vs disconnected. But that's a good suggestion; I'll hook it to the manifold source and see if it affects the idle.

[BigBird05]

Do you have a hand vac pump. Hook it up and look at the vac advance to see if it moves and holds or does it leak down.

[tim_mc]

I left my Mity-Vac at my dad's shop, so used lung power to make sure my vacuum advance diaphragm was working and the link wasn't binding after switching out the canister. You might not get much of a gas taste if you use a new section of vacuum line.