vacuum advance miss

[JFORRESTERSR]

I have a 1972 k5 blazer with newer engine, not sure what year but it was blue so I am guessing late 70s. It has the hei distributor with the coil on top
heres my issue it has a miss at idle but when I disconnect the vacuum
advance it goes away. My first thought was a bad vacuum diaphragm
but then I put a vacuum pump on it and it held so I am not sure where to go
i did a tune up when I got along with a 1000 other things and the old looked like it had not been changed in 100 years my first thought is to replace the whole distributor there pretty cheap but I thought I would ask around first in case it was something stupid I was missing

thanks for the help

[geezer#99]

You might have a vac pot that has too much total advance.
I had one that added 26 degrees.
You can buy and install a simple limiter that’s adjustable.
Like in here.
http://www.crankshaftcoalition.com/w...EI_distributor

Or get lucky and find a pot with less advance.
Or install an adjustable one.
Or adjust the one you have if it’s adjustable.
Or buy a complete new distributor and still have the same problem.

[RustyPile]

What is your base timing setting?? Could be your base timing is too far advanced and the additional timing produced by the vacuum advance brings the timing to an unacceptable point.. Never, ever throw parts at a problem before the root cause of the problem is found...

[rpmerf]

I have one of these:
https://www.summitracing.com/parts/crn-99619-1

It limits the amount of travel the advance has. Here is what I do.
Start the engine.
Disconnect the vacuum advance's vacuum line and cap it. Note the advance
Reconnect the vacuum advance's vacuum line. Note the advance.
Subtract these 2. That is how much your vacuum advance is adding, by setting where it is starting. My engine liked about 10-12* on top of 16* base timing. This makes my total at idle is 28*. If it is more that that, rotate the limiter to push the plunger further. Should be about 2* per notch. Note that when you set the limiter, it raises your base timing, so you will need to lower your base timing.

The real test for vacuum advance is a cold engine. The choke makes the engine run rich, and it needs less advance. If it is good at part throttle on a cold engine, the vacuum advance should be good.

[Sheepdip]

Originally Posted by RustyPile

What is your base timing setting?? Could be your base timing is too far advanced and the additional timing produced by the vacuum advance brings the timing to an unacceptable point.. Never, ever throw parts at a problem before the root cause of the problem is found...

Good advice here^^^^^^^^^^ a miss will show up under stress/load, check the condition of your cap, rotor, wires, plugs etc. including the clocking of the rotor it could be the advance is causing the "spark" to take the least resistance path.

[68 P.O.S.]

What rpm is your idle set at? How much vac at idle? Your vac level may not match your vac advance operating range and its causing the advance to come in and out creating a miss Or maybe even your idle could be set too high and its causing the mechanical advance to come in and out creating the miss as well. Definitely give us your timing numbers, initial and total. Do you have vortec heads on that engine?

[Wrenchbender Ret]

Hook the vacuum advance hose up to ported vacuum. That way it won't advance at idle. It will advance at part throttle.
George

[68 P.O.S.]

You want it hooked up to manifold vac so the vac advance will work as designed. Your engine wants the added advance at idle and cruise for best idle cooling, throttle response, engine efficiency, and fuel economy. Hooked to ported you’re sacrificing these. The vac advance needs to be restricted to 10-15 degrees, mechanical advance springs need to be changed so total timing is all in by 3000 rpm, and the vac line connected to manifold vac. Timing should be at 36 degrees total, 32 if using vortec heads.