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Old 06-28-2008, 09:15 AM
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vacuum advance sticking?

I think I have a bad or sticking vacuum advance. When I suck on the hose I have to suck really hard to get it to move. No you can not come over. The first time I did this it would not move I moved it with a screw driver and now it moves but takes allot of effort. It does not to have a leek. The reason I ask is at 5600-5700 rpm it looses power pops or backfires and then the power comes back and goes to 6300 no problem. It only does this in gear and not when I rev. The second thing is and this has been going on for some time. I set the idle at 1000 (800 in gear) drive it and some times it is at 1300 rpm when I get back. Next week it is at 1000. Any ideas?

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Old 06-28-2008, 09:31 AM
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Quote:
Originally Posted by tomthecomic
I think I have a bad or sticking vacuum advance. When I suck on the hose I have to suck really hard to get it to move. No you can not come over. The first time I did this it would not move I moved it with a screw driver and now it moves but takes allot of effort. It does not to have a leek. The reason I ask is at 5600-5700 rpm it looses power pops or backfires and then the power comes back and goes to 6300 no problem. It only does this in gear and not when I rev. The second thing is and this has been going on for some time. I set the idle at 1000 (800 in gear) drive it and some times it is at 1300 rpm when I get back. Next week it is at 1000. Any ideas?
Your screen name surely fits.

When I read the first couple of sentences I was ROFLMAO!

If you can suck strong enough to pull enough vacuum to really move a vacuum advance unit much; you would have some great lungs (and be very popular)

Under hard acceleration under a load there will be very little vacuum (if any at all) and the vacuum advance will add little or nothing to the overall timing. This is why it gets disconnected and plugged when actually checking / setting the timing.

Your loss of power could be caused by several other things such as lack of air, incorrect timing, plugs, valve float, over revving, etc. A stock GM HEI is pretty much at it's limits at the RPM range you posted.

Your idle speed is high enough that some mechanical advance is more than likely being induced. This will cause you to set the initial timing incorrectly. If you can't get the engine to idle around 650-700 RPM in gear; then set the Total Mechanical Timing so that it is "all in" between 2500-3000 RPM and let the initial fall where it ends up.
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Old 06-28-2008, 10:25 AM
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No I don't want to come over! LOL

My thoughts are the distributor is binding up, not the vacuum advance. I have come across this problem before.
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Old 06-28-2008, 11:38 AM
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Quote:
Originally Posted by tomthecomic
I think I have a bad or sticking vacuum advance. When I suck on the hose I have to suck really hard to get it to move. No you can not come over. The first time I did this it would not move I moved it with a screw driver and now it moves but takes allot of effort. It does not to have a leek. The reason I ask is at 5600-5700 rpm it looses power pops or backfires and then the power comes back and goes to 6300 no problem. It only does this in gear and not when I rev. The second thing is and this has been going on for some time. I set the idle at 1000 (800 in gear) drive it and some times it is at 1300 rpm when I get back. Next week it is at 1000. Any ideas?
Idea:
First your carb throttle shaft could be worn and binding the butterflies.

Your irregular idle could be caused by the centrifugal advance not coming back to zero easily. Did you check the spark advance while this is happening?

The centrifugal spring needs to be a tad tighter so the advance does not begin until 1300 rpm. Using one tight spring and one loose spring set with slack at the post to delay its tension. This gives the overall tension of both springs the same as 2 equal springs so the total advance comes in by the same rpm, but the intial beginning starts later. This should stop the irregular idle if it is advance related.

Vacuum advance cans come in different total degrees of timing, some adjustable, and they also come in different vacuum values for total advance, and different vacuum values to begin the advance. This has to be coordinated to your engine combination.

A person can suck about 3" of vacuum. Most cans don't move at that value. Screwdriver movement should take a lot of effort. There is a stout spring in there for vacuum to overcome.

Popping. Are you sure you are not sucking air at the fuel tank on hard acceleration, and the carb gets temporarily lean when the air bubble gets there. It also sounds like wires or ignition module or coil breaking down.
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