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Vacuum advance - advanced question

271 Views 3 Replies 1 Participant Last post by **DONOTDELETE**, Apr 11, 2000

Discussion Starter · Apr 10, 2000

Should the vacuum on the distributer advance be a smooth rise when accelerating? I was checking it with my gauge and noticed that if I stomp on the gas it rises a little, but mostly just bounces around at about 10. If I gas it slowly, it rises nice and clean up to 20. Is this normal? Or does it mean I have a dirty carb? Or maybe even a leaking manifold?

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Discussion Starter · #2 · Apr 10, 2000

Anyone have a guess?

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· #3 · Apr 11, 2000

A slow rise is normal. The quality/brand of the distributor will effect how much the timing jumps around at idle and under slight acceleration. I use a mallory unilite 9000 w/ electric advance, very cool setup. with it you can set up the advance to specific degrees at specific rpm's, works great for bottom end torque.

/wwwthreads_images/icons/cool.gif Big Ed
'88 YJ, 4" susp,3" body,33's,283 Chevy V8,TH350,4.11's,D30,D35c

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Discussion Starter · #4 · Apr 11, 2000

You have two things causing your timing to advance...the mechanical and vacuum. The Mechanical is 'supposed' to be a smooth increase based on engine RPM only. Disconnect your vac advance and then watch the timing while you increase the RPM.

With the vac advance hooked up, you will have full vac advance at idle or with just a little bit of throttle (if using ported vac). As you increase load, the vac advance goes down....this is to reduce pinging. It will be hard to see this in your driveway since you can't really put any load on the engine in park...in fact, you will most likely not be able to see the timing mark with the vac advance hooked up.

Hope this helps....

John......southern CA
84CJ7, 3"lift, 32"BFG, 4.10's, ARB Locker, Solid Axle's, Durabak

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