the stats, 8ba with 400 miles. pertronix in a bubba gm dizzy, new epoxy pertronix coil, new wires, new ngk plugs, new stromberg carb. while setting my timing the timing mark will jump around a bit. the engine sort of stumbles when this happens. the mark always jumps to the advance side. i really dont know what to look for anymore and its driving me insane. any thoughts? thanks.
What does the vacuum gauge tell you? I've read on here that you can diagnose just about anything with a gauge and a spark plug reading. I'm no expert though.
Mine did that when my plugs were dying.. I had my carbs set up too rich and it was killing plugs..I think what happened was the resistance in the plugs caused some sort of cross firing in the cap? might be wrong, could be the pertronix.. thats just a guess. I would have a chat to Bubba, he'll no doubt set it right.
I had one (Pertronix) do that on me. Worked fine, removed it to swap motors, put it back in and when I tried to time it, the mark moved around like the sequential tail light on an old Cougar. I gave up and put the points back in.
I've seen timing marks bounce around with a bad harmonic balancer too, but that should not effect the way it runs.
i just hooked it up to a vacume guage as was suggested. at idle which is set to about 500 the needle bounced between 17 1/2 and 19 1/2. hmmmm?
Could be bad bushings in the distributor shaft. (this next one's a guess, as i know NOTHING about Pertronix or other electronic ignitions)....but maybe wrong air gap between the 2 doo-dads that make up electronic crap?
I'd call bubba and ask him, especially about how early the advance starts to come in. If it's already picking up the weights, that can cause the timeing to fluctuate slightly.
IF, you had the distributor out...then you might be off a tooth..just sayin...could make it impossible to time correctly.
Too bad he don't have one of those old Indian motorcycles with the throttle on one handlebar and the distributor adjustment on the other handlebar....LOL....Great little trick..You advance or retard the timing on the "fly" sitting at a light etc...nice little LOPE...LOL
the dizzy is tight and timing is dead on. (except for the jumping around) ive been breathing gas fumes all day. is there any way to test the pertronix? thanks for all the help, i really appreciate it.
I'm not familiar with the term 'bubba' on the GM distributor. If it has the aftermarket Pertronix mounted on the old point plate it could be the worn point plate bushings moving erratically. Worn mounting plate bushings will cause timing issues visible with a timing light.
Bubba is our resident expert on here for ignitions. Do a search on here (GMC Bubba). He is in Vegas this week and may be hard to contact. If you don't find a solution by this coming Monday contact him @ 317-487-9460 after 9am EST
Does it do it with the vacuum advance disconnected? If so, look to the mechanical advance. If you have a missfire, that will cause the engine rpm to fluctuate which will cause the timing to jump, rather than the advance causing the engine to stumble.
Check the clearance between the Pertronix module and the magnet or distributor lobe thru the entire rotation of the shaft. The only time I had this problem was with an Accel distributor. The reluctor was so eccentric that it was clear (maximum clearance) at one angle and would scrub the pickup 180 degrees later. (Accel was little help in solving this, saying I was basically wrong. Jegs took it back even though it was out of their return window- that is why I will buy Jegs..)
i had the same thing happen to me as RR.it was so out or round it wasnt funny.i took it back to summit and bought a msd.
How are your sparkplug wires run? If they are too close together you can get induction (is that right?) firing. Larry T
This. Can you borrow someone's electric timing light with a digital rpm readout to confirm the idle is as dead smooth as possible. With vacuum unhooked the mechanical advance could be flinging out just a touch- check your springs, etc for snugness/tension to rule this out. I agree to check that the engine/plugs/wires are changing the timing instead of other way round rick
thanks, ill have to give him a call and have it tested if it comes to that. no more vacume advane on this dizzy. i had them running through those aftermarket looms. tried to run them stock , still problems. ive tried 4 sets of wires everyway possible. i have a good light that shows rpm. its around 510 and goes to 525 the back down around 510 again and again give or take. i wish it was that easy, this thing is killing me! im starting to think its something else like maybe a valve, i dont know anymore. here is a video just for fun. its not the best quality but you can see the mark move.
Flathead curves start advancing right off idle, circa 500 rpm...likely your distrib is just starting to advance at your idle speed. Vac should be disconnected and plugged while setting initial timing.