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09-28-2020, 01:06 AM | #1 |
Senior Member
Join Date: Oct 2015
Location: Brownsburg, IN
Posts: 1,072
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Timing is Bouncing Around
I have a pertronix small body electronic distributor and a 40000 volt flamethrower coil in my truck. When I check or set my timing, the timing mark at idle is not constant and steady. The mark is fluctuating maybe a degree most of the time but also randomly bounces approx 10 degrees every 10-15 seconds. This is causing the tachometer to bounce around also. This bounce shows up with the vacuum advance hooked up or shut off and carb vacuum line plugged.
I'm also seeing this timing bounce when you increase the engines rpm's. Say at a steady 2000 rpm's, the timing mark is way advanced on the harmonic balancer(like it should be), it holds steady and then I see that random 10 degree bounce. With a vacuum gauge hooked to manifold vacuum, the vacuum needle is constantly moving back and forth just a very small amount( maybe less than 1/2 inch of vacuum). I think the vacuum movement is caused by the timing bounce that fires a cylinder at the wrong time. What I've done: new distributor cap, new rotor, new plug wires, ran a new 12 volt supply line to the distributor, checked the distributors grounding, new coil. The pertronix distributor is basically new with maybe 400 miles of use on it.The trucks original sbc engine was completely rebuilt during the trucks build. When driving the truck at speed, the engine feels good, you only notice the tach bouncing some. However, at idle you can feel a miss that is random and more than likely in concert with the timing bounce. What is also wierd is that sometimes this issue will disappear for 5-10 miles and then start again. When it disappears, the tach is perfectly steady and you also feel nothing happening at idle. Any suggestions?
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1972 Blazer Highlander 1971 GMC K1500 SWB (sold) |
09-28-2020, 07:48 PM | #2 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,477
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Re: Timing is Bouncing Around
Might try disconnecting the alternator and see if the problem goes away. Thinking if some aspect of the charging system is sending voltage spikes, they might be resetting the distributor control module and it takes it a couple of revolutions to reboot and figure out proper timing again.
Same thought for any other systems in the truck that might draw lots of power. Never hurts to double check the battery cable connections and any junction point connections. |
09-29-2020, 04:18 PM | #3 | |
Senior Member
Join Date: Oct 2015
Location: Brownsburg, IN
Posts: 1,072
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Re: Timing is Bouncing Around
Quote:
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1972 Blazer Highlander 1971 GMC K1500 SWB (sold) |
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