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Timing retards with rpm, distributor walk?

2K views 5 replies 3 participants last post by  rmcomprandy 
#1 ·
I recently noticed some pinging around 2000-2500 rpm. Today I checked my timing and something odd noticed as rpms go up my timing advance decreases. I am not seeing the mechanical advance and its actually decreasing until I get up to around 4000 rpm then it suddenly goes.

My setup is at 4500 ft elevation, 79 ford f150 3.50 gears 35 inch tires. 9:1 compression d3 heads. I recurved the distributor when engine was swapped and had 38 total in by around 2500 rpm. I have the duraspark 2 system and using a 13L slot (26 degrees mechanical) 38 total with 12 initial.

Has ran great for years up until recent pinging. Bad gas? I added 91 octane when down to 1/2 tank plus booster and same problem. I usually run 85 octane and always run non ethanol.

This timing decrease is really bugging me. Is my distributor walking?
 
#3 ·
I did disconnect the vacuum advance and plugged it.

Over the weekend I played with the springs and still could not get it to have any mechanical advance. I swapped in a known working distributor and it works and runs great.

I supposed it was detonating because spark was to late at that rpm.

All the internal parts seem to function fine, I rigged up a driver for my electric drill and while spinning the distributor it appears that the weights spread out like they are supposed to. I wish I had a test machine.
 
#5 · (Edited)
It does move freely and snaps back just fine. I have it setup with spring tension just like the working distributor and still cannot get it to advance with rpm. When I intially installed it 3 years ago it worked great.

I hate to use a different distributor because the faulty one has the 13L slot that I want to use. But I fear that if something is wrong with the weights that if I swap it into the other distributor it won't work. I am very mechanically inclined and this baffles me.
 
#6 ·
Magnetic impulse trigger systems usually retard about 1 degree per 1,000 RPM or so, if the distributor is totally locked with no compensation.

I would say that the mechanical advance system within your distributor is not working correctly.
The only times I have ever seen "pinging" be more prevalent with the advance NOT working, happened because of the extra HEAT created in the combustion chamber created by the retarded ignition timing.
 
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