AMC 258 HEI conversion timing issues.

[CheemsK1500]

On my '82 Jeep Scrambler, I just rebuilt the AMC 258, and installed a GM style HEI distributor and a Motorcraft 2100 carburetor. I seemed to have gotten the timing perfect initially, but sometimes it seems to change. The distributor seems tight and does not seem to be moving. What else could make timing inconsistent? Thanks.

 

[SirRobyn0]

On my '82 Jeep Scrambler, I just rebuilt the AMC 258, and installed a GM style HEI distributor and a Motorcraft 2100 carburetor. I seemed to have gotten the timing perfect initially, but sometimes it seems to change. The distributor seems tight and does not seem to be moving. What else could make timing inconsistent? Thanks.

Well a sloppy timing chain, but generally you'll be able to actually see the timing bounce around when your watching it on a timing light.

If the mechanical advance is sticking that can make it appear to change.

If the gear on the distributor or the camshaft is worn.

That's what I can think of. Is this one of the no-name HEI distributor conversions or is this a Davis unit?

 

[CheemsK1500]

Well a sloppy timing chain, but generally you'll be able to actually see the timing bounce around when your watching it on a timing light.

If the mechanical advance is sticking that can make it appear to change.

If the gear on the distributor or the camshaft is worn.

That's what I can think of. Is this one of the no-name HEI distributor conversions or is this a Davis unit?

This is whatever unit JEGs sells (This engine came with a different HEI unit of unknown origin when I got it). The timing chain is new and the timing light does not show evidence of bouncing. Are the Davis units better quality?

 

[CheemsK1500]

It's also worth noting that the subtle timing changes are not instant. They only seem to happen after extended driving sessions.

 

[SirRobyn0]

This is whatever unit JEGs sells (This engine came with a different HEI unit of unknown origin when I got it). The timing chain is new and the timing light does not show evidence of bouncing. Are the Davis units better quality?

Davis vs JEGs? No idea. As I have zero experience with the JEGs units. Certainly Davis would be better than no name brand brought off E-bay. I know Davis is good quality.

It's also worth noting that the subtle timing changes are not instant. They only seem to happen after extended driving sessions.

So how much change is there, and does it advance or retard. Will it go back to where it was after the engine cools off? What about the balancer pully could it be slipping?

 

[CheemsK1500]

The cam shaft and distributor gear are both brand new, I am also currently running a vacuum advance. I'll check the balancer pulley. Thanks for the suggestions @SirRobyn0

 

[SirRobyn0]

@CheemsK1500 I just hope something I said in there will be helpful....

 

[AyWoSch Motors]

On my '82 Jeep Scrambler, I just rebuilt the AMC 258, and installed a GM style HEI distributor and a Motorcraft 2100 carburetor. I seemed to have gotten the timing perfect initially, but sometimes it seems to change. The distributor seems tight and does not seem to be moving. What else could make timing inconsistent? Thanks.

My 82 CJ-7 has a 258 AMC as well, and i too HEI swapped it. I seemed to get it left first shot, runs pretty good. Could just take another stab at it.

An inconsistent idle will do that also. Try holding it at a RPM above idle, and see if it moves.

 

[75gmck25]

One item to check is to make sure the idle is very low (maybe 600-650 or lower), with vacuum advance disconnected and plugged when you measure base timing. Some aftermarket distributors start mechanical advance early and it ramps up quickly. A small difference in idle speed might be enough to start getting some mechanical advance.