350, lopey cam, Holley 750 DP, Delco Dist w/ electronic pickup no vaccume advance.

It runs fine (it seems) until you hit 3500+ RPM, then it loses power, runs rougher and pings. Does not ping at lower RPM even when lugging the motor.

Timing is set at 12* initial, up to 32* at 3000ish. Then at higher RPM the timing mark jumps around very erratically and looks like it might jump substantially higher than 32*.

The distributor shaft has no lateral play in it, so it doesn't seem that the bushings are worn.

Anyone got any bright ideas? I'm stumped! :mad:

Have you looked inside the distributor for loose parts or connections?

Onovakind67 - Yup, it all looks fine to me. The electronic pick up under the mechanical advance can be wiggled just a little bit, but the mounting plate it's on is secure, so that doesn't seem like a big deal.

I should mention that I'm running 10 inches of vaccume at idle. The needle jitters between the 10-11 marks. But when you speed up the motor, it smooths out. So it doesn't seem like a vaccume leak.

Are the bushings on the advance weights in good shape. I didn't pick up on what distributor you were using, but HEI ignitions in particular are prone to wear out the bushings and allow as much as 50* advance :eek:

Could spark plugs be fouled? I say this due to a problem I had. I chased the same problem around for days. Would run perfect up to 3500 to 4000 or put the car on a load. Over that rpm the car would break up real bad, ping, ect. Finally changed the plugs after replacing many other things. The old plugs were also new but had flooded them durring a startup. Just a thought.

Just read that post again. This shouldnt cause your timing to jump up so high. I would lean back towards the dist.

Back a few years ago we was troubleshooting a buddys late '70's dodge van. It idled dead smooth and ran great at low speeds around town. Get it out on the highway and it bucked and snorted like crazy. We suspected a fuel delivery problem, and changed the pump, filter, and removed the sock out of the tank. That didn't help. The very first thing we did was checked the distributor for excessive play...it had seemed very plenty tight. We ended up replacing the distributor and that fixed all the problems. Apparenetly it doesn't take much wear in the shaft bushings for them things to get sloppy at higher speed. You might try another distributor if you have access to a known good one...or take yours and have it run on a distributor machine.

I'm using an old Delco distributor. The bushings on the weights seem solid. The MSD cap doesn't fit quite perfect and you can wiggle it just slightly on the distributor base. But it doesn't look like it moving at high RPM.

Is there anything BUT the distributor that can cause the timing to jump around like that?

timing chain?

Definitely could be the timing chain. What does the timing do then you try this....hold it at a steady 2500 or so, the let off. When you let off does the timing jump quite a bit then settle down?

Also, I have no idea how this affects the timing mark, but years ago I had similar problems. The thing would break up at higher RPMs. This was "back in the day" and I was running graphite suppressor core wires. The header heat would break the things down in no time. I swapped wires and the problem was solved temporarily. My permanant fix was to go to solid copper core wires. I never checked the timing with a light so I am not sure if the wires breaking down would cause the mark to jump, but it sure made it run bad. Just a thought....

If its a converted point type distributor then I would suspect worn shaft bushings, but if it's an HEI It's possible that the rotor isn't lined up with the terminal on the cap. The pick up can be off enough so that at low rpm the spark makes the jump but a higher rpms it causes eratic spark. The only way to fix this is to losen the vacuum can screws and move the mechanism one way then the other to see if it clears the problem up. There is a way to diagnose this but it means destroying a cap to do it. Cut a large hole in the cap near one of the terminals so that you can see the terminal and rotor clearly. Then with a timing light start the engine and observe the rotor to see if it's early, late or ok. I think by the way you describe it, this problem is distributor related.
Hope this helps.
Jim

Greg O - it sounds like a gear drive so no timing chain. And the wires are recent high performance, though I don't think solid core, so I doubt they're the probelm.

GMJim - I think it was a points-type converted to electronic. The shaft doesn't seem to have an side-to-side play, so is there another way to check the bushings? The rest of what you describe sounds just like changing the timing??

Thanks, but still stumped . . .

Luke

One place to smooth the timing mark issue is to remove the oil pan/pump and polish the pressure relief plunger with crocus cloth.

The timing chain stretch is a good idea cause every time the lifter is on the opening ramp, stretches the chain in one direction. Lifter gets over the nose, and down the closing ramp, stretches the chain in the opposite direction. Multiply this by 16 and you can imagine the effect.

The distributor is at the other end of the camshaft and the cam twists to some degree. Thus, the dist timing is affected.

Now add an oil pump and a sticky relief valve and this adds more load onto the chain. I did this to my engine and the timing cleared right up.

Luke
I think before you get into the engine to solve this problem, I would swap the distributor with a loaner. If you have an HEI it's easy to drop in (IF it fits)and check if the problem is still there. Another thing to consider might be the general condition of the cap and wires (cross fire?). I would also suspect the conversion parts. The module or controler for the distributor might be going wonky at higher rpm?
Jim

:D Love the word, GMJim "wonky" graemlins/thumbsup.gif


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