72 Jimmy w/HEI Wont Stay Timed!

[gweav3]

HELP!!

My 72 Jimmy with a stock motor-except for the HEI from a later year stock motor-wont run.

I just replaced the entire wiring harness with a Painless #10206 for 67-72 GM trucks. Now I have replaced the HEI with another stock unit (not MSD, Accel or the like) and it wont run worth a damn. I timed the motor to 10 degrees BTC with the vaccum hose removed (and plugged). I confirmed I was at TDC of the # 1 piston and nothing mechanical changed. All the wiring works and I went overboard on getting everything grounded.

The motor will run for a while (but lacks power) but then starts to misfire as it warms up. The distributor cap also wobbles and I dont know what that is about. I have run out of ideas so....

Any suggestions?

[bigsnookmaster]

I would go buy a new aftermarket HEI dist. They usually run somewhere about $150, and they are plug and play. Money well spent. Don't buy a real cheap one though, get a Mallory or Accel. My old truck ran 8.40s in the 1/8th with an old HEI, changed the dist, and it went 8.0s, with that one thing changed. Good luck!

[gweav3]

So do you think the problem is specifically related to the stock HEI or might I have bigger problems? I was thinking maybe bad ground, stretched timing chain etc. The motor has 92K miles...

[gweav3]

Oh, and I forgot to mention.... While I do have the distributor on the right 'tooth' of the cam gear and the #1 piston at TDC, its doesnt exactly match up with the #1 mark on the distributor body/#1 tower on the cap. I suspect an old timing chain but the truck did run when I took it apart....

[BIGKEV]

The balancer may also be "slipping"

[bigsnookmaster]

I would try to move it one tooth, and see what it does. Timing chain could be a possibilty, but I would almost think it's a worn out dist. See if you have a buddy that you could borrow one from and find out. That's what I did when I picked up 4tenths. I could feel a difference in power right away.

[BIGKEV]

Easy way to check timing chain is to pull the plugs and distributor cap and move the balancer side to side, you can tell it there is slack by watching the rotor for movement in relation to the crank.

[gweav3]

Thanks!!! I didnt think about the balancer. Everytime I get into figuring out a motor I end up rebuilding it and I didnt want to do that this summer. The ideas definately help!

[BIGKEV]

what you need to do, is find the absolute tdc of the #1 piston, and mark that on your pulley.

The best method in my opinion is teh "dead stop" method. No special tools or gauges are needed, and the result is accurate.

You see, one problem is that the piston stays virtually motionless while the crank turns over the tdc position. The crank keeps turning, but the piston doesn't move until the crank starts pulling it back down. This brief moment of piston wait, or "dwell", or "death", can last for several degrees of crank rotation.


The solution is to find the middle (in crank degrees) of the piston "dead" spot. Here's how:
You need to fabricate a simple piston travel stopper. Take an old spark plug, and knock the porceline center out. (bench vise, hammer, steel punch or something similar). Cut out the side electrode too. Then drill and tap the remaining metal part of the plug for a suitable bolt size, i use a metric m10 size.

Now find a long bolt that you can turn in and through the plug. You will need to have maybe an inch of bolt tip exposed on the other side, and enough threaded part of the bolt visible on the bolt head side for a jam nut.

This may take some trial and error, but next you need to "guess" how much of the bolt tip you want to protrude into the combustion chamber. Adjust the bolt accordingly, and lock it in place with the jam nut. I would also file the bolt tip round, because it will come into contact with the piston in the next step.

Now, screw your home made dead stop "sparkplug" in, while the piston is at or near bottom dead center. Nice and snug, almost as tight as a regular sparkplug.
Now slowly start turning the engine clockwise, until you feel the piston hitting your dead stop.. If the engine can turn a full turn without stopping, screw in your bolt a little deeper and lock it in place again.
Anyway. You have now managed to make the piston come to a complete stop against the bolt coming through the spark plug. Mark this position on the pulley. One sharp, thin mark right right on the outer edge.
Do not move your dead stop "spark plug" at this point.
Slowly turn the engine over the opposite direction, until the piston again meets the dead stop bolt. Mark this position on the pulley again.

Your true tdc is precisely half way between these two markings. Measure with a flexible tape measure along the outer lip of the pulley, or count the degree marks if you have a degree pulley to find that half way spot, and file a notch or otherwise mark your true tdc on the pulley, and rely on that in all your future adjustment procedures.

[chevytruckluver]

Originally Posted by gweav3

The motor will run for a while (but lacks power) but then starts to misfire as it warms up. The distributor cap also wobbles and I dont know what that is about. I have run out of ideas so....

Any suggestions?

If the distributer cap is wobbling, you need to look into that first, most likely it is not setting down on the distributor correctly. If is off that much it may be misfiring. The only thing that actually touches inside the cap is the top center of the rotor.
Good luck

[BIGKEV]

Wow, how did I miss that part. Pull the cap and check the rotor. If the rotor is good , replace it and cycle the engine by hand. If the rotor wobbles, pull the dist, and check the wear on the gear. If there is a normal wear pattern replace the dizzy!

[my67c20]

Had the same problem. What i found was that the piece of #%$^ chrome holdown was snug up against teh manifold and had just enough pressure to make me think it was locking down the distributor. With the engine off see if you can rotate the distributor. Went back to the stock holdown problem gone.

[frednoah]

Originally Posted by my67c20

Had the same problem. What i found was that the piece of #%$^ chrome holdown was snug up against teh manifold and had just enough pressure to make me think it was locking down the distributor. With the engine off see if you can rotate the distributor. Went back to the stock holdown problem gone.

Had issues with the timing jumping around on my truck also. Previous owner had a cheap chrome holddown that looked tight, but I could still turn the distributor by hand.