Engine Troubleshoot Puzzle

Superman · 07-30-2009, 08:28 AM

Fact it, this is something many of us find really fun. Troubleshooting a motor that's not running right.

I've got the motor in my car now. It's an '83 motor in a '78 car, so rather than transfer the computer and the extra 14-pin plug, etc. to the new tub, I've placed the fuel distributor and WUR from the '78 engine onto the '83 engine. Along with the sensor plate box.

I've got the engine usually starting okay. It idles fairly smoothly. But there is a hesitation when I blip the throttle and the engine misses pretty badly throughout the range. But again, it idles fine. It can be driven around the block, but runs too poorly to be driven further.

When I pull each spark plug wire from the distributor, there is a healthy arc that jumps a fair distance (at least an inch) and the engine idles more poorly than before. Or so it seems. Hard to tell. When I hook the timing light up to each spark plug wire, there is steady flashing. The wires are in good shape (I built them myself) and each shows the proper 5000 ohms resistance. Last night, as I was starting it, I heard snapping noises while it was turning over. Arcing, it sounded like.

I tried reversing the wires from the "green wire" at the Hall Sender in the distributor, and it changed the ignition timing but does not seem to have impacted how the motor runs (after I fixed the timing, of course).

The injectors have been cleaned and the spray pattern is good. I did a volume test of the fuel system and all injectors are delivering just about exactly the same volume, regardless of sensor plate position. The sensor plate is, I believe, properly positioned in the sensor plate housing bore.

It is a substantial mystery to me why this engine is missing so badly. I have had experience with secondary ignition gremlins (we all have, I think) and this is acting somewhat like that. Missing under load. Not missing under no load. Except that the motor misses badly even when I'm simply operating the throttle at the engine by hand. Again, it seems to idle fine. So.......I'm going to change spark plugs. perhaps there is one or more cracked spark plug insulators, or something like that. I can also swap out CD boxes and coils, and will do so. And perhaps that will solve the problem. But for some reason I doubt it. Spark plugs, may be. CD box, probably not.

Any guesses/suggestions?

T77911S · 07-30-2009, 09:37 AM

air leak- you know somebody was going to say it.
fuel mixture. set it as rich as the idle can stand.
timing. is the advance working properly?

i lilke the idea of the 78 stuff on the 83. more reliable.
the 78 WUR has a vauum enrichment. is that connected and connected to the correct port?
the thermo time valve, if you still have one, is it working. you should be able to blow thru it when it is warm
of course- control pressures- goes along with mixture
cracked dizzy cap

i think it was mca that had a problem with a new set of wires he put on his car. it would run bad with the new set and good with the old set. try another set if you have one.

how does it run at a steady speed? say 3k rpm.

DUK · 07-30-2009, 09:46 AM

Last night, as I was starting it, I heard snapping noises while it was turning over. Arcing, it sounded like.

Did you try and look at it in the dark and see if you can find where the arc is coming from, if there is one? Is the coil arcing or the distributor? Just some idears.

Superman · 07-30-2009, 10:35 AM

Great ideas, guys. I will check them out.

I've already chased down several intake leaks, and I don't think there is any leaking going on now. Ignition advance IS working properly. vacuum lines are hooked up properly.

I will need to check the rest out. Perhaps a new cap and rotor to go along with the spark plugs.