Timing A GM 250

[I Wanna Be A Bear]

Okay, so my first mistake some months ago was buying my '81 CJ5 that had a GM 250 in it. Its not a bad engine, just a royal pain to get information and parts for. About 3 weeks ago, I did a block exchange rebuild with a cam upgrade. It was one thing after another but we finally got the engine in and bolted up.

Now, I don't know much, if really anything, about timing. While the engine was out, we brought the first cylinder to top dead center on the compression stroke. The we threw on the distributor with it pointing to the first cylinder and dropped the engine in. It didn't turn over. We then bumped it back around to line the notch on the harmonic balancer up with the 0 on the timing cover, dropped the distributor back in, and it still won't start. Even when we turn the distributor ever so slightly while cranking, still nothing but back fires up through the carburetor and not even consistently. We re-timed it to 10 degrees before TDC as some nova board said to do, and still nothing.

The question:
Does anyone on here have any knowledge about timing on a GM 250 that myself and my buddy appear to lack? Or maybe just a few other ideas to try?

I've heard everything from 10 degree BTDC to 8, 6, 5, and 4 BTDC.

I'm sick of driving a Camry and need my Jeep back.
Thanks in advance.

 

[Old Dog]

I'll start with, I know nothing about a GM 250.
But when you get backfiring it usually means your 180 out.
Make sure your on the compression stroke when you line up
the timming marks.

 

[I Wanna Be A Bear]

Yeah, initially when timing it, we were 180 out. Stuck my finger in the first cylinder and bumped it until I felt it pulling in, then reset the distributor after bringing it back up by shoving the Jeep back in reverse a few times. The fun part is finding the sweet spot I guess. I think I'll just try 4 degrees BTDC. If that doesn't work, I'll probably do what I should have done a long time ago: Sell it and buy something I haven't the slightest clue how to work on until I can afford to own a toy like that.

A guy at AutoZone once said to me, "You need to have at least 5 grand in the bank at all times if you're going to have a Jeep as a daily driver." I asked why and he replied, "Its called buying another vehicle." In retrospect, he was probably right about that for a kid in high school without a job and little experience with working on vehicles such as myself.

 

[priceg]

I am going to go through the whole process because I don't know what step you are currently on. Cylinder location is 1-6 front to rear.Firing order-153624, base timing is 8 degrees before top dead center. Distributor rotation is clockwise. From the passenger fender looking at the distributor, #5 is at the 6 o'clock position, and #3 is to the left and #1 is on the right. This is cylinder locations on the distributor cap. Knowing the #1 position on the cap, get a marker and move in a clockwise direction from #1 and mark the cap,153624.Some better quality caps come marked. Now, with the cap marked, get #1 cylinder up on the compression stroke. You can do this one of to ways. The easy way, put your finger over #1 and rotate the engine counter clockwise until compression is felt,at the same time, looking for the balancer timing mark to come around to 0 degrees. The more work way, pull the rocker cover, rotate the engine untill valves are closed on #1. Since #5 is the second cylinder in the firing order, the intake rocker will be moving up as #1 moves toward compression. Now that the engine is in the correct posture, you want to install the distributor with the rotor pointing towards #1 on the cap, witch is at the 5 o clock position. The engine may need to be rocked with the crank bolt, to get the distributor to sync with the oil pump drive. To verify you have it in time, The rotor will be pointing exactly at #1, the balancer will be exactly on 0 degrees. Tighten the distributor for now, and reinstall plug wires in the correct order. If all steps have been done correctly, engine is successfully timed. To set base timing, disconnect vacuum advance, engine warm at idle, rotate distributor untill the balancer mark hits 8 degrees. Good luck and may the force be with you.

 

[CJ]

Before anyone asks, I cannot find where / if this was copied.

GREAT post! Thanks!

 

[I Wanna Be A Bear]

I am going to go through the whole process because I don't know what step you are currently on. Cylinder location is 1-6 front to rear.Firing order-153624...

Well I'm going to give it another go here in about an hour if the whether holds up, gotta love spring time in Colorado. We've timed it to 0 at first top dead center at least twice. Once was definitely 180 out and I don't know why its not turning over now that that particular problem has been addressed. But I'm going to take your word on the 8 degrees once its running. And hopefully today is the day because tomorrow makes 3 weeks.

Thanks guys.

 

[I Wanna Be A Bear]

Got it! Now I just have to stop the oil leak where the mechanical fuel pump used to be.

 

[priceg]

Glad you got it. This whole operation is the same for a AMC 258 i6 / 4.2l liter except for the cap orientation. On a AMC 258 i6 / 4.2l #1 is at the 6 o clock position with #5 on the left of it.