Finding TDC

[PlatonicSolid]

89 GMC C3500 454 7.4L TBI - Installing new distributor. I see my current distributor has plug cables 45° off from spec. Spec would be moving them all 1 place counter clockwise per the manual. Figure I should make it right when installing the new distributor. If I line up balancer timing marks @ 0 (timing setting for this truck is 4), shouldn't that = TDC for cylinder 1? Pic with cap removed is with timing marks lined up @ 0.

 

[Schurkey]

89 GMC C3500 454 7.4L TBI

- Installing new distributor... ...If I line up balancer timing marks @ 0 (timing setting for this truck is 4), shouldn't that = TDC for cylinder 1?

Yes, that WILL get you to TDC #1 IF (big IF) the damper is in good condition AND correct for the timing pointer being used.

You might be at TDC-Compression (correct) or TDC-Exhaust (NOT correct.)

Therefore you need to verify that as the piston is coming up towards TDC, that both valves are closed--and therefore the cylinder is developing compression pressure. Most guys put their thumb over the spark-plug hole to feel for compression, OR verify the position of the distributor rotor in relation to #1 on the distributor cap before pulling the old distributor out, OR stick a "whistle" or other indicator in the #1 plug hole, so that it makes noise or pops the cork out as the piston comes up.

Note that at TDC-Exhaust, both valves are ALMOST closed, so it's hard to tell from the position of the rocker arms which TDC you're at unless you're watching them move as you turn the crank. Neither rocker arm will move as you come towards TDC, or pass TDC if you're near TDC-Compression. If you're near TDC-Exhaust, the exhaust valve will be closing, and the intake valve will be opening, with both open just a little on overlap.

 

[PlatonicSolid]

Thank you Schurkey. I figured it out and got the distributor in the correct position now. I put a compression tester on #1 and just barely kicked the ignition to confirm I was on the compression stroke. Unfortunately the new distributor didn't solve my idle problem. So I can't set the timing since it won't run.

 

[Schurkey]

What is the fuel pressure? Does the fuel pump prime? Is there spray from the injectors when the engine is cranked?

Do you have spark to the spark plugs, and is it strong enough to reliably fire a spark-tester calibrated for HEI?

Are the plugs fouled? Spark TO the plugs is essential, but then that spark has to actually jump the plug gap, not bleed-off down the deposits on the porcelain.

 

[PlatonicSolid]

What is the fuel pressure? Does the fuel pump prime? Is there spray from the injectors when the engine is cranked?

Fuel pressure was 3 lbs at prime before I replaced the gas tank and fuel pump and 3 lbs at prime after. Old fuel pump was measuring 7-8 lbs running (had to keep foot on gas to keep it running). New fuel pump seems to have made things worse as it hasn't run at all since. I believe there is still spray from the injectors, I'll double check (rebuilt TBI installed and confirmed functional prior to fuel pump replacement).

Do you have spark to the spark plugs, and is it strong enough to reliably fire a spark-tester calibrated for HEI?

Replaced coil at the same time as distributor. I don't have a spark tester, but I find it unlikely that spark is the issue.

Are the plugs fouled? Spark TO the plugs is essential, but then that spark has to actually jump the plug gap, not bleed-off down the deposits on the porcelain.

Replaced plugs and cables a couple weeks ago. I'll have to inspect the new plugs and report back. They have roughly 2 hours of engine run time on them.

I'll post a video of a start attempt later.

 

[PlatonicSolid]

I got it to run a little bit a few times, but it's always erratic.

Running Video 1
Running Video 2

This only works a couple times, then it doesn't start again. I have several videos after above where it just doesn't start.