Novice mechanic needs help 1) finding TDC 2) some troubleshooting advice for won't idle/runs rough on 620 with L20B engine

[Ricky Harline]

Background:

Truck's engine performance has been slowly deteriorating-- first it ran a bit rough, then VERY rough, then would only run for a few minutes, then would only run for a few minutes with significant throttle, then did not run at all. After rebuilding the carb (no problems or blockages found) we are back to it runs if significant amount of throttle is applied. It sounds really rough and unhappy, though. Checked the air gap on the distributor, it was towards the wider side of in tolerance so we moved it down to the middle of the tolerance range which made no difference. 

 

Issue 1 - finding TDC: 

I want to check the engine timing and valve lash. Looks like both tasks require finding TDC. OK, no problem, I don't know shit about shit but I've found TDC on a couple cars in the past, was always really easy. Apparently L20Bs have a notch on the crankshaft pulley that lines up with a neat little gauge mounted on the engine block. PROBLEM. Cannot for the life of me find that tiny notch. Pulley is old and rusty and has lots of little pits in that could or could not be the notch. Both the GF and I shined a flashlight at the thing while the other turned the engine over multiple times and we cannot for the life of us find the damned thing. So uhh... how do I go about finding TDC now?

 

Issue 2 - what the hell should I be doing/looking for/working on: 

We started with the carb because a guy that knows a lot more than me said it looked to him like the carb wasn't putting out enough fuel. Was hoping to find some sort of major blockage in the carb, but we didn't. We pulled off the fuel line and it's spouting fuel fine, so that isn't the problem. 

  

Other than the engine timing and valve lash adjustments, what is likely to be causing such problems? It seems unlikely to me that engine timing or valve lash could lead to rapid deterioration of engine performance such as we've experienced. I'm happy to do it, I'm sure the truck needs it as it hasn't been well maintained, so what else should we be looking for? The GF is moving soon and if we don't get it to the point where it can make a 2.5 hour drive in the next 30 days we're going to have to pay a boat load of money to get it towed that distance.  😬

Would really, really appreciate some help. Thanks, y'all! 

Edited July 24, 2023 by Ricky Harline
english is hard

[datzenmike]

Timing is not going to change like a points distributor. The EI distributor, one set, changes very little because there is nothing to wear out.  The timing mark Take valve cover off and turn engine by hand (is best) till the first cam lobe is at 10 and the second one back is at 2 o'clock as viewed front in front of the radiator. Engine is now close to TDC. Now look at the pulley for the mark.

 

Carb: just make sure the mounting bolts are snug.

 

It's very possible that the valve lash needs checking and adjusting. As it's going to be near impossible to warm the engine set it to the cold lash specs of 0.008" intake and 0.010" exhaust. The easiest way is to set to TDC and adjust the following...

Both valves on #1 cylinder, intake on #2 and exhaust on #3. Turn engine one full turn and line up the mark on TDC. Set exhaust on #2, intake on #3 and both valves on #4. Done and without farting around bumping the starter.

 

 

 

[Ricky Harline]

Rad, thank you. Do you think the valve lash could be the problem causing it not idle and run rough? Or is it likely that's just a start and we'll have to figure out more stuff from there?

[datzenmike]

Yes, tight valve lash means the valve may be prevented from closing properly.

[Ricky Harline]

Thanks, appreciate you as always, man!

 

Will hopefully get this job done soon! Expect either a very grateful appreciation post if we get it running or a very anxious and worried post if we don't! =P

[datzenmike]

No worries. It's often a guessing game based on information and clues and a process of elimination.