Ok guys, the TPI to carb swap went well.

Only problem I have is a dead miss.

Here is what I have checked.

no vaccum leaks, Dist is in right. Timing is 12* inital and 36-38 total. New cap, rotor, plugs, wires, and accel coil.

I have fire at all the plug wires, no crossed wires, and the plugs are gapped at 45.

The carb is a holley 750 vs and is tuned fine. The motor has a dead miss at idle, but smooths out some when I get on the throttle. It has the stock cam, but idles like a lopey cam even with the timing set. If I get on the throttle fast, it back fires through the carb.

Is it possible I have a valvetrain problem? Maybe a broke spring, bent push rod or something?

I am going to pull the plugs and clean them. Kinda black soot on them. Then try it again. If that dosent work, I will pull the valve covers and inspect the spring, rockers and push rods.

I just dont know what to do. I thought about a worn cam lobe, but the motor only has 60k on it and the heads and bottom end is stock.

more is in the sig about the motor. Sorry for so long, but I am trying to rule out everything.

thanks.

 

OK, let's get logical here.

At least one cylinder is not running.


Step one: determine which one(s) it is. Wait until the motor is stone cold. Start it up. Let it run for 30 seconds, then shut it off. Squirt water on the header tubes a couple of inches from the heads; see which one(s) are cold. Should be like real obvious.

Step two: think about all possible things that will cause the particular cylinder(s) as observed, to be dysfunctional. Forget all about "what you've tried", "what new parts it has", etc. etc. Instead, use Occam's Razor: "The simplest explanation that fits all the facts is probably the right one". This means, if #5 & #7 are both cold, check to see if the plug wires are swapped; if #4 is cold, pull the #4 plug; if #3 & #5 are both cold, do a compression test for a bad head gasket; if #6 is cold and its compression tests REAL high, pop the valve cover and see if the #6 exhaust valve is moving; etc. etc.

Step three: if you haven't figured out yet, post your activities and observations, and we'll see if we can analyze it.

 

well, I tried the water on the headers trick (forgot to mention) and seemed like #4 and #6 were cold. I checked the wires and they were not crossed.

I am going to pull the plugs, check and clean them. That way I can rule that out. Then I will go on to the valvetrain. I hope there is nothing bent or broke, but I guess that gives me an excuse for head work. lol.

 

#4 & #6 are adjacent center cylinders. Therefore there is a very high probability of a bad head gasket.

I'd suggest starting with a compression test, rather than fooling with the valve train. It's pretty hard for a single valve train malfunction to take out 2 cylinders, but very easy for a head gasket to do so. I would estimate that the odds of a blown head gasket causing that malfunction are at least three orders of magnitude greater than the odds of a valve train malfunction causing it.

"The simplest explanation that fits all the facts is probably the right one". A very wise man, that ancient Greek named Occam.... do you realize, he said that even before TPI was invented!! That was a really, really long time ago.

 

Well, I had thought about the head gasket. I wont to make sure it is #4 and #6. I will check them again.

Although, I have not found any water in the oil.

 

If the fire rings are destroyed in between 2 cylinders, it will not cause water in the oil. Blown-out fire rings between 2 center cylinders on a cyl bank are far more common than failures that exchange water & oil.

Try the compression test. Listen to the 2 cylinders with a piece of hose or something while you're doing it; if you hear compression coming out of the #6 intake port while testing #4, and compression leaking into the #4 exhaust port while testing #6, you've found it.

 

good point. I will check each cylinder with the water on the headers again, just to make sure which ones, then go from there.

The pita is, I just had the intake and all the accessories off, not to mention I just put the headers on. If it is the head gasket, look like I will just have to tear it down again. Oh well.

 

ok I did the compression test.

Here is how it went:

#1-170
#3-170
#5-165
#7-170

other bank

#2-165
#4-170
#6-165
#8-165

I check the plugs and all had black soot, except for #6. The tip of the electrode was white. Not the whole thing, but just the flat surface of the plug.

So what now? How does the compression test sound to you guys? Would this be a sound motor to build on?

I just want to get it going and run it, but I dont want to put a cam in it if the compression is not good.

What should the compression be at on a stock 92 L98?

 

ok, I went back and checked the intake for vaccum leaks, adjusted the carb air screws and reduced the fuel pressure down to 5 psi.

It is still backfiring through the carb. and has a dead mis.

I have noticed when I hook up the timming light something odd.
when the light is flashing, there will be a pause every so often. This is usually when the motor will mis or stumble.

I something maybe wrong with the dist? It seems like for a second it is not firing. Is it possible it is the pickup coil? What about the gear on the dist? It is from a older gm and is not a new gear.

Dont know what to do. What could be causing it to mis and backfire through the carb that I have not checked?

Thanks