F-134 Popping Through The Exhaust

Fired up the Jeep (1968 M38a1) today to run for fuel. Seemed to start and drive fine, but I noticed some popping noises when at the stop lights. No noticable decrease in power or throttle response. Engine is F-134, with a 0.060 shaved cylinder head, pertronix electronic ignition conversion and a Weber 32/34 carb.
Got it home and pulled the plugs, they look ok to me, but I swapped in a new set anyway - no change.
Grabbed my compression tester, all cylinders are even at 120 psi, BUT.... cyl #1 and cyl #2 lose pressure - I can watch the gauge reading drop.
Engine has maybe 1000 miles on a complete rebuild.
At this point, I am suspecting the head gasket or burned exhaust valves??
Any thoughts?

 

Complete rebuild? Did you retorque the head bolts after first run ? If you didn't do that, a blown head gasket is a very big possibility.

 

Popping in the exhaust can be a bad exhaust valve.

 

Yes a bad exhaust valve can produce popping, but a bad valve isn't indicated with 120 PSI. It could be valve adjustment is too tight on the two cylinders losing pressure, could be an air leak in the manifold, or could be a bad head gasket.

My thought is, if the engine is producing 120 PSI each cylinder, a bad valve is unlikely, loosing pressure on two of the cylinders is what made me suspect the head gasket which is why I asked about the re-torque, something often omitted, especially if the engine was rebuilt by someone other than the owner.

 

Well, I am 99% sure I re-torqued the new head bolts after the initial break in. However, I did have an overheating problem last summer, due to an air pocket in the cooling system. Coolant temp was over 240 degrees a couple of times....
Looks like it's time to pull the head and see what's actually going on.

 

My '56 CJ5 F-head has always had a 'popping' rhythm as if from one of the four cylinders. But it has always done that, and runs fine.

A vacuum gage on the intake manifold might offer more info.

 

Well, for mine, this is something new. The fact that cyl #1 & #2 are not holding compression pressure worries me. When I set the valves last, I set them all at 16 thou, both intake and exhaust. Has been running strong with no noticable issues since.

 

A head gasket leak between them would not be unusual. But I'm not sure why that would make the exhaust pop, given your compression readings.

Is it possible the carb is running rich, and/or not shutting down effectively at idle, or has a vacuum leak at idle?

 

I believe your adjustment is wrong on one or the other. If I recall they are something like .016 and .018, but don't have my manual in front of me. Just something to double check.

 

Correct. Intake is .018 exhaust is .016

 

Are you sure? Just asking, I was thinking that should be the reverse,, the hotter valve ( exhaust) would have the bigger clearance, I think. , humm.. can't find my book,.
But the values seem right at .016 and .018

 

Think about it some more. Setting the "lash" with more "clearance" means the valve opens later, isn't opened as far, and isn't open as long as it would be with less lash.

The exhaust valves on the F-134 are the same as used on the L-134 where both intake and exhaust (same sized valves) are set between .014-.016 ( I go for .016). The intake valves are much larger on the F-134 than are the exhaust which factors into flow design.

 

While counterintuitive, .016 exhaust and .018 intake, is, in fact, correct.

 

Assuming the clearances are set cold, the intake would require more clearance because the push rods will lengthen (.002"?) when hot and use up some of the clearance.

 

No fair using logic.

 

x2,.

 

Popping in exhaust is usually unburned fuel getting ignited.
I'd check and see if you're not real rich at idle.
Does it pop when you shift gears? As in Vroooom pop pop, vroom.
or just a random pop in exhaust at idle?
Verify timing also. If it was steady pop pop pop at idle I'd suspect ex valve, if not look elsewhere.

 

Yep, I was thinking about L-head engines, the F-head motors are 0.18 on the intake. The push rods for the intake valves will get longer when they heat up reducing the valve clearance.