Maybe a broken valve spring??

 

When is the last time the carb was rebuilt? Rough idle, but runs good sure sounds like the idle circuit. Is your float adjusted properly?
Dave

 

Thanks for the replys.

Carb rebuilt last fall.
I thought it might be the idle circuit too so the other day I temporarily switched to another good carb just to rule out any funny carb issues. Still acted up.
The float's adjusted so fuel is at the bottom of sight glass.

Mikegh what's the best way to find a broke valve spring?

Any other suggestions??

 

The motor seems to have a strange squealing sound almost like a vacuum leak?..Does it smoothe out if you put your hand over the top like a choke?..It sounds kinda sluggish when you rev it?

 

You actually have about 15" of vacuum which is very good. If you had a broken valve spring you would have a miss at low revs. Easiest way to see is to remove the valve covers and look. Unless the engine has had a major over rev it is unlikely to be a valve spring. The compressions are reasonable for a stock motor and good that they are all relatively even. I assume it is an automatic. Check your idle speed with it in gear. It could just be a low idle speed. A vac. leak will normally cause an elevated idle speed due to false air leaning out the mixture.

 

Yeah I had the air cleaner off during the video which could be the squealing sound. I haven't tried putting my hand over the carb to see if it straightens out. When I blip the throttle it revs good.

I've got the idle speed turned up to keep it running in gear. I'm really at a loss here.


Chris

 

Base on those compression values it is not a valve train problem. You have a problem with the idle circuits or a vacuum leak. Do you know that the other carb was good?

 

The other carb was an autolite 2100. I did find a vacuum leak around the throttle shaft so I put the holley on. The Holley is less than a year old, I only ran it for maybe 2 months.
Same symptoms with both carbs. Maybe the damn spacer is cracked or something.

 

bumpity bump

 

Think it needs to be a hands on solution now.

 

Update:
I removed all vacuum lines from the engine and plugged them.
Looks like #3 cylinder is dead/ very weak. When I pull the plug wire off it, there is no change in the way the engine runs.
I took it further by swapping plugs and plug wires on that cylinder. No improvement. I then removed the distributor, reclocked it so the wires were in a different spot on the cap and pertronix module, to make sure it wasn't nothing crazy going on there, no difference.
Then I removed the valve cover and checked out the springs & pushrods. Everything looked good there. So I brought #3 up to TDC and applied compressed air to look for any air escaping through the carb or exhaust. Everything checked out good.
Then I rechecked the compression, 120 psi. Then I removed plugs 2 and 4 and rechecked compression again to see it the head gasked wasn't blown between the cylinders. 120 psi.
The only thing left is a blown intake gasket on the valley side, which I havent pulled the intake to check yet.

I'm abt to run out of perseverance (sp?)


Chris

 

Burned valve

 

Sounds like you might have a cam lobe going flat. I had about the exact same symptoms on a 302 once. I just "knew" it couldn't be the cam and it worried me to death until I finally tore it down. Good compression, leakdown checked OK, just wouldn't fire on one cylinder. When cranking the engine over rocker motion on the affected cylinder looked the same as the others. The only way to check without teardown is to mount a dial indicator and measure the rocker motion as you turn the engine by hand. The better way is to use an extension on the indicator, pull the lifters and measure the cam lobe directly. I didn't do that. I got fed up fiddling and just pulled the cam. I found part of my problem was four other lobes were also wearing down. Not knowing what lift I was looking for I compared to the other cylinders. Because I had multiple problems I was getting readings all over the place. That engine had been running fine for like 10 years, well maintained. Just all of a sudden decided to start eating cam lobes apparently.

 

Wouldn't my compression readings be low if it were a burned valve?

 

Ok maybe it is a burned valve. :shocked:

I took a slip of paper and held it up to the pass. side tail pipe (true duals) and it sucks in sporadically. Drivers side blows out evenly. In everything I've read this indicates a burned valve or a blown head gasket.

But I can't for the life of me figure out how every cylinder on the passenger side has good compression and no real audible leaks when compressed air is applied. To me if a valve is burned or not seating or bent or whatever you would be able to see that in a compression test.?.

Like I said before, when I pull the plug wire off #3 there is no difference in the way the engine runs. So I paid close attention to #3 and the surrounding cylinders, and they all act/sound the same when doing poor boys leakdown test.

Has anyone ever seen anything like this?

Chris

 

Screw it.... I'm gonna pull the engine.