cylinder 6 acting up

[corvairdoug]

Hello,
140 PG engine, less than 600 miles. #6 cylinder not having any affect on the engine running. Pull the plug wire, no change, just runs rough.
Well, upon further inspection, we've found some things... #6 exhaust push rod is bent, and really dark; also worn where it goes through the guide plate. We switched out the push rod, but nothing changed as far as how it runs. Compression on the cylinder is higher than the other 5, by more than 10%. Readjusted the valves, but on that one, it never "bogged down" when we tightened the nut, and we could to turn it in 2-3 turns and it didn't change how it ran. It also seems to ping at low rpm. The plan is to pull the engine; pull the left head, send the head to a well known shop for inspection and whatever work may be needed. If there's nothing going on with the head, we might swap it and see what happens.
Is there anything else we should be doing or are missing? It's a really odd problem.
thanks!

[cnicol]

Before pulling the head, I suggest performing a leak-down test. This will confirm or condemn the cylinder and valves. One thing I've noticed on 140s: Corner cylinders under the secondaries often misfire due to a lean condition caused by the secondary carburetor throttle plate either not fully closing or not sealing. There's also a possibility of a defective spark plug or spark plug wire. One Corvair I worked on had an electronic distributor with a bad magnetic reluctor on one cylinder; that was fun to find. (took an oscilloscope).

[Dennis66]

X2 on the leakdown test. Also (saying just in case) be sure the pushrod guide isn't in backwards for #6. There should be a stamped U facing out. The push rod slots are angled and if backwards, they can bind on the rods. Dennis

[erco]

You can remove the head(s) without removing the engine, just lower it a bit at the rear and take out the carb studs.

[corvairdoug]

We're pulling my engine to swap with a "loaner" to get me back on the road until mine is done...

[joelsplace]

If you have compression why are you going to take the engine apart?
You have an ignition problem. Bad plug, wire, cap or something strange like the reluctor mentioned above.

[Frank DuVal]

Compression on the cylinder is higher than the other 5, by more than 10%. Readjusted the valves, but on that one, it never "bogged down"

If it never bogged down, is it because it never fires? See Craig's explanation for lean condition/no spark. Pull #6 plug wire from the plug, stick a spare plug into it and lay it on the top cover/linkage/some metal of the engine. Start engine, is the plug sparking?

10% high is not much. Red herring usually.

Not a dropped seat, since you have compression.

Were both rockers moving the same amount? No worn lobe, right?

Ignition or fuel. Fuel, Air, Ignition needed to run.

[joelsplace]

Good point on the flat exhaust lobe. I forgot that one.

[jimbrandberg]

What do you think a correlation could be between a bent pushrod and a flat lobe?
I agree that I like to do as much testing as I can before "going in".
Reading between the lines it might be that someone else put the engine together 600 miles ago and they're furnishing a loaner engine while the problem is being resolved.
Jim Brandberg
Isanti, MN
CorvairRepair.com

[manvair]

Lifter temporarily jammed in the extended position? I'd pull both lifters off that cylinder, disassemble and compare them.

[joelsplace]

I can't see any connection between a flat lobe and bent pushrod. I guess whatever bent the pushrod would put more pressure on the lobe but it would be such a short time that I can't imagine it would hurt it.

[corvairdoug]

Well....... leak down test was done, passed with no problems. The guys took the secondaries off, and they're both soaked, as if dumping fuel. They used 2 block off plates, and the car is back to its old self, running well. They'll go thru the carbs and let me know what's happening.

[jimbrandberg]

Drippy Rochester HV carburetors are the bane of my existence.
Jim Brandberg
Isanti, MN
CorvairRepair.com

[Frank DuVal]

Well....... leak down test was done, passed with no problems.

Of course! You had good compression...

Yep, too rich will also cause a miss.

What do you think a correlation could be between a bent pushrod and a flat lobe?

I wasn't thinking there was a correlation, just a "what were the rockers doing while adjusting them" question. All the same?