235 flat cam symptoms

Here's the story- my 235 has started to idle rough, one cylinder not firing very well, esp. at low rpms. Checked spark plugs, adjusted valves statically, replaced plug wires. This was rebuilt w/ stock spec. cam about 15000 miles ago. I didn't know enough o add zinc the first 8000 miles or so. So what are the chances the cam has lost a lob? this has electronic non-vacuum distributor and a newer 2 barrel.

 

A vacuum gauge is handy for this. Try it one at a time on two full vacuum ports (sometimes you have to to see the problem).
You're looking for high, steady vacuum. 17" hg or higher. More importantly, does the needle flick? A slowly roving needle 17-19 is fine. I'm talking about a quick needle action.
Time to get a vacuum gauge.

 

Pull the cover and watch the valves. If one isn't moving as much as the rest it will be easy to spot.

 

No more spring pressure than those engines have flat cam in not very likely. Pull the cover any flat lobe will have a bunch of valve clearance.

 

You must not've liked my answer on your earlier thread?
http://www.jalopyjournal.com/forum/showthread.php?t=827633

I still think it might be a vacuum leak at the base of the carb. Did you check like I suggested? If it was a flat lobe, it would probably be sneezing out the carburetor.

Joe, and in case you don't know...you don't have to start a fresh thread. Just bump the old one by posting on it.

 

My carb is a holley weber Pretty new I'll check for vacuum leaks. And I have a vacuum gauge installed,Runs off the intake manifold. Tomorrow I'll pull off the valve cover and check the action while it's running.

 

So is the vacuum high and steady, then?

 

Ohhhhhhhh...ok. Holley Weber carb...not a Rochester.

Of course, disregard everything I said except maybe the possible vacuum leak.

Sounds like you know that though. Good luck.

 

Of all the early Chevy sixes I've ever torn down (a lot), I've seen one that had worn cam lobes, and the whole engine looked like it had never had an oil change. Like someone else said, it's a three minute job to remove the valve cover and watch the rocker arms.

 

The vacuum is bouncing between 15 and 20 at idle and is bouncy all thru the rpms. The rocker arms all look to be moving about the same amount. So I'm thinking the cams fine. Couldn't find any vacuum leaks around the intake and carb. base. No vacuum advance or wipers. I've got NGK plugs, with the little skinny electrodes, gapped to spec. What I'm I missing?

 

Vacuum is bouncing? It should be high and steady.
Sounds like a valvetrain problem.

 

Ditto, what SmogGuy said.

4TTRUK

 

OK, but everything looks good with cam, everything's working good with the rocker arms, push rods, valve springs. Where should I be looking?

 

Leaky valve check compression.

 

Well, compression check showed 100- 125 on #'s 123 & 6. Only 30- 35 on 5 & 6. Could it be the head gasket is leaking between the 2 low cylinders? Or burnt valves on both cylinders.

 

Either or, but if I had to bet money on it, I'd say that when you remove the head, you'll find a head gasket with a chunk missing.

 

That's what I found. A piece blown out between the 4th and 5th cylinders.

 

Well, at least that's an easy fix.