Engine problem - maybe broken cam
 

Just got back from a shortened drive. At idle, the engine made a sudden loud "clack", followed by severe knocking/banging. The sound eased up pretty quick, and the engine kept running. I shorted the front two plugs and it doesn't seem to make a difference. Shorting the rear two plugs causes a noticeable drop in RPM. Seems like the front two cylinders aren't firing. Is camshaft breakage common?

I'll do some more troubleshooting and pull the head soon.

Re: Engine problem - maybe broken cam
 

Re: Engine problem - maybe broken cam
 

What Y-blockhead said.

Cam breakage is very rare. Before pulling the head I'd recommend looking inside the valve cover and doing a compression test. You may end up removing the side timing cover and looking at the cam gear. I check everything I can before pulling a head.

Re: Engine problem - maybe broken cam
 

Is the engine idling now the same as it did before the "bang"?

Are you shorting out both front plugs at the same time, then both rear plugs at the same time or one at a time?

I can't see any reason to pull the head now. What are you expecting to find if you do?

Re: Engine problem - maybe broken cam
 

Take out the plugs and make sure there is compression in each cyl. You may find one or two with no compression. If so, drop the pan and look for broken pieces. Good luck.

Re: Engine problem - maybe broken cam
 

I agree with doing a compression test first.


This may give you a direction to go.


I also agree, not a broken cam.


My opinion,


Chris W.

Re: Engine problem - maybe broken cam
 

The model A&B cam shafts are forged steel not cast iron like most later cams. Its not likely that a forged cam could or would break .

Re: Engine problem - maybe broken cam
 

Sounds possibly like a broken rod.
Pull the first two plugs and make sure the pistons are moving up and down.
If not, drop the pan, go from there.

The original cams are tough, we have welded on them and back ground the base circle way down into the cam, nothing broken even up near 7k revs.

J

Re: Engine problem - maybe broken cam
 

Wouldn't a broken rod continue to make noise as the engine was run?

Re: Engine problem - maybe broken cam
 

If a broken cam caused the front 2 cylinders to quit firing, what is operating the valves on the back 2 cylinders?!

Re: Engine problem - maybe broken cam
 

Once the interfering parts are pounded up and away from the rotating crank there will be no more noise. It depends on where the break occurs. Typically the piston is shoved up into the top of the cylinder.

I sure hope this is not the case, but OP description of "the engine made a sudden loud "clack", followed by severe knocking/banging. The sound eased up pretty quick, and the engine kept running" describes a typical rod separation.

Could be flywheel or clutch but that wouldn't explain the front cylinders not responding. Flathead motors can't drop a valve into the piston either, so that is out unless this is a OHV motor, OP didn't mention that.

I wish the best of luck to the OP here, take the time to properly diagnose and let us know what happened.

John

Re: Engine problem - maybe broken cam
 

More than likely the problem is a broken piston skirt . The original pistons had a split skirt , all the way from the oil ring to the bottom of the skirt . I've seen a few model A engines drop a piece of piston skirt into the dipper tray . When this happens , the rods hit the broken piece, make a hell of a knocking sound and then sort of settle down . The engines would still run but skipped on the broken piston cylinder . It could be something else but I can't see a forged model A camshaft breaking or a connecting rod . Model A engines unless highly modified are low compression low RPM engines .

Re: Engine problem - maybe broken cam
 

Time to drop the pan...pulled the valve cover and all seems normal there. Piston skirt or rod sounds likely...it was a lot of bad noise, not something small.