The Stovebolt Site Forums The Shop Area Engine Shop 216 problem

The local machine shop called me today, seems he machined and rebuilt a 216 for a guy with a 50 Chevy truck.The engine has never run correctly since the rebuild.He asked me to ask around for opinions.
Here goes
The engine idles as smooth as a Swiss watch , starts instantly hot or cold,and runs perfectly at low speeds but as soon as you push on it, the engine pops back through the carb very badly.All cylinders have proper compression, the cam timing and lift are spot on,A million things have been changed, ignition parts, carb swapped off a good running motor etc etc. The head was pulled and checked out for cracks . A few local garages have looked at the truck, you know, the garages with old time mechanics.Distributer checks out fine, even a new distributer grounding wire was put on.It has enough fuel flow and fresh fuel in the tank.
The only outward clue is the #6 plug gas fouls, not hint of fouling on the rest of the plugs and no black smoke comes out the tail pipe. No sign of blue smoke either.If nothing else, a lean condition can cause popping out the carb.Typically,ignition disorders cause popping out the exhaust.
I would think that several very experienced older mechanics would not miss anything. The owner of the truck is threating to sue the machine shop!!!.Have any ideas? something that may been overlooked? Thanks.

Tony, I'msure someonechecked this already but ask the guy if he checked the vacuum advance mechanism. Does it spring back when turned by hand and does it advance when he gives it the gas? Just grasping at straws. Jacques

I had this same problem with a f*rd truck with a factory rebuilt engine. Where ever it is today it still has it. I changed everthing too. The only thing left was that I thought it had a slightly bent intake valve that hung up from time to time. I was NOT going to pull the heads so I sold it.

Speaking from experience in this sort of stuff- I would first eliminate the simple things- weak breaker arm spring, breaker points improperly adjusted, wrong type of spark plugs, (old engine) excessive carbon and lastly if they are using the original valve springs, then I would check #6 for weak valve springs. This last one is usually what I have found to be what is happening. If it is running good at idle and misfiring when you rev it up, I would anymore just go straight to the valve springs.

Tony, did they figure out what was wrong?

I haven't heard anything yet.Just imagine, a simple 216 engine problem has baffled 3 good local garages, a machine shop,several old farm tractor men and a Harley builder.

Tony,

My 52 dumper ran popped back through the carb and had similar running characteristics until I spent a weekend going through it. Biggest problem was an intake valve hung open, but the timing was way too retarded as well. Also was running lean which didn't help any. Have they checked to make sure the intake manifold gasket isn't leaking?

Mine still runs lean, but it is a carb problem that is easily fixed by leaving the choke on a little till I have a spare afternoon to rip into it again.

Found an intake manifold that had burned a small hole thru from the exhaust hot box ,,,, 'caused lots of problems ..

check and make sure your distributor isnt to retarded. Mine popos too. It drove me crazy. I had a great idle but twice a second it would pop. Drove me f%$king nuts. Ended up buying a new dis. hope you figure it out.

the last 292 I had would pop a little at idle sometimes. Nothing major, but it drove me nuts. I happened to be in the right position one day to notice a little day light between the manifold and head. New gasket took cured of the problem. Mike

This engine is completely rebuilt,everything new ,pistons, bearings or babbit,cam,valves ,springs etc. The engine had the exact same problem when it was originally started up and used dual carbs for induction.Then a stock manifold was installed,3 different carbs, same problem.Like all you guys suggest, its some minor thing probably.I don't know nothing, I'm just a electrician,but the machine shop thought I might know a "secret" problem about these engines.

Tony, we all know you do know the secret,,,but you are not suppose to tell secrets now are you?
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1.Weak or broken valvespring
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2.Bent or sticking valve.
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3.Valve adjusted too tight.

If they have checked these things-weak breaker arm spring, breaker points improperly adjusted, wrong type of spark plugs- and everything is as you have aid, it can be nothing else but a weak valve spring. Did they check the "new"(?) valve springs before installing them? Sometimes ven new ones are weak. What cam is in it? May need to double spring it for a high-lift cam. But I will guarantee that it is the spring. That is all that will give these symptoms.