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I readjusted the valves on the 51 200 yesterday and it now runs much quieter. The valves had been set wide after an overhaul. Now with the engine much quieter, I can now hear an exhaust leak. The manifold gaskets were replaced as part of the overhaul, and there was no exhaust noise prior to the overhaul. My guess is the gaskets may have slipped, or the connection to the exhaust pipe is bad. Any suggestions as to how to track down the culprit?

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A mechanics stethoscope, run the engine and using the 'scope "listen" at each join. Failing a mechanics stethoscope a DIY can be easily had from a length of rubber tubing, not as precise but still effective.

It's not likely that a gasket "slipped" as they are held in place by the studs and really can't slip. The gasket or gasket ring between the manifold and exhaust pipe is a good bet to check, that's a fairly common point of leakage. Just feel around it with your hand while the engine is running, you should be able to feel the leak.

When you had the manifolds off, did you have them surfaced or at least checked with a good straight edge? Warpage not uncommon after all this time.

If the intake manifold is bolted to the Exhaust manifold then u mite have to loosen the bolts that hold them together. Tighten the bolts at the block, THEN tighten the bolts that hold the 2wo manifolds together.

thanks guys for the advice. I did check the manifolds with a straight edge and they looked good. I'll work on this tomorrow following all of the suggestions.

I've also found that re-tightening the bolts two or three times after running the motor will help with exhaust leaks. Do it hot, if you can, but be careful not to gronk on it too hard, naturally. Seems like I always find a loose stud or two when I do this after I've had a manifold off. Good luck.