finding exhaust leak?

[whans]

I have an exhaust leak somewhere down below, and I have been unable to pinpoint it. When the engine is running, exhaust smoke rises from it-- it seems to be coming from the front of the engine somewhere, although that is of course the easiest place to see. It may be crankcase fumes-- if I turn the crankcase breather tube away from the air intake, the amount of smoke increases. I recently replaced the first section of exhaust pipe, where the water is injected, so I don't think that's it. It was very badly corroded and may have been contributing to the smoke, but it did not solve the problem.

What is the best way to hunt down the source, and/or what are the most likely sources?


Thank you.

[Don Moyer]

Whans,

It sounds as though your smoke is probably from blow-by. Here's a recent technical note we prepared on the subject:

Blow-by refers to both burned and unburned gasses escaping between the piston and cylinder bore from the combustion chamber (blowing-by the piston, hence the name) during normal engine operation.

All engines have a little blow-by, but excessive blow-by is an indication of worn rings, worn cylinder bore, or both.

There's a couple things you can do to reduce blow-by:

1) Check compression. Compression should be over 80 psi in all cylinders, or sufficient to blow past your thumb if you try to hold it over a spark plug hole while someone turns the engine over on the starter. Compression in the range of 60 to 70 psi can frequently be improved with Marvel Mystery Oil (MMO) treatments. MMO treatments are simply a matter of squirting 5 or 6 squirts of MMO into the spark plug holes of affected cylinders and then running the engine to burn off the MMO. For best results, the MMO should be squirted away from the manifold side of the engine, toward the bore. The MMO will free up partially stuck compression rings, for better sealing
against the walls of the bore.

2) If your compression is good on all 4 cylinders, check the ignition timing under load. To do this, you simply loosen the distributor hold-down bracket and slowly rotate the distributor a small amount in each direction to look for an increase in RPM. If the engine RPM increases, tighten the bracket in that location.

NOTE: If the engine has not been tuned up for several years, you might start by replacing the points and condenser (or at least cleaning the
points) before doing the timing check.

3) If the exhaust from the engine has a bluish look, a potentially rich fuel mixture would be indicated. While a richer than normal mixture would not directly cause or contribute to the severity of blow-by, it can lead to sticky rings, which would in turn aggravate a blow-by condition. A rich mixture can usually be leaned by adjusting the main jet (on an early model carburetor), or by the installation of an adjustable main jet (in the case of late model carburetors).

4) If you're not already doing so, start using Marvel Mystery Oil (MMO) in your fuel at a rate of 8 to 10 ounces per 10 gallons. MMO has a good track record in freeing up rings and valves and thereby improving the overall health of your engine.

5) In the case of engines that are otherwise performing well, and still produce objectionable blow-by, the blow-by can be greatly reduced (and frequently eliminated) by installing the crankcase venting kit produced by Indigo Electronics. You can call Indigo at: (800) 428-8569.

Best regards,

Don Moyer

[NickD]

I had a similar experience with fumes in the cabin. Then I accidentally broke my rubber breather tube which was cracked and taped. When I found a simple copper elbow plumbing fitting and installed it directly over the carb, the fumes disappeared. Nick