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Mysterious Engine Burndowns


Some issues seem to have come to light more than others, over the years.
Frequent is the notion of a fan cooled twin having a “hot” side, that coincidentally is the farthest from the fan.
This old notion has been used to explain away hundreds of seizures. It is logical but not likely, that the cold air
across the first cylinder is not as effective at cooling the second cylinder. It is more likely that the second
cylinder is actually running hotter.

Many sleds have arrived at our shop with staggered plugs. Usually that is a sure sign of major problems.

As a rule there are two culprits here.
One is a leaky seal, the other is crank deflection.

A leaky crank seal is blatantly obvious as it leaks oil and gas.

Most times you have to split the cases to instal a seal, so read on about deflection.

To run this test you need a dial indicator and a degree wheel. Both of these are now available from China and
incredibly cheap.
To start remove the plugs and starter housing exposing timing marks. We just need T.D.C. marked on the mag side.
Install the degree wheel on the flywheel and the indicator in the mag plug hole.
Rotate your engine clockwise and determine T.D.C. on the piston crown. This should correspond to T on
your flywheel and timing mark. If it doesn't line up do a double check. If it still is out, mark a new line with a scribe.
Set your degree wheel and counter to exactly 0 degrees. Move the indicator to the other PTO plug hole.
Rotate your engine 180 degrees and check to see exact T.D.C. occurring.
If it is happening sooner or later than 180 degrees, that is deflection, expressed as thousands of an inch of piston travel.
You can express the deflection in degrees by moving the crank and reading the pointer.

It doesn't matter though, it is all BAD. If your reading shows B.T.D.C., that is ignition advance,
A.T.D.C. is retarded.

From these figures you can determine a hot cylinder problem or the reverse. Some older sleds
could skip a crank out of phase just by breaking a belt or by being stuffed with rope for a really nasty
clutch removal. In the dark ages the solution was to run a colder plug on the hot side. The real and
only solution is to pull your crank out and have it straightened. In the days of the 77 SRX we routinely
pulled the engine after every second race.
Come to think of it they were never ever true. Every time the crank would be out one way or another. It was easy
horse power for us and the basic reason our sleds did so well. Drags could be easier on the sled but ovals liked to
twist that light crank. It was a fact of life that you may have to accept as we did.
My advice, for what it is worth, is to degree out your engine before you race it. Never expect a straight crank.
It is almost a given in racing that an absolutely straight crank is the only way to achieve true performance.
While you have your plant out and easy to work on, do a quick pressure test to determine primary compression.
Plug your intake and exhaust ports and hook a small pump to your pulse line. Use a low pressure gauge to
introduce a pressure of about 3 or 4 pounds. A reasonable engine should hold that low a pressure for a long time.
If not check the gaskets, find the leak and fix it. P.S. it really helps if you have the plugs in the engine.
The problem is the worst when a Siamese coil is concerned. Some guys that set the old points systems with dial
indicators could compensate but it is still all bad. A very few thousandths will cause a dramatic
increase in cylinder temp and most likely lead to detonation. Try for zero and you will have peace of mind at the track.

See you there!!
GORD HEALY
SSRA #100

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