1995 Speedster, Port engine low compression

[Bruce '95 Speedster]

Hello all,

I had checked the compression a few days ago due to issues with the port engine running poorly, and it seemed fine. I had tried to test run the boat, and the port engine was still running poorly. It would start out of the water fine, but backfire a little bit. In the water, it would barely run.

I checked the compression this morning, and the rear cylinder of the port engine has zero compression. The front cylinder is fine.

Both pistons move properly, and inspection through the spark plug hole does not indicate a hole in the piston.

I am reasonably mechanically adept, but am wondering:

Is there a common cause for a loss of compression?

Would a rave valve be the cause?

Seeing as the piston moves up & down properly, can I assume this is a top-end problem that I can fix without removing the engine entirely?

Thanks!

Bruce

 

[derek]

95 speedster is 657X motors. no RAVE valves. what causes the rear cyl to fail sooner than the front is crank torsion. under load, the front cyl is pushing on the crank. heavy boat-good grip on the water. the crank will flex about 1 or 2 degrees. the ignition is up front. doesn't know rear piston is coming up a little late. result; timing too advanced on rear cyl under heavy load which is worst poss time for too much advance. burns up the top of the piston. when you pull the head you will find the outside of the piston has left the building down to where there are hunks of top ring gone. you will probably need a head too. the stuff flying around in the chamber before it went out the exhaust will have done a number on it. check the rear of the rear cyl. part number. it will be 380 or 382. last 3 numbers. 382 is a 657x cyl. 380 is standard 657. don't mix them. x cyl is 1 mm taller. right now on eBay there is a 657X cyl and piston for sale. .75mm oversize. 39 bucks opening bid-no bidders. closes in less than 2 days. closes sunday evening aug 3. you could probably get by with a standard 657 head. X heads are hard to find. use thinnest base gasket possible. the 650 head has a combustion chamber that is 1mm taller than the x head. so, tomorrow, pull the head and see what you got.