Motorcycle Repair: loss of compression, correct valve clearances, piston rings

Question
unfortunately i have a 1991 dr 350,so i'll take any advice i can. the bike had great compression when i got it months back. a couple of days after a good wash no compression. thought it could be compression release,but all ok. so tore down to check valves. ok again,rings ok. put back together with new gaskets. started right up,but had less compression,so drove it that way. now same thing no compression and wants to backfire a little with no compression. so i tore it back apart and still have not found anything that i can tell is wrong. am i missing something here ? any thoughts will help. thank you.  

Answer
Jim, hard to believe that the bike lost compression after a wash job, unless you filled the cylinder with water and bent a rod when you tried to start it.

Leave the valves in the head and pour some solvent down the ports with the head tipped up on edge and see if it leaks. If not, then you don't have to pull it apart again.
You should be able to see shiny seat marks on the valves and seats, if everything is seating properly. Do check the valve guide clearances if you do remove the valves. If they were excessively loose, the seats would be wide and irregular.

If the piston rings have a normal end gap and the cylinder walls show no signs of seizure or irregular wear, then the piston/rings should be okay.

I would check and double check the camchain play and stretch as well as the tensioner. Cam timing is critical to compression and if the chain were excessively worn, it might jump timing and suddenly be very late with the valve openings, which directly affects comprssion readings. Make sure that your cam timing is correct when reassembling the engine. Some of the "automatic" tensioners can back off under certain circumstances.

Be sure that you have correct valve clearances when you reassemble it all and either diconnect the compression release or be sure that it is not affecting the compression of the bike.

If you still have problems, have a shop do a leakdown test to see where the compression is going.

Bill Silver