help compression

hi afyer my second recent rebuild new piston rings chain bearings etc etc i was driving along today road use approx 50 55 mph a sudden uh and the engine stopped pulled clutch in and free weeled safe to side of the road seems to have lost about 50% of its compression

trailered home and took the cam box cover off all 5 of the cam followers are moving up and down fine when kiked over any ideas pls

its like hard to find top dead cent for a good kick,,,this happened after i installed the piston but full comp came back after a few miles

some one must no it was running great till today ive done about 100 miles on the new engine no probs starting etc well stubborn but there 2nd 3rd kik

well are you sure you had enough oil? that's the first thing to check because lack of oil could seize her right up. another thing is i had the exact same thing happen to me, i was givin er and she lucked up out of nowhere, and wouldn't start for nothing. so i took it home and checked over the whole motor and everything was in spec and didnt change anything. so since it was all apart i took apart the carb and cleaned it up and blew out all the jets and put the carb back in and she started 3rd kick. there must have been a spec of dirt in the carb or something. all it takes is a tiny tiny tiny spec of dirt and the bike wont run. so i suggest cleaning the carb out and seeing if that makes a difference. -Wes

Did you check the decompression valve may be sticking

what bike is this?

thanks for the replys lads

johncerekid the carb????? my compression has all but gone could the carb affect this ..

mikeprosgula yes the decomp working fine i can use my fingers to engage disengage it

cjard its a 2000 wr 426

once again thanks for thr replys going to have a look in today and post another thread about valves there my concern i think i hope im wrong but fingers crossed eh

wes i just did an oil change the day before and added approx 1.5 litres of 10 40 semi i belive it was a good oil not the cheap rubbish

regards peter

well heres the problem whats left of my new piston what caused it now plenty oil water ok rad still full


I had a piston in an XR650L do that after I didn't properly gap the rings (thought the shop did it) and it finally got hot enough that they seized shut and wrecked the piston.

I probably had 60 miles on that bike before it happened.

Just a thought.


the heads away to be relined plated waiting for it coming back water ways fine lots of oil to cams etc my only thought now is the oil pipe that feeds the piston skirt area could this be in any way restricted a blockage yes but the mechanic said if it were blocked the gudgeon pin would be the first to go not piston

i would like to get to the bottom of this b4 a rebuild costing too much

regards peter uk

reet getting some where the squish must be spot on after a head and barrel skim too close the piston to cyl head this over heats and causes all sorts of bova so i need a squish test does any one no the gap thanks

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