CR250 engine noise

[cr250racer]

I have a CR250 2002 model. It has developed engine noise at full throttle when at high revs like in the top end of the power band. It is a loud metalic rattle sort of noise. I have checked for play in all the berings, small end, big end, mains etc no faults found. When it makes the noise it fills like it is also loosing power. Could it be detonation? the bike is completely standard to my knoledge apart from possibly a topend rebuild the previous owner might have carried out. the only thing i can think of is maybe the power valve is rattling has anyone had any troble with there power valve rattleing inside its housing, it is all ajusted up tight. It also starts very well when cold even with out the choke on. I run 95 ron petrol. I thought that detonation occours when the engine runs lean not rich am i wrong? thanks for ant help which you can give me because i dont know where to go from here thanks john

 

[cr250racer]

also the petrol i put in it has been in a can for about a month, can petrol loose its octane level, could this cause my problem thanks john

 

[j_freak]

Yes, old gas has a lower octane than fresh fuel. Other than that I can't help you

 

[SULLY]

sounds to me like detonation. When the mixture entering the combustion chamber is too lean (not enough fuel in the air/fuel ratio) it will self detonate before the correct time(spark). If it is happening at wide open throttle that is most affected by the main jet fuel circuit. IE those components that are being used to deliver fuel for wide open throttle. Your Airscrew or idle jet will have no effect on wide open throttle.
So...you need to be looking at the main jet, needle part number, needle jet, and what clip position the needle is set on.
I don't know what yours should be set to but There are plenty of threads on this site to helep with this. Do a search on CR250 '02 jetting.
Also be sure that your float bowl is clean, and that the float height is set correctly (will have an effect if it has been badly set, but usually does not get changed unless someone has been messing) Also check for Air boot leaks around the carb.

 

[cr250racer]

Found the problem i put fresh petrol in it, and it sorted it runs like a dream. The petrol was only a mounth old ive used alot older petrol than that in other bikes ive had and never had a problem. the hondas are obviously a bit fussy

thanks john

 

[robbaker450]

Fresh petrol

My 04 kx250 2 stroke has had the same problem with detonation because of petrol.The first thing i did to solve it was buy racing fuel 100 plus octane.The second fix was this fuel went off quickly and i buy it fresh on race day. I have owned 4 strokes for 3 years and never had a problem and MANY bikes before that and none have ever needed race fuel.