RM250 Detonation

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Feb 23, 2004
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#1
I have a 2003 RM250 that i am rebuilding the top end on, not because it blew up, just because i felt it was time. The bike was running fine when i took it apart.

The top of the piston and the cylinder head have some tiny crater like pinholes and some burn marks and a local mechanic has told me I am having detonation problems.

My question is: how do I cure this so that I don't blow up my new top end? I race but not in a pro or A class, so i am willing to sacrifice a small amount of performance to get reliability and long life.
 
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#2
Could be that the jetting is way to lean or the fuel is old or not high enough octane. The timing could be not set correctly too much advance will cause more heat in the cylinder, because it fires closer to tdc. Was the deck or mating surface of the head cut, or maybe the base of the cylinder was shaved. Hit me back with more specifics and I and probably others will help you. :)
 
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#3
No, the deck surface has not been cut nor has the timing been changed.

I have messed with the jetting a little bit...it was leaner than stock when i got the bike and i changed it back to stock. Should I go even richer?

I run 92 octane and have been told i should run 94 and I will start doing this. Do i need further octane boost?

I run Bel-Ray MC1 mix at 50:1. I have occasionally used cheap Chevron mix and run it at 40:1.

I have also been told to run two cylinder base gaskets and this should help the detonation?

Other things...Bills Pipe, FMF Powercore silencer, Vforce reeds and cage on the bike
 

Chili

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#4
When you say stock jetting did you go back to the "USA" stock or the "Rest of the World" stock settings? 03 RM's in Canada came with unbelievably fat jetting to the point my kids RM125 would barely run, if you are running the jetting that the bike shipped with there is no way it should be detonating. When we got my son's 125 leaned out to where it ran it's best we were dealing with some slight detonation. I was too cheap to run race fuel simply becuase he rode way too much and it would have broken the bank so we opted to use Torco Mach Accelerator in the fuel and it worked like a charm.
 
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#5
i'm running Canada spec jetting...if i remember correctly, i think it is just one size richer on the main than US spec.
 

Chili

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#6
Hmmm, the 125's were 10 (460 versus 560) richer :yikes: on the main and 2 richer on the pilot. I'll have to see if I can find the specs somewhere.
 
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#7
Put the stock pipe and silencer on and see if it still detonates. Check your spark plug for how the timing is set. Or just chug race gas in it!