I have an 87 250R, bought it completly stock. After putting a new keihin 39mm carb on I've blown 2 engines. The first, piston melted on the exhaust side. Bought another engine off ebay that was fully ported and polished with hot rods rotating assembly. Went up on the main jet from 160 to 180 and left everything else alone. Rode it for about 20 min, started rattling real bad then died. The ports on the intake side were cracked and it needs a new sleeve. The piston rings themselves are welded to the piston on the exhaust side.
Ok, it starts and idols well and runs very well, but keeps poping pistons. The next step I'm going to do is move the needle up all the way to richen it up. Is there anything else I should do, like mess with the timing or anything? Any help would be appreciated.
Thanks,
Luke
87 250R- Hot rods rotating assembly, boyesen reed cage, cr head, keihin 39mm carb, fmf full exhaust.

 

read the cylinder break in sheet on this page.. ctracing. it will tell you how to read the spark plug to jet the carb properly. sounds like you are running way lean.

 

If anyone else has a similar combo could you tell me what your jetting is?

 

i may be way off base here, half a sleep, over simple, or on crack (not really) but what kind of plug, ignition, etc are you running? what type of gas? race gas??? if you dont have high compression i just dont think you need it for your setup. mine runs on 91-93octane, 32:1 premix with a stock carb, boysen radvalve reed cage, full FMF system as well with Q silencer; runs excellent and has for the 5yrs i have owned it, and 17yrs before i got it. its only been rebuilt once.

from the factory, the 86-88 engines had the same stator and a little weaker spark and specified a br8es plug. the 89 engine had a higher output stator, more spark and called for a br9es plug (which isnt as hot). evidently they just wanted a little more spark or they would not have called for the weaker plug with the higher output stator; i know it seems counter productive....but thats what honda did. im gonna go out on a limb here and assume that the engines you bought were not stock and may have an aftemarket, higher output stator that maybe and evidently isnt gelling with your electrical and jetting situation.

on the jetting it does sound too high; why go UP if you are blowing up stuff?

on the first time - are you sure the bike was stock when you bought it? and ALL you did was change the carb?

 

Yes, the quad was 100% stock when I bought it. I first did the reed cage and exhaust, couldn't get it to run all that well, then went to a new carb. I am jetting up because if I'm melting pistons I'm sure I'm too lean, therefore I should give it more gas/oil mixture to richen it up.

 

I'm also running an 8 plug, and am sure the timing is stock. I have no idea about the timing issue if someone could lead me in the right direction it would be much appreciated.

 

Ok got the engine back together this afternoon. I moved the needle clip all the way to the bottom raising the needle up all the way, and it's still not smoking at all out the exhaust. So my question is the needle effected by the main jet or piolet jet? What I mean can I get more fuel in the mid-range by jetting up on the main or piolet jet? Also is the idle air screw used throughout the powerband or just at idle? I know that I can cut off air with it but will it help throughout the power range. I'm going to wynoka oklahoma this weekend and would like to have it squared away and not pop another piston. Any knowledge would be appreciated!
Thanks,
Luke

 

Anyone?

 

It sounds to me like you are pulling in air from somewhere else.180 jets on a 39pwk with little motor work is way too rich even for sea level. Id go over your intake boot real good to make sure its not split. Then Id check your base gasket

 

Ok thanks man I appreciate it. Your right with me being plenty rich up top but I think I'm too lean on the piolet, so even when I let out it doesn't really get the chance to cool the engine. I'm learning something new every day and it's been fun so far. Hopefully I can get it close enough to not blow it up at the dunes this weekend. Thanks again,
Luke