250R higher compression motor

[nj250R]

I had a guy tell me that if i built a motor like this it would be nasty. Let me know what you guys think.

1. Buy a Trx 86-87 Bottom End
2. Trx 88-89 Upper End with the head milled
3. Atc 86 piston
4. 38mm PJ CR500 carb with 168 main jet
5. 89 Trx CDI unit
He said that this would make a high compression motor

[tw05tr0k35]

WRONG !!88-89 barrels are 5mm taller to accomadate the longer rod (125.3mmL for 85-87 and 130.3mmL for 88-89, same piston) This combo would not make a high compression motor.!! Sorry for the misinformation. I stand corrected, the part numbers are different but close. I was wrong.

More compression does not always work in a two stroke engine. In a MX style motor where it is not overrevved (much) and needs bottom end to pull you up jump faces more compression is generally tuned into the motor. Top end motors generally use less compression than a torquey motor. Trail tuned motors somewhere in between. If you use a high compression motor mainly for trail riding there can be issues with overheating on long runs with the r's up. If you can get a motor builder to tell you what they generally do then the consensus seems to be for a gasoline burning motor 9.5:1 captive for a MX motor and 8:1 for a top end motor. Simply milling the head is not the best thing to in some cases either. You can, by closing the squish gap tune in more MSV than required and this can lead to a chronic overheating problem too. Stock CR's are a little weak, but consider buying an aftermarket head or having your stock one reshaped or, at the least have your dimensions input into software and get a good idea of what changing the squish gap will do to MSV.

[jeffatc250r]

88-89 barrels are 5mm taller to accomadate the longer rod (125.3mmL for 85-87 and 130.3mmL for 88-89, same piston) This combo would not make a high compression motor. More compression does not always work in a two stroke engine. In a MX style motor where it is not overrevved (much) and needs bottom end to pull you up jump faces more compression is generally tuned into the motor. Top end motors generally use less compression than a torquey motor. Trail tuned motors somewhere in between. If you use a high compression motor mainly for trail riding there can be issues with overheating on long runs with the r's up. If you can get a motor builder to tell you what they generally do then the consensus seems to be for a gasoline burning motor 9.5:1 captive for a MX motor and 8:1 for a top end motor. Simply milling the head is not the best thing to in some cases either. You can, by closing the squish gap tune in more MSV than required and this can lead to a chronic overheating problem too. Stock CR's are a little weak, but consider buying an aftermarket head or having your stock one reshaped or, at the least have your dimensions input into software and get a good idea of what changing the squish gap will do to MSV.

Cylinders are all the same hegiht from 85-89. The difference for the rod length is taken up with the location of the piston pin location. 87-89 lower ends use a long rod.

[KASEY]

88-89 barrels are 5mm taller to accomadate the longer rod (125.3mml for 85-87 and 130.3mml for 88-89, same piston)

this is bad information!!!! Wrong all atc and trx cylinders are the same,,,,,,,,

[audioworks04]

That is correct the jugs are all the same height, but the porting and port timing is different between the atc and the trx also different between the early models and the later models.

[JohnR.]

Yes, the jugs are interchangeable but definitely NOT the same. I have had the stock jug off my 89 TRX sitting next to the stock one off my R and they are quite different. There is a totally different bridge set up IIRC.