Whats a long rod 250R engine?

 

The 85 and 86 year motors had the shorter rod while the 87-89 motors had the longer rod.

 

If you have an '86, you can change to the longer rod by running a spacer plate under the cylinder.

I have heard opinions on both sides about whether this makes for a better high revving motor and better low end torque. The longer rod causes longer dwell. The piston stays near top dead center longer which is supposed to help high RPM. The long rod also reduces the rod angle which is supposed to help torque because the rod is pushing down straighter on the crank. Often, in two strokes, you lose at the bottom what you gain at the top. I have yet to hear what the negative points of the longer rod are. One thing I wonder about is if the dwell is increased, this means the linear speed of the piston must be increased at 90 degrees crank rotation. Is this good/bad/irrelevant? Port timing would be altered. Does the top of the exhaust port need to be raised in order to increase the difference between the top of the exhaust port and the transfers? If the piston is moving faster, the time for the exhaust to exit before the transfers are uncovered would be decreased.

 

The longer rod also reduces the angle of the rod as compared to the shorter one, and should help with top end life.

 

One more thing to ponder upon ( volume verses velocity )
Going to a longer rod lowers your primary compression ratio ( crankcase compression ) if you opt to go with a spacer plate. You can build this motor using the piston designed for the longer 130.3 rod, and if you do you don't have to read the rest of this because it won't apply ( use the late model 250 R piston and no spacer plate is needed )The longer rod with a spacer plate will increase crank case volume, this added volume is a good thing for a long strong intake cycle. This makes for a broad but lower power band, but the trade off is a slower velocity at higher rpm's. The porting can be altered to make this less of an issue but I won't go into that here. This example is for the 250 R and not all motors. The 250 R has an acceptable rod ratio with ether rod, and not all motors would fall into this category. ( both rods are over a 170 rod ratio )( 170 being a good acceptable rod ratio ) This is just one way to look at the long rod verses short rod debate. There are far more things to take into account when choosing between the two, but I felt inclined to but this on the list as one of them.

 

Harry,
How does this affect the motors when displacement is increased to 350 strictly by bore increase? What is your opinion on the 350 in general, and what size carb do you reccomend for general purpose riding and occasional sand drags. I'm putting this in an ATC. I'll probably be running a different dome for drag racing to enhance performance through the use of race gas. I'm having a hard time getting a solid answer from the local performance shop I ordered the jug through and who is taking care of the case machine work.

Thanks

 

kciH
I'm guessing that you have a 79 mm piston making the motor 353cc's. This motor would be a good revving motor and great for drag racing. I would be a little concerned with the strength of the cylinder with that big of bore especially if you are going to run hi compression. Cooling, transfer port tunnels, gasket sealing, would also be a concern as you push the motor for drag racing. I still think if done right it would be fast. I would try to keep the piston weight down and I would definitely go with the longer 130.3 rod, this will help the piston live. I would also consider ceramic coating the piston top this will help heat displacement. The carburetor I would use on this motor would be a 39 pwk . You better be investing in at least a 10 inch swingarm on that three wheeler before you go anywhere though. My recommendation: hold on real tight!!!!!!

 

I get my FL350 pistons ceramic coated on top and teflon coated on the sides by Swain Technologies.