GRM 125cc Pipe Pull Question

[Al Peffley]

For some reason, when I pull the pipe on my Rossi 125cc motor the rpms peak at about half pull and then at full pull the motor bogs down. It does not matter how slow I pull the pipe. Another 125ccH driver said he is having the same thing happen to him when he pulls the pipe. I have owned a 350cc Konig, a 250cc Yamato, and a 250cc GRM PRO motor with pipes and never experienced this situation. I tried changing pipes and it still does it. Can anyone explain why the motor is doing this? Maybe the pipes have the wrong section dimensions for my motor setup? Both pipes came with the motor. They are definitely GRM 125cc factory pipes because they have the dorsal guide blade on top for Rossi's pipe ring. I don't know of any other PRO motor that uses that pipe ring. I am using a new OEM cylinder and 38mm VHSB carb. Weird...

Al

[nicf14]

Your over propped, try a smaller prop

[Steve77t]

agree!

You are either over proped or the set up is way off. The motors you ran before have little to NO comparison to any 125. A really good 125 makes less torque that you do when you tighten the spark plug. Always keep that in mind! Steve

[bill van steenwyk]

both replies you have had are very good ones............

.....but here is one more thing to check. If you are not absolutely sure of the gear ratio in the lower unit you are running under the motor, check it the old fashioned way by counting the revolutions of the flywheel versus the prop shaft.

I was sold a lower unit at one time that was supposed to be a 12-15 and turned out to be a 12-16. The motor acted exactly the same way with the prop I was trying to run which should have been right for it.

Fortunately it was fast enough to win the first year I ran it but that winter at the suggestion of someone a lot smarter than I, (Harry ZAK) I checked the gear ratio and found that I had been snookered with the purchase of a brand new unit. It had also had the correct gear ratio (what it was) stamped out and what I thought it was stamped on it, so the seller knew what he was doing.

A really crappy thing for a dealer to pull!!

[Al Peffley]

Prop, Gears, & Setup

I am sure the prop I was using was mismatched after talking to Craig. I am also running too lean, even when I run a 38mm carb with a power jet on it. I will change gear ratios and the prop size during my next test session. Thanks for the responses. Not much I can do about the torque shortfall, except live with it I guess; seemed to be much more fun squeezing the throttle to full open and pulling the pipes on a whole 250 motor than half of one - LOL.

Al