My friend and I installed a 11-1 piston and a stage one cam, using the stock springs, when the engine is cold, it starts fine, but once the engine is hot, it wants to back fire, or load up, not sure which way is best to explain, anyway, is their a fix to the problem, like shaving the cam some on the weights, have you had this happen, and what would your recommendation be?? The spring, plunger are installed correctly but still dont know what causing this, has anyone else had a similiar problem and what did you do to correct it??

thanks

 

What do you mean it wants to backfire or load up?? You need to post more details. Does he have the carb boots on all the way? What is he jetted at and what altitude? It might just be a lean problem, or maybe he didnt adjust the valves properly.

 

Sound like you need to rejet just the needle by just lowering the needle by one slot.

I had that same problem that it was loading up and backfiring on the mid range. So I adjust the needle of the carb and the problem went away. It just running a little too rich on the mid range.

Hopefully that will help you.....

 

Did you guys use the stock decompression pin? If you did, it might have torn it up.

 

Carb boots are on all the way, jetted 160-165, 3000' elevation, runs good, the problem is when you first start it, when it's cold, it will start fine, after running and the engine gets warm, you start it, it wonts to back fire or load up, this is the only explanation I can give, yes, he used the stock pin/plunger/spring, could their be a way to install it wrong?? I will have him recheck the valves....

thanks....

 

The pin shouldn't have anything to do with it bogging, but if it came with a new pin you may need to put it in instead of the stocker. But I would try what desracers said and lower the needle, it will lean out the mid range, but by any chance when you say backfire do you mean its stumbling or misfiring?? If so its the mid range being to rich.

 

I think your idle ckt is rich... Recheck the air screws on the bottom of the carbs

One thing you definitely dont want is coughing and backfiring on start up or take stock on yamaha one-ways.

 

Is it only on start up? Did you bore for the piston or just put it in? How about check piston to wall clearance?

 

yes, it only happens during start up after the engine has been warmed up, do you know if the make another spring to replace the stock one, will move clip and mess with the air screw tonight and see if that helps, just don't wont it acting up and knock the boots of the carbs or worse, the only other explanation I can give is it's like kick starting a motorbike, you know how sometimes the kick starter will come back at your leg, something like that, other than that, it runs fine...the cylinder was honed to except the the piston, don't know the clearance of piston to wall..

thanks for all the help......

 

Has this machine been rejetted after the cam and piston? I installed the same components and wound up going up 10 sizes on the mains...and 1 on the needles. It could be very lean if not.