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McCulloch Bp1 runs too good


McCulloch BP1 Runs too good. I acquired a BP1 a while a go along with a assortment of other chainsaws. I finally decided to do something with these guys. So i am starting with the BP1. First I took it apart and cleaned it up. Reassembled it with no problem. The thing started on about the 12th pull. Great!!!:):):):) Now for my problem it went to full throttle and stayed there. (love the sound). I looked down the carburetor throat and the butterfly was closed all the way. How is it i get full throttle when the carburetor butterfly was at idle position. I figure there must be some sort of way the thing is getting air to run, any ideas? I did not disassemble the cylinder part of the engine.


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Well... if the throttle is closed, you're not getting 'full throttle'... you're just getting lots of RPM...

Are you absolutely, positively certain that the throttle is completely closed? If so, then there's gotta be... not only an intake leak... but a fuel leak as well... like... a crack in the carbeurator someplace.

If it were me, I'd pull the carb off, look at the gasket really close, and look at the idle stop, and see if mebbie it didn't get cranked in a bit and left there.

It doesn't take much flow to race the engine up to speed... but if you put a load on it, it'd fall flat quick.

Remember that a two stroke running lean will spin pretty darned fast under no load... not good for 'em, but they will do it.

Also... check the spark plug and combustion chamber for deposits... IF there's lots of crud in there, it's displacing chamber volume, AND insulating the head from combustion heat. These two things cause the chamber pressure to rise very rapidly at TDC, and make any sharp edges really hot, which causes preignition... it becomes a high-speed diesel (like a glow-plug model engine), as a result, it's acting like the timing advances a whole lot, once it's running.

If you think this might be the case, get it started, and once it runs up, shut off the ignition... if it keeps running, it's preignition.


I think the diaphragm is probably stiff and the needle & seat isn't shutting off the gas, kind of like running a fuel injection carb. With both pistons sucking gas, might be possible. The BP-1 s were recalled because the bottom piston ran too hot, and getting proper carb responses. The man that designed that saw was ahead of the times.
I used one in competition in the small saw class, always had to take the spec charts with me to prove I wasn't cheating. Nothing could come close!

Andrew Mackey

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One other possibility: Are you sure you assembled the carb to the engine corretly? Some saw carbs allow the throttle to go over square to the bore. When you assemble it to the engine, it gets locked in this partial open throttle position,and the engine then runs away as the throtle cannot close. The other thing would be - did you take the throttle plate off the shaft? it can be mounted reversed and posibly upside down which would not allow it to close the bore completely


I looked at the throttle plate, think that i where my problem lies. it binds on the bottom and has a gap on the top. Also made some new intake gaskets. You would not happen to know the high setting I'm out 1 and 1/2 turns. don't think it is possible to adjust when it is all together. I will update you guys when i fire it up again. thanks for the help.

---------- Post added at 05:52:25 PM ---------- Previous post was at 05:51:04 PM ----------

I think andrew a few post down hit the nail on the head. Thanks for the help.


Checked the throttle plate, new gaskets just won't idle unless around 5000 rpm is what the things low speed is. I made a video but don't know how to post it.