jb_dallas

Member
Feb 17, 2009
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First off let me say that I know this topic has been beat to death and I have read many of the mix threads, but they dont address my specific question. I have a 1982 RM 125. I just installed a .080 over wiseco piston. The cylinder was in good shape. I expected more power now compared to the worn piston I removed, but the powerband doesnt seem to hit as hard as I expected. I am running stock jetting and 30:1 non syn oil. I am getting alot of spooge, a wet plug, and alot of smoke. I noticed some mix oil in the crankcase when it was apart also...could that be burning off? I replaced the crank seal, so that shouldnt be an issue. Shouldnt the bigger bore and higher compression piston require a larger jet? This is exactly the opposite of what I would expect. My air screw is turned slightly over 2 turns. I bought some synthetic oil and the guy at the store told me to run it at least 40:1. Should I re-jet, change the mix, both?

Thanks
 

Rich Rohrich

Moderator / BioHazard
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Leave the mix alone and fix the jetting.
 

jb_dallas

Member
Feb 17, 2009
498
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By Fixing the jetting, do you mean go to a smaller jet? Wouldnt that starve the larger bore high compression piston and make it difficult to start cold? If I go to a larger jet, wouldnt that make the smoking/spitting problem worse?

Thanks
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,838
16,902
Chicago
jb_dallas said:
By Fixing the jetting, do you mean go to a smaller jet? Wouldnt that starve the larger bore high compression piston and make it difficult to start cold?

That's not how it works. A larger bore, or higher compression does not automatically mean you'll need richer jetting (larger jets). That's why they call it tuning, you have to TUNE the jetting to your new combination. Here's some good info to get you started : http://www.chicagostories.net/ericgorr/EG_Carb Tuning.html



jb_dallas said:
If I go to a larger jet, wouldnt that make the smoking/spitting problem worse?

Yes.
 

jb_dallas

Member
Feb 17, 2009
498
0
If I am already running stock jets, it seems a smaller jet would starve the engine for fuel. It sounds like a trial and error procedure.
 

_JOE_

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May 10, 2007
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The volume and velocity of air coming through the carb as well as the flow out of the engine will effect jetting. Typically a bigger bore will require more fuel but that bigger bore will also draw more air through the carb which may result in more fuel being drawn up through the carb requiring smaller jets. Reguardless, if the bike is smoking heavily and feeling flat the jetting is rich.
 

jb_dallas

Member
Feb 17, 2009
498
0
My Clymer manual talks about moving the e clip position on the jet needle to make the mixture leaner or richer to change mid range performance, should I start there? What are opinions on this procedure?
 

jb_dallas

Member
Feb 17, 2009
498
0
That is some good information. My reeds are worn, I noticed one isnt laying flat...maybe that is the cause or one cause of my problem. I guess I should start there.

Thanks
 

jb_dallas

Member
Feb 17, 2009
498
0
I am replacing the reeds because they are worn, but I am confused on how a leaky reed could mimic rich jetting. The bike is not hard to start, usually starts first or second kick.
 

jb_dallas

Member
Feb 17, 2009
498
0
I replaced the reeds with Boyeseen Power reeds and moved the needle up a notch, it made a huge difference. I have some tuning to do yet, but it is closer.
 

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