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With the new RB head, I seem to be getting a lot more spew, black crap out the back of the tail pipe.

Running 40:1 "smokeless" premix.

Any solutions to this??

Thanks

do a plug check, you might need to rejet your carb. maybe you might need to service your top end. if your kips is not working or worn piston and ring contribute to more carbon.

a little is somewhat normal. Is your silencer cleaned? did you rejet after getting the head from rb? do a plug shop test to verify your jetting.
Note: don't jet by the amount of spoo you get / don't get, I only mentioned it because it might be part of the issue. the only way to jet right is to do a plug shop test, Inda has a great tutorial in the jetting section.

...."a lot more spew..."

With more efficient combustion due to the RBD head mod you may just be heating up the goop that had accumlated over time and you are now pushing it out the exhaust tip. I agree with check out the jetting and replace the packing (if you have a non-OEM silencer). More efficient combustion should lead overal to less spew - but of course you need to be jetted somewhat close to get the correct burn rate and the maximium power out of the engine.

I have the original stock silencer, with 525 miles on it, I'm sure it's never been repacked.

Jetting
is 40 P/J
55M/J
stock 1172 needle
boyesen reeds
FMF pipe
RB head

Yup... heating up the old spoo in the pipe and it will be nasty for a while.... mine is starting to calm down but for a while it was an insane amount.

wow interesting! so the RB head runs hotter?
Still using the B8ES plug?

I"ll try 50:1 in the next premix if that will help?

thanks

changing to 50:1 will only richen up the jetting setup if i am not wrong. why not trouble shoot one thing at a time then it will be more accurate. thats my opinion.

Less oil will richen the jetting, now I'm really confused.

I would think it would be just the opposite if you're putting
less oil in the mix, there would be less to burn.

Gas/air mix is what determines rich or lean... less oil only makes the mix richer.

correction 155M/J

ALso, throw the 'smokeless' **** away, and get a good fully synthetic 2-stroke oil. Stay at 40:1. You may find you can go leaner on your MJ depending on where you live.