Kornfused RMX

[atomarc]

Hello All,
Brand new to the site. Hope someone can point me in the proper direction on my questions. A '91 RMX 250, FMF Fatty, Rad Valve, Keihin 36 pwk carburetor and a port job. This bike was purchased for my son in a not running condition. We replaced reeds, plastic, pressure test engine etc..ran great and. A outing to the beach seized the engine. LA Sleeve did sleeve and supplied piston etc. Assembled, broke in at 32:1, ran great...NOW, at 40:1 it blubbers off idle, isn't snappy and has poor throttle response! Keinhin came with #55 idle jet, #188 main jet and needle on 2nd clip from top.. I have no clue what I am doing. I am going to take carb back to specs it was supplied with; #55 idle, #165 main and ?? on clip position. I purchased new Boysen pro reeds although old ones look ok, I guess! Why did this thing seem to go from ok to funky?? I, obviously am not a M/C guy. :bang:

HELP, Stuart.

 

[cnielse5]

What is you elevation.
from the sounds of it it sounds like the bike ran ok until you changed the gas/oil mixture. when you go from a 32:1 mixture to a 40:1 you are actually making the bike run richer. Sounds backwards right? well the way it works is, that your carb always allows the same amount of fuel into the intake (unless you change the jet size) and so when you mix your gas and oil at a certain ratio (say 32:1) that means you have 32 portion of gasoline and 1 portion of oil that is in that set amount of fuel charge going into the intake. so when you lean out you premix gas/oil (to say 40:1) you actually are makeing the fuel charge richer. (32:1 had 32 parts gasoline compared to now 40 parts in 40:1 mixture) I know confusing, but hopefully you can understand what I am saying.

Now for the short answer.
The bike is running rich and causing it to burble. lean it out. you altitude makes a diffrence.
if you have blubbering at low end end move the clip up a notch and maybe mess with the main (go down a step or two i.e. 185, 182

 

[atomarc]

Cnielse5,
Thanks for the info...I am at sea level. We originally ran the bike on 50:1 and it ran and idled great. I am going to try and lean it out as you suggest, but I am a "how come" sort and I don't understand why it has changed from fast to blubber. I think the power valve system is a "work or no work" kind of thing isn't it!. I don't know if I described the condition in the proper terms. When you crack the throttle, it isn't snappy..it's like a car that is timed too slow. Is this a lean or rich condition? I keep thinking that when the engine seized maybe a crank seal got too hot and failed and I am pulling air from the un-oil (?) side. This bike seemed like it was, as they say, a rocket..I would swear that at the break in period on 32:1 it was fine, then , with only changing the ratio. it got funky. I sure don't want to lean it to a molten lump!

Stuart.

 

[elf]

Switch back to 32:1 and try it. Then you will know for sure if changing the gas/oil ratio is what caused the problem.

 

[sick 96 250]

the new top end can make your jetting change because it combusting the fuel differently then when the piston was worn a little. Every time I have redone a top end on my RM's jetting almost always needed to be redone. Sounds like it is semi rich on the top end. I doubt it is because you went to a 40:1 mix, thats what i run and I have ran 32:1 as well and there shouldnt be that big of a difference. Check fmfracing.com for jetting specs. Although they use a 50/50 mix of race gas and regular their specs always seemed to work for me when i ran 93 octane. Good Luck and let us know how it turns out.

 

[atomarc]

I am not sure why this Keihin 36 which calls for a 165 main jet could work on this bike with a 188 jet..isn't this a huge jump. I have seen posts on changing idle jets and main jets, but when does a person change the jet needle...if thats the correct name of the thing with the clips on it. Also, my son said the bike took off on him without the throttle being twisted, is this bike possesed! :debil:

Stuart.