mikuni tm38 fs carb on 250r

[3wheelrider]

hi everyone,-i'd like some input/help on this mikuni carb (tm 38 fs) going on 85 250r stock except for exhaust(dg) and no lid. local shop sold(ordered) this guy this carb saying it was great on this bike and how it was pre-jetted! he brought it to me to install. i didnt know he was going to sell him this damn thing and not the keihins he had there in stock. -anyway, needless to say jetting is so off (bog badly) as soon as you give it gas. moved the needle up and it helped some but still bogs). has a 22.5 pilot 230 main. put the clip back in the middle now. need to rejet. 22.5 looks like about a 40 on a keihin-am i right? pilot must be off several sizes i imagine. main too maybe. needless to say, i have no jets for this and i dont have the drillout kit which i dont like. -oh, did i mention the fuel inlet and the idle screw is on the opposite damn side.not sure if the fuel line is going to be an issue since it has to loop back up and over the carb to the other side.-sucks. has anyone used this carb on this bike? is the slide/needle ok. is this even a good carb for 2 stroke?

[dcreel]

Yes, it is a good carb for a 2 stroke. Back in the 80's it was the carb to use. I use the 38mm flat slide on 2 of my Tecate's and a 41mm on my other Tecate. Rocky Mountain sells the jets for $2 each. Buy some smaller size mains and see what happens.

http://www.rockymountainatvmc.com/pr...FamilyId=24377

[phil53]

I have a 38mm mik FS that I used for awhile on my 86 250r, it was suppossedly pre-jetted also. Problem I had with it the jet-needle was a Q2 and needs to be a Q6 for a 250cc engine. Verify what needle and jet-needle you have in there. I overheated mine really bad in the midrange and warped the deck of the cylinder. I had a surge at about 1/4 throttle or so. I eventually got it worked out and then swapped to a 36mm TMX. I never had any issues w/ the fuel line. It will haul when it is jetted right. Where you at in miami? i have a few jets for it laying around somewhere...

[3wheelrider]

hey thanks for the replies guys. hmm smaller main? i figured it was lean on pilot and or needle. it wont rev up -just bog. gonna have to try some different size jets smaller/bigger see what happens.-hey phil53 nice to see another local member. do you still ride that r? we gotta ride, i have one too and a 185s with some balls too. hey i might have to take you up on those jets to try ,-that would be greatly appreciated!- i'm between hialeah/miami lakes/opa-locka. would like to meet.

[phil53]

I haven't riden it to much lately. It needs some work, damn ethanol in the fuel messes up everything it stays in contact with. The carb for the 86 is sitting on the shelf waiting for me to put it back together. The 85 is all in pieces waiting to be put together. Where is there to ride locally lately? I am by the gables.

I looked at my 38 FS and saw I had a 300 main jet, take the carb apart and slide out the jet the needle slides into under the main and double check the #, it will cause it to lean out the midrange. What shop did the carb come from?

[3wheelrider]

from M.A.M. (grant) -near you. gotta check on the needle #-still. next to metro-west prison. its not that great for these bikes but just somewhere to ride without any hassle-so far.

[phil53]

That is where I got my carb from, if there is a problem with it, take it back to Grant. He will get the right jet-needle for you. The vendor he gets it from sells those carbs for 125cc bikes, sounds like the same thing I went thru. I went back with the 36mm so I can use the airbox. I will send you a PM w/ my #, I havent been out to metro-west in a long time. I am not that big on mud, that is why I tend to not ride in the summer.

Part # 37 in the link ---> http://www.sudco.com/Diagrams323728/exptm.gif

It is called a needle jet, I got confused.

[3wheelrider]

oh -now i got you. i'll have to check that # on that (q2) or (q6). thats something i wouldnt have thought of.i was looking at the jet needle #.i think your right on thanks for the tips and diagram. hey- ya dont need any help puttin that 85 together do ya?-if so let me know.

[3wheelrider]

That is where I got my carb from, if there is a problem with it, take it back to Grant. He will get the right jet-needle for you. The vendor he gets it from sells those carbs for 125cc bikes, sounds like the same thing I went thru. I went back with the 36mm so I can use the airbox. I will send you a PM w/ my #, I havent been out to metro-west in a long time. I am not that big on mud, that is why I tend to not ride in the summer.

Part # 37 in the link ---> http://www.sudco.com/Diagrams323728/exptm.gif

It is called a needle jet, I got confused.

hey man seems youre right on. this thing is set up for a 125cc i guess. grant said it should've came with a 35 pilot/290 main he said. having him get me those jets which will be alot closer along with the correct needle jet #389-q6 and jet needle rf40. the rf41 is a leaner needle. thank you so much for the tip.

[3wheelrider]

well, got the standard needle (rf40) ,the q6 needle jet, 35 pilot, 290 main. now it does more than 5 mph! o.k, it revs when starting, then goes down then up eventually.- went to a 40,45,now 50 pilot. its now better but still unsteady idle ,just not as bad. but im wondering if 50 seems like a very large number already. could be sucking air thru left crank seal maybe? it runs perfect except for idle at times.remember-this is a (38mm) mikuni- not keihin sizes............not sure if it should be that large. any idea anyone? hey phil53 -what pilot jet did you have on yours? can you check please?................ i have a 55 pilot 170 main on my 85'r with stock carb -but its a keihin...yeah its got work!

[phil53]

I just pulled mine apart from up on the shelf, it had a 45 pilot and 300 main. How is your intake boot and fitment? What pre-mix ratio you running? I use 40:1 with klotz techno-superplate or yamalube2r or a motul oil(forget the name). Set the air screw at 1.5 turns from fuly seated and adjust pilot jets from there.
I remember specifically buying a upp 38mm intake boot for this carb because it was such a PITA to insert in the stock boot. I have never seen a crank seal leak on my 250r, if you suspect a leaky crankseal leak do a leak down test.

[3wheelrider]

I just pulled mine apart from up on the shelf, it had a 45 pilot and 300 main. How is your intake boot and fitment? What pre-mix ratio you running? I use 40:1 with klotz techno-superplate or yamalube2r or a motul oil(forget the name). Set the air screw at 1.5 turns from fuly seated and adjust pilot jets from there.
I remember specifically buying a upp 38mm intake boot for this carb because it was such a PITA to insert in the stock boot. I have never seen a crank seal leak on my 250r, if you suspect a leaky crankseal leak do a leak down test.

has new gaskets on intake, boots fine, has no airbox lid. yes it can accept this carb with the stock intake and you dont have to lose the box. (just use a heat gun). i just wanted to know if i was ,in the ballpark with regards to the pilot jet. gonna try 2 more sizes up on the pilot again. its not sputtering yet -(accelerating). i dont have a tester made up. thanks for the reply. ....hey ,you gettin yer' own ready yet?

[phil53]

I decided to pull the swingarm and axle out of the 86 to grease everything and clean it up a bit + I have a new LS axle for it

What problems are you having with idle or off idle to be going up so big on the pilot?

[3wheelrider]

anyway, well -it revs a little high when you 1st start it(cold only) for about 15 seconds or so then comes down for a while then goes down a little to lower and stalls eventually-figured its still lean on the pilot. if i lower the idle then it doesnt rev up (cold ) but as it warms idle drops to low.subtle changes with either screws have sudden large changes. adjustments not helping. neither is going up on the pilot now (55) wont take more (had 22.5!). main (now 310) ,(had 230) . needle clip in middle notch. (correct needle now). nothing makes it go away. hate this carb but it runs perfect except at idle.(wouldnt go on 1/4,1/2/,3/4,/or full throttle before changes. oh yeah, using supertechniplate 4oz.per gallon.( 32:1). 93 octane. boots fine too.

[phil53]

I wouldn't worry about it idling high at start, as long as it comes down. Set the pilot screw at 1.5 turns, Take it out and ride it, get it warmed up, do plug chops at 1/2-3/4 and full throttle to make sure it isn't lean anywhere.
Now try adjusting the idle, & then the pilot screw, if it is more than a 1/4 turn from fully screwed in, or more than 2 turns out, the pilot is either too big or too small. Use freshly mixed gas & oil. drain whatever was in the tank before going out and riding. I had alot of problems when I would ride every week and not change the fuel out in my tank, I got very inconsistent jetting results, spent more time working on it then riding it.

Read this link, it is for keihin carbs, but the operation is the same on a mikuni and may help you troubleshoot what is going on.
http://www.duncanracing.com/TechCent...arbJetting.pdf