xr650l mikuni tm38 verses stock CV carb

[bentbike]

I recently bought a beater '94 XR650L that was uncorcked but otherwise stock. I has a completely stock CV carb and a Mikuni TM38 flatslide kit with it. I know a pumper is a great choice but is too expensive for now. Is it worth putting the Mikuni kit on the bike? My old '96 XR600 ran decent with a well jetted non-pumper stock carb. Hmmm...

[goblin127]

Put the mikuni on and give it a try. I'm running one on my L, and am very happy I did over the stock one.

[Beyond The Pale]

As you suspect the stock CV carb isn't the greatest thing for performance. You can make it work better with a set of changes known as "Dave's mods." The non-CV carb will work even better, but only after you get the jetting sorted out.

I'd say put that TM38 on there, just be prepared to spend some testing time to get the jetting right.

[bentbike]

I've messed with CV carbs on other bikes and they just aren't that great. I've read the posts regarding the carb on this site and they confirm my belief. I searched for a while for info on the TM38 and found very little info. I suppose I should give it a try. Does anyone have one all set up with some jetting info? I'm decent with carbs but it would be nice to save some time.

[ghrati ghoti]

I've messed with CV carbs on other bikes and they just aren't that great. I've read the posts regarding the carb on this site and they confirm my belief. I searched for a while for info on the TM38 and found very little info. I suppose I should give it a try. Does anyone have one all set up with some jetting info? I'm decent with carbs but it would be nice to save some time.

Interested in why you would say they not so great. My CV has no flat spots, hesitation, stalling or starting problems.

[martinfan30]

Interested in why you would say they not so great. My CV has no flat spots, hesitation, stalling or starting problems.

they work good when jetted right. if you are doing cam/piston/full exhaust/maybe even pod filter, they hold it back a bit. then you need a flat slide pumper to realize FULL power potential.

[XR650L_Dave]

Interested in why you would say they not so great. My CV has no flat spots, hesitation, stalling or starting problems.

The CV carb can be tuned to be flawless, and in many cases gives better smoothness and control (important to us XRL trials types) but a manual slide carb will have faster throttle resonse than a CV.

The stock CV carb is fairly large, so I don't think you'll get much HP boost out of a carb swap.

Dave

[martinfan30]

The CV carb can be tuned to be flawless, and in many cases gives better smoothness and control (important to us XRL trials types) but a manual slide carb will have faster throttle resonse than a CV.

The stock CV carb is fairly large, so I don't think you'll get much HP boost out of a carb swap.

Dave

you're prob. right. maybe its the difference in throttle response that makes it feel more powerful.

my fcr is a 41mm, and i really feel that top is much better, i know it is.

or my subconcious is trying to justify the 500.oo price tag! lol

[bentbike]

It seems hard to get decent throttle response from a CV carb. It also seems more difficult to get them to run right when you put in a hot cam. Maybe has to do with the change in vacuum caused by the different cam but I'm not sure. Anyway that is why I was asking for all your opinions before I spent a bunch of time messing with the flatslide or jetting the CV.

[goblin127]

At 150ft above I am running a17.5 pilot and a240 main a6fj41 needle(clip in 3rdfrom the top groove).Air screw is about 1 1/4 turns out. TM38-85 Mikuni flat slide carb.

[bentbike]

Thank you!

[dmvmechanics]

i've got the flatslide pumper on my 650r i love it. it might not be the most cost effective mod for a 650l but once you get it all set up it is nice.

also, with the kick start it is substantially easier to flood. especially if your used to two strokes.

[bentbike]

I just measured the ID on the intake side of the carb and found it to be 40mm. Shouldn't it be 38mm for a TM38?

[martinfan30]

It may have been bored out.

[goblin127]

Mine came from XRs only, I belive theres has the adaptor fited to the air box side and the intake sidewas stock.

[zodran]

+1 on being bored out, more than likely what you have is the 41mm mikuni performance kit. Mine came from white bros, I think it's just a flatslide 38mm bored to 41, jetted for our application and a new throttle assembly.?

[bentbike]

How do you like your White Bros setup and how do you have yours jetted? Mine sounds like yours. It has the adapter on the airbox side of the carb and a gunnar gasser throttle assy.

[zodran]

I like it just fine, I had some issues with a modded CV carb and the mikuni was the solution. As much as I'd like a pumper jetting changes for altitude take a few minutes.

I'm running the same main jet as what came in it, 250 main, the needle all the way up, and went back to the original pilot jet. For Colorado I'll drop to a 230 main and South Dakota and Moab a 240.

[bentbike]

I agree I would like a pumper but I just don't want to spend the money right now with this carb lying around. I would rather do a cam and high compression piston in the near future. Also, someone put a 220 main and a 25 pilot in mine. What pilot are you running? Goblin 127 gave me his jetting. As soon as I get yours I'm going to jet mine near your two specs and go from there. It should save me some time. Thanks

[zodran]

Sorry bentbike, I just had the magnifying glass out trying to read the pilot jet number off the one I've been hauling around with the mains in a film canister. I just can't make it out, something .5

I meant to say when I got the mikuni was 99 or 2000 or so. At that time the only pumper was the fcr and it's still probably the best. Mikuni has a pumper now, but my bike is nearing the end of it's time with me and I just don't want to put any more money into it. I had a cracked weld in the steering head heliarced recently and I'm coming up on 30,000 hard miles.

You should be very happy if you go ahead and do the cam and piston.?