mikuni tm38 jetting

[Kristian]

I have been struggling with the jetting on my sport which has a stock motor with a mikuni 38mm flatslides and a two brothers racing 2 into 1 exhaust. I believe the carbs and exhausts came of of the same bike but that bike may have had a modified engine so was probably a little thurstier than mine. when I got the carbs the main jet was a 210 (still installed) and the pilot was a 30 (recently changed to a 37.5 with marginal effect) not sure of needle jet and needle right now. when recieved the needle was at lowest position (leanest)--do race bikes tend to have big mains and lean needle position?

my main problem is with idle--I don't like futzing with the throttle to keep her alive when I'm trying to concentrate on b raking--and rapid transition from idle (bogs down).

even with the large main the bike performs well once the main circuit has taken over and the plugs show a dry mid brown color (a little rich perhaps but the least of my concerns).

I noticed that ducinthebay has big 60 pilots in his 36mm delorto's. Is my pilot still too small.

If someone could tell me what they are running successfully with a similar setup I would have a good starting point (notwithstanding differences in environmental conditions).

I am hoping that larger pilots and another step leaner with the needle position could be the answer--I think the idle is currently dependent on my main system which consequently has to run a little richer.

What range of positions should the slide be within at idle?

Any information would be appreciated.

P.S. I just picked up a 99 monster rear rim which looks like it will fit my bike without modification. I'm excited about this because I can now run matching Bridgestone bt016's and take the old girl to the track. Metzeler and Avon also make high performance matching sets in 130/70/16 and 160/60/17.

Thanks for any advice on the way,

Kristian

[Maico88]

I do not have a Ducati with Mikuni carbs, but I recently put TM 38 flatslides on a 750cc 2 valve BMW 75/7. The TM 38 cabs come with the following jetting.

Main jet # 230
Needle jet 389 Q-2
Needle 6FJ 40
Pilot jet 22.5

I am currently running the following jetting
Main jet #230
Needle jet 389 P-6 ( slightly leaner than stock)
Needle 6FJ40
Pilot jet 30

The bike starts, runs and idles well.
I am running the needle at its highest (richest) setting
and the idle mixture approximately 3 turns out (out is lean, in is rich)
Hope this is some help to you.

[ducinthebay]

Kristian,
Jet numbers do not match from one company to another. So a 30 pilot from Dellorto won't be anything like a Mikuni 30. If you find an old tuning guy in your neighborhood, he might have a pin set, or drill set to make jets. The old guys didn't buy jets, they just soldered them and redrilled them. Imagine tuning an old TZ750 with 4 Lectron carbs.

If you have a nice tan plug, you are definitely in the right ball park. Is there an idle air mixture screw on that carb?

As far as your rear wheel, yes Monster and SS wheels with 17mm axles will fit straight away onto your swingarm. Get the brake rotor, caliper, and caliper hanger also. It all slips right on and you now have a 17" wheel and modern rubber. There are 4.5 and 5.5 rims, and either is fine. I have the 5.5 rim and run a 170 rear tire to help it turn in quicker. If you intend on running those 160 tires, get the 4.5" rim.

Cheers,

[Kristian]

Thanks for the info duc in the bay and Maico88.

It's helpful to know that manufacturers use different numbers--I guess standardizing would have made things to simple for us all!

I will wait for a little while to see if I get a response from someone with tm38's on a sport before disassembling again. At this stage, based on my suspicions and the info you have provided, I will go back to the 30 pilots on the assumption that I am running rich (i neglected to mention that turning out the air screw does slightly improve the idle). In addition I will drop the needle back down to it's leanest position so that I can open the slide more for idle to lean it out. When my needle was in the middle position I definitely ran to rich (any rider behind me would tell you so)!

Thanks again for the info.

duc in the bay, does changing the brake require a new brake line as well (most of the calipers hangers on monsters and ss's seem to hang below the rotor rather than
above it like the sport).

[ducinthebay]

Yeah, some caliper mounts are above the axle, and some are below. If you get one that sits above the axle, then you might be able to use the same brake line. If its below the axle, you will certainly need to get a new brake line. But since your old brake line is now 20 years old, you may want to replace it anyway.

There is one super cool option that is below the axle and has a brake rod that attaches to the engine. It reduces brake hop. It came on the 900ss superlights.

Cheers,

[higgy]

The old guys didn't buy jets, they just soldered them and redrilled them. Imagine tuning an old TZ750 with 4 Lectron carb

The way I see it, if you can remember what a TZ750 was and that it had Lectron carbs that makes you an "old Guy"

[Mc tool]

The old guys didn't buy jets, they just soldered them and redrilled them. Imagine tuning an old TZ750 with 4 Lectron carb

The way I see it, if you can remember what a TZ750 was and that it had Lectron carbs that makes you an "old Guy"

A what ? with what on it ?

[Mustang505]

Hi Kristian
i have Mikuni 38s but the BSDT type so my info won't be much help. I have had quite a bit of hassle getting these set up to run nicely as well. Still not right.
Are you going to run a 16" front and 17" rear? I have throught of this but have yet to try it. I currently have a BT 016 front and the BT38 rear but a more sporty rear would be good for the track as this is where my sport is detined.

[Kristian]

Hi Kristian
i have Mikuni 38s but the BSDT type so my info won't be much help. I have had quite a bit of hassle getting these set up to run nicely as well. Still not right.
Are you going to run a 16" front and 17" rear? I have throught of this but have yet to try it. I currently have a BT 016 front and the BT38 rear but a more sporty rear would be good for the track as this is where my sport is detined.

Hi Mustang505,

I'm thinking that the 16" front with a 17" rear should be fine as a number of new sport bikes are set up this way--thus the availability of the tires. Also the diameter of the rear is increasing by 1" and the front, being a 130/70 instead of the original 130/60 means a diameter increase of 26mm which is very close to an inch. The only change would be in overall bike height. Any of the rears substituted for the original 160/60/16 make for about the same, or greater, height increase than the 17" wheel with a 160/60 tire fitted.

180/60 =216mm tire+406.4mm wheel=622.4mm
160/60 =192mm tire+431.8mm wheel=623.8mm
(tire height being eg. 0.6x160x2 to get tire portion of overall diameter.)

I did see a link to a site that listed jet set up for the carbs you are running it's on this forum somewhere under mikuni jetting. from memory it suggested 140 main and 37.5 pilot lists needle and needle jet to.

As far as my carb issues are going: I did move the needle down to it's leanest position--will check plugs after first short ride for too much heat--and left the 37.5 pilots in place. Bike idles much better but I think it's still rich at idle as it bogs with a fast transition from idle. This is probably normal unless idle is set higher than stock specs--do race bikes have high idle? Anyway I will put the 30pilots back in at some point and see if there is any improvement. This will mean a return to the exact set up that the carbs had when I bought them--it didn't work then but there was probably another cause for that--maybe a blockage.

When I do this I will note the needle valve. From memory the needle is 6fj40. If my problem is solved I will note the jetting here for others to reference, although it seems that there are not to many people running the tm's on their sports.

[Kristian]

finally got around to changing the pilot jets back to the 30's. I would say this is an acceptable setup but at this stage I am having trouble getting the idle to stabilize it seems to want to climb intermittently (to lean now?).

main 215
pilot 30
needle jet 389 p-8
needle 6fj40 (set at lowest leanest position)