Mikuni TM34 jetting

voelser

voelser

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Hey guys,

I've decided to replace my old BS38 carbs with some shiney new TM34's. Obviously they will need re-jetting, so I'm trawling the internet for information, and have already trawled through here (but maybe I've missed things) so I thought I'd see if you guys have any knowledge to share.

The engine is on it's second oversize (so slightly increased capacity), no airbox but pod filters instead, unbaffled exhaust. The BS38's are set up as follows: PJ:25, MJ: 140, NJ: Z8, stock needle (unsure what clip). Runs fine albeit a bit rich on startup and low... revs.

From the information I gathered so far the most common suggestion for the TM34's is as follows:

PJ: 25, MJ: 190, NJ: P6, Needle 6FP (middle clip).

What do you guys think?

My jetting chart. Baseline of current setup on the left, stock Mikuni setup, a reference chopper (which also runs rich on startup) and some setups I've gathered from research:
Jetting.JPG


Pic of the bike:
12491791_1013338338709583_4665190549827388271_o.jpg
 
I'm running TM34 but as a linked unit and also chasing info on here about jetting. Mine come jetted for the xs650 so will we see on Saturday what I'm running
 
I’m in Bermondsey. Is your linked unit the one from Topham.de? I got my carbs from the US when I was there. Saved me a bundle. Just ordered some jets and will be doing so e testing in the next few weeks.
 
Moonman, Voelser is using straight TM34 carburetors. They don't bear much resemblance at all to your 4-stroke pumpers, which are actually part of the radial smoothbore design group. Your jets will run much smaller due to major differences in flow characteristics, and your jetting won't be useful to him at all. But it would be very interesting to know what brass Topham installed; I'm looking forward to seeing your jet list. I've found that for streeet tuning, 2 mm. of venturi diameter in carbs of the same series doesn't make a big difference; so here's what's worked best for me in TM36/68 pumpers on a 700 cc. motor running 8.8:1 static CR, Shell #1 cam, ported head, and corrected 1.75" headers with straight through glasspack mufflers, and K&N filters. Altitude approximately 600 ft. ASL.
PJ: 17.5. MJ: 135. NJ: P-6. JN: as supplied, 6DZH6-50, clip at #3. MAJ: Open. PAJ: 1.0. Slide: as supplied, 4.0.
 
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Voelser, that's a very tasty looking ride--congrats! The "chopper" jetting is not going to work. When VM and TM carbies come from the factory they're set up with 2-stroke jetting, which is richer throughout the range than a 4-stroke motor can use; it takes more than a change of main jets to tune them for a 4-stroke motor! If I were doing the tuning, I'd start with the Smedspeed baseline with P4 NJ. They know the XS650, they know how to tune, and they've worked with the same fuel you'll be using. Bear in mind that the TM carburetor is an entirely different animal from the BS38, and that the use of much smaller PJ and NJ and much larger MJ is normal.
 
voelser said:
I’m in Bermondsey. Is your linked unit the one from Topham.de? I got my carbs from the US when I was there. Saved me a bundle. Just ordered some jets and will be doing so e testing in the next few weeks.
Click to expand...
No mine are from Allen's performance up north bit same as tophams setup...I tried tophams first and they couldn't sell to my because of licencing issues with other company.
 
I agree with grizld as I had a set of TM34 from SmedSpeed on my wife's bike and he has them in the ballpark setup already for pods and headers and hers ran great and wasn't really rich.

Grizld thanks for your input on jetting. Will have a look tmrw and get back to You, your help is appreciated as not many people have done the linked setup so im in the dark on these...however the bike starts on first kick and runs fine but the plugs are slightly wet after....buuuttt i havent been able to go for a few miles...to test properly with fresh plugs.
 
Thanks for your replies Grizl1 and Moonmanz!

Grizld1: Thanks for the compliment! I appreciate that. That the chopper jetting doesn’t work I discovered. This was set up by a garage after’tuning’. Which is why I am tackling this myself now. It runs really rich at startup, so rich it kills the plug in the left plug (although I suspect there is another issue here). And if it does run and you ride it it surges dangerously. So still a work in progress and I’ll post pics of that once I tackle that.

But I thought I’d first tackle the XS I’ve put up here as it needed some TLC anyway. Already put a hydraulic clutch on the bike and will finish the rewiring this weekend (I’m much better at it now). As for the jetting, that is next. Is your suggestion of using Howard’s (Smedspeed) setup relevant for the chopper or both bikes?
 
Grizld1, as you can see below I did my research already on needle and needle jet sizes based on the information I received from the various sources. I've already put my order in to Allens Performance for the P6 and 6F9 jet and needles as it was the most common touted baseline and they seemed leaner than the P4 and 5F3 combination. So I thought I'd try that first.

If you thinks it's valid to get the P4, 5F£ and airjet combo in, I can order those as well of course.

full
 
As you've seen, there's no single "right" jetting recipe. Some combinations work, some work well, and some don't work at all; carburetion is by nature a series of compromises. If I already had the P6/6F9 combination, I'd try to work with it before buying any more brass. Yes, I'd start with a known baseline in the chopper carbs and work from there, and the Smedspeed baseline looks as good as any.

Re. needle dimensions, I can't use the charts to make more than a wild guess at what will work, and sometimes what works for one guy won't work for somebody else with a similar motor under similar conditions. Gary Hoos tunes VM34 carbs successfully with the 6DH4 needle. When I've done roll off tests with the 6DH4 it's thrown a lean signal rolling off to 7/8 throttle and a rich signal rolling off to 3/4. 6F9 needles in VM carbs pass both tests for me. Why does the 6DH4 do weird things for one guy and work fine for another? I have no idea.
 
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Thanks for your reply. That's along the lines for what I thought. I'm setting up the cafe recer/tracker first and will use the P6/6F9 combination as a start and see where to go from there. Once that one is up and running I can use some of the knowledge (and left overs) on the chopper.

As for the needle chart, I made that to understand the differences between the different types so it would help understanding what I may need..;)
 
I thought I'd put up a bit of an update so it might also benefit others. After spending time rewiring the XS I finally got roound to put the new carbs on. Based on the research I did I jetted it as follows: PJ: 25, Needle: 6F9 (middle slot), NJ: P6, MJ: 190.

Popped them on and hit the start button and it ran straight away! Easier to start than on the old carbs. Idle was high and I had to turn the air screw out 3 1/4 turn to bring it down to acceptable levels. So PJ may be a bit high. I replaced it by a 22.5 and turned the airscrew out 2 and a bit turns. Maybe that can still step down a bit, but I'll keep it like this for further testing.

Took the bike out for a few runs through central London and it all feels pretty good. Idles well, pulls well (more than before), but obviously it needs more than that. So once I've worked out some kinks and the weather gets better, more road testing and Dyno for finetuning. But so far it all looks good.

Biggest kink: the throttle pull out of the top of the carbs and the fuel taps of the tank want to take up the same space! The only was I was able to do some runs was by slightly tilting the carbs, which is a bit crap for the floats of course. working on some solutions, but open to ideas!
 
Yep, clearance is usually an issue with cables pulling the slides through the carb tops. Mikuni offers angled carb top cable adjusters, and those can be pointed inward, but IDK if that will yield enough clearance with your tank. One solution I've used is to lift the back lof the tank a bit; not ideal with a stock seat, but yours won't interfere with the back of the tank. Extended carb mounts might move the carb tops back far enough to clear the petcocks and would yield a bit of perflormance gain as well. And finally there's what I always end up doing, which is fabricating mounting plates for thinner profile aftermarket petcocks.
 
Cheers. I have ordered an angled cable adjuster as a first attempt. I think they may just about clear. I may get back to you on the aftermarket petcock mounting..haha
 
Haha...yes, the cover was a private joke, but very nicely done by Tony the Engraver (find him on FB)...
 

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