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FAQ - TM40 Upgrade

FAQ - TM40 Upgrade

Q: TM40 jetting and throttle cable question

I'm about ready to go ahead with an order for the TM40. Info on your site indicates you can set it up for modifications I have already made to my bike. I have a 2006 DR650 with the airbox top cut out and a GSXR muffler with Two Bros. mid pipe installed. I have ground down the header weld as well. I want to use the stock choke lever and I have a taller handlebar mount & bars. The stock throttle cables are a little on the tight side in this configuration. Anything else I need to tell you before I place my order?

A: Our standard jetting setup will be just right for your mods. You should have no problem keeping your choke in the stock location either. We do offer a remote choke assembly that puts the choke in an easy to reach location. If your throttle cables are tight you might consider inverting the throttle housing so the cables are running under the bar. That will give you a couple additional inches of cable length. That is all we need to get your carb headed your way!

Q: TM40 cable routing

I'm having trouble routing the TM40 throttle cables. It looks like the elbows bend the wrong way. Did I get the wrong cables?

A: You've got the correct cables. See the picture below for the correct orientation of the cable elbows.


Q: TM40 jetting questions

I put the TM40 on my 2007 DR650 about 2 weeks ago. I also installed a FMF Q2 and put a 2" hole in the top of the air box and removed the snorkel. Pretty much like the other responses I've read, the bike runs great with the TM-40. It has improved throttle response, and I don't have to shift as often because the bike is much more tractable across a broader RPM range. I've got a 14 CS sprocket on, and the bike pulls hard up to an indicated 100 mph. I'm not sure where the rev limiter kicks in, but it winds up fast to 100 mph and wants to keep going. I'm getting about the same mileage as I was with the stock BST. The bike will go just over 100 miles before it hits reserve, which if I figured right is 42 - 45 mpg. Most of my riding is stop-n-go on surface streets, and I ride fairly hard on the throttle. But the mileage has been the about same on highway trips.

I think the TM-40 carb may be a tad rich. The exhaust smells like it did with the BST, no gas odor. But, the bike did need a little choke for about 30 seconds to get going in the morning with the BST. With the TM-40 the bike starts INSTANTLY with no choke. There is a slight blip on initial throttle and between shifts, other than that, it rips.

Do you think I need to make adjustments, and if so, where do you recommend that I start? Personally, I'm reluctant to start changing settings for effect, because it runs very well as is.

I'm interested in the Twin-Air filter for the DR650. Does it flow any better than the stock filter? I don't want the K&N filter. Sorry about the length here. I thought it's better to tell you what I know, than for you to have to ask later. Thanks for your help, I'm very pleased with the TM-40 kit.

A: The blip between shifts should be addressed. It's getting a little bit too much fuel at the initial opening of the throttle. What it needs is more gap between the top of the accelerator pump rod and the white nylon arm that actuates it. This can be adjusted with a small phillips screw on the upper right hand side of the carb. I'd also suggest lowering the needle (raising the clip) one notch to improve fuel mileage.

Starting easily with no choke is definitely a sign of being too rich at idle. Screw the mixture screw in until you can just notice the idle start to drop. You want the screw set right at the cusp of idle speed drop.

The Twin Air filter flows much better than the stock filter - on par with the K&N. Only change one thing at a time though! Then test ride thoroughly between changes so you can gauge the individual changes on their own.

Q: Do I need new throttle cables with the TM40 kit?

Are the throttle cables supplied with the TM40 carburetor kit for the DR650 the same as the stock Suzuki cables, or are different throttle cables required for the TM40 conversion? The reason I'm asking is because my DR650 is less than a year old, and the throttle cables should be good for some time. If different throttle cables aren't required, how much do they add to the cost of the kit?

Also I have some reservation about cutting open the top of the air box. I've discussed tuning the bike with a mechanic I've had work on other bikes, and he recommended removing the air box and using a pod filter. Other than avoiding water crossings (which I do anyway), is there any other reason to run a modified air box?

A: The stock cables are too long to work on the TM40. We get Motion Pro replacement cables and modify them to work with the new carburetor. The reason people modify the airbox is the DynoJet jet kit is designed to work correctly only with a modified airbox. The airbox doesn't have to be modified to run the TM40. My own DR650 has an unmodified airbox with just the snorkel removed.

I would not recommend using a pod filter. I don't have any jetting test data for that setup so you would be on your own as far as finding a jetting setup that would work without the airbox.

Q: What is the correct TM40 jetting setup?

I just picked up a TM40 from a local riding buddy who took it off of a Harley. I need some jetting advice for initial setup for my typical riding of sea level to about 7000 feet. I'm also planning to do the GDR, so I'll need to get the jets for altitude also. The meat of the Colorado/Wyoming part is around 8000 feet +/- with a few passes going over 10,000 feet. Currently the bike is set-up with the stock exhaust and the snorkel removed from the airbox and 1" hole added in the top. I've searched and people with similar setups have used anywhere between 127.5 and 140 for the main and 17.5 to 20 for the pilot. The 17.5 pilot seems to have a pretty wide range so I'll probably go with that one. Will the 17.5 work OK at altitude or should I pack a 15 just in case?

For my standard riding elevations, I'm thinking the 132.5 or 135 is a good starting point. For the GDR, maybe start the ride on the lean side at 130 and pack something leaner to swap if it starts bogging. How small do I need to go at those altitudes?

A: One thing to be aware of if you are using a TM40 off a Harley is that the accelerator pump nozzle is way too big. You need to drop it down to a 40 or 45. As for the other jets, a 150 main and 22.5 pilot works fine for me from sea level to 9500 feet. No carb adjustments needed. That's with open airbox and FMF Q4 exhaust. The key to acceptable running at altitude is to jet on the lean edge of the 'good running' envelope at your home elevation. I doubt the 17.5 pilot will work very well for you. The TM40 works really well with stock equipment, but it functions much better with a different silencer (breathe better) and the top cut out of the airbox allowing it to breathe better.

Q: Mikuni TM40 or Keihin FCR-MX?

 Some people are putting the Keihin FCR39 on their DR650s.  How does the FCR compare with the Mikuni TM40?

A: Each carb has both good and bad attributes when compared to the other. The FCR is a more advanced carburetor but that also makes it a more complicated piece of hardware as compared to the Mikuni. The Mikuni is very simple and straightforward to make jetting changes. The FCR has the 'coast enrichener' that will decrease (but not eliminate) the annoying popping on deceleration. The TM40 will pop just like the stock BST. The TM40 (kit) is a brand new carb and therefore a known quantity. Most FCR conversions will start out with a used YFZ450 carb of maybe questionable history.

If you can score a good deal on a good used FCR then you can do the whole conversion for less money than the full TM40 kit. In my opinion the FCR is more suited to the guy who is more of a hands-on tinkerer. Someone who is not afraid to delve into a used carb, modify and re-jet it and evaluate the success of his efforts.

The TM40 (kit) is probably a better choice for the guy who feels comfortable swapping out carburetors but doesn't really want to become intimately familiar with internal carburetor parts.

Q: What should jetting be with exhaust & airbox mods?

Do you have a standard jetting you would recommend I start with once I've installed the exhaust & done the airbox mod? I live at approximately sea level and won't normally be going any higher than a max of about 4,000 to 5,000 feet (I live in the Northeast and do travel up to Mt. Washington/Green Mountains from time to time and do frequent the Berkshires).

A: We haven't done much jetting work with the stock carburetor. With a cut airbox you will need a DynoJet jet kit. Probably a DJ160 main and needle in 3rd or 4th notch. Again, we have not done much jetting work with the stock carb since I switched over to the Mikuni TM40 pumper.

Q: What is the aixbox mod I read so much about?

My Dynojet kit instructions say to remove the side cover from the airbox. It's really noisy with that cover off though. Is there anything else I can do?

A:  Yes. the intake honk echoing off the left side cover can be annoying. Instead, you can leave the airbox lid in place and cut out the top of the airbox. I used a knife that I heated up with a torch to cut out the top and the upper sections on the sides. Note pictures below.


Q: I removed the airbox snorkel, what's next?

I have a 2007 DR650 and just recently removed the snorkel and am ready to do other modifications for additional power. What should I do next?

A: The simple, easy next step is to shim the carburetor needle up by about .030". Just removing the snorkel is only a benefit if you also raise the needle. If you want to install a DynoJet jet kit you will need to not only remove the snorkel but also cut out the top of the airbox (or remove the airbox side cover). This allows the motor to get more air.

Q: How much to shim needle after removing snorkel?

I recently removed the snorkel from my airbox and shimmed the needle 1.5 mm. Is that right?

A: That is a bit too much, should be .75 to 1 mm (.030" to .040"). More than that and fuel economy will suffer.

Q: How to tell if jetting is OK?

I have a new (to me) 1996 DR650. The airbox is cut open. I asked the fellow I bought it from if the jetting had been revised and he said that he did not do the airbox modification, that the fellow who owned it before him did the modification and revised the jetting. What I want to know is if there is a way that I can check to make sure that the jetting was revised.

A: If the bike runs decent then the jetting has been revised. A bike with a modified airbox and stock jetting will be just about impossible to ride. That said, you still want to take it apart, inspect everything and make notes as to how it actually has been set up, for future reference. A lot of folks doing the rejet will end up with a setup that is too rich. If it runs OK but you aren't getting close to 50 mpg during moderate riding, then the jetting setup could use some work.

Q: 17/42 gearing OK with the TM40 kit?

How well did it pull the 17/42 gearing with the flatslide carburetor? That seems pretty tall. Can you go from the 13 to 17 using the same chain length? The biggest reason I have stayed with the CV carburetor, besides lack of funds, is the altitude change around here. In a day’s riding it is possible to have 5,000 to 6,000 feet of elevation change and the CV carb seems to handle that pretty well.

A: Yes, you can go from 13 to 17 on the countershaft sprocket with the stock length chain. 17/42 gearing works better with the flatslide but the stock carb will pull it just fine as long as you don't let the revs drop too low. It puts 5th gear right where my imaginary 6th gear would be. The TM40 isn't bothered by altitude changes. I've run mine from sea level to 9,500 feet without a hitch.

Q: Is a  snowmobile carburetor the same as a TM40?

I have seen a few sets of snowmobile carburetors on eBay, are these the same carburetor as your TM40 kit for the DR650?

A: No! These are not ‘pumper’ cards. These are designed and built for 2-strokes and do not have an accelerator pump. They also do not come with all of the parts necessary to make installation on your DR a snap. You DO NOT get the steel adapter ring bonded to the carburetor body for a firm, reliable fit in the stock air boot; new cables; a wide range of extra jetting for fine tuning; extended fuel screw and detailed instructions, all of which are included in our TM40 kit. We also include a knob type choke (even though the stock cable choke fits perfectly). This carburetor even has a vacuum port for the stock fuel petcock and is pre-jetted for the typical performance modifications of a pipe and modified airbox, or we can set it up for your particular bike before we ship it out.

Q: What is a 'pumper' carb?

Just so I understand, can you explain the difference between a 'pumper' and regular carburetor? Second, what dimension on a carburetor determines the size of a carburetor?

A: The 'pumper' designation refers to the carburetor having an accelerator pump that gives a small squirt of fuel any time the throttle is opened. With a pumper carb you can usually run leaner jetting (better fuel economy). Without the pump you usually have to run the needle a bit richer to be sure to get the desired throttle response. Second, the size of the carburetor is determined by the size of the opening through the body near the needle.

Q: Are the carburetor adapters really necessary for TM40?

I want to install a pumper on the thumper, and need a kit. I was wondering what the difference is between the ProCycle and Kientech kits besides the adapter that you include in your kit. Do you really need the adapters? Are they made of steel and if so, is there any problem with rust?

A: I believe the adapters are necessary to avoid damaging the intake boot. You can clamp a TM40 into the intake boot by squeezing the boot down tight with a hose clamp but I have seen this done on other bikes and eventually the boot will tear along the edge of the clamp. Torn boot = vacuum leak. The intake boot is very difficult to replace because Suzuki holds it on with phillips head screws and those screws are secured with locktite.

Our original idea was to ship the carburetor with a new boot that fit the TM40 perfectly. Then I realized the trouble folks would have had trying to get the screws out and decided to use the adapters.

Q: How do I get more MPG from my TM40?

I purchased a TM40 kit from you awhile back and I'm trying to fine tune it, but before I do I figured I should seek someone more qualified before I just start guessing. My average MPG is about 42-43. I just put on an older model DR Dakar fairing and it bumped it up to about 45 mpg. Any advice on where I should start to improve that into say the 50+ mpg range? I know here in Oklahoma the wind has been ferocious lately and that affects mpgs, but the bike seems like it's chugging down the gas, I'm guessing too rich maybe? I know it's hard to tell without actually riding it, but if there is any information that I can give you for extra help please let me know. The setup on the carb is the same as yours (that I do know). I didn't know if I need to adjust the air screw (if so which way?).

A: The first place to start is to drop the needle one notch (raise the clip). Take it for a nice long test ride and pay special attention to how it responds to small throttle movements in the 35-45 mph range. If the drivability is fine drop the needle (raise the clip) one more notch or if that puts you on the top notch move the nylon washer from below the clip to above the clip. Repeat the test ride and repeat lowering the needle. At some point the needle setting will be lean enough to cause surging in that 35-45 mph range. You want the needle setting to be rich enough to eliminate the surging.

If you are still using the OEM Suzuki air filter you might consider replacing it with the Twin Air filter. The aftermarket filter will lean things out just a tad.

Q: Will I get better mileage with TM40 conversion?

I know this is probably a dumb question, but I'm interested in the TM40 carburetor kit. I've read all of the threads that I can find on this subject but still haven't quite found the answer I am looking for. My current setup on my 2001 DR650 is stock exhaust and a Uni air filter with the backfire screen removed. I like the bike quiet. I understand that the throttle response is much better with the TM40, but I'm not real hard on the bike and pretty easy with the throttle. With all of that out of the way, will the TM40 kit help my gas mileage?

This is the area that I would like to improve on now. Also, I don't know a lot about carbs and jetting, do I need to change anything if I were to purchase this kit from you guys with the setup I have now? Not sure how you guys set up the carbs before they leave, if you do at all. I've seen different prices on these carbs, but yours sounds and looks more like a whole kit, correct?

A: I have been able to get better fuel economy with the TM40 but the improvement is small. In relatively sedate riding with the stock carb I would get 50-52 mpg and with the TM in its current tune I have gotten nearly 56 mpg - pretty good but it would take a LOT of riding to pay back the cost of the carb kit. I don't yet have data on how it compared in more aggressive riding.

I am with you 100% on keeping the bike quiet. My setup is stock airbox w/no snorkel and stock muffler. I did remove the anti-backfire screen but it makes no difference in airflow unless the airbox is opened up. I'm running the K&N filter but our testing shows the same jetting works with the Twin Air filter.

Whenever possible we ship our carb kits set up as close as possible for the buyers modifications.

We sell this as a complete kit for the DR650 including throttle cables, adapter sleeves, extra jetting and installation & tuning manuals. We also sell the TM40 by itself for those folks who can and want to do their own fitting and tuning.

Q: Will I lose MPG with the TM40 kit?

I’m looking for a moderate power increase but don't want to sacrifice reliability/mpg too much. I'm afraid if I go to a larger carb, I'll suffer on the MPG side of things. I do have a pretty conservative right hand, but would really like some more power (who doesn't like to play, right?).

I am getting around 120 miles to reserve stock and would like to keep it above 100 after modifications, if possible. I don't really want to add an aftermarket tank as the extra fuel would only add more lbs to the bike, which I would like to avoid like the plague. I do about 65/35 street/dirt and I just want a little something more out of the bike.

What supporting mods do you have for your TM40? And what mpg do you get? I'm weighing my options here.

A: Proper jetting will not result in more fuel consumption. The reason you hear of people losing mileage after rejetting is because it is easy to jet too rich but more difficult to jet 'spot on'. The TM40 isn't any bigger than the stock BST40 - they are both 40mm carburetors. The TM40 doesn't really make any more power (though it does feel like it). What you get with the pumper carb is instant and controllable throttle response. On my own bike I actually gained 1-2 mpg with the TM40 over the stocker. The reason for this is you don't have to compromise the jetting setup as much as you do with the stocker. The TM40 can be set to run on the lean edge of good running at cruising speed but still give the necessary squirt of fuel for good response.


I typically average around 52 mpg. The best I've seen is 57 and very enthusiastic riding will get my down to 46-47. I am currently running a modified airbox, K&N air filter and full FMF system with Q4 muffler.

Q: Does the TM40 have issues with altitude?

Should I go with the DynoJet jet kit or spring for a TM40 carburetor for my 1991 DR650. My concern is that I have heard the TM40 pumper carburetor is not as forgiving to huge changes in altitude as the vacuum operated carburetor.

A: While this is theoretically true it is not necessarily the case. My ride last weekend took me from 400 ft. elevation here at home up to 9550 ft. at the top of the Steens Mountains. The TM40 worked flawlessly the whole way. Yeah, the bike was down on power way up there in the thin air, but there were no hiccups or blubbering at any time. It ran like a top.

Q: Are CV Carburetors better at dealing with altitude?

I remember reading a few years back (when someone else was putting a FCR on something) that a CV carb was better at dealing with altitude? Like riding from sea level up to 8000 feet. Is this true?

A: I would attribute this to the fact that the OEM standard jetting in most CV carbs is borderline lean to start with. Start out too lean at sea level, jetting will be pretty close at say 4-5000 feet and only a little on the rich side at 8000 feet. Jet your flatslide the same way and you would have similar results. I have ridden my DR650 equipped with the TM40 from sea level to over 10,000 feet with no adverse affects.