Mikuni TM 40, TM 42 and TM 45

I see that Allens performance list TM 40, TM42 and TM45 as options for the 650R anyone try the bigger ones or best to stick to the TM40?

I can see a big bore kit and hot cams in a year or two and I'm about to splash out on a new carby now, what are your thoughts? A new FCR is so much more to buy, with a baby on the way I'm lucky to be getting a carby at all at the mo. Keep in mind I mostly ride a 2000 CR500R that is not standard.... and I like top gear wheelies.

I don't mind sacrificing a little bottom end power for a decent kick in mid to upper end.

Everybody uses a TM40 or some FCR, the latter more difficult to fit/tune but may give a little more.
You want advice on a larger TM, and the word 'everybody' above means you'll be on your own here.
So, we all (everybody) are waiting in anticipation for you to do all kinda experiments and then inform us on what you have found, and if substantially better we all will chuck our carbs and invade on the source for the new/better one!

Mind though, somehow some of the above logic does sound illogical to me, someone somehow must have tried already what you want to do..... but hell, go for it anyway!

if your mtr's stock the tm40 is the go, a few mods tm42, completly hotted to hell = biggest baddest carb you can fit in it

Hey all. Looking at either the Mikuni TM40 or TM42 and wondering which would be better for my ride. Bike is a 2001 with high flow air panel, batwing, White Bros E series exhaust plus larger header. Jetting doesn't scare me, but the thought of purchasing something I will have to upgrade in the future IF I decide to do some engine mods does. I was going down the TM40 route until I read what Temporary Kiwi had to say. Anyone more knowledgeable than myself (read everybody) have some pros and cons pertaining to these two carbs willing to chime in?

the trouble with selecting a carb is that the bigger you go over stock, is it will require more higher states of tune to make use of it ! ,ive had a 40mm fcr with substantial mods and yes it was good, but as soon as i went above stage 1 cam tuning , it became a limiting factor, along with the stock airbox, i then got the fcr bored out to 43.5mm and built a much higher flow airbox to enable a stage 2 + cam, now trying a stage 3 webb cam and 695cc mtr.
so IF you are going to the expense of higher states of tune then go for the tm42mm, id be surprised if the 45mm carb would be needed unless your planning a seriously expensive engine program. on the plus side the xrr loves to rev with appropriate engineering, and it actually becomes economical on fuel- i get 16km/l easily, have fun

I have the TM40 and am very happy with it.
It is a simple carb (unlike the FCR) and has a pumper, so it beats the std carb hands down and it's bigger.
I bought it because I wanted 'better', didn't want an FCR (for which less info is available) hence bought the TM40 - for which an abundance of info exists when paired to this bike of ours.

Anyone trying to eek out more than 'std' performance (ie hot cams, bigger bores, bigger valves, special pistons, maybe a supercharger or turbo) will have to do his own homework, and I've heard about injection systems used already..... good luck, and post a few pics if the engine goes BANG please

Thanks for the help guys, I really appreciate it. I went ahead and ordered the TM40. Figure if I get around to modding the engine I can burn that bridge when I get there. It is going to be a minute before I can install it as my schedule is crazy (and it hasn't arrived yet) but in the meantime I thought I might get a jump on the jets. As I mentioned before, I have a White Bros E Series exhaust and header, with a wrap on it, uncorked airbox with high flow panels, and batwing. I mostly ride single track around 1000 feet, although its been to Baja twice now. Any ideas on a starting point for jets? Thanks again. Oh, I'll get some pics up here ASAP

BuRP wrote:I have the TM40 and am very happy with it.
It is a simple carb (unlike the FCR) and has a pumper, so it beats the std carb hands down and it's bigger.
I bought it because I wanted 'better', didn't want an FCR (for which less info is available) hence bought the TM40 - for which an abundance of info exists when paired to this bike of ours.

Anyone trying to eek out more than 'std' performance (ie hot cams, bigger bores, bigger valves, special pistons, maybe a supercharger or turbo) will have to do his own homework, and I've heard about injection systems used already..... good luck, and post a few pics if the engine goes BANG please

ive had the pleasure of tuning a few different flat slide carbs- fcr's, tm's, rs38's ive found the fcr's the easiest to work with, cant say i found the rs38 mikunis that way, and that goes triple for most cv carbs. but i am a m/cycle mechanic so probably makes it abit easier than for some

Stepbender,

you're coming right bru?
Here's a link to some info on here :
http://www.xr650rforum.com/t1576p50-mater_e-s-project-beasty

Baby steps. After about 3 million jet change combinations, screw fiddlings, and tank removals I found that the adapter on the exhaust side of the TM40 was loose. Thanks NicheCycle. For a temporary fix I hit it with some liquid gasket and popped it back on, although I am not sure that's the best way to go. It did change things a little however. I am able to idle with the choke on fairly consistently (although it surges a bit) but when I take the choke off it pretty much dies right away. Giving it a bit of throttle keeps it alive for a bit but surging is pretty bad and it dies eventually. Last known setup is 25 IJ, 1.2 AJ, 140 main, Y4 NJ and a Y1 JN. Have the pumper rod out for now. All jets are new and clear. Air filter is good (UNI with support screen, properly oiled), Just adjusted the valves again to cover my, err, bases.  Float level adjusted from 19mm to 20mm. Going to purchase some new premium fuel tomorrow to rule that out. So far no leaks detected with gas burner, or WD40 but I am going to shoot some starting fluid around tomorrow anyway. Son of an idle circuit! If I can just get that dialed in I can start making real progress (in theory) and retrieve a bit of my sanity.

I bet you'll be 'on point' with
- a new intake rubber (they all perish over time)
- an AJ of 1.1 (to go with your 25 IJ)

Keep 1 thing in mind though,. and that is that this carb IS a pumper-carb ..... meaning that, with a removed pumper-rod (which is very much correct to do for tuning!) the engine will 'stumble' if not die when throttle is give a notch too quick, in fact it must do this!
The TM40 uses less fuel than the OEM Keihin ('strue!), part of the reason being the idling circuit (which is active to around 1/4 throttle) is leaner..... and that means that, without active pumper, it will stall easier!

So, if you have the above you're on the right track!

Regarding your "idling circuit from hell" - no squire, this is a stupidly-simple one, in fact they don't come any simpler: IJ, AJ & screw! (Sure, the throttle-screw's also there but any carb has this for it needs one)
Proof in point is the std Keihin, which in addition has a choice of mixture-tubes (emulsion tubes, like the 68 & 68S) plus a completely separate leaning-off membrane-driven circuit which is activated on overrun..... which is veryvery often disabled because it does diddly squat running-wise but may cause problems!
The TM40 is a stupidly-simple carb (remember, old Harleys have them!    ) however it works superb AND is a big improvement on the XRR >> thus a no brainer, no offense pls.
For those preferring the FCR-carb and having one...... geezzz, goodluck jetting that carb!

You'll get there bru, and then you'll smile wiiiiiiiidely!  

I've never said this specifically here in any TM40 thread/subject but I use a 25 & 1.2 idling jetting, however keep in mind I live at 1500m above sealevel (and it's warmer and dryer here than in most places).
It follows that at sealevel (where you are, 200m HASL is forkall) you'll need a bit richer >>>> perhaps the 27.5 & 1.2 ? If more then put the 1.1 in....


This is another TM40 thread, dunno if you've read it:
http://www.xr650rforum.com/t2093-another-tm40-jetting-thread#14147