Hold onto your helmets ladies and gents...

I installed the Mikuni HSR 40mm carb yesterday. Other than adding a 22.5 pilot jet, and a 145 main jet, I used it as it came out of the box. I also removed the cable operated choke and used the pull choke from the TM36.

I'd already blended the inner half of the Kaw intake manifold carefully with a Dremel disc to match up to the 40's ID, and left the intake attached to the carb for the 2 months it's been sitting here so as to stretch it out a bit. It's still a stubborn fit in the OE intake; maybe if I Dremel'd a flared edge at the entrance to the intake, the carb would slide in easier.

Right away I could see the AP was squirting too much fuel in, as it bogged bad when the AP kicked in; even on the stand. A little research online revealed the orig AP nozzle (probably intended for a Harley) was more than likely a 70.

Luckily I had extra nozzles on hand, and a size 45 is what's original for the TM36-a good place to start, no? Though I could have taken the nozzle out of the TM36, it was easier to use the new one with a new O-ring rather than take apart the TM36 just for that. Testing it on the stand now reveals no more bog when whipping the throttle open! I have a 35 and 40 nozzle if I need to go leaner.

Other than that little glitch, the bike ran like a raped ape! My a/f meter showed numbers within acceptable range-13's and maybe 14s at various part throttles and WOT. It just SCREAMS at WOT as I go through the gears. (14/48) No indication at all of being over-carbed at all!

It's virtually the same external size as the TM36; I think the bell mouth is like 1mm larger OD or some other small difference.

Next ride I'll pay more attention to the a/f numbers and see if there's room for some fine tuning to be done. Especially if this 10% ethanol gas I use requires 12.2 at WOT for max power.

After installing the smaller nozzle and pull choke this morning, the bike started cold very well and idles perfectly. The idle mix screw is aprx 1 3/8 out from all the way in after adjusting it via the a/f meter yesterday. Which says the pilot jet is just about right.

Damn, I ordered a bunch of extra needles and air jets thinking getting the monster 40mm to work on the 250 would require a lot of changes to the Mikuni factory jetting, and I ended up not needing them!


Is it quicker now? It ain't no slower, I came back from the test ride feeling like it was running quicker, but I'd been more concerned with the a/f ratio than quickness.



The next step with any of these carbs now that they're tuned for good jetting is to time them each over a short sprint straightaway and see what the stopwatch says.

Then the MCM... wonder how well the cam mod and 40mm will work together??

 

My congratulations on the carb working. How does it pull down low? That was where the 38s were a curse on the two strokes, not to mention big carbs on car engines. How does it compare to what might be experienced when running trails or dirt roads working the low middle?

Are you planning any dyno runs to see what the power curve has?

 

No sign of peakiness or loss of low end. It felt just like the TM36 as far as I can see and there just wasn't a hint of losing low end to gain top end. I haven't done any low speed plonking or picking my way through a technical area with the TM36, but first impressions are the low end is essentially the same for the two carbs.


I'll take it out again today for a few km on less traffic'd Sunday roads to look at a/f ratios over various rpms and see if there's room for a notch on the needle or slight main jet adjustment to get the numbers even better.

I thought it'd be much harder to dial in; Eddie over at TT (resident guru) who makes high HP DZR400s had warned an XR250 owner that the same carb was way too big for a 250. So I was prepared for the worst..

I think low gearing can overcome many things, but this 14/48 sprocket combo isn't that tight. In fact it's feeling more and more like it might be time to switch a 44T in the back.

The initial test ride a few days ago was to check a/f numbers which I just assumed would be way way off , but instead all were acceptable except for the AP nozzle shot-which I fixed.

There aren't any dynos here in the Philippines. Maybe in Manila, but it's 500+km away which would include crossing to many islands on the way if by land.

 

40mm carb factory specs: (mine)

Main jet (N100.604) 165 (147.5)

Pilot Jet (VM28/486) 17.5 (20)

Needle Jet Y-6

Jet Needle 9DJY01

Air Jet 1.1

 

Will tell you I'm running a 15/39 on the road right now. Seems like it would be good for a road rider, makes 5th a good gear for 50-60 mph, 6th holds it's own unless pushing a serious headwind. I will go to a 13 shortly which will be about on par with the stock 14/43 (or whatever it was on the S).

 

This is an S (D Tracker) that came with 17" wheels and 14/39 gearing IIRC. Sprockets are hard to get for it here, but shortly after buying it I had the hub hole on an XR200's rear sprocket opened up a few mm at a machine shop for less then $5 to fit the Kaw hub. I have acquired both a 13T and 15T for it since then.

I like the 14/48 for around town etc, but as the power goes up, the mechanical advantage isn't needed as much.

Raining today-further a/f testing of the 40mm will have to wait for another day. I hate to keep applauding the a/f meter, but especially in this case of an untried carb combo, it's really great to actually know that it's within acceptable range rather than assume is just because it's not melting plugs or flooding itself etc. - And to be able to pass on accurate info regarding this carb's jetting in this untried application.

I'm anxious to get some timed testing done. Imagine the max hp possibilities for bigger bores and the MCM if testing proves it's quicker with the 40mm. I mean it is carburating clean, with no apparent severe loss of low end, but I suppose it could behave very well and yet be down on power compared to the 36 if it's just too large a carb.


I did manage more than WOT pass thru the gears during the initial test ride; once past the AP bog it just took off like a rocket.

 

Just got back from a longer ride.

Though the idle a/f ratio is good, just a bit off idle sounds a little woolly/seems a little rich so I am going to drop the pilot from 22.5 to 20 and see what happens while I am changing the main jet. Though it did start almost instantly this morning in 85 deg weather with the choke.

In the end I may instead want to back of the AP kick in point to a setting a little further away from idle.

WOT is a bit lean for 10% ethanol with a/f readings in the 13s and sometimes 14s. I have a 145 main jet in it; have on hand both a 147.5 and a 150 (and larger)-I think I'll try the 150 since I'm a bit far away from the 12.2 optimum.

All other rpm ranges seemed fine, usually 13s. It pulls hard at WOT including thru 6th gear, though except for when heading for the 1-2 shift and maybe nearing the 2-3 shift doesn't seem inclined to hit the rev limiter, nor did the 36mm carb. I may be just shifting at the right time rather than revving it too much.

So, it's definitely compatible with even the little 250 engine. It runs perfectly, pulls from lower rpms in 4th on the hwy very well. it's not like you'd have to be shifting all the time to keep it on the pipe at all.


I'll get the jetting changed then focus on some sort of timed testing.

 

The 20 Pilot jet seems to be better than the 22.5; a nice crisp response from the throttle when I hit it.

The 150 main jet showed mostly low 12 a/f numbers at WOT, but dipped into high 11s. Low 12s is max hp a/f ratio when using 10% ethanol. High 11s might be in the "rich torque" a/f area, or I might try the 147.5 before it's all over.

I took a look while it was apart and the needle that came in the 40mm has the C clip in the middle notch.

- Maybe someone with a KLX250 big bore can borrow a 40mm pumper from a friend with a bigger bike like a DR650 or 450cc on/off road m/c who has added the big 40mm carb. All you'd have to do is buy some jets; it's not like you're going to hurt the carb by trying it.



I noticed the float bowl gasket on the 2 Mikunis are expanding to larger than wanted when you remove the bowl for a jet change, maybe because of the ethanol in the gas here. Dipping them in hot water seems to help some but it's really getting annoying. I'm going to have to research what to do about it.

 

Keep the updates coming! This is awesome. Can't wait to hear about the timed results.

 

I dropped the main to 147.5 from 150 and along with nice 12.6 a/f ratio, it's pulling the front once again on the 1-2 shift. I backed off the AP actuator plastic a bit to cure a small flat spot when I rev it quick. Odd they say aim the AP at the needle but much of the time it's hitting the slide..

I could drop AP nozzle from 145 to 1 or 2 sizes smaller next time I have the carb off, but it's responding fine now that the AP kicks in a little later.