36mm Keihin on an 03 CR 250 Test

03 cr 250 woods bike

v-force, torq spacer, fww, fmf gnarly

So I finally got the 36mm bolted up.

quad vent air striker 40-P 162-MJ stock needle for that carb on the 3rd clip

2-t out on the AS

at 2500f 80 to 90 degrees.

I wanted to jet the bottom a little richer and see how the plug looks after some time in the trails.

Bolting it up was a minor pain, best way was to remove the top sub frame bolt and rotate it down and away, attach the rear boot then set the carb into the front boot when bolting up the sub frame.

It fit into the boots but it's doesn't sit in the intake boot as well as it should. It doesn't leak and seems solid but I want to try an intake boot off of a 2000which looks to angle more towards the left which is what is needed.

Took it for a short test ride, around the block, it seemed to run much cleaner with almost no sputtering, I got on it to see how the power valve responded and it was fine...came on the pipe hard and strong.

I rang it out 5th pinned for a few seconds and again very clean.

I then bogged it around and snapped it open, very clean pull until it climbed on the pipe.

seems torquer down low.

I will take it for a shake down this weekend and see if it needs any fine tuning and report any power differences

Just to ask, what made you decide on the smaller size carb?

My thinking with the smaller 36mm would be that it would possibly give a little extra low-end response from the conceptually faster initial air speed of the smaller venturi. On the flip-side, you may get a little less top end power since the smaller venturi will not have as much air volume passing through the intake tract when wide open.

It is great to hear that the cleaned up jetting has helped the way the bike runs. I think that is the main key for consistant power, both in temp/altitude and powerband wise. :thumbsup:

Just to ask, what made you decide on the smaller size carb?

My thinking with the smaller 36mm would be that it would possibly give a little extra low-end response from the conceptually faster initial air speed of the smaller venturi. On the flip-side, you may get a little less top end power since the smaller venturi will not have as much air volume passing through the intake tract when wide open.

It is great to hear that the cleaned up jetting has helped the way the bike runs. I think that is the main key for consistant power, both in temp/altitude and powerband wise. :thumbsup:

I have been off of 2 strokes for the past 5 years and although I have years of MX experience in compairson I am new to trail riding.

The local trails I ride are very tight, techinical, wet, muddy, greasy, with lots of elevation changes, better to have clean torquey power down low and on thru the mid range.

I got the bike cheap, it was in cherry condition, already set up for trails and I found that I actually enjoy riding this bike in the tight stuff more-so then my 450-X...so, in this case I don't mind spending a few extra bucks testing a new carb that may or may not benifit the ride experience.

The stock mikuni was ok, I had it jetted pretty close but there was a fine line between running rich and pinging as well as the sputtering issues.

So, I read that a 36mm was a fix and I wanted to give it a try...I'll know more tomorrow if it was worth it.

I have been off of 2 strokes for the past 5 years and although I have years of MX experience in compairson I am new to trail riding.

The local trails I ride are very tight, techinical, wet, muddy, greasy, with lots of elevation changes, better to have clean torquey power down low and on thru the mid range.

I got the bike cheap, it was in cherry condition, already set up for trails and I found that I actually enjoy riding this bike in the tight stuff more-so then my 450-X...so, in this case I don't mind spending a few extra bucks testing a new carb that may or may not benifit the ride experience.

The stock mikuni was ok, I had it jetted pretty close but there was a fine line between running rich and pinging as well as the sputtering issues.

So, I read that a 36mm was a fix and I wanted to give it a try...I'll know more tomorrow if it was worth it.

For your type of riding, it seems like the 36mm could be a good choice. If it does what I think it should, that sounds like the ticket for your needs. I hope it works out good for you.

I got a chance on Sunday to spend the day thrashing the bike in the same trails to see if the carb swap was worth the time, money and effort.

Here's what I found, on a scale on 1 to 10

1. Power improvement, I'd have to give it a (6) it is a little more torquey and it did clean up the sputter issue, but the power gain wasn't what I would call night and day, the difference was not enough alone to make it worth the swap.

2. Ease of install, this got a (2) the 36mm was tough to install and get seated into the boots...and it never did fit completely into the intake boot. Another problem was removing the float bowel...the carb sits further behind the frame rail making changing jets tough...then there's the issue of re-seating the carb back into the boots again.

Accessing the needle was easier as the Keihin has a screw top and the slide can be removed without tilting the carb.

In a nut shell:

Although the stock carb did sputter some it never hindered the ride experience nor did it cause any performance issues...such as fowling plugs, floging or stalling... the bike still performed as well as it did with the 36mm

"IMHO" I felt that the swap was not enough of a gain to offset the cost and effort.

One thing that was a major issue was the fit, even with a V-force and reed spacer it still did not sit into the intake boot well and getting it there was a pain. Yes, I did get it seated and it was leak free but if I wanted to change a jet or had to loosen the boot bands it ment that I had to go through a lengthly process to reseat the carb again.

My advice is to stay with the stock carb, spend some time on jetting and save your money.

One thing that was a major issue was the fit, even with a V-force and reed spacer it still did not sit into the intake boot well and getting it there was a pain.
Good info to have. BTW, did you try a small amount of grease on the carb outlet bell?
Good info to have. BTW, did you try a small amount of grease on the carb outlet bell?

No, but it wasn't that I couldn't get the boot over the bell of the carb there were 3 problems, first the carb is shorter, second it doesn't face the intake correctly, the right side of the carb where it fits into the intake boot is just bearly in. Third thing the carb is tucked a little more behind the frame rail and that makes it a real pain to work on the carb...once you loosen the bands it's a fight to get it back into place.

I'm sure if I played with it for a while I'd find a sequense that would make things easier...but in compairson the stock set up is super easy to jet and remove the slide without removing the carb from the boots.

I still have it mounted, I plan to ride a few more times and then switch back to the Mikuni and will most likely leave it in.

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