Well, I got out in the garage today and started working on putting the new Mikuni TM33 pumper carb into my KLX250S. I know I was excited to see it get here yesterday, but after putting it into the bike, touching the start button and having it fire right up with no adjustments, well . . . . . now . . . . . I'm . . . . . STOKED!

I took my time doing the install as I wanted to make sure everything went very well. I must say the instructions that Bill Blue, while short, were very sweet indeed. It went just as he said.

Before I started I did take some pictures of the carbs - side by side - for some comparisons. Here they are:



In the above pic, you might note that the adaptors that Bill Blue made for this application not only made the carb a 34 mm diameter carb, but it also made the carb the same physical length as the Keihin CVK34.





The big differences on the Mikuni and Keihin carbs are the Mikuni "PUMPS" (squirts) a charge of faw fuel into the air path whenever the throttle is opened. The throttle plate on the Mikuni is also directly coupled to the throttle on the handlebars. When you open it, the throttle plate opens in direct proportions to your hand movements. The Keihin opens a throttle plate that allows vacuum to open the actual slide that let's air/fuel into the engine.

Here's a couple of shots that really show what I'm talking about.



In the above shot, the throttle is completely opened. You can see how much air flows through this carb before the slide is raised by the vacuum. The engine vacuum only opens as much as the slide as it can use. This is a good way of meeting emissions requirements, but does not give you throttle response.



In this shot you will notice that the throttle plate is wide open. Also that 'tube' that is sticking up is the accelerator pump nozzle. This is where the raw shot of gasoline is injected into the air stream whenever you open the throttle, even very slightly.



This has nothing to do with this. I had the throttle open a camera in my hand and I just had to get creative.




First without a carb. Finally the carb in place. Once again, nothing had to be cut, welded, bent, hammered, altered or anything else. It is a simple, bolt-in upgrade. How does it run? I will find out very shortly. My wife is out shopping and I'm watching my Grandson. He was helping me wrench on the bike. Once the wife comes home, I'm going out playing. A performance report will be following.

 

Hey Marty, this picture is a work of art, I will name it "carbel tunnel" (photo by Marty)

 

The old carb has a throttle censor that works the electronic advance, as i understand it. With the new carb how does the timing advance work.

 

Ol Guy,

Yep, I was thinking about that too. However, it does not seem to be noticed with it just hanging loose. I had talked to several others about this and they didn't think it would be noticable.


Okay, I just got back from my check out ride for the new carb. A bit disappointed in the performance. It is jetted a bit on the lean side - at least on the top end. But that is an easy fix.

How do I know this? Well when I left the house, I had the airbox lid off as I was playing around with that last week. As I got to the dirt road across the road from me, I whacked the throttle. I immediately got this feeling like the first time I put a 1050 Dominator on the race car. There was this loud roar, but nothing was happening. A big noisy BOG is what happened. Dang that was sure disappointing. But if I stayed out of WOT it ran very good. So I went back and put the airbox lid on.

On the second ride, I noticed it was surging on the WOT so I think I'm still a bit lean. So, back to the house and install the stock snorkle. What a difference. The bike just literally jumps up and flies now. The scary part is that there is going to be a whole lot more once I get the jetting corrected.

In comparison to stock, this carb is a whole lot better. When you open the throttle something happens. Even though it is not jetted correctly - very hard to do if you don't know everything about the bike it is going on - I'm happy with where I am and where the bike is going.

Once the carb is dialed in - even though it will pull to redline right now with no hesitation - something it would not do with the CVK on it, I feel that this will be the thing to do with the KLX250S.

Stay tuned for a tuned carb report.

 

Hey Marty I'll give you $50.00 for your stock carb. What do you say? Ok I had to at least try and put a smile on your face right?[8D]

 

Deej,

I'm NOT down, quite the contrary. The bike is pulling almost as hard as it did with the CVK on the bottom end with the small snorkle in and the airbox cover on. It runs WOT throttle now with the small snorkle and the airbox lid on.

What I'm looking forward to now is getting the right jets in the Mikuni carb as I have a feeling the bike will really wake up. When I was running with the airbox lid off, at partial throttle it was quite impressive. Only when I went WOT - which points to the main jet being a bit lean, is when the disappointment set in. But once the jetting is right on, I'm sure I will be smiling bigger than I ever have!

 

Great news, Marty!

I'm glad that you were able to get into the "performance zone" by playing around with the airbox cover and snorkle combinations that you tried. It will be interesting to see what the jetting turns out to be in your final configuration.

BTW - Did you do any "plowing" in your wife's garden like Nobrakes first did?

 

Congrats on getting it installed and your first test ride Marty. Do you have some spare jets to richen it up or do you have to order them? Sounds like the Mikuni will work fine and I'm glad to hear no modifications to the frame are required for the KLX250S as was reported on Planet KLX for the KLX300. I'm sure once you get it flowing more fuel and you can remove your airbox lid again, it will be an even bigger difference.

 

I think the planet KLX procedure allows you to keep the two wire throttle arrangement. If keeping two wires means removing material from the frame, I may go to the one wire set-up as well.

 

Hey Marty congrats, may I ask how much the carby cost you all up?