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Discussion Starter · #1 ·
I picked up a 2000 M750 to be donated as a raffle prize at a local rally that benefits a charity for kids with cancer. The bike has had all sorts of issues with the carbs.

I was going to crack them open and begin cleaning and such. Looking at rebuild kits it almost seemed a better idea to watch Ebay for running carbs. Silly me I recently was able to purchase a set of 38mm Mikuni DUC B132 K5Y1 carbs that came stock on the M900, brand new never ran. A quick search without running out of time at least let me know that all the Monsters used the 38mm carb so I went for it. I'm having difficulty finding complete jetting info, I have found one source that might be correct http://www.postdiluvian.org/~mason/moto/jetting.html . If it is then my jets are all screwy. Not sure how much a difference going up in the main jet 7 points will really effect the bike, especially here in TX were it gets extremely hot.

The bike does not idle at all. It runs great when you romp on it. Getting it started is a little difficult. I'm guessing the pilot jets are clogged and that may have something to do the bike not idling. I am certainly willing to clean the original M750 jets in my ultra sonic cleaner and install them in these carb bodies but I figured I'd ask around if anyone else here has swapped out the M900 carbs for the M750. I'm really wanting to avoid pulling a bunch of parts off of the M750 and leaving them around to mix in with the many other torn apart bikes in my garage. I'd like to make a quick direct swap when I change them over. I'm not sure how much tweaking may have already been done to the original carbs so I can't trust that I can pull them down, clean them and then have them run correctly. Probably need to be stripped fully and then synched. I was hoping that these new ones would be ready to go with just a little work.

If anyone has any experience with this swap, any advice about working with the originals, or has some clean slip on cans let me know.

Thanks, Mike.
www.harvestclassic.org
 

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Discussion Starter · #2 ·
Ok of course it's Saturday and I've bought all the corals that I saw at the local reef stores earlier this week, woke up this morning and helped get my buddy's MG running again and then came home to post on DucatiMonster about how I want to wait to tear into this project. An hour later and the carbs are off the bike and torn down. Sorry.

Well then here is the latest update,

Carb body reads 38 B240 H951
starter jet 70
needle 42.5
main 148

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Holy cheeze whiz, I'm flummoxed. I've been listening to gremlinradio.com for hours and my head was already swimming in crazy electronica beats but WOW this was an eye opener. My understanding is that I should have opened these up to find a main jet of 132.5 . So the only difference between these carbs ATM is the 140 mains in the B132 carbs and a pilot jet labeled Y-2 instead of Y-4.

I realize that the mains are not used until 3/4 throttle but why on Earth would I have 148 mains in the original carbs? The M900 did not even use mains that high.



Uh oh, Wait a minute, I have a thought. This just hit me a like a ton o bricks. From previous research on this site, could these be a kit like FactoryPro or Dynojet? If so some things I've read say that the mains are labeled 10 higher than Mikuni. So then would they be 138 Mikuni mains? Arghh I thought about this when I noticed the adjustment screw cover had been removed. This could be difficult if a kit had been installed. Yeah the airbox has been all cut up and a K&N filter added, damn thing had holes punched through it for some reason.
 

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Discussion Starter · #3 ·
Ok so another hour has passed. I "lightly" installed the new carbs. Didn't hook everything up all the way to them. I then hit the go button and vroom they idled perfectly. Just like i wanted. However at about the 3/4 throttle position they start to fall off. Sputtering and a little backfire. Not sure if it's overjetted or needs the ones that were in it. I did however tear apart the old ones to start cleaning and certainly found missing o-rings and a broken top cap. Nothing that can't be fixed but hey I'm off to a good start.


Just wish I hadn't gotten started late on a Saturday. I might be able to get jets from a horrible dealership on Monday, or wait until Tuesday for a good dealer to open.

Comments? Suggestions? Please feel free to comment/help away.
 

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Hi Mike,

I'll throw my 2c in here. I have recently installed a factory pro kit in my wife's 2000 M750. I can verify that the kit does not come with anything like a 148. It came with a 2 each of mains 135, 137.5, 140, and 142.5 (stock was 132.5) and a pilot of 45 (stock is 42.5). Their numbers seem so close to stock that I doubt they are using their own +10 numbering scheme - perhaps Dynojet does. Dunno.

One way to tell if you have a factory pro kit, or at least something not stock, it to look at the jet needles. The stock ones barely have any taper on them at all. The factory pro ones have a very distinct taper going down to a much finer point that stock. Also, most people would do what I did and take the opportunity to replace the needle jets with better stainless steel ones - the stock ones are a brass colour.

I can also verify that the 900 and the 750 use precisely the same carbs, just with different jets. I have a 93 Superlight and it had the same carbs - this is backed up by my haynes manual.

For reference, I have removed the airbox snorkels and gone with 45 pilot jet and 140 mains. It is possible I could go down to 137.5 on the mains though - the bike doesn't spend much time at higher revs. One the same basis I could have stuck with the stock 42.5 pilot jet - but I have no complaints about the 45.

I have been running with the fuel screws 2 turns out, but during winter (Melbourne, Australia) here it just wasn't enough, so now with 2.75 turns out things are better down low.

The bike certainly idles, no problems there. It all feels okay, but when I feel rich I might get it redone professionally - I'm a bit of a desmoron!
 

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Discussion Starter · #5 ·
Cool, sounds like I'm on my way to success. I'm going to finish cleaning the original carbs today and put them back on the bike just to see how it goes. Doing a leak down test showed me that the butterflies are at least very close to be synched. Not a scientific test by any means. I've been showed over the years a way to back the set screw all the way off so the butterfly's are closed, then fill with solvent to the same point. After that you adjust the idle screw until the butterfly's just open, verify that the solvent leaks out at the same time. It's just a quick bench test. Need to run around and find some tiny o-rings shouldn't be a problem. They look just like the ones for Amal air screws. I also have the ones I need in the 4 Keihin 28mm flatslides I bought for racing Derbi GPR50's but would rather not steal them if I don't need to.

I will still plan on buying appropriate jets from DucAustin on Tuesday for the new carbs. I can't find any jet labeled as Y-2 for the pilot. Can someone describe it for me? Is it the one on top of the CV? I did not tear down the choke mechanisms, is it under the choke?
 

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Discussion Starter · #6 ·
Well after stripping the carbs and boiling them, the originals have been tuned back in and are running like scalded assed apes. Pure enjoyment. It will be tough to donate but well worth it.


New M900 carbs for sale.
 
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