tri z 38mm main (need help)

[denn]

Hey guys, my tri z fuel pump took a crap. Just for kicks i put my jp racing airbox gas tank on and while I was at it I threw in a 38mm mikuni carb. The carb is from an 85 yz 250 and even though my bike is pretty much stock (Stock bore,No porting, Jemco pipe, reeds) I jetted it to the wrench report for starters. I have the Q-0 needle jet, 3.0 slide, 50 pilot and the stock tri-z jet needle. I tried the 350 main but it seemed like it wanted more so I kept going up. I am at a 460 right now and it still seems like it will go another size or 2. Does anyone here think that that is a bit big for a 38mm carb on the Z. It is a mikuni VM and not a flatslide, but I don't think that makes much of a difference. Has a bunch more pull. I have a 6F16 jet needle laying around and thought about throwing that in also. I am just concerened about the main. The bike idles awsome and there are no air leaks or anything. Any input would really help.

Thanks denn

[denn]

Hey guys, my tri z fuel pump took a crap. Just for kicks i put my jp racing airbox gas tank on and while I was at it I threw in a 38mm mikuni carb. The carb is from an 85 yz 250 and even though my bike is pretty much stock (Stock bore,No porting, Jemco pipe, reeds) I jetted it to the wrench report for starters. I have the Q-0 needle jet, 3.0 slide, 50 pilot and the stock tri-z jet needle. I tried the 350 main but it seemed like it wanted more so I kept going up. I am at a 460 right now and it still seems like it will go another size or 2. Does anyone here think that that is a bit big for a 38mm carb on the Z. It is a mikuni VM and not a flatslide, but I don't think that makes much of a difference. Has a bunch more pull. I have a 6F16 jet needle laying around and thought about throwing that in also. I am just concerened about the main. The bike idles awsome and there are no air leaks or anything. Any input would really help.

Thanks denn

Anybody......

[the otha tri-z]

I don't have much help to offer because I'm somewhat in the same boat. I just put a Mikuni TMX38 off of a 2002 RM 125 on my TRI-Z. It runs very well but mine seemed to be a little on the lean side as well. It currently has a 430 main. I just picked up a 450 and 460 from the shop this morning. I will try those out and see if I have any luck.

[denn]

Like I said i am up to a 460 and its still pulling HARD. I really think i can go up to 490 no prob. That scares me cause when i first put it on I had the 350 main in it and was doing full throttle spurts. It seemed to run good also. Every increase in size it just pulls harder. If it wasn't raining I would throw the 490 in there and see how it takes it right now. I Have jetted bikes before and understand the concept of trial and error. What raises my eyebrow was the fact that on the wrench report they came up with a 350 main. Unless they were testing this thing 5000 feet up, I figured i'd be somewhat close not 12 sizes off!!

Denn

[the otha tri-z]

I agree 350 does seem a little small even at 5000 feet. Well hopefully somebody else chimes in on this to help out. Just wondering if you are using the stock intake boot with that carburetor? I am but it's stretched out to the max.

[denn]

I tried streching my stock boot and it worked like sh*t. Depending on what set up you are using you have a few options. 1) sprock racing intake which works great but you lose the boost bottle. 2) if you run a clamp on filter a 1989 yz 250 is the same except again no provisions for a boost bottle and it is angled. 3) if you plan on keeping the factory set up, boost bottle and all a 1981 yz 250 intake is the HOMERUN. It is excatly the same except the boost bottle fitting was sold seperate that year. No biggie, you can use a 3/4 pex coupling and be good to go. They are discontinued of course, but speed and sport started reproducing them and now carry them on hand in there store. They are $115 dollars and will definetly solve your problem. I think they may sell the fitting too. Here is the link check it out. #2 is the manifold and #4 is the pipe fitting.

http://216.168.60.165/speedandsportinc/Results1.tpl?command=search&db=yamaha.db&woModeldata=YZ250%20H&SKUomsort=1&cart=125043708356330&max=50&Title=Yamaha%2520Parts&Switch=search&ViewToken=T

Also The 82 and 83 it250 intake is even better than the yz cause the fitting is already part of the manifold. Speed and sport has those also, the are part# 25y-13565-00. I think they have 3 on hand and they are $150. These intakes are fine if you want to be super original but for that price, i'll throw the boost bottle away and get a sprock intake. It up to you. Hope this helps

Denn

[clutchcargo]

If you run a sprock intake, you won't "need" the boost bottle. It makes a big enough difference for it to be worthwhile.

[the otha tri-z]

I agree those Sprock intakes are very nice. So I think I might be putting my stock carb back on. This past weekend I was able to make it out to the dunes and give that Mikuni 38 a try and to be honest it was to much carburetor in my oppinion. My motor is pretty much stock like yours is Denn, but I would be curious what Jetting you ended up with and if you think it's to much top end. I ended up going with a 460 main. I just needed a little more bottom end out at the dunes. I have read that a lot of people like using either a flatside or roundside off of a 250r, so maybe I will look for a good deal on one of those and try it.

[denn]

I wound up witht the same deal, a 460 main, or at least thats where i'm at now. I haven't messed with it since. I don't think the jet needle is right but it ssems to be working for now. I used the stock z needle cause its richer but I think its too rich in the mid. No to pat myself but I ride real fast so I don't mind the low end loss a bit especially if I can get the Z to rev a bit quicker. Im looking into a different flywheel, a smaller one that will let the Z wind up a bit faster. For you in the dunes, that sand can lug the bike a bit, you need some bottom end or a real fast rev to get the bike in the powerband. The flatslide 250r carb I herd works real well on the Z. You have to change the throttle cable though. Its a 34mm so its a bit bigger but not too big. Im really just playing games with mine, mixing and matching stuff that I got laying around till I find a set up that I think works well with my style. I have like 3 38mm carbs from YZ's so i figured why not try one. I'll tell you what though, the JEMCO pipe works well with a bigger carb. Pulls real hard. Anyway, hope this helps a bit.

Denn

[ceaserthethird]

Well jettings kinda a pain, but you need to check spark plugs what your looking for is a Dark to Medium light Brown color.

I would read jetting 101