1998 YFM400 Kodiak 400 Bogs at 3/4 throttle

[Does_This_Look_Bent?]

Hey everyone, I just replaced the carp on my 98 Kodiak 400 with PN#4SH-14101-00-00. It starts and idles perfect, but once it’s good and warmed up, it bogs out at 3/4 throttle or more… it sounds like it’s bouncing off a rev limiter haha… I tried the air fuel screw, all the way in and out 2-1/2, and it made it a touch better… tried 2 turns and no different. Am I missing something?

[Mech]

It sounds like the main jet's possibly the wrong size, or the needle in the slide is wrong.

Are you sure this carb is made and set up for your bike ? Is it a genuine yamaha carb ?

[Does_This_Look_Bent?]

I ordered it from Niche Cycle, it says it’s the replacement for the discontinued Mikuni Carb… it came with 3 nozels, a 105,125,135… the mikuni that came off had a 105… I hate carbs and don’t know much about them lol

[Mech]

Hmm. Well 3/4 throttle is about where the carbs changing from having it's fuel metered by the slide and it's discharge tube, and being metered by the main jet, which is what those sizes you mention will be. I think that the simplest thing for you to try is to take the top off the carb and pull the slide and it's needle out, and then change the groove the circlip is in the needle. If you raise the clip on the needle it will make it leaner, and if you lower the clip it will make it richer. It's easy to do as long as you don't drop any bits.. haha. Try it either way and see if it's better or worse, then move it again based on that first test. You can do that easy and undo it if wanted, and if that cures it then the main jet doesn't probably need any other adjustment, but if you pull the whole carb off and change the main jet, you might need to start adjusting the slide needle afterwards to get that right for the new jet. Try the easy thing first.

[Does_This_Look_Bent?]

Thanks for all the info! I think I just realized something… mine is one of those odd 1999 but VIN says 2000, and the site doesn’t list a 99 carb. The 98 carb came with a 145 nozzle installed!! The one that came out of my old carb was a 105!!! Are the nozzles interchangeable? Can I take the original Mikuni one and put in my Niche one?

[Mech]

Not sure what you are calling a nozzle. There is a long brass tube the main jet screws into the bottom of, and at the top of that tube is where fuel discharges into the air stream. The tube is mostly called an emulsion tube. The slide needle drops into the emulsion tube. You can sometimes change the emulsion tube, but it has to be matched to the slide needle to work properly. Sometimes the threads are different on the jets and/or tube.

If it was my bike(or job), I'd do up the original carb.. It's set up just right for the bike.

The easy thing for you to try now is to change the slide needle height and see if it makes it better or worse.

[Gwbarm]

I think he is referring to the main jet size , you may be too rich, what do your plugs look like. If you are referring to Niche Cycle on Ebay they sell Mikuni carbs, since it came with the different jet sizes it is for many applications including the rare carb on your Kodiak. Is it a Mikuni . I would try installing the 135 and see if it helps and go down from there. Was it bogging with your old carb? I wouldnt throw my old one away. 

[Mech]

Yeah I think you're right. I suggested those numbered parts were main jets earlier.

DTLB..  Those are called main jets, and service manuals and parts places will all call them that.. If you read on the internet about tuning your carb, they will mention main jets. That's them.. So you know.

[Mech]

If it pulls strongly up to the 3/4 mark where it starts to falter, then runs out of go abruptly, then it's likely to be main jet more than the slide needle, but if it's weak from 1/4 to 3/4 then runs right out of steam, it's almost certainly the slide needle. A bigger main jet may cure it's shortness of breath at 3/4 if it's the needle, but it may not run well at full throttle and it may stumble accelerating. I could make an educated guess by riding it for a bit up and down hills and along some flat..  But...

The slide needle position is the easy thing to change. That will do something and give some indications about what to do next. It's easier than changing main jets.

Oh..  The jet threads need to match the emulsion tube threads, which needs to match the carb body threads, so you have to be careful if changing parts from one make to another, and, not all jets are measured the same way, or numbered with the same system.. You need to check them by eye or gauge to be sure they do seem the same measure.