1999 M900 Carb Fuel? Issues

[ducpainter]

Do the mixture screws only control the mix before the mains pick up? I am thinking of fattening the mix for the time being, since I am only at 3 turns out.

Yes, the idle mixture only affects the lower throttle openings.

If you normally ride at low throttle settings I'd give it a try. IIRC my bike was set at 3 1/2-4 turns out.

[Dellikose]

Thanks DP - I do usually cruise on lower throttle, so that should help me out.

[koko64]

With an open airbox that's fully chopped you need 160 to 165 main jets (Factory Pro or Mikuni), or 170ish to 175 DJ main jets. That jetting relates to V 900 heads, so if your bike has W heads then my experience may not be relevant. Maybe the previous fouling was caused by carb flooding and contaminated your data? I wouldnt want to thrash an air cooled motor showing off white plugs. I would add some fuel.
I would want the plugs more tan or light brown coloured than the pics. These old air cooled motors dont mind an extra dollop of fuel for cooling and response.

[Dellikose]

Thanks koko. I have the V heads, so I will take your advice and reinstall the 150s at the very least.

I am a bit confused though...Factory Pro has the CRB-D12-3.1-TI kit here. Its description states that it is For when you remove the airbox and run individual filters OR no airbox lid. When you add this kit to the cart via PayPal, the description is CRB-D12-3.1-TI,M900,145,147.5,150. Those mains are what I have in my kit, and are much smaller than what you are suggesting. Is Factory Pro not including the correct size jets in their kit?

[koko64]

I have run 155 mains with a drilled airbox, 160-162.5 with pods and 162.5-165 with fully open airbox. Maybe our fuel down here is the difference. Those plugs looked lean except for the porcelain tip. If they are white or cream coloured deep down on the porcelain, then the main jets are too small.

I would buy some sets of main jets. Some have preferred DJ needles with Factory Pro springs. Some are even moving back to oem springs if they have had slide flutter issues. There is room for personal preference with how you want the power to come in at different parts of the rev range. Main jets are cheap enough to try.
Ime 150 jets work with the snorkles removed or the airbox drilled to some extent. Because Im way over here, its hard to account for different fuels. Plugs dont lie, however.
Curious to see how it works out.

[ducpainter]

This is how my airbox looks...



Ran 165 Dynojet mains,dynojet needles and springs. Mixture screws about 4 turns out.

Always 87 octane.

[Dellikose]

Regarding ordering new mains, these look to be the correct part: Mikuni N208.099 Press-In Style Main Jet for Ducati and Yamaha BST Carburetors

I am thinking of getting 160 and 165, based on everyone's option. It's funny, I compared the jets which I removed when I installed the FP jets, and they look larger than the largest jet in the kit (150) but they do not have any markings on them. Pretty stupid of me to miss that, and I guess that the previous owner already bumped up the mains at some point.

Also - here is a guide I have been using to help me tune the carbs: Carb Worx

Do the carbs need to come off to replace the mains, or can I just remove the bowls and get to it that way?

[koko64]

Unfortunately yes (unlike FCRs). It would be very difficult doing it reliably without removing the carbs and flipping them to get at the jets. The main jet retainer can be fiddly to firmly support the jet. You need to clearly see what you are doing. Not impossible but very tricky with carbs in place.

[ducpainter]

I did my Dyno jet kit with the carbs in place.

It wasn't easy, but it was probably easier than removing them.

Plus, Bruce told me to do it that way. If I got into trouble taking them off the harassment would have been unmerciful.

While we're on the subject of stock vs flatslides, I miss my choke already and it isn't even cold.

[koko64]

  Its the throttle cable set up I hate. D at least can remove the pods to get a clear shot from the front of the bike. Put a rag over the front cylinder fins..

More accel pump squirts needed as the year goes on. I got up to six pumps with up to three starting attempts in our "winter". Im betting 10 pumps and five starting attempts in your Winter. We could run a book on it.

[koko64]

This is how my airbox looks...



Ran 165 Dynojet mains,dynojet needles and springs. Mixture screws about 4 turns out.

Always 87 octane.

I ran mine for awhile with a similar set up with a drilled airbox. It ran 155 FP main jets which would roughly equate to 165 DJ jets.

[Dellikose]

Well I borked this pretty good. I was installing the 165s and stripped the bolt that holds the bowl on to the carb. Literally two finger pressure and boom, totally stripped. It's the one that bolts the bottom of the bracket that holds the throttle cables in place. Think it can tapped for one size up?

I hope that that wasn't a $250 screwup.

[Speeddog]

http://www.timesert.com/

[Dellikose]

Those should be included in the jet kit

[koko64]

I recently put some little M5 helicoils on an FCR for the float bowl screws. Worked well, but you gotta put them in at the correct depth when you don't have a blind hole to seat them. Over tight float bowl retaining screws are pretty common.