1999 M900 Carb Fuel? Issues

[Dellikose]

Well, I still have the same issue even after all of the cleaning. The bike idles decent, I still can tell that it is a bit off, but when very slight throttle is applied it will bog and backfire through the carb. When I pull the plugs, the vertical cylinder is fine, but the horizontal is white. It smells a little like gas, but not soaked as if the plug was not firing.

I ran a compression test just to ease my nerves, and came up with 125 psi on both cylinders - so at least the piston isn't cooked. I also checked the pick up resistance and came up with 105 ohms on both cylinders. One last thing I checked was the gasket between the intake manifold and head, no issues there.

I'm still leaning (ugh) towards fuel starvation.

[Dellikose]

Here is a list of all that I have done to try and fix this issue. I am going to check the valve clearances and then pull the carbs once again for another cleaning, and to make sure that no more grit is inside.

New Parts

165 Mikuni Jets and o-rings
K&N air filter
Rebuilt fuel pump with Mikuni kit
NGK spark plug wires
NGK spark plugs
Fuel filter
Fuel pump vacuum line
Fuel lines with heat shielding
Manual petcock

Checked

Cleaned carb
Set floats to 14mm
Cleaned carb vacuum pod filters
Timing and belt tension
Vacuum leaks
Carbs synchronized
Cleaned all gas tank vents
Pick up resistance
Switched ignition modules
Carb slides open evenly

[ducpainter]

Have you verified both slides are lifting?

[Dellikose]

Yes - I forgot to add that to the list. I ran the bike without the air filter on last night and saw both slides lift evenly as I applied throttle. I was also able to see the atomized fuel come from the needle valve. One other thing I did was to spray a little carb cleaner down the 2 small ports (idle passages I assume since they are before the throttle plates) on each carb and the bike would stumble a bit.

I also noticed that it coughed out of the carb on both cylinders with the slightest amount of throttle, but rev fine once I got past the initial tiny application.

[ducpainter]

What does this cough feel like when riding?

Is it pronounced?

[Dellikose]

Yes, it is pronounced. It feels like a complete miss.

What is a good testing procedure for the fuel pump? Just disconnect it at the carb and aim it towards a bucket and crank? What sort of flow am I looking for?

[ducpainter]

I don't think it's a fuel pump problem with regard to the cough. If you can fill the bowls it should respond to throttle. Fuel delivery issues would occur at wide open throttle.

That said a typical Mikuni diaphragm pump puts out ~35 l/hour.

What are your fuel screws set to?

[Dellikose]

I am currently at 4 turns out from seated.

[ducpainter]

I am currently at 4 turns out from seated.

Open/drilled airbox with cans?

[Dellikose]

Open air box with stock cans.

[ducpainter]

Open air box with stock cans.

Have you tried screwing them in a half turn or so.

I'd ride it too instead of just trying to work everything out while the thing sits and gets hot from not moving.

[koko64]

That low cough is in the  idle mixture and pilot jet zone. It's either a rich stumble type cough or lean hesitation type cough (or out of synch cough). Turn it in as DP suggested to see if the cough is more or less pronounced.

[Dellikose]

Thanks guys.

I will turn the idle mixture screws in (clockwise) - this should lean the idle mixture, correct?

I have the carbs off the bike now, so I will bench sync them to get a baseline and then sync them again with the bike running after they are reinstalled.

[koko64]

Yes. Try that and see if it improves response. Try half a turn at a time.

[Dellikose]

It's getting a little better as I get the carb dialed in.

One question I have is that my vacuum taps on the intake manifolds are not the same size. One is 5mm and the other I had to drill and tap to 6mm a long time ago since it got stripped. Would this imbalance my manometer when I try to sync the carbs with the bike running?

As for the backfiring mentioned earlier in this thread, here is a still from a video I took. You can see the orange flame behind the slide on the left (horizontal) carb. From everything I read, this is a lean condition. That's what my spark plug says too.