M900 Coughing Issue

[ChrisK]

Other than the four hose clamps I need to tighten down on the intakes and manifolds, where else would there be an air leak?

[ChrisK]

Actually, there's an elbow nipple that comes down from the bottom, front, left corner of the airbox. Also, there is a rubber hose that is sitting unused near the left pod, with the other end being down near the bottom of the bike, ending in a sort of 'reed' fitting, almost like it's pinched at the end kind of. Is that supposed to hook into the airbox?

[ducpainter]

Yes, but that's not the issue with the bike.

That's the airbox drain.

It can leak air from a vacuum hose. Either the one to the fuel pump, or the one to the vacuum petcock.

[ChrisK]

Okay I buttoned it all back up again, this time I took extra care to really torque down every screw, hose clamp, bolt, etc. It got better but it's still terrible. A leak somewhere makes the most sense to me. With the choke pulled it runs better. If I let it sit at idle for 20 seconds or so with no choke it will have enough gas in the bowl to rev the engine. But, if I release throttle and immediately rev it again it leans out and is starved of fuel. As I back the idle mixture screws out it gets slightly better but they're already almost the entire way out. So, what else would be causing too much air/not enough fuel?

[ChrisK]

Also, if I place a piece of paper over the air filter covering 3/4 of it, it runs great. Granted that's with the idle screws turned way out.

[ducpainter]

How much fuel in the tank?

Have you verified the vacuum line to the pump is connected

[ChrisK]

Plenty of fuel and vacuum lines are securely connected

[ducpainter]

Don't take this the wrong way.

It wasn't broke, other than a light cough, until you 'fixed' it.

New parts shouldn't make the thing barely run, so it must be an adjustment or assembly error.

You say the diaphragms are right as well as the o-rings being in place.

If you've verified fuel delivery from the pump and made sure there's no kinked lines...

I'd re-visit the floats.

[ChrisK]

No offense taken, I'm nearly 100% sure that this is installer error. I'm just trying to get opinions on what I could have messed up to cause it. I believe it's probably something I did that is preventing fuel flow... and like you said, I think at this point the float levels are the best bet. Since I've messed with those and I believe I've confirmed that nothing else is kinked/improperly connected.

[ChrisK]

Well I've had the carbs out of the bike 4 times over the weekend now, so I'm getting pretty damn good and taking them out and putting them back in.

I'm going to rattle off my thoughts on it if you guys would like to confirm my theories and point me in the right direction.

The first time, I switched to the biggest main jets I had available to me, and I set the float levels to the recommended height of 14 mm, moving from the 11 mm they were set at. So I went to a jet that sprays gas faster, but changed the float making it so there was less gas available. The bike ran way too lean.

The second time, I went to the smallest jets, 165's and moved the float level back to the low end, making more gas available. The bike ran really rich.

Now, I've slid in the 170 main jets and set the float levels to a compromise, at 12 mm. The bike still starts and runs rough. Cracking the choke a little bit helps it run better.

That said, since cracking the choke restricts airflow, does that mean it's running lean? If so, should I maybe consider moving the needle position up? In theory to allow more gas through?

Any suggestions?

[ChrisK]

Now that I think about it more, adjusting the needles upward, so more fuel comes through, makes more sense. The needle jets are supposed to affect performance at 1/4 to 3/4 throttle, correct? The bike idles alright and when I go WOT it absolutely rips. It's when I'm just cruising between 30-60 mph in 2nd-5th gears that it seems to run lean and snap, crackle, and pop. So, in theory, wouldn't adjusting the needles make that mid-throttle range perform better?

Somebody? Anybody?

Also, for anyone exploring the world of carburation for essentially the first time like I am, this is a great link: http://carbworx.com/Carburation.html

[Howie]

Go back to 14mm. on the floats, raise the needles 1 notch and give it a try.

Yep, needles control partial throttle, mains control full throttle, high load.  Mixture screws and pilots control idle and just off idle.

[ChrisK]

Okay thanks, going to ride this evening since it's finally beautiful here. But hopefully I'll have time to do it tonight, I'll report back. 

[ducpainter]

Go back to 14mm. on the floats, raise the needles 1 notch and give it a try.

Yep, needles control partial throttle, mains control full throttle, high load.  Mixture screws and pilots control idle and just off idle.

I'm going to add that with the CV carbs all the circuits affect each other.

While the main jet is solely responsible for the wot circuit, it also affect the midrange, and to a lesser extent the idle. Same for the needle. It is responsible for the midrange, but fuel does flow through the main jet and therefore the needle jet at idle.

None of the circuits shut off at a particular throttle position.

[ChrisK]

Thanks for your input. Different jets do take precedence at different points in the throttle range, however they are all working in conjunction at all times. Good to keep in mind.