1999 M900 Carb Fuel? Issues

[samuraibiker]

i am no duc expert...absolute noobie actually...but i have been riding vespas for 30 years and those photos of your plugs look like you are running way too lean....

[koko64]

Yes on the lean side. If the tan section was over the whole porcelain it would be better.

[Dellikose]

I had some quality time yesterday with the carb.

First things first, I mentioned that I stripped one of the float bowl bolt holes. I may have been lucky - the bolt that Factory Pro provided in the kit is too short to go through the throttle cable bracket and the bowl mounting surface. So while I did strip a few threads, there may be enough of a bite with the original, longer bolt. If not, it's Timesert / Helicoil time.

I replaced the jets with 165 Mikuni jets. I had the choice of either 160 or 165, and I chose 165 due to a couple of reasons. First, I measured the jets that were in before I started any of this work, and found them to be around 162.5. The way I measured was to place them on a 1.5 mm allen key and see how much slop each jet had. Second, and the most important, was based on the experiences of the commenters in this thread.

I did notice a bit of corrosion between the float bowls and the carb body, along with the gaskets. I cleaned these up and reinstalled. I also adjusted the floats to 14 mm because I was there. They were close, but not perfect.

So now I just get to put everything back together and play with it when the weather breaks. I will fine tune, sync the carbs and post some follow-up pictures.

[koko64]

How did you go?

[Dellikose]

I'm still waiting on the weather...still have to put the carbs back on. What's the best way to attach the throttle cables? I always remember it being a bear in the past (which is why I try to leave the carb on as much as possbile).

I also noticed that one of the vertical cylinder valve cover gaskets were leaking. They are the paper kind, so I will be replacing them with OEM metal.

When I put everything back together I will be replacing the original spark plug wires with NGK Racing wires too.

[koko64]

Getting the cables on is a pita for surem because you need three hands for the second cable. One is easy, but the other requires careful routing not to rub and rotating the throttle pulley. Try and lock it into place with a small Allen key or something lke that. Mechanics gloves help with pinched fingers.

I remove the cables from the throttle housing end now days.

[Dellikose]

Update

Everything is back together and running. Unfortunately, I experienced the same issue about an hour into a ride yesterday. Everything was great, and then the engine started bogging at cruising RPMs. Every time throttled was applied the engine would just bog down. It got worse and worse until I was forced to pull over. It would idle fine, but any throttle would just bog until it would stall itself. I turned it off, removed the air filter (which I just replaced as I thought the previous one may have been clogged) and it ran just fine and took full throttle without issue. Maybe I unkinked fuel lines when I lifted the tank...I don't know. I only had about a mile to make it home, so I ran it without any air filter for that short time.

Here's what I have done this winter which related to this problem:

• Replaced the internals in the vacuum fuel pump
• Upped mains to 165 Mikuni OEM jets
• Added heatshields to fuel lines in case they were collapsing
• Replaced the fuel filter
• Replaced the air filter (K&N open airbox)
• Replaced spark plugs

I plan on balancing the carbs soon...besides that, what other items should I be looking at?

[koko64]

When it bogs, runs on one cylinder or do both bog?  Could a coil be dropping out when hot? Can happen.

How much oil in the air filter?

Maybe you did fix a kinked line.

[beethoven]

As an Aussie don't know your weather but any chance the carbs are icing?

[koko64]

How cold is it there?

[Dellikose]

It feels like both cylinders are bogging. I am not 100%...I will try to determine that the next time it happens. But there are no abnormal vibrations like one cylinder is dropping.

The air filter was brand new, right out of the box. The pleats looked the normal pinkish red - nothing excessive or dripping.

It was around 60 F / 16 C when I was on the ride.

Another thing I saw come up searching the forums were clogged vent lines. Is there any good way to test these to see if the little filters have become clogged? Suck on the end and see if I get a mouthful of gas?

[ducpainter]

Fuel starvation?

Open the cap and see if it gets better

[Howie]

Also confirm your fuel pump delivers sufficient fuel.

[Dellikose]

Fuel starvation?

Open the cap and see if it gets better

I opened the gas cap when it happened, and I did not have a "whoosh". Also, I can draw through the gas tank vent lines with no problems.

Also confirm your fuel pump delivers sufficient fuel.

I will add this to the list and check back.

[Dellikose]

This is starting to get frustrating.

I was letting the bike idle while I was cleaning out the garage. About 5 minutes into idling it was warmed up, and I cracked the throttle a couple times and it revved up fine. A few minutes later, I tried to rev it again and it bogged and died.

I drained both bowls, and only got ~ 2 beer bottle capfuls (extremely scientific measurement) out of each bowl. I figured that I found the issue. For shits and giggles, I waited a few minutes and started the bike back up, turned it off and checked the bowls again. This time, fuel kept flowing out and did not stop. I'm guessing that it's a fuel pump issue, because it should be closed with no engine vacuum on it right? I have almost a full tank, but I wouldn't think it would force it's way past the pump membranes just by gravity.