906 Starts from Cold at Wide Open Throttle

[PhillyPaso]

This one is new to me. My U.S. spec 1990 906 starts from cold at runs up to full speed. Without any throttle input, and with the throttle fully closed, the tach reaches 8,000 RPM as I'm twisting the key to kill the ignition and avoid any expensive sounds. Scary.

I had recently replaced the excess fuel return line that runs from the front of the Weber, just under the float bowl, around to the fuel return feed pipe at the very back of the tank. A test ride around the block after the replacement was completed showed the bike operating normally. No issues whatsoever.

The bike sat for a week and a half before starting again this past weekend. When I did, the bike zoomed from cold to 8,000 on the tach. After checking my pants, I consulted higgy. But we're at a standstill. I'm asking you, the collective ownership, for advice on what I should look for before I start disassembling things.

After removing the airbox and velocity stacks, I started the 906 again, while looking straight down the intakes. Here is what I observed:
- Twisting the throttle opened and closed the butterflies evenly and normally. No issues, hitches or odd occurrences
- Starting once more, the butterflies stayed fully closed while the bike revved out of control
- After shutting down, I checked the throttles again - operated normally
- The accelerator pump works normally sending little jets of fuel into the venturi at WOT (engine off when tested)
- Gasket between carb body and cylinder intakes appears intact, but it's hard to see with the Weber still mounted

Please let me know what you think I should look at. I'm stumped. (Note: I've never, ever worked on a Weber carburetor before. Please be patient).

[Mc tool]

8000 rpm with no load would only require the throttles to be open a wee bit ...so best make sure the throttle crank is resting on the idle speed screw. The choke also has an air bypass to act as a fast idle ( allows air thru the choke assy to mix with the extra fuel the choke provides , this extra air/fuel is not controlled by the throttle ) so a slightly sticky throttle and a choke bypass could see a rise in revs ........but 8 grand is up there all the same . Maybe not so much choke ....have you tried it with no choke ?
Its not hard to get cables snagged whilst refitting the air box either

[PhillyPaso]

Mc tool: thank you for your insight. FYI, it has been warm enough to start the bike with no choke. Subsequent restarts were also accomplished with no choke. Interestingly, the problem occurs with the airbox on or off. Though I was careful to see how the choke cable was routed into the channel provided on the airbox when it was on. Choke cable works the leaver through the range of motion.

With the airbox off, the carb received a thorough exterior cleaning with a can of carb/choke cleaner. The bike was restarted, and again raced to WOT.

I will check to see whether the idle speed screw moved. But I cannot see how it moved with the bike parked and the tank on. The 906 performed normally one time, after the fuel line repair.

[Mc tool]

Re the idle screw, I was meaning that if the throttle shaft crank was not resting on the idle speed screw that the throttle may be sticking slightly open ........which would be evidenced by the crank not resting on the screw.
Another thing to consider is that these engines dont carburett well at low speed with the air box cover off......so running it with the air box off wont help but I dont think thats the issue here .....just sayin .

[PhillyPaso]

*****

[PhillyPaso]

Mc Tool, Thanks for the insight. Higgy has been also helping me. A quick update from (my) Sunday shenanigan's.

I looked very carefully at the throttle stop. I checked its operation with a feeler gauge. No play, no gap, and the throttle stop made full contact in position with the throttles closed.

Jumping ahead, I was able to see that a missing Choke Spring Retainer Split Ring was allowing fuel to move from the choke to the intakes. I replaced it with a spare split ring that held the spring in place and checked the choke operation. Everything felt and looked OK.

Higgy suggested that I coat the old top cover gasket with anaerobic sealer. I did. I reassembled the top half of the carburetor and I let the sealant cure for 24 hours. The next day, I started the 906. The bike started and raced to 8,000 RPM from cold.

I've since ordered a Weber 44 DCNF rebuild kit. But that will take a wile to get to me (Southeastern Pennsylvania, Philadelphia suburb, USA).

Higgy & I think that air and fuel is getting into the mixture. The fuel leaks from the bores in the choke spring tubes certainly looked like the culprit. I thought fixing the previous owner's oversight and sealing the gasket would help. Alas, it's not the only culprit.

[higgy]

https://www.youtube.com/watch?v=LFnYvgSiQQg

[PhillyPaso]

Extra special thanks to @higgy for making the trip to my humble garage.

In the end, it was a bad top cover gasket in combination with the left hand main jet that got unscrewed, somehow. The wide-open main jet allowed too much fuel into the left-hand venturi. After that, it was off to the races.

Two new O rings on the Idle Jet Holders (screwed-in heads) finished the job. No air leaks. Bike starts, idles, and roars when I twist the throttle. Thanks also to Mc tool for your sage advice. You started me on the right path.

I have to close for now and go riding...

[higgy]

We also solved his brake light Issue

[PhillyPaso]

We also solved his brake light Issue

Wait... What?!?

Thin thing has brakes?

[Mc tool]

Brakes , when I got my Paso I thought the brakes were great if a little underpowered but good feel . In the wet I learnt that the 1st (sphincter tightening ) couple of rotations after grabbing a handful of brakes wipe the water off the discs then its business as usual.
Then I got me a 1997 900ss desmodue ( looks like a superlite ) and the brakes on that made Paso look pretty average.
Pleased you got your Paso sorted.