I think I found the cause of my unstable idle

[MarkM]

95 Skorpion:

I bought the bike late in October, rode it 2 weekends, and it has sat since, until I start a new insurance policy for it (soon!).

Rear brake over heated, power was low, and idle would be intermittently unstable, revving high. My ex GF reported loss of power at cruise, only recovering after slowing to first gear speed. That never happened to me, in riding 3 tanks of gas worth.

Just went looking for air leaks the other day, nothing obvious externally. Pulling the diaphragm cover shows the issue:

I set the pinched area into the groove, have retained it with hemostats (fancy clamps) for a couple of days, and will see if it will relax where it is supposed to be. Otherwise I'll try a little bit of 3M (#77?) tack spray (applied with a toothpick) to keep it where it belongs.

Hopefully this finds a few more horsepower with a carb secondary opening well.

Cheers,

Mark
San Carlos, CA

[DAVID THOMPSON]

nothing beats a cv carb for being a big pain
in the xxx
dave

[Anders]

That is the effect of unleaded gas swelling the rubber diaphragm...

You should probably get a Tunebike carb kit (or make one yourself) to compensate for the swelling, search the forum for "Tunebike" for details.

[MarkM]

That is the effect of unleaded gas swelling the rubber diaphragm...

It might be swollen, but it was misinstalled. Take a close look at the full resolution picture. The diaphragm was doing nothing and the slide couldn't lift properly at all.

On my bike, a helper had taken off the tank and done an external visual inspection for air leaks. I was thinking the next step was carb removal for cleaning (could still need that), but I decided to pop off the diaphragm cover first since it is so easy. The mistake was obvious.

On my 4-carb bikes, I reach in to each carb and lift the slide (expecting to feel the air resistance), and let them drop. The drop should be slow and damped by the same air resistance. A snap shut shows a split or mis-fit diaphragm like this one. When I bought my first MaximX Yamaha, one of the slides was completely stuck shut.

M^2

[Anders]

The swelling tend to make it hard to install correctly due to dimensional changes.