Carburettor frothing

[Storkfoot]

It’s a Mikuni TM24 on a GT186. The other day, I thought I had the carb jetting somewhere near. I was happy that the previous issue of an air leak at the inlet manifold had been resolved with the CamLam inlet rubber you can see in the picture on the post below.

viewtopic.php?f=7&t=14534&start=20

I am happy that the carb is not touching anywhere on the engine or inlet manifold and is “suspended” by the rubber mount.

If I rev the scooter hard on the stand, I can see petrol pouring out of the overflow. It’s an overflow that comes from the carb body above the float chamber. There is no surge contraption in a Mikuni like on a Dell’orto. I am also happy that the float chamber is set at the correct level. I have had a couple of backfires too.

On the road, running is erratic and seems to run better downhill than uphill.

My suspicion is that the rubber is too short and effectively is acting like a solid mount, causing frothing in the float chamber. I can’t pull it any further off the inlet manifold to give a bigger gap between the carb body and inlet manifold.

I am going to ask Cam Lam if they have a longer inlet rubber but has anyone any opinions or thoughts on what I have said please?

I haven’t yet rechecked the timing but the scooter ran well with this static timing on the previous PHBH30.

Ps. I have used a compressed air can to blow through every drilling and Jet on the carb too. The other thing that has changed on the engine is the exhaust. A Ron Moss Avanti clubman replaced the old MB/bum Clubman. This allows the engine to rev higher but I have not noticed increased vibration through engine and frame.

[coaster]

I could very well be wrong but I thought that fuel frothing was a problem affecting high reving engines with solid mounted carbs. Could it not be float or float valve related?

[Storkfoot]

I could very well be wrong but I thought that fuel frothing was a problem affecting high reving engines with solid mounted carbs. Could it not be float or float valve related?

The float and float valve are set as Mikuni direct and I have tried altering the height +/- 1mm. I have also tried two different floats. I am thinking that the float is vibrating too much in the chamber as well as the fuel

[hullygully]

"On the road, running is erratic and seems to run better downhill than uphill."
how is it on the flat?
cos when I travel to Scotland with a std tank, with a Gori 200 c/w 27mm Delly, the above happens to me, I take it as fuel level starting to get low when its a steep incline?

[Storkfoot]

"On the road, running is erratic and seems to run better downhill than uphill."
how is it on the flat?
cos when I travel to Scotland with a std tank, with a Gori 200 c/w 27mm Delly, the above happens to me, I take it as fuel level starting to get low when its a steep incline?

It isn’t great on the flat but then the jetting isn’t spot on yet (as you know, you can only Jet a carb when everything else is okay). I’d need to do some more miles to be certain that it is noticeably worse going up hill.

That doesn’t explain the fuel coming out of the overflow pipe when revving on the stand though.

[Storkfoot]

By way of update, I have realised that revving to full throttle on the stand is probably going to give you the maximum vibration you can possibly have, causing the fuel to spill from the overflows. That probably happens on lots of carbs.

On the road at full throttle, of course, the vibrations are not as bad. I have managed to reposition the rubber to give a 9mm gap between inlet manifold and carburettor and this has helped reduce the fuel spilling out of the overflow pipes because it allows the carb to flex a bit more.

As regards the poor running, I think this was more to do with the jetting. I have weakened the slide and richened up the atomiser and main jet and it is running much better, with no difference between up and down hill. I am not quite there but not far off.

Thanks for your help again.

[hullygully]