Scooterotica

Right, 200 Mugello on a small block, 28mm Dellorto PHBH, Clubman pipe.

Most of the time the scooter runs a dream. It starts easily will attain 65MPH and is a joy to ride.

Every now and then, when I try to start it. It goes into a sort of burbling mode where it won't quite pick-up and I have to hold it a max throttle just to keep it burbling. Sometimes after a minute or two, it will spring into life and all is well for miles/days.
Sometimes it refuses to pick up, so I whip off the ram-air filter. It fires into life. I replace the ram-air and ride off for miles/days. Until the next time.

At the moment, I am bamboozled as to what the hell is going on.

All suggestions welcome, though I suspect I may have tried most of them.

It’s not a minder bender, it’s standard issue on scooters with bellows and filter. You probably have good/safe jetting, combined with a little spitback from the piston ported inlet. The excess fuel is pooling in the elbow of the bellows and the scooter doesn’t pickup off the throttle until that clears.

Try drilling a hole in the outer corner of the bellow elbow to allow the excess fuel to drain out. Provided your jetting is not excessively rich, this should do the trick.


Sent from my iPhone using Tapatalk

Nice one Drunky............I'll give it a go

Hmmm.

I personally wouldn't put a hole in the bellows without ensuring that that drain off was also filtered, but concede that was once a standard thing on pre GP bellows.

However, I do believe that with a tuned engine, then the standard bellows can be inadequate, as I have found to be the case on even 'only' 175 engines.

Even with homemade, large elbows/bellow & huge, homemade foam filters, there can still be an issue of 'gassing up' causing starting problems, even though the filter does not feel in any way damp or saturated, particularly with piston port engines that have large inlet timing such as the Mugello & Rapido. I should also add that there is no pooling of excess fuel in the lower elbow when I have experienced starting problems.

It's an issue that I have been having for some time with a Rapido & I've found the best solution, so far, is to have such a long (vertical as possible) bellows between the carb & the filter that it places the filter up near the frame air scoop. Currently, I am running two elbows to gain some distance between the filter & the carb.

I've done the work advised by 'Drunky' I'll report back after the next time I use the scooter.

Hopefully, that will be a solution.

However, I'd be concerned if a standard bellows on a bigger carb with a tuned engine that needs so much more air/fuel DIDN'T cause a restriction, though that may sound a contradiction!

The CSA (Cross Sectional Area)of the standard bellows is a constriction, IMHO, & my belief is that a greater CSA is necessary.

I don't know an awful lot about the requirements of filtered air, but motorbikes & karts often have a huge plenum chamber for some reason or another, which makes me think that has to be something to emulate as much as is possible.

The pipe I run between filter & carb is a flexible 60 mm diameter just for a 30 mm carb. Compare that to the standard bellows......

As said, common problem but manageable so long as you turn the petrol off shortly before turning the engine off. Then leave it turned off when you restart.

Good advice above. This happened on my GP. When it burbles turn petrol off and after about 30 seconds it should start revving and be quick to turn the petrol on.

i noticed when i had my bellows horizontal it would occasionally flood, as described. I altered my bellows to a vertical position and it was much worse. I've now fitted one of the Chiselspeed panel filters and this seems to have cured the problem.........i hope.

O/K I've ridden 120 miles today. It only did it once (outside the pub I stopped at, naturally)
This is an improvement, but I made only a small drain hole, for fear of wrecking the 'jetting'
Tomorrow I'll check the plug. If it still looks O/K ? I'll enlarge the drain hole.

Thanks again 'Drunky'