This is a mindbender ...........

Anything related to Lambrettas... ask tech questions, post helpful info, or just read and learn.
Daggs
registered user
Posts: 962
Joined: Wed Jan 07, 2009 1:23 pm
Location: Addington Kent
Contact:

Thu Jun 21, 2018 9:07 am

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.

User avatar
drunkmunkey6969
Moderator
Posts: 4264
Joined: Sun Jan 04, 2009 1:42 pm
ebay user ID: sc00terotica
Main scooter: '69 Lambretta GP
Location: North Yorkshire
Contact:

Thu Jun 21, 2018 9:44 am

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

Daggs
registered user
Posts: 962
Joined: Wed Jan 07, 2009 1:23 pm
Location: Addington Kent
Contact:

Thu Jun 21, 2018 11:02 am

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

Warkton Tornado No.1
registered user
Posts: 87
Joined: Mon Oct 17, 2011 6:16 pm
Main scooter: Lambretta SX200
Contact:

Sat Jun 23, 2018 12:17 pm

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.

Daggs
registered user
Posts: 962
Joined: Wed Jan 07, 2009 1:23 pm
Location: Addington Kent
Contact:

Sat Jun 23, 2018 12:45 pm

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

Warkton Tornado No.1
registered user
Posts: 87
Joined: Mon Oct 17, 2011 6:16 pm
Main scooter: Lambretta SX200
Contact:

Sat Jun 23, 2018 2:10 pm

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......

User avatar
coaster
registered user
Posts: 3688
Joined: Wed Jan 07, 2009 12:35 pm
Location: London and Norfolk
Contact:

Sat Jun 23, 2018 11:56 pm

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.

User avatar
Timbo
registered user
Posts: 2244
Joined: Tue Mar 03, 2009 2:46 pm
Main scooter: SX200 and GP230 RT
Location: Luton area.....
Contact:

Sun Jun 24, 2018 8:15 am

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.

mick1
registered user
Posts: 1544
Joined: Sun Dec 04, 2011 7:43 pm
ebay user ID: saxonleaf
Main scooter: Li Avanti 225
Location: York
Contact:

Sun Jun 24, 2018 12:02 pm

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.

Daggs
registered user
Posts: 962
Joined: Wed Jan 07, 2009 1:23 pm
Location: Addington Kent
Contact:

Sun Jun 24, 2018 8:16 pm

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'

Post Reply Previous topicNext topic
  • Information
  • Who is online

    Users browsing this forum: No registered users and 15 guests