Unsolved petrol issue

[Revvers]

Hello. I've put a couple of posts on here regarding a problem I had since getting my scoot back from the painters. Initially I found that they had left bead in the tank which obviously blocked the fuel pipe and stopped the engine. I cleaned that out but have an ongoing problem. At high(ish) revs, the scoot just hits a plateau and starts to lose power. If I open the throttle nothing happens and I'm going along at half revs. I pull the clutch in, rev like buggery and power is restored. Someone on here said I had an air lock but I don't know how to get rid of that. Also, 2 new petrol caps have blocked up causing more problems. The only other thing changed at the painters was the exhaust. I changed a Clubman to an Ancillotti. The carb filter inside the 'banjo' cap looks clear
So, have I an air lock and if so, how do I clear it? Why do my petrol caps keep blocking? Do I need to strip the carb down? Should I have re-jetted for the Ancillotti ?
Sorry for going on but this has me stumped and is very dangerous on the road as you can imagine
Thanks

[grandpa]

Hi Revvers,
Is your fuel pipe getting kinked when you sit on machine .Is it a different pipe i.e softer
Just a thought
Grandpa

[Donnie]

You seem to be saying you havent stripped the carb? I hope Im wrong as fine blast particles will get past the banjo filter and ruin your rings!

[IaninDorset]

Yes, sudden loss of power is something to be worried about as cars don't see us riders enough as it is and also an unreliable ride takes the fun out of scootering when you obviously care for your machine and should reap those rewards.

Blocking petrol caps is something very unusual, by definition they are fairly set and forget parts but something is causing something in the tank to get into the breather part and lock up the air flow which allows your fuel level to drop, without that, you have a vacuum and that is probably the sudden power loss as the vacuum is retarding the fuel itself from flowing naturally into your carb. The outside air pressure also gives the fuel a little "push" so it flows evenly, less fuel means more air pressure so the small mass of fuel still flows per se.

Have you used a tank sealer at any time? Another thought comes to mind is water in the fuel stations tanks, water rides on top of the fuel and develops into a lovely rusty scum as it sloshes inside of there which could be causing some of it to get into the cap and block it. As water gets drawn into the carb though it blocks jets and the gauze filter reducing flow at the engine end, air can then be drawn back up the pipe as the vacuum carb side fails and cause air to block your fuel pipe.

Edit: Donnie also brings up something there, if any of them sharp blast beads get past the quite slack fuel gauze filter in the carb and through the jet, your rings and bearings will likely be torn to shreds causing a major engine disaster and you wouldn't want a bearing fail at top speeds as it will lock the engine solid

[peejay]

please make every effort to ensure that your tank is spotless as others have said. blasting medium will destroy your motor in minimal time

[Revvers]

I've ridden with the cap slightly off as the engine stops when the cap is fitted. The petrol pie is softer and longer than before, I'll have to check that. Looks like I need to strip the carb. Something I haven't done before!
Thanks for the advice chaps, I hope I can sort it and that I haven't buggered anything up yet

[MickYork]

Stuck rings may be an option.....

[Revvers]

Hi Mick, would it still start and run ok most of the time if the rings were stuck?

[IaninDorset]

If it were a Vespa the culprit I would be looking at is the main and idle jets being partially blocked as Spaco/Dell Vespa carbs have a sliding off of the idle to main which occurs with the idle turning off about 2/3rds throttle, dunno if Lambrettas have this same sort of setup.

[sunrisemac]

Have you tried doing a plug chop after it has started misbehave?, if not try holding the throttle open for a short while, when it wont rev out, then do a plug chop and see if the colour indicates weakness. If it does then fuel starvation may be the problem which could happen after a bit of hard use has slowly dropped the float level in the float chamber until it runs weak. Knocking it out of gear allows the fuel level to 'catch up' until it happens again. Try checking the hole above the float needle to make sure it is not restricted.

[Revvers]

Carb off methinks Thanks Guys

[coaster]

Carb off methinks Thanks Guys

Right decision, not a difficult job just be methodical and lay parts out as you remove them. Count the number of turns on the mixture screw and buy a can of carb cleaning spray to clean the outside and then to blast through all the internal channels and jets.

[IaninDorset]

You can get most replacement parts under the vintage motorcycle section of Dellorto.co.uk, the springs and jets I ordered from there arrived next day and spot on parts too