fuel blow back whats the cause?

[brianc]

hi,had problems with starting when engine was hot,turns out im getting fuel blow back from the carb,its sitting in the hose and when I try and start the engine again its flooding,ive tried smaller pilot jets but its still doing it,ive resorted to drilling a small hole in the bottom of the hose,when the engine is running I can see fuel spray from the hole,obviously I don't want that,idealy I want to stop the blow back,any ideas on cause and a cure??

brian.

[johnd]

Engine? Carb? Exhaust? Ignition?


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[Spanish Fly]

Your posting in the Lambretta section so I deduce that your machine may well be one. Other than that you provide no clue that would allow someone to assist you in your quest for a cure.
What colour is it?

SF

[Yanker]

Yhe colour is most likely 'wet'? Your float needle is pooped, replace it (that is with a new one rather than remove it then put it back in again) then try again.....

[brianc]

sorry for the lack of info,its a lambretta gp 150 with a tuned 175 barrel, mahle piston,22mm jetex,42mm exhaust,electronic ignition,il try changing the float needle as above post suggestion.
brian.

[Monty]

I have had a summer of this and other pain in the arse problems. Similar set up but with a JL and a muggy. At one point on tick over or the last few strokes as you switched of it was pouring out. Did the hole trick and then had fuel dripping out onto a hot exhaust. I stripped it down 3 times. New rings. New piston and rings. Bigger squish. Pissed about with the float, needle and jets even though the same jetex had been fine for years. Tried another jetex 26mm same. I had always been a fan of the ruddy things as being easy to set up even though they are like Marmite. Most hate them. In the end I dumped it and bought a 26 mm Dellorto. I have now seen the light. It starts first kick on choke. It rides from low revs and pulls through the rev range and is not on or off like the jetex. The only drawback for me was loosing the airbox and the induction blaah. Its better with a remote filter but you get used to it.

[Chris in Margate]

Often caused by a mis-matched tune; overly ported, over carbed, wrong pipe.
Got to run open mouthed with "tea strainer" filter to avoid unspent fuel to blow through and stop engine bogging.

[brianc]

chris,i tried running it with one of those little aftermarket k +n type filters and it was fine because it was soaking up the fuel,but I really wanted to find a cure and use the standard hose and filter,maybe the k+n type is the answer,can you recommend a decent one ? in the meantime il try replacing the float needle in the jetex as previous post suggests,


brian.

[Chris in Margate]

From experience I would say it's the state of tune and not which type of carb or filter.

[sean brady scooters]

sounds like its got extended inlet timing duration hence blowing back a bit at at low revs ,not a prob on an open carb but always on filtered systems .
if its just the piston skirt that has been shortened then you could just fit a new piston to cure it.