Air bubbles in petrol pipe/is it causing fuel starvation

[rps123]

I am using a brand new PWK Keihin Carb 33mm long range tank over sized breather in cap and cam lam fast flow rear exiting fuel tap and i'm getting air bubbles in the clear petrol pipe sometimes up to 2.5 inch long. I have been having problems recently with the motor seizing at 6000+ rpm in top gear it will run all day under 6000 but ask it for more power and after 5miles ish of sustained 6000 +rpm it trys to nip. It as already wiped out 1 piston and is on its way to no2. Its been on 2 different dynos and is set on the rich side. Could the air bubbles coming into the pipe be causing fuel starvation and how do i stop it. Help needed asap. Cheers

[Andy Pickering]

Now thats really intresting is that, im also running large tank and camlam rear exit and have been having the same symptoms of late..take it steady and its good to go all day long, whup its ass and it gets red hot and led to another slight nip at 70mph last weekend.. ..when i came to a stop and checked the plug it was choccy brown but I did notice my clear fuel pipe full of bubbles..which led me to beleive i was going onto reserve, when i checked the tank it was half full..

I have noticed this morning that the rear exit tap has been rubbing on the engine mount which could cause vibration to transfer to the tap..I am going to lift my tank higher to give a bit more clearance..its parked up now for a couple of weeks until it gets dynoed at Sean/Dans..

[rps123]

i only changed to a cam lam tap to try n remedy the problem as it was faster than my fast flow tap n didnt kink the fuel pipe as much, i can see the bubbles come from the carb back up the pipe?

[minotaur]

13l tank, 30mm bgm pwk a cam lam rear exit fast flow and i have up to an inch long air bubble in pipe

[Andy Pickering]

That is odd...I do think my problem is caused by not being set up right but that is odd..Ive just turned mine onto reserve and the bubbles have gone yet plenty of fuel in the tank, infact about half a tank, i might put my normal fast flow back on and see if any bubbles appear on normal tap position, I know Eden is running a rear exit tap but from memory sure he runs it always on reserve..?

[rps123]

Yes i always run on reserve as well, ive run out of ideas of why im nipping at 6000rpm plus everything in engine is brand new and since i wiped out the 1st broady piston after 4 nips ive had the barrel honed to 4000 clearence on the new piston and changed the full ignition side fitted augusto changed the fuel tap, big breather in tank cap, changed end can on devtour everthing to try and remedy the prob which always looks like a fueling prob damage always round exhaust port, had it dynod twicw after 1st dyno it tried to nip again at 6000 rpm after 5mins on a W27 plug so had it checked on another dyno and confirmed it was very safe jetting wise so BR 10 plug in onto motorway 6500rpm 5mins tried to nip yet again. Changed the barel and piston and same story. The only odd thing is the air bubbles up to 2.5inch appearing after short journeys could these be causing the fuel not to enter the float bowl properly thus causing fuel starvation when i need more flow at increased speeds

[soulsurfer]

If it's fuel starvation the plug colour will show as lean directly after the nip up if you kill the motor, de-clutch and coast to a stop.

[Andy Pickering]

If it's fuel starvation the plug colour will show as lean directly after the nip up if you kill the motor, de-clutch and coast to a stop.

Agreed..thats why Im certain mine is due to incorrect set up carb side, still bloody odd though this air bubble lark..

[rps123]

Plug colour after nippin is always leaner than the jetting would suggest nice rich colour normally

[soosh]

A pwk carb needs the float needle jet drilled out to 3mm.I had the same issue with mine on my GP.
Its down to the flow rate coming into the carb.One drilled,you may need to get a cotton bud with a bit of autosol and polish the seat up if it doesnt seal.