WOT fuel starvation phbh28

[tonydevon]

just wondering if WOT on the dyno or sat still in garage ha ha ha ha, would require less fuel than WOT on the road maybe up a slight hill.

this relates to my bike engine carb, on dyno it seemed ok, setup really well and the main problems now are with the actual engine not the jetting. low compression ratio, huge squish

however I went out last night to try and run it a bit and test for its first proper run next weekend, local A road, long slight uphill section, its a piece of crap soon as you hit a hill, thats something I have to address, but after a period of WOT it died, no lock or funny noise, just went BLAAAARRGH, clutch it and coasted to layby, checked not siezed, checked fuel in tank, all good, turned key and it fired right up

will be double checking that float didnt get disturbed during jetting on the dyno, but was wondering if the float valve could be a problem, has a 200 in it, will change to 300 out of interest, but couldnt get my head around if the engine actually empties the float bowl quicker out on the road under load, than on the dyno etc

carb was very cold

also while educating me anyone explain why some float valves are sprung and some arent?

[Speed Demon]

200 sounds small for a 30hp motor, normally look at 350 for that. The problem might be slow flow out of your tap, a vacuum forming in the tank if the breather is too small (or there isn't one) which you can check by listening for a sucking noise after running when loosening the cap, or simply that you have reduced the 'fall' from tank to carb that the bike had in your set-up.

[coaster]

It will definitely use more fuel on the open road as the dunk won't account for headwinds, gradients and rider weight etc. 200 is also too small for the fuel valve, for a motor putting out your level of power, you wi need a 300. I'd also check the fuel level, you can bend the tag a little to affect the float height. I had similar issues with a pwk that I had operating at an extreme angle which would die after about 10 seconds of WOT but would recover by the time I'd coaster to a halt.

[tonydevon]

many thanks guys for the advice but also explaining things, I like to know answers but also why

the cap on the tank is just the stock lambretta one, that has a tiny hole in the handle loop bit, it is very small, so will enlarge that a bit, put the 300 valve in and double check float height

the fuel air ratio on the dyno was spot on, really good, and went well on there, just seems that as soon as you put it under load it dies, I was thinking more engine related but 160psi compression is healthy, engine is good, then last night got me thinking of fuel starvation on the road, hope that this might improve things a little, also got an electric pump coming, just to try and make sure that theres supply to the carb, bike ones rated about 2-3psi so have to wait and see if the float valve is overpowered by it or not, if it works fine on normal bike carbs I cant quite see the problem.

I have 8mm hose from tank to carb, the tap is an inline straight jobby with 8.7mm bore, the fuel gushes out as a solid bar of fuel, so flow isnt an issue.

think I might put the hose into a spare tank and run it full open tap, see if it slows down, that would help pinpoint a breather issue on the tank

carb icing is something else thats been mentioned elsewhere, never known anything about it myself, but last night was pretty cool, had been raining during the day, and I have never noticed it before on my old lammy GP etc, but the carb was very cold, like it been in the freezer??