750 F2 carb problem?

[sonsman]

Seventy five miles into a ninety mile ride my F2 went on to three cylinders. Got home and number one spark plug was oily but sparking nicely and then realised that the carb on that cylinder was piddling petrol from the overflow. I did the usual thwack to the carb body but no joy. I am assuming it will be carbs off and a clean out but before I do that anybody else had a similar problem and what was the outcome?

[royhall]

Had a lot of these problems over the years. Has the bike been standing with fuel in the carbs. If so it will be a carb strip and clean, be careful if you change the float valves as some of the pattern parts are rubbish. Another common problem is dirt in the fuel from deposits in the tank. Best thing to try first is an inline fuel filter in the fuel delivery pipe. Put that in then drain the carbs and let some clean fuel run through. That has fixed a few of these problems without the need to remove the carbs. Good luck.

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[neat street]

Hi, Could be a couple of things, but either way, I would suggest carbs off. the float needle could have some goo stopping it from sealing when the float is up, damaged float needle or seat, float not adjusted correctly (on a CB750F1 it should be at 26mm). These would be my first checks. you may be able to just take of the bowl and let the fuel run, gently lift the float and see if it cuts off? if not then check the seat or crud around the float needle/seat. if it does seal, then possibly suspect float height (could also be a small hole or crack in the float so not working as it should?
I hope this helps?
John

[royhall]

It's an F2 so will be hard to get the bowl off without taking the carbs off. Not likely to be goo as you were okay for 70 miles, I would try the fuel filter first as resealing the carbs on old rubbers is not easy and new rubbers are not available.

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[sonsman]

Thanks guys, the bike has been in regular use so I don't think it will be gummed up or stale fuel from being laid up and it's been running fine over the couple of years I have had it until now. I'll let you know how I get on.

[westfieldandy]

more likely a dodgy float valve, you cant always free them with a tap with a spanner.

andy

[onethumb]

I had to strip and rebuild mine and three of the float valves were stuck and needing replacing.

[sonsman]

An update on the F2 carb front: a motorcycle tech I know who has been in the game since the late 70's spotted me fettling in the garage as he was passing by and reckoned with the right tools the no 1 floatbowl should come off without taking off the carbs - he was right. I took the bowl off, dropped the float out and the needle valve and gave everything a good checkover and clean and reintsalled - still the petrol peed out! Looks like the carbs will have to come off after all but I sent off for a new float valve from that nice Mr Silver to be on the safe side.

The good lady is away visiting family so before dismantling the carbs I dropped the float bowl again to try the new float valve - straight away comparing the old and new one it was obvious that the operation of the old one was nowhere near as good. Install the new float valve and bingo no more gushing petrol. it fired up easily and a quick ride showed everything back to normal! Time for a celebratory beer.   

[onethumb]

Good news.