SOHC.co.uk Forums > CB750

Accelerator pump

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royhall:
Here we go again with the dreaded F2 carbs.

Just re-furbed a set with as many new parts as are available. Carbs themselves are perfect, problem is now the accelerator pump.

It has a new diaphragm kit fitted and the non return valves are all clean. If I press the pump stem with a screwdriver they pump petrol perfectly, and the pump lever arm is free to move. But when I open the throttle its like the spring that pulls the lever arm across is not strong enough to operate the pump. So no petrol squirt.

Any ideas anybody?

martin_uk:
The throttle return spring is pretty strong so difficult to replicate the normal operation when you quickly open throttle.

Suggest fitting the throttle cable and trying with the throttle control.

UK Pete:
I hate F2 carbs they are terrible to set up
it sounds like you need a stronger spring on the lever arm or a softer one under the diaphragm
Pete

royhall:
Yep, I think you may be right about the bottom spring Pete.

Am mid changing diaphragm and spring back to the original one that was previously okay. Now waiting for another packing set as the Cruzimage float bowl seal has decided that now its seen petrol its going to grow by 10% so it can never be put back into the groove.

Hate those Cruzimage kits and come to think of it, the whole F2 carb setup as well. If I keep the bike I think I will convert it to K series carbs. :-\

"O for the simple things in life." ;D

UK Pete:
I have a Rickman cb750 with the F2 engine, last week i stripped the carbs and attempted to get the thing going, i had the carbs off over 6 times and was getting really pissed off, but i won the battle in the end and had the bike running and ticking over, thats a first for me to get a decent tick over with f2 carbs, now i have to replace the valve guides and seals, another bloody f2 thing,
I have heard that enlarging the pilot jets in these carbs can cure lots of the rough running on start up, but i cant finf the link to the post where i read it,
In theory sorting out the lean running on these should make the bike a whole lot better
pete

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