SOHC.co.uk Forums > CB750

K7 carb help.

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hairygit:
Not really that much difference, mainly jet sizes etc due to ever tightening emissions laws. Irrelevant really, as you will want to jet to suit the Imp engine, you may need to use a bit more throttle to pull away due to the lack of accelerator pumps in the other models, but Imps are like Honda's, they thrive on revs. K3/4/5 can be bought relatively cheaply from places like D.K. spares as they import a lot of bikes from the U.S. and break a lot of them.

zippy!:
Brilliant info, thanks again, it really is appreciated.

Just an update, got the carbs off last night, and took the float bowls off. Attached some tube to the mains one at a time, and blew up, each appears ok.

Do the same with each of the smaller (pilots/idle?) and they are all clear.

Blew through the fuel feed, and floats are all working ok. Wonder if I have unblocked something in the process.

What I did find was a couple of pin holes in the inlet manifold, and that at the inlet on no1, there was light showing through the gasket when I shine it through, maybe a bit warped. This will not help at all. I will address the manifold next, then pop it all back together and check the timing. Not sure if / how that would have gone out but worth a check.

Thanks again, Gav.

royhall:
The manifold problem could probably be the culprit. If its not totally sealed it will draw air in and lean out the mixture. That will cause the baulking that you talk about.

zippy!:
I'm on with it now. I will get there!

Gonna try getting something down the small ports (2 little pipes) on the air intake suddenly of each carb (not sure what they do mind).

K2-K6:
I wouldn't disagree with the above but hope to add a little to help.

As hairy pointed out, many don't like the PD carbs as if they stand around not being used much they can give problems related to blocking of the small jets and require cleaning to get them working properly.
I like them though and if given a clean supply of fuel and regular use they are nicely accurate carbs I feel.

Saying that, they are fussy and difficult to alter, even on their original engine if you modify the intake system. I don't know what you've got in the way of filter hardware but they would originally have had bell mouths fitted inside the original airbox. I believe these are vital to making them run properly and think you'd be very compromised without them. Also, I feel they benefit from an airbox/plenum even if it was to be a small one supplied by a large bore intake hose leading to a remote filter if it can't be fitted direct to the carbs.

The fuel supply for a car I'd consider a fuel return system in which you go inline from the tank, pump, filter, then T off to the carbs but continue the fuel line into the pressure regulator and onwards back to the fuel tank. This will give you a pressure regulated supply to all the carbs inlet valve / float chambers as opposed to effectively coming from the current system with regulator before the carbs and any fuel after that point being variable in pressure and quantity delivered to each carb.

But most fundamentally I think you'll run into setup difficulties with this particular carb setup as it maybe giving you a mismatch that's hard to get out of.
I'll put a further post to cover that statement.

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