SOHC.co.uk Forums > CB750
Carbs Install
Martin6:
Yep, the oil checks were done on the centre stand. Sumping is a possibility. But I checked it first time within only a couple of days of it last being run. I'll change the oil before I start it next and see what it looks like. :)
Is it possible fuel can drain into the head without troubling the overflow pipes?
Odd Job, yes, I scrupulously checked the float heights, then rechecked, before reassembly. I could be doing it wrong, but I followed the two YouTube videos posted by Spanner Rash, on his CB750, which helped me understand the procedure in the Honda manual.
I think another carb cleaning day is in store...
McCabe-Thiele (Ted):
We did have a member here a while back who accidentally left his fuel tap on - the petrol drained into the sump iirc causing damage when he tried to start it.
I assume the petrol went into the inlet manifold via an open valve into the cylinder head / piston area - drained into the engine. Are you sure your fuel tap is closing fully?
Martin6:
Ted, Yes, the tap seems OK, no leaks when I have the tank off. I did leave the tap open for about 5 minutes, when refuelling.
McCabe-Thiele (Ted):
Might be worth disconnecting the outlet - fitting some pipe to check if the valve is passing internally overnight.
K2-K6:
"Pilot jet blocked again? But it does idle and starts OK without throttle when warmed up."
Two elements to this circuit that can run you round in circles.
Assuming that the fuel jet isn't blocked, it can just flow full to that capacity without impediment. Its mitigated by the air supply which is modulated by the screw.
If this air route is blocked, then you'll see no response to changing positions, just that the idle mixture is fixed at any speed according to venturi vacuum only, so not full rich when starting but not low like it should be.
So it'll work in a fashion but without the idle mixture graduation it should be able to use in smooth tickover as will be permanently rich, not seen as you get higher revs as it acts normally there (idle jet is maximum flow) but lumpy when you try and let it idle.
This also forces you to use more slide lift just to keep it running (to counteract the rich idle mixture) and again keep chasing your tail with this in never being able to set a competent position for general running.
So very detailed clean and blow through/verification of air passages should bring these to working correctly.
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