Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: ColinHH on March 24, 2022, 12:42:19 PM
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CB750K4 1974.
Bike is running great but I noticed that the petcock was still delivering petrol even in the off position. It's only a drop every second or so but overnight it loses quite a lot. So I bought the gasket with the 4 holes in it and fitted it hoping that would solve the problem, but no it still leaks. I've tried everything but can't get it to seal 100% so assumed that this is probably normal. So the next problem is that one of the carbs loses this petrol through the overflow, so I thought I need to adjust the float level. Same as on my BMW R100 i.e bend the little tag slightly upwards to get it to shut the gas off a little earlier. However, I am either flooding the carb or it is dry. The very slightest of movement on the tag gives either too much fuel or none at all. I can't believe that it is so sensitive! I am moving it what seems like 10ths of a millimetre. and just can't get it right. Am I missing something? Could it be anything else? I have flushed the fuel inlet jet and confirmed that it does shut of fuel when the valve is put in the closed position. HELP please, I have been playing with this for 2 days now!
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My thoughts on the fuel tap is that it should not pass any fuel in the off position - not sure if your tap is original with new seals or a replacement unit. The DS seal kit I understand uses two seals in place of the original one I stuck with my original tap seal as it was still good.
As regards the floats levels again are you using originals floats & valves or aftermarket replacements - a lot of aftermarket carb parts can cause flooding so is the needle & seat good?
Quite a few old posts about getting a good seal from the needle & seats.
I am taking it as read that you have cleaned all the carbs / good fuel hoses I use an inline fuel filter to keep the very small stuff out.
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Split in the copper tube or the JIS screws don't have the right washers to seal ;)
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Thats the screws holding the tap to the tank inside the float bowl, generic fibre washers are too thin and spliit, genuine parts are expensive and people think they dont matter as its only a fibre washer
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OK, after a couple more hours I finally found the problem with the carbs overflowing. I hadn't seated the bowl gasket properly on the side that I couldn't see. This meant that some of the gasket was overhanging on the inside which intern was stopping the floats from coming all the way up. Refitted the gasket and no problem. However, the stopcock is still leaking. It is a Genuine Honda part and genuine 4 hole gasket but it still drips in the off position. With regards to the washers on the two screws holding the petcock to the tank, how can that make the taps leak? Wouldn't any fuel leaking through those washers just go into the petcock bowl and then enter the petcock in the normal way? My understanding is that when the switch is in the OFF position the plate is being pushed against the two exit holes and inlet hole so no fuel should be getting through?
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The screws holding the petcock in position go into the tank, so fuel seeps down them but is supposed to be stopped by the fibre washers from entering the bowl, if the washers leak it means fuel is getting into the bowl even if the fuel tap is closed, hence why it’s a drip and not a flood.
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The bowl is after the 4 hole rubber and tap in the direction of flow
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Bryanj,
When I was referring to the bowl,I meant the bowl at the bottom of the petcock, not the bowl on the carb. The bowl on the petcock (where I think the fuel would flow into from a leak from the two screws) is before the 4 hole seal isn't it? I will check the washers tomorrow though and see how it goes.
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I was talking about the bowl on the tap and its after the seal with holes, trust me i found the problem 30 yrs ago
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We are on about the bowl on the fuel tap. If the washers are not sealing correctly then, the fuel is bypassing the fuel rubber and running in to the bowl so, it doesn't matter if you have the fuel tap turned off ;)
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Just a tip, if it’s like the 500/4 tap, I find a bit of (lithium in my case) grease on both sides of the spring washer prevents the lever getting stiff to turn.
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Thanks all.
Problem solved. It was the two fibre washers holding the fuel tap to the tank. I've put standard fibre washers in for the short term whilst I await delivery for the OEM part.
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Bryan,
You were lucky. New washers didn't work for me. I had to re-face the tap body where those screws tighten down onto as they had corroded to the point the washers could stop the leak. All good now on the 51 year old petrol tap now though.
Just for the benefit of others who are still wondering why they still leak.....
Cheers,
Jerry
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Yeah, it's always the simple, but not necessarily obvious, things! Mine is only a baby at 48yrs old.
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49 and 46….still working….after two new seals, two bodies and two seals,etc..triggers broom😀😀
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Makes me feel good when people listen eventualy