SOHC.co.uk Forums > CB500/550

Fuel draining but carbs not overflowing

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Oddjob:
Try this. I’m assuming the 550 is the F1/2 model not the 550k3 version?

If so, remove all the vacuum screws from the inlet manifolds, the same screws where the vacuum adapters fit. Refit the carbs, remove the spark plugs as a just in case and turn off the kill switch. Now refit the auxiliary petrol tank. If the petrol is going into the engine, as we suspect, it will pass the screw holes in the inlet manifolds and you should see it dripping out of one or more of them. That will at least tell you which carb is overflowing. Kick the motor over after and see if any fuel is ejected from the spark plug holes, hence why the kill switch is off so no spark is generated whilst turning the engine over.

The 550k3 model not having the screws on the inlet manifolds means you’d just have to rely on the spark plug holes to see if any fuel is down the bores. No danger of an engine lockup with the plugs out but of course don’t do this if they are fitted.

kevski:
Compression needs to be tested on a hot engine, your problem at present is either float levels too high, leaky float valves, fuel tap not working properly in fact you can have all of these problems, fuel should always be turned off when you switch engine off. Don't  strip the top , drain oil, replace filter and oil, do float levels and needles rebuild or replace tap.

Oddjob:
If the engine has been left for some time, it's also possible the rings are stuck in their grooves on the piston. This would allow compression to escape quite quickly and also allow the passage of fuel into the crankcase. I'd try and run the engine for a few weeks to see if they'll release over time. As Kev says it's better to test the compression whilst the engines hot, so 2 birds with one stone type of thing.

Outrunner:
Stripped the top end today, looks like we have found the issue of the low compression. The cylinder head gasket was blowing on cylinders 1&2, as we lifted the head off the gasket was already in 2 parts. The exhaust valves need a good clean up and possibly a relap for all 8 valves. The pistons and rings all appear to be in good order.

There was oil around the head stud at the front between cylinders 1&2, going to check the head isn't warped.

Bryanj:
It is rare for a 500/550 to actually blow a head gasket, you need to get the gasket surfaces checked and possibly refaced

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