Fuel draining but carbs not overflowing

[Outrunner]

If I have an auxiliary fuel fitted with the main tank off. If I fill the auxiliary tank it drains to fill the carbs, but continues to drain somewhere until the tank is empty, adding more fuel it continues to drain, the carbs are not overflowing.

Has anyone experienced similar and fixed the issue?

[Orcade-Ian]

It goes without saying that you MUST NOT crank the engine, either with the electric or kick starters until you have found the problem - doing so could land you in a whole heap of trouble.  The fuel has only 3 possible routes if the float chamber screws are not leaking, out through the overflows (you have discounted those) upstream back into the plenum chamber (that should have a drain anyway) or downstream through an open inlet valve and into the engine and thence via the rings into the sump oil.
Has the oil level risen in the sump?  Can you smell this missing fuel?

Ian

[Outrunner]

Thanks Ian, I'll take a look on Saturday

[deltarider]

Make sure your fuel supply - whatever it is - is not higher placed than where your tank usually is. Float needles can not withstand the extra pressure.

[Outrunner]

Carbs have been removed, flushed and refilled while on a bench, angled 2.5 degrees forward as per when fitted on the bike, and they are not overflowing/leaking.

While the carbs are out, stripping the head and barrels to have a look in there too

[Orcade-Ian]

Did you check for any increase in oil level?  It seems a shame to disturb the top end and barrels - quite apart from the expense of seals/gaskets - have you had it running before and if so, was there any suspicion of all not being well?  I would be inclined to change the oil (the missing fuel could have ended up there) remove the plugs and then crank it over for a while until you get oil pressure, refit the carbs and enjoy it!

Ian

[Outrunner]

Ian, there was petrol in the oil, now drained, still not sure how it got there.

Yes it has been running

As far as I know, the bike has only done 18000 miles, the compression is on the low side, max 90 - 130psi with the throttle wide open, so we are going to have a look at the valves and rings.

[Orcade-Ian]

The fuel got into the oil from open inlet valve(s) and washed the bore(s).  Does the compression improve after a small squirt of oil down each bore?  If you haven't already started dismantling, it might be worth trying a run - no need to fit all the airbox and stuff for a quick test - just the carbs.

Ian

[Outrunner]

The compression was around 90 dry and 130 with the squirt of oil when I tried it a while ago

[Murf]

If your comprehension tester has a longish hose between the fitting that screws into the spark plug thread and the pressure gauge, this can add more volume to the cylinder and have the effect of decreasing the compression ratio thus causing a lower compression reading. The smaller the cylinder volume the bigger the effect.  So just something to look out for.   You could do a leak down test which would give you an indication if the valves or rings were leaking.
 I would say there is always going to be some valve leakage on an old engine.
Checking for relatively even compression readings across all cylinders might be more relevant in checking the health of the engine.  Check you have no tight tappet clearance as a first step.

[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