NC30 petrol in oil

[AdrenalineJunky]

Hi everyone I was wondering if anyone can help me out.

I have just rebuilt the carbs on my NC30 and also done the valve clearances. When I first put it all back together and ran it at idle for a while to check it all worked, it seemed fine. I came back to the bike the next day and there was a strong smell of petrol and the engine wouldn't turn over even though the starter motor, solenoid etc were all fine. I removed the oil filler cap and lots of petrol vapours came out, drained the oil and it was full of petrol.

So then I took the carbs off and checked the float heights, and they were all way out, too large, because I hadn't done them properly. So I made a tool out of a bit of card and got them down to ~6.8mm all round. Put it all back together with fresh oil and started it on Tuesday, it ran fine, I rode ~40 miles on it and left it overnight. Then on Wednesday I did about 90 miles and it was all good, made good power, no flat spots. On Thursday I did another ~80 miles and got 47mpg, so I was happy. I noticed a very small bit of white smoke on startup but this soon disappeared and wasn't present when I when I stopped for petrol or when I got home.

But now, after not using it yesterday (Friday) at all, I smelt the same strong smell of petrol again, and again it won't turn over. Again there is petrol fumes coming from the dipstick/sump. I left the bike with the fuel tap in the "off" position so I don't really understand how this is happening...

What might be relevant, but I hope is not, is that the bike overheated a couple of times a year ago when the cooling system wasn't working properly (I'd just bought it). It started and ran again after it had cooled down though, and I have fitted all new radiators and new fan switch and that has sorted the cooling problems out.

I read some internet stuff that suggested maybe worn valves or wrong clearances (but I just did the clearances so don't think it would be that), or worn piston rings or bores (but the white smoke clears up when the engine's warmed up and I would expect that if it was burning oil that it wouldn't clear up), or a warped or cracked head. I don't know how to check for these things or even if they are likely or not. Because the bike was running fine, then it stood for about ten months, and I did the carbs and valves, and now this is happening.

If anyone has some experience of this or similar problem or some advice I would be very grateful for some help, because this is preventing me using my bike and costing me about £30 in oil every time it happens

[hunter]

The fuel in the oil i would say is worn float needles,They are not sealing in the seats and allowing fuel to enter the
float bowls,the fuel will then run down the ports passing the open valves and running down into the sump.
Get soe new needles.
The white smoke on start-up is normal.

[AdrenalineJunky]

Thanks for the advice hunter. I will do that and see if it works. Do you reckon I should also replace the valve seats?

[hunter]

Most of the time you will get away with just needles,

[arsey30]

The float needle valve comes in a set with the seat, but likely only 1 is leaking so find which one.
The fuel tap must be leaking in the off position too, or no fuel would run into the carbs.
Has the tank been modified to HRC diaphragm alteration.

[AdrenalineJunky]

Hi arsey30 (great name)

Thanks for the advice. Do you know how I can find which one(s) the leaking comes from?

I don't think any mods have been done to the tap. I am pretty sure it doesn't leak in the off position because I've had the tank off the bike and full of petrol for days at a time recently and it hasn't leaked. And the tank is not empty even now when there is enough fuel in the cylinders and sump to stop the engine turning over; if the tap was leaking I'd expect it to just run petrol until the block, carbs, and fuel pipes were full and no more could flow from the tank, or until the tank was empty. I had a GS500 Suzuki ages ago and once I left the tap on "prime" and when I came back to the bike, the entire contents of the fuel tank was a big flammable puddle on the ground... That hasn't happened here.

I think what's happening is:

a) a float (or more than one) valve got stuck open
b) then that the float bowl(s) filled up too far with petrol
c) when the engine is running the excess fuel is burned
d) but when it's turned off the excess fuel goes into the cylinder(s) and floods it, and some goes into the crankcase/sump

Thanks again for all the advice so far.

[arsey30]

If the tap shuts off, check for petrol in the vacuum hose, the tap diaphragm could be punctured, fuel will then go straight into the head, over time.
It is possible for the engine to run [poorly] with a leaking float valve, but it would have to leak badly to fill the engine between stopping and turning off the tap.

Remove the air box, you may see that 1 carb is wet, if not remove the carbs and empty each one into a jar to see if they have the same quantity of fuel in the bowls.
Refit bowls and hang them up.
Fill with a funnel pushed into the fuel hose and watch each one for leaks

[AdrenalineJunky]

Cheers, I will do those tests.

Funny thing is that the engine was running great, making loads of power all through the rev range and giving 47mpg (would have been >50mpg if I hadn't been caning it along B-roads). So maybe petrol going down the vacuum hose is what's happening here...

[AdrenalineJunky]

I took the hoses off, there was indeed a small amount of fuel in the hose from the fuel tap vacuum union to the head. Cheers arsey30, you were spot on with that diagnosis!

I took the tap off and took it apart. The diaphragm appeared to have developed a "lip", and on the side where the the vacuum union is, there was a small amount of fuel at the place the "lipped" bit of diaphragm would sit. I've tried to show both these things in the photo below:



I also took off the airbox - there was some fuel around the crankcase breather tube - and looked down into the carb passages with a torch, while moving the slides up out of the way with a screwdriver. They were all dry except for the carb for cylinder 1, which was wet with fuel inside it.

Then I took off the carbs and got a bit of a shock:



Cylinders 1 (bottom right in image) and 3 (top right) were flooded all the way up the manifold above the valves! No wonder it wouldn't turn over! Cylinder 2 (bottom left) also had a bit of fuel sloshing around above the valves, and cylinder 4 was clear. This might be due to how the bike rests on the sidestand...

I took out the sump plug and while waiting for the oil/petrol to drain out, I drained the float bowls and then refilled the carbs through a funnel via the fuel pipe. When the carbs were full I drained the fuel out of the hose, and then drained the float bowls again, noting the amount of fuel in each one - they were all pretty much the same. I checked the float height and smooth operation on carb 1, as I expected it was fine. I did them all less than a week ago, so I didn't expect them to be out of order.

The fuel above the valves hadn't drained out, so I siphoned some of it and mopped up the rest with rags on the end of screwdrivers inserted into the manifold. I took the plugs out on cylinders 1 and 3 to let the rest of it evaporate overnight.

[AdrenalineJunky]

So my understanding of what has happened is this:

1) the diaphragm in the fuel tap isn't sealing properly;

2) when the tap is in the "ON" position and the engine is running, the engine sucks some liquid fuel directly from the tank into cylinder 3 via the vacuum hose take-off in the cylinder 3 inlet manifold;

3) when the engine is running some of this excess fuel is burnt in cylinder 3 but some (most) of it makes its way past the piston in that cylinder and gets in with the oil;

4) the circulating oil/petrol emulsion gets everywhere as the engine rotates;

5) since oil and petrol don't mix, and oil is denser than petrol, once the engine is turned off the emulsion starts to separate as the oil sinks to the bottom of the block/sump and the petrol floats on top of it;

6) The petrol has nowhere to go except up through any tiny gaps between the piston rings and the cylinders, so eventually it will end up on top of the pistons in the combustion volume, locking the engine.