Fuel coming out of crankcase ventilation hose?

[Sammy_1101]

Hi all,

Title more or less says it all. I have a 1979 GS850G, and have just noticed a lot of smoke and some unburnt fuel coming out of the crankcase ventilation hose. I've read a few posts saying about float needles being stuck open, but I recently rebuilt my carbs and changed the needles also (I will do a float level check soon, but as of now I assume they're fine.) Compression in each cylinder is also not too bad for a 79 bike (around 110psi for each), so I'd think the piston rings are ok, but could it possibly be valve stem seals? Or anything else? Has anyone experienced similar problems before?

Thanks in advanced for the replies

 

[Brendan W]

Do you mean liquid fuel or just a smell?
Is it possibly condensation and not fuel at all.
What is the oil level like?
If you suspect that a lot of fuel has been draining into the crankcase you should consider your oil is toast and change it.
Loads of threads here about this and how to stop it.
Main culprits are defective fuel tap and float valves.

 

[londonboards]

And defective needle valve seat O rings.

 

[Agemax]

And defective needle valve seat O rings.

A 79 GS850 will have VM26's, no O rings on valve seats, just small gaskets.

 

[Sammy_1101]

Do you mean liquid fuel or just a smell?
Is it possibly condensation and not fuel at all.
What is the oil level like?
If you suspect that a lot of fuel has been draining into the crankcase you should consider your oil is toast and change it.
Loads of threads here about this and how to stop it.
Main culprits are defective fuel tap and float valves.

Alot of condensation yes, but also neat fuel coming out too. Oil level hasn't really increased, atleast not a noticable amount. Will double check my fuel tap and float valves, but I have no reason to think they are faulty. Petcock was inspected not too long ago and carbs were rebuilt with new float valves too

 

[tom203]

Check/test your petcock again....it's the most likely culprit...

Try this... remove both lines from petcock: get some clear vinyl tubing that fits snugly on petcock vacuum fitting. In "on" and "res" fuel,show flow when you suck on this tubing- otherwise it should not. Any signs of fuel in the tubing means the petcock diaphragm is likely leaking.maybe just when bike is running but maybe at alltimes.​

 

[Sammy_1101]

Check/test your petcock again....it's the most likely culprit...

Try this... remove both lines from petcock: get some clear vinyl tubing that fits snugly on petcock vacuum fitting. In "on" and "res" fuel,show flow when you suck on this tubing- otherwise it should not. Any signs of fuel in the tubing means the petcock diaphragm is likely leaking.maybe just when bike is running but maybe at alltimes.​

Ok, thanks. I'll give it a go. How would the fuel get to the crankcase ventilation tube though unless some seals or rings are faulty? Is there another way it can bypass the cylinder and into the crankcase?

 

[allojohn]

Heavy fuel/oil vapor should be the worst you get through the breather and that's not uncommon. Check the float valves again, it's possible fuel is passing out the back of the carbs to the airbox - I've had this happen.

 

[Brendan W]

You could put the bike on the sidestand and set the tap to PRI.
If fuel starts coming out the airbox you have a problem

 

[allojohn]

It'll do it on the center stand as well and fuel will also flow out of the overflow tubes...

 

[Sammy_1101]

I should probably mention this here, as I forgot to before. My bike has no airbox, it has pod filters. So the fuel cant be coming out the carbs and into the airbox. It has to go into the engine and out the vent hose.

 

[allojohn]

I should probably mention this here, as I forgot to before. My bike has no airbox, it has pod filters. So the fuel cant be coming out the carbs and into the airbox. It has to go into the engine and out the vent hose.

The airbox side of the carbs is lower than the engine side, so it's still going to flow into the pods. Unless your pods are weird...
It's almost impossible to have fuel dump into the engine with VM carbs, the overflow tubes prevents this. Where is your crankcase ventilation hose positioned - can you post a photo?

 

[Sammy_1101]

The airbox side of the carbs is lower than the engine side, so it's still going to flow into the pods. Unless your pods are weird...
It's almost impossible to have fuel dump into the engine with VM carbs, the overflow tubes prevents this. Where is your crankcase ventilation hose positioned - can you post a photo?

The hose comes out of the top of the valve cover and runs down the back of the engine so that anything coming out of the hose will just spill onto the ground. Cant post a picture right now but I will get one as soon as I can.

When I turn the petcock to prime, no fuel comes out of the overflow, thats why it leads me to believe that it somehow is leaking out of the cylinder before its ignited.