Carburetor Vent Discharge - MYSTERY SOLVED!

[mountaineerxii]

11/7/17

I posted the below earlier in the year and have finally figured out and solved the mystery of the oil/gas drops under the bike after idling for long periods of time.

A bad "new" carburetor kit in #2 carb caused me to have to pull the carb bank back out after only putting a couple thousand miles on the bike since the main rebuild. In the disassembly process, I realized that I had routed the two carb vent hoses (the clear pvc ones) down and under the airbox (not the box with the filter in it) and down to the ground, rather up and over the airbox and down to the ground as the factory routing was, and what others have confirmed their bikes to be setup too.

Routing the vent lines up and over the airbox solved the issue, no more drips coming from the vent hoses after long periods of idling.

I finally figured out why. When the bike idled for long periods of time, everything around the bike got as hot as the cooling system would allow. On shutoff, some intake valves are always randomly open and the cylinder walls are coated with a thin layer of oil. The carb float bowls also sit right over top of a very hot engine casing. So what was happening was that after I shut the engine off, the carbs and engine would go into heat soak, evaporating some fuel out of the float bowls up thru the carb vents and evaporating some high boilers out of the cylinder oil film out thru the intake valves back into the carbs and up thru the carb vents. These would condense in the vent tubes, then gravity run down and drip out of the vent hoses onto the ground. Routing the vent hoses up and over the airbox before going to the ground allowed these condensables enough time to cool off, and then to run back down into the carbs, rather than onto the ground. Going down the road there were no drips because there was enough airflow thru the carbs, around the external of the carbs, and enough pumping action from the engine to keep everything flowing thru the cylinders.


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BELOW POSTED EARLIER IN THE YEAR


Have been test running a 26k mile, '97 xii since doing a meticulous engine top end & carb rebuild. After shutting the bike off after running the engine for an hour or so, I'll get 4-5 drips of an engine oil/gasoline mixture out of both carburetor upper vent (5/16" PVC) tubes (not the 1/8" PVC ones off the bottom of the float bowls). Can't figure out why, and was wondering if anyone else has seen this before?

Here's where my line of thinking has been going:

- Has one of the new intake valve seals come off, allowing lots of oil mist to get into the intake and back up into the carburetor? - No oil smoke on cold start and no oil smell in exhaust, not to mention that it is coming equally from both vent tubes (carbs 1-2, carbs 3-4).

- Is there excessive blo-by from the crankcase vent, leading to the carbs ingesting lots of oil? - Air box connected to carbs is bone dry, along with air box catch can

- Is oil thinning too much and excessively getting past intake valve seals?- a possibility, but Temp gage appears normal and don't smell burning oil in exhaust. However I am using 3.5qt of 10W-40, instead of 20W-50.

One thing that remains a possibility if I had another bike or good pictures to compare to. When I rebuilt the bike, someone had obviously had the carbs out before. The vent tubes (5/16" pink PVC) appeared to be routed haphazardly and very short (~4"), and such that their ends were pointing up. I didn't like that, and the hoses were hard as rock, so I replaced them with longer PVC, routing them up and over the air box, and then down the chassis to terminate just above the center stand support, where all the other vent lines are routed & pointed down to the ground. If the factory hose routing was also short with the ends pointed up, then liquid discharge would naturally drain back into the carbs, yet it could still vent gasses. I couldn't find any pictures in the service manuals or online as to the routing of these 5/16" PVC vent lines, and didn't have another bike around to compare to, so I don't know what was factory routing.

Thanks for any help, this forum is great.

[triton28]

The routing you have used is the correct route for the 1/4" ID carb vent hoses. The stock PVC hose measures approximately 9" and terminates about 4" down the back of the lower air box.
I also ran mine down to the lowest elevation around the center stand hose clip bracket, but I haven't noticed the problem you describe regarding oil/fuel drips.
I don't however think the valve stem seals are contributing to the described problem as any bypass would go into the cylinder head intake ports/combustion chambers.
Also one would think that if the internal crankcase pressure was too great, it would show up as an oily residue in the lower air box catch tank.
The oil thinning out with heat build up is normal and should not contribute to the problem you describe.
Not having been present for the rebuild it is hard to offer any concrete possibilities. Hopefully other forum members may be able to describe a cure, but at least you now know that the routing is OK.
Dave