Carbs overflowing into crankcase

[hannibal]

I recently got my bike back together and did a 60 mile break in ride. Upon returning, I noticed some oil leaks from the stator cover and oil filter cover. No big deal as I planned to change the oil anyway and this would be the perfect time to clean up the mating surfaces on the stator cover and fix the O ring on the filter cover.

I put the bike on the center stand and realized the oil level was too high. Once I shook the bike, I realized the oil was way too liquidy (I made that word up). I've dealt with overflowing carbs before. The first time (last summer), I replaced the float valve needles but forgot to tighten down one of the float valve seats. Off came the carbs again to tighten the seat. I did have to rig up some clear tubes in the bowl drain to measure fuel height but once that was set, I had no problems.

I took the carbs off when I broke the motor down. I didn't touch them at all and just reinstalled after the motor was back together. I'm hoping the issue this time is something as simple as a stuck float. I did not tap the bowls while the carbs were on the bike since I never ran it after noticing the fuel overflowing. I've already pulled the carbs and sprayed out the valve seats in case some debris found its when in there. My dilemma is whether to change the oil first and clean up the leaks before trying out the carbs again. I'd hate to fill it with fresh oil only to have the carbs overflow again.

As I'm typing this, I'm realizing $15 in oil isn't worth risking 5 months and $300 of work on the motor. So oil change first, then test out the carbs.

 

[Boriqua]

The two things that come to mind in my own experience is a faulty petcock or somehow one or more of the floats are out of adjustment. I would suspect the petcock first and its the easiest thing to diagnose. If you do tear into your carbs again check your float heights.

 

[allojohn]

That shouldn't happen on your bike. Your overflow tubes in the bowls are blocked.

 

[hannibal]

I forgot to add the petcock was replaced last summer or the summer before with a K&L unit. The tank sat inside with fuel in it all winter and I never saw a drop leak from the petcock. Fuel flows on prime but not on "On" or "Res".

My overflow tubes are clear. I have the bowls sitting next to me and can see through all four when I hold them up to the light. I can blow thought the rubber tubes that attach to the overflow tubes. In fact, I've never had fuel come out the overflow tubes. Perhaps this means my gaskets are too thin and the tube sits higher than it should. The bike is parked on a slight hill with the front pointing down. The incline is so small I can't imagine the overflow tubes failing to do there job in that position.

Float heights are in spec at around 27mm. Again, last summer had the carbs on and off many times to figure where to set float heights so it would not overflow.

 

[allojohn]

Yeah, try setting float height closer to 24mm see what happens. '78 spec is 24mm and they're essentially the same carbs apart from jetting.

 

[hannibal]

Lower float height means higher fuel level. Last summer, I kept having to adjust the float height higher (fuel level lower) to get the fuel level within spec (2.5-3.5mm below bowl mating surface). If anything, I'm tempted to adjust float height even higher.

I'm going drain the oil, clean up the gasket surfaces and the oil filter cover, and try to get rid of these oil leaks. I'm hoping it was just stuck floats but it's hard for me to believe I went 60 miles with a stuck float. If the carbs continue to overflow, I'll rig up the clear tubes again to measure fuel height and keep adjusting the fuel height lower.

 

[allojohn]

With a higher float level the float valve will close earlier - that is not happening. You've eliminated the petcock so the issue is the float valve/s is/are not closing - it's probably just one valve. If the float drops too low the valve can drop down and stick open. If possible take the bowls off while still on the engine. I replaced the screws with allen bolts to make this easier.

 

[hannibal]

At this point, I've opened every cover or surface. All JIS screws have been replaced with Allen heads. Good point about the valve and floats opening too far which explains the upper limit when setting float height. With the carbs right side up, I have noticed that 2 floats sit lower than the other two at full open of the valve.