On my 1986 FLTC I don't have a tank vent. I've had issues with float overflow and tank pressure. I thought I had a vent and replaced the tubing with new, it turned out to be the drain hose for spillage when filling up.

So can anyone tell me for sure if my gas cap should be a vented cap? I think so, but what it came with (bought used with 3 other owners) is a cap that looks like a radiator cap. It has a raised center and an inset covered center hole, possibly the vent portion.

My overflow troubles seem to occur after riding when the tank is full or agitated and the bike is hot, or when it sets in the hot sun. Seems even overnight in the garage it'll drizzle out the gas remaining in the line after the shutoff valve is turned off (vacuum operated)

I've already been over and over the carb and still want to polish the valve seat (non replaceable).

I've already replaced the float and valve with HD parts, and adjusted it with no luck. I'm going to make certain the valve is not hanging up or dragging in the bore.

I'm certain I set the float without the valve fully closed, so the measurement is off. It doesn't show an actual picture in the service manual, it's just a diagram. I understand how to hold it so the float can swing free on the level, on it's side, but I think I should be pushing against it to be certain the "valve is fully closed" (seated) as is written on the diagram.

I can't find any specs in the service manual about the cap. It does not have a vapor tank emission setup.

Because I always get a whoosh when opening the gas cap, I figure if it is a vented cap it's probably worn out and not working correctly.

 

Mine does not look like it's vented, either. It looks like you said, a radiator cap but without the vent mechanism. I have 2 overflow lines, one is connected to the bowl of the tank center console, the other is connected to the neck of the tank, under the console. I have experienced an occasional fuel leak when I pull into a gas station on a hot, sunny day, filled the tank, then pulled over to the side parked in the sun for a few minutes before taking off again. The cold gas from the underground tanks at the station expands enough in the hot sun to overflow out of the neck drain, and since it's not in the best shape, some of it runs down the outside of the tank under the console and drips out over the left rocker box then down the front side of the clutch hump on the primary before running to the ground over the small inspection cover. It's only done this twice so far, but I am also a lot more careful when filling it now, and stop when the fuel reaches the bottom of the neck.

Are you SURE it's coming from the carb? I would tend to think the neck overflow line on mine equalizes the pressure between the tank and the atmosphere. Maybe yours is kinked or plugged?

 

It must be vented but they do fail. I drilled a small 1/8th hole in mine & no problems now. No leaks. You can get a small piece of brake line & push it into the drilled hole if you feel more at ease with a tube on it.
Hatch.

 

Thanks for all of your suggestions.

I'll be checking this morning with the dealer about a new tank cap.

 

Mine's not vented. It's the original one. It has "wings" on the top to grab on to.

 

As so often happens I found some suggestions going through the forum. I applied the variables and came up with my cure.

The gas tank cap is a vented cap and works well. It relieves / releases positive pressure build up in the tank. The cap actually is marked "Pressure Vacuum". I was able to adapt a hose to the center opening and blow out the center and heard the air escape from around the caps outer ring.

The "pressure" that I heard when removing the cap is vacuum. So the tank does normally have a vacuum in it. Why? I can't be sure. But a vacuum in the tank means it's not forcing gas through the line and into the bowl ... and out the overflow.

As I mentioned above my question on the float's position when measuring the float's bottom to the edge of the carb body where the float bowl attaches. Having it sideways with the pin on top the float rests against the closed valve... HOWEVER OEM float valve also incorporate a small spring loaded pin in the center of the valve that must be collapsed too. This is a difference of a few millimeters to set the float correctly. I had to stare at the diagram a dozen times to figure out the measuring tool they were using was actually pressed against the bottom of the float, and you read the measurement with the float valve seated, AND the pin collapsed. Again it was a few millimeters difference.

So far no overflow leaking. I test drove it and held high RPMS going about 80 and the bowl never dried out, nor did the engine falter until it hit the rev limiter. It's set at 16mm the way I explained above.

Before I let the float just rest on the pin on the valve and that was enough to have the overflow. I'm checking right now to see if it burps out any gas from the overflow but so far so good.

I think I'll button it up and take it out fully assembled and just check on the freeway to be certain I can accelerate hard and get up to speed without a stall from an empty bowl issue. I've had that in the past when trying to get this right. It's been a few years of focusing every now and then to stop the overflow issue.

 

Well I'm back, and I'm done for the day!

The bowl ran dry of gas several different times on the e-way while accelerating, so the float is now too far the other way. AND it's still allowing gas to drain out the overflow when sitting.

Just when I think I have it figured out ... I don't.

I've spent too many hours on this issue!! If the carb didn't function at all I'd toss it but it runs the bike perfectly the only glitch is the float valve not seating correctly.

I'll have to tear into it again and reset it back, or try to find a happy point in between.

Wish I had bucks just to replace it with one that doesn't leak out the overflow when setting. Even the new (used) vacuum operated petcock allows gas to drip when the motor is shut off. I double checked it and sure enough right at the petcock, with no vacuum so it should be "shut off" it still drips before connecting it to the carb. The only way it stops is in the completely off position.

 

The petcock might be at least partially the problem. If it doesn't close fully with no vacuum. then it might not be fully opening with vacuum, either. Once the bladder goes bad or the mechanism sticks, it doesn't work right either direction.

 

Johnny, set the float pretty much level with the float just touching the spring loaded pin not compressed, hold it sideways and tilt it, till the float seats the needle.

Try cleaning the seat with a q-tip chucked in a screw gun or drill, use a new oem needle.

Make sure your fuel line is not deteriorating and getting in the needle valve, I'd change it anyway if it has any age on it.

 

That original carb was a total POS. If you have the bucks, dump it and get a CV or a S&S. But, if you insist, keep messing with the stuff as suggested above. And that bike didn't come with a vacuum petcock. Most people with vacuum petcocks seem to be replacing them because they don't work. Usually don't open, not don't close.

I suggest a Pingle. Get it from Jireh and they aren't too much.