Fuel Starvation

[hudson]

Hey gents - another problem just started on me, leaving me very frustrated. It appears that there is some sort of venting situation going on with my 71 /5. It just happened out of the blue one day and I can't remedy it.

The bike runs perfectly and I rode much of the summer. New Mikuni VM32 carbs and fuel lines. The fuel lines have the clear cork type filters in between the petcocks and carbs. I put these on a few months ago and all was good. New Karcoma Petcocks. (BTW,Valve clearance, Carb synch, timing & spark good).

The problem. I filled up on gas one day and the fuel doesn't seem to want to down the line to the carb. Opening gas cap does not affect anything. I tested fuel flow from the tank, through the petcocks - good. Then the flow through the petcocks with attached fuel lines & small filter = all great flow. I hook back up to the carbs. Turned gas back on to let it fill up bowl. I can see the fuel go down the line and pass through the clear filter. then removed bowl. Gas in it and floats good. I double checked float needle and arms while fuel flowing out. Works great. Turn petcocks off, put bowl back on. Removed bowl drain screws. Turned gas back on and it pours out of the bottom of the float bowl. Also drained the tank completely and put in gas again.

Still thinking tank venting, so I put a new gas cap on (the one with no lock). No difference whether closed or open.

As of last night, I turned on the gas, it filled the line and the float bowl. Seems ok. The clear filter showed full of gas and when engine started they kept full. Took for a ride near my house for a test. Died on me. Obviously I was just running on the fuel that was in the floats bowls. Turned off bike. Tapped the little fuel filters, a few bubbles came up and then the miniscule amount of fuel in them dropped down the line. This is happening on both sides.

With bike either running or off, to get fuel back down the line, the petcocks on, I have to take the float bowl drain screws out quickly, dumping gas out everywhere, and then screw back in quickly and the fuel comes back down the line and fills the bowl. Like I was bleeding the line or something. Again, tap on the little clear fuel filters, a few bubbles rise up and the fuel drops down again and no more gas flows down- whether the bike is running or not. Repeat...Repeat...Repeat - same results.

There is some kind of vacuum or venting problem and I can't get it resolved. Never happened to me before all the years of riding.

Searched internet and not seeing but only a few posts with some similar symptoms, but no solutions given except that tank venting is the cause. What else am I not trying? This is driving me crazy!

[SteveD]

Have you removed the filters and tried it?

[hudson]

Will try this next step. I don't know what else left to do. I guess I skipped over that because the fuel flows right through them. How long do these type of filters last? I only had them on a couple months. It's so weird that it just happened out of the blue...

[jagarra]

All your tests were done with the motor not running, maybe the motor creates some vacuum through the carbs to assist in the flow. I would replace the fuel lines without filters, maybe there is some fuel line deterioration internally that is shutting off the fuel when the motor is running. I am thinking it closes when the vacuum is present, but is open for your test sans vacuum.

[Kurt in S.A.]

How does the motor create a vacuum thru the carbs to assist flow? Seems like the engine sucks gas up the venturi which lowers the float which lowers the needle which allows gravity to feed fuel into the bowl. Is that the vacuum process you're talking about?? In the end, isn't it gravity that flows the gasoline?

Kurt in S.A.

[jagarra]

I don't know for sure, but the only difference is that the motor is running when it runs out of fuel and stops flowing and it isn't when it flows correctly. So any other theories?

[hudson]

Agreed since gravity fed, so this is a mystery to me. It seems that before startup, whether engine is cold or warm, the fuel flows down the line and fills the carb bowl. Engine starts and idles fine. Regardless it runs out of fuel in the bowls.

Here is another experiment that I did (I had 3 large fans blowing on the cylinders), although it may still be frowned upon:

turned petcocks on and fuel flowed into float boal. Turned off petcocks. Started engine and let it idle until it finally stalled due to all the fuel in the bowl used up. It idled for around 5 minutes or so. Turned off bike. Turned on fuel. Again filled the float bowls, this time leaving the petcocks on. Started engine, I let idle for longer to see if it will still run after the above 5 minute idle prior. I let it go to about 8 minutes. It still would have kept running, but I turned the engine off. Turned off petcocks. Felt that maybe all is well.

Next morning, before I touched anything - first thing I did was check the float bowls and they were empty, (even though I didn't let the bike run out the night before since I shut the engine off myself). I was expecting fuel to be in them still. I should have checked them last night beforehand to see any level of fuel that remained, but didn't think of it at the time.

I have had this bike for over 6 years and it never had any issues like this. Trusted it all this time. Now I don't feel like I can trust it to do any laps around my block!

I will put new, longer fuel lines on since the filters will be out of the equation. I am scratching my head though because the flow of gas down the petcocks through the line & filters flows great into a separate gas can. But maybe vapor is being trapped from escaping them? They have been on though for a few months and worked perfectly. I'll give it a shot...

[Zombie Master]

So your fuel taps have screens themselves. If there is some crap at the bottom of the tank it could be blocking there. When there is little demand (idling ) there may be enough getting by, but when you start riding you are using much more fuel. I'm not considering the fuel lines because they are new. It seems to me you have a problem when you need more flow.

[SteveD]

I've had a circumstance where the in tank tall filters clogged the taps. There was a small cork in the top of the gauze to help keep its shape that had disintegrated, in both taps. The bike would run badly on one cylinder occasionally, then be fine, then it'd stop only to start again in a few minutes as the bowls slowly refilled. Took me ages to work that out.

It might be worth emptying the tank, removing the taps and inspecting them, maybe retrograde flush them.

At the very least cleaning the top end of the system will negate that as a cause.

[hudson]

Thanks Steve - I did replace the petcocks (which included the tall "straws " to suck gas as well as the tall screens that they slide in to). Tomorrow I am going to remove the small, inline external fuel filters and see what happens. If not go, I am going to relook at the petcocks and tank despite the strong flow of fuel that comes out of it. Will keep everyone posted in case someone else happens to go through this annoying situation.