Odd problem, cant quite fix it (fixed)

This is from another web article and is more Motorcycle specific:

"The tank to carb fuel flow is gravity fed. So, the only pressure in the fuel line, comes from the fuel volume in the tank and the height difference between the carb's float valve and the highest level of fuel in the tank. The difference in fluid height creates "Head pressure". This head pressure gets lower as the fuel level decreases in the tank.

The carbs are vented and the float valves are at the top of the float chamber. The fuel level in the carb's float bowl chamber is always lower than the float bowl valve tip and seat. When the float valve opens, air can go back into the fuel line from the carb's bowl chamber, as fuel trickles into the carb float bowl chamber.

The stock fuel line routing allows these small bubbles to rise to the fuel tap and bubble up into tank, keeping the fuel line relatively clear of large bubbles. Honda routed the lines so there was a constant "fall" of liquid along the entire length of the fuel line. This allows the fuel to accumulate at the bottom of that small tube vessel and air to rise and occupy the top of it, where the fuel tap is. Bubbles can be trapped by long lines that loop down and back up to the carbs. If the lines are long enough and the amount of air excessive, the force of the bubbles trying to rise back to the top of the fuel level surface in the tank rivals or exceeds the head pressure force attempting to let gravity feed fuel to the carbs.

The result is a stoppage or reduction of fuel flow to the carbs. When the carb bowl level drops below that of the pilot jet tube, the pilot systems starves for fuel and the mixture leans out, causing loss of power.

Complicating the matter, is the saddle hump in the fuel tank, and the stand pipe, which is used before "reserve" is selected. Fuel on the far side of the hump stays on that side, unless there is fore and aft motion of the fuel in the tank, which sloshes it over to the other side, where it can feed the fuel tap inlet. Also, the head pressure into the stand pipe becomes lower as the fuel level in the tank approaches the height of the stand pipe.

The short of it is that your long loop of fuel line may well be causing your loss of power."

This seems to be pretty accurate for the problems experienced with feeding the Carbs on these bikes. About 30" of height will create 1psi of pressure. So as Patton says, the decrease would not be huge in pressure, since it would be under 1psi. But as you can see, with a Gravity system, using the wrong filter, and/ or incorrect fuel line routing. the Carbs may be starved for fuel.

The web article says: "When the float valve opens, air can go back into the fuel line from the carb's bowl chamber, as fuel trickles into the carb float bowl chamber. . . The stock fuel line routing allows these small bubbles to rise to the fuel tap and bubble up into tank, keeping the fuel line relatively clear of large bubbles,When the float valve opens, air can go back into the fuel line from the carb's bowl chamber, as fuel trickles into the carb float bowl chamber."

Here's why I think that's incorrect:

The motorcycle carbs I'm familiar with all have float bowl vents whereby air escapes from the float bowl (and without forcing its way through the float valve against incoming fuel and ultimately into the fuel line).

With a properly functioning bowl vent, there are no "bubbles" created by air backing up into the fuel line. And no such bubbles in the fuel line blocking fuel flow into the carb.

So long as the fuel line isn't kinked, long loops of fuel line are perfectly all right, provided the loops remain below the tank.

To provide sufficient room to accommodate in-line filters, longer looping lines are often necessary and commonly fitted without causing any problems.



Ample fuel flow here:



Good Fortune!

I would recommend to Test the flow rate from your petcock, before and after a added in-line filter to check the restriction. you don't want to make your carbs lean out from a restricted fuel filter.

Motor Head wrote: I would recommend to Test the flow rate from your petcock, before and after a added in-line filter to check the restriction. you don't want to make your carbs lean out from a restricted fuel filter.

+1

took the tank lid/door off, complete disassemble, seemed clear but I have heard a slight hissing sound sometimes after I stopped.
Looked up the filter I have, supposed to be for zero pressure lines i.e. lawnmowers, generators etc.
Field report later

Medina wrote: took the tank lid/door off, complete disassemble, seemed clear but I have heard a slight hissing sound sometimes after I stopped.
Looked up the filter I have, supposed to be for zero pressure lines i.e. lawnmowers, generators etc.
Field report later

It's far better to actually test run the bike with the fuel cap ajar.

If problem doesn't appear with cap ajar, but appears with cap snapped down, then there's no doubt. Then it's time to attempt clearing the vent.

Good Fortune!

Ok, now I'm convinced its the speedometer...everytime I hit about the 100 mile mark on the tank of gas the problem appears...so logically, I'm replacing the speedometer.

Tonight I started to wait for it starting around 93 miles..yup, at 102 cough cough sputter, I was prepared, spare key already in tank popped the lid, no difference, coasted to the side and 3 minutes later,..vroom.
Anyone here old enough to remember the ping pong ball trick? ping pong ball in gas tank floats till its about 1/4 full, gets sucked to the intake and it dies...starts again a little later.
I'm going to gut the fuel line/filter/inline petcock and re-do it all tomorrow.
I've got it down pat now, coast to side of road, four ways on, and act like I'm talking on the phone

where is the vent? looking at the manual, its a hole under the rubber gasket, partially hidden by the large spring? If thats it, I wired it, hit it with 60# of air today.

Yea the vent is in the cap, can disassemble and clean. But sounds like that's OK. Have you checked for spark, when on the side of the road and won't fire?
Otherwise you could take a screwdriver with you and open the bowl drain screws and check for flow.

Medina wrote: ...at 102 cough cough sputter....

If not already done, try switching petcock to reserve position.

Good Fortune!

Motor Head wrote: Yea the vent is in the cap, can disassemble and clean. But sounds like that's OK. Have you checked for spark, when on the side of the road and won't fire?
Otherwise you could take a screwdriver with you and open the bowl drain screws and check for flow.

Have not checked for flow, will do. Haven't checked for spark as I'm assuming (trouble I know) its fuel related. I instantly pulled the enrichment all the out and it fired, engine raced to 6K and died.
Odd..aint it.

Patton, I'm running it in "prime" position, vacuums been plugged, using an inline petcock till winter and redo the carbs- some "pro" shop in Indy worked over twice. But thats exactly how it feels- that not enough gas in tank to reach primary, time to switch to reserve.

good news is last two tanks have averaged right at 40mpg, quite pleased with that. expected 35ish

The stock vacuum petcock on Prime, OK. But this In-Line petcock, what it the internal diameter? Is this a 1/4 turn valve? Can you look and see the D of the valve when switched on? I'd check out going flow, each piece. Use a container and a stop watch. Get some flow checks done.
Just thought that you said the Vacuum Petcock had been gone through. Is it possible that the tubing that is pressed into the valve body is miss installed, the Normal "ON" is in the "PRIME" spot? Or when it re-fires, it doesn't shut down again?
Do you know if the Tank Has been Lined with Chemical for Rust Treatment? Find anything in the filter? Blow through it backwards in to a clear glass bottle or similar.

Patton wrote:

Medina wrote: ...at 102 cough cough sputter....

If not already done, try switching petcock to reserve position.

Good Fortune!

Patten's Other Site :laugh: Just jabbing at ya....