Curious Fueling Issue

Bike (850 MKII w. fresh Amal Premiers) has been running great past few thousand miles. Last two rides, been having a bit of idle drop below normal 1k rpm when coming to a stop light. I have a transparent filter on each carb feed from tank and while watching these at idle, I can see a stream of fine bubbles coming up through the filters...more so on the right side one. Also, blipping throttle just to 1/4-1/3 open, the fuel level in the filters plummets suddenly, leaving nothing but air in the filter body. Thinking maybe a possible leak in the hose connections or perhaps the float banjo/float bowl gasket permitting air into the fuel line? Cannot detect any fuel escaping from float bowls or connections. Running at high rpm has not had any issues at all...loads of power as always, even with heavy/rapid throttle input.

Thinking to drop the bowls and check al the lines and gaskets. Other ideas?

Fuel tap screens in the tank may be clogged.

concours said:

Fuel tap screens in the tank may be clogged.

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Always possible but issue remains when turning on either tap (reserve side rarely gets opened so it would've had to clog without much fuel passing through).

Tornado said:

Always possible but issue remains when turning on either tap (reserve side rarely gets opened so it would've had to clog without much fuel passing through).

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Don’t know that fuel passing through it necessarily effects clogging - it may clog from particulate by being immersed- just a thought.

I guess the question remains...where, when the filters show full of fuel, does air come from when blipping the throttles? This happens even with full tank so cannot be from taps drawing air down from above? Air must come up from carb side to replace fuel as level in filters drop. Should be a standing pool of fuel in the bowls regulated by the float/needle valve. Don't understand how air can get down and out banjo into fuel lines from there unless there is a leak somewhere?

Possible gas cap venting issue. You can check by repeating with the gas cap opened.

baldy said:

Possible gas cap venting issue. You can check by repeating with the gas cap opened.

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Yes I tried that too...no changes when opened...level in filters does not suddenly re-fill if low when I open tank cap.

Air might come in thru the tickler?

Air might come thru the carb bowl gaskets .
Check the tightness of the 4 awkward to get at screws ?

Also check the O rings on the pilot mixture screw and the premier pilot jet.

Ignoring filters and bubbles:

When you shut the throttle and revs sink to below normal idling rpm, then recover, that can be a sign of a rich idling mixture.

Can also be carbs out of synch.

Tornado said:

I guess the question remains...where, when the filters show full of fuel, does air come from when blipping the throttles? This happens even with full tank so cannot be from taps drawing air down from above? Air must come up from carb side to replace fuel as level in filters drop. Should be a standing pool of fuel in the bowls regulated by the float/needle valve. Don't understand how air can get down and out banjo into fuel lines from there unless there is a leak somewhere?

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Air bubbles UP from the crab bowls, which are vented to atmosphere.

gortnipper said:

Air might come in thru the tickler?

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Torontonian said:

Air might come thru the carb bowl gaskets .
Check the tightness of the 4 awkward to get at screws ?

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But then there's normally "air" in the space between the fuel level and the top of the float chamber.

Ok, so "air" would normally be displaced by fuel vapour but either air or vapour would have to travel down through the float valve against fuel flow to reach the fuel line when that air/vapour could easily vent from the tickler with no restriction. The cylinder vacuum would also tend to draw the air towards the carb throat in my opinion.

concours said:

Air bubbles UP from the crab bowls, which are vented to atmosphere.

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Yes, "air" rises through fuel so that "air" (fuel vapour?) would again have to travel downwards through the float valve and also against the fuel flow to get from the float bowl to the banjo and into the fuel line which seems unlikely in my opinion.

Tornado said:

Also, blipping throttle just to 1/4-1/3 open, the fuel level in the filters plummets suddenly, leaving nothing but air in the filter body.

Click to expand...

What I believe you are seeing is more likely to be a pocket of fuel vapour, not air, possibly caused by the filter restricting the flow to the carb.

Tornado said:

Thinking maybe a possible leak in the hose connections or perhaps the float banjo/float bowl gasket permitting air into the fuel line? Cannot detect any fuel escaping from float bowls or connections.

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Due to the head of fuel pressure then in my opinion, fuel would be far more likely to leak out, not air leak in unless there was a sufficient restriction to the fuel flow between the filter and carb.

Tornado said:

Always possible but issue remains when turning on either tap (reserve side rarely gets opened so it would've had to clog without much fuel passing through).

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Well, then, don't pull them & check them.
Let us know what the problem is.

L.A.B. said:

But then there's normally "air" in the space between the fuel level and the top of the float chamber.

Ok, so "air" would normally be displaced by fuel vapour but either air or vapour would have to travel down through the float valve against fuel flow to reach the fuel line when that air/vapour could easily vent from the tickler with no restriction. The cylinder vacuum would also tend to draw the air towards the carb throat in my opinion.




Yes, "air" rises through fuel so that "air" (fuel vapour?) would again have to travel downwards through the float valve and also against the fuel flow to get from the float bowl to the banjo and into the fuel line which seems unlikely in my opinion.



What I believe you are seeing is more likely to be a pocket of fuel vapour, not air, possibly caused by the filter restricting the flow to the carb.



Due to the head of fuel pressure then in my opinion, fuel would be far more likely to leak out, not air leak in unless there was a sufficient restriction to the fuel flow between the filter and carb.

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Could air/vapour be drawn down from the idle circuit path into bottom of bowl and then out banjo? Not sure what pressure could cause thst, as you say, the low pressure should be above the bowl drawing this upwards at all times.

Perhaps a tiny leak, such that liquid fuel seeping out is almost immediately evaporated and not seen/felt as wet.

Today I will remove banjos and tank lines, replace the existing hose clamps with better fitting types (actual fuel injector rated types that better conform to the small diameter fittings rather than the standard hose jubliee type on now).

Tornado said:

Could air/vapour be drawn down from the idle circuit path into bottom of bowl and then out banjo?

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I would think not as any air/vapour that somehow managed to enter the idle drilling (in red) near the bottom of the float chamber would be drawn directly into the horizontal gallery connected to the mixing chamber and idle/transition ports,...

...the drillings of neither bowl nor body are connected to the banjo inlet and/or needle valve.
Even a leak at the gasket wouldn't allow air/vapour back into the banjo because the fuel entry is not positioned at the float bowl/gasket face.

I had the same "problem". I recently put a resprayed petrol tank on my Commando
and was worried about any particulates remaining in the tank, so I used clear petrol
pipe and clear fuel filters on both sides in an H type configuration. With the cross
piece being lower than the float bowls.

I observed air voids within the fuel in and around the fuel filters. On one occasion the bike
was a bit difficult to start. I kept the filters in for a hundred miles or so, but was a bit worried
about the air voids. I then removed the filters and made the H pipes such that the cross piece
was below the carb bowls such that any heavy partitulates and water would accumulate in the
low cross pipe. This eliminated the air voids. The filters were clean on removal so no particulates
came from the tank and there have not been particulates or water observed in the low cross piece.

My other bikes all have black rubber petrol pipes and no filters so I cannot comment
on "air" voids for the past 40 years !

But I now quite like the appearance of the clear pipes and that I can see the fuel and
any "problems".

Do people have problems that necessitate fuel filters ?

I have never found particulates in my float bowls.

Staytite said:

Do people have problems that necessitate fuel filters ?

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With a filter on each fuel tap and another in each banjo then there shouldn't be any need for a third filter, however, my Commando and T140 would suffer from the occasional flooding carb with sediment getting into the float bowls. Later I noticed the banjo filters were not reaching right to the bottom of the banjo so particles were able to pass under the filters.
After shortening the banjos so the filters bottomed out, sealing the gap, then I don't think I've had a carb flood since.

Ok i've R&R'd the banjo and fuel lines to the tank. Replaced the standard jubliee clamps with proper fuel injector rated types at every connection. Also gave all float bowl screws and drain caps a nip up snug. One or two bowl screws seemed to need a bit more of a nip than others. And the drain caps took a bit of nip easily.

All back together and after a twenty minute highway ride, idle seems back where I like it at around 1k, no dips below at any point so far. Have not seen any fine bubbles in fliters and no dramatic drops below filter when blipping throttle.

Today is not as warm as last few outings where these issues were happening. But may not be a factor.

Tornado said:

Ok i've R&R'd the banjo and fuel lines to the tank. Replaced the standard jubliee clamps with proper fuel injector rated types at every connection. Also gave all float bowl screws and drain caps a nip up snug. One or two bowl screws seemed to need a bit more of a nip than others. And the drain caps took a bit of nip easily.

All back together and after a twenty minute highway ride, idle seems back where I like it at around 1k, no dips below at any point so far. Have not seen any fine bubbles in fliters and no dramatic drops below filter when blipping throttle.

Today is not as warm as last few outings where these issues were happening. But may not be a factor.

Click to expand...

Good. The carbies are being shaken violently and everything needs to be retightened periodically.