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Old 02-16-2008, 09:08 PM   #1
N.H.Sledder
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I'm new to this board, and hope some of you can help with a couple of hopefully minor problem this sled is giving me. Just bought it this winter, new to the sport, and hoping to tweak this old, but good, sled back into good shape so my son can ride it safely.

Just had the carbs cleaned by a local mechanic, but still notice air bubbles in the fuel line ahead of the fuel pump (from tank) and in the lines that feed the carbs. Not sure if it's a bad pump, or air getting into the lines through cracks (can's see any), or some "internal" line within the gas tank I was told was also an option. How can I get the air out?

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Old 02-17-2008, 01:19 AM   #2
63November
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Air in the lines is not a problem. If the flow is greater enough they'll probably get pushed through. Since fluids seek their level and air is much less dense than fuel, those bubbles seek the higher areas in the lines. If it bothers you, switch to black fuel hose and you will neither see nor know if bubbles exist. That's worked for me on my 97 RMK for 22,000 virtually trouble-free miles; previous machines of similar type all had air bubbles in the lines.
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Old 02-18-2008, 09:01 AM   #3
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I, personallly, would not risk starving a carb for fuel. Also, air in the fuel pump can tear the diaphram and strand you on the trail. With bubbles between the tank and pump, check the clamps on that hose and the condition. It is simple to replace. If that is all ok, then you will probably have to pull the tank to get access to the the outlet fitting to replace the pickup hose inside or that fitting.
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Old 02-18-2008, 10:52 AM   #4
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Quote:
I, personallly, would not risk starving a carb for fuel. Also, air in the fuel pump can tear the diaphram and strand you on the trail. With bubbles between the tank and pump, check the clamps on that hose and the condition. It is simple to replace. If that is all ok, then you will probably have to pull the tank to get access to the the outlet fitting to replace the pickup hose inside or that fitting.[/b]

Fear not, you cannot "starve" a carb because of bubbles in the line. The fuel simply moves past, the bubbles jiggle around and sometimes get pushed partway toward the carb, but then slip by and seek the high level where they were. Fuel flow will still be normal. Ice on the other hand, can cause starvation of the carbs.

As for air and the fuel pump diaphragm??? That needs a logical explanation and defies all experience. We start with air in the fuel pump every fall after the machine has been drained for the summer, the fuel lines fill with air every time a machine runs out of gas; fuel pump diaphragms will last for many years and thousands of miles. It simply isn't a concern nor worth time spent trying to "correct."

If your lines don't leak fuel, if the pump is pumping fuel, and if the carbs are filling properly, don't worry about the air bubbles.
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Old 02-19-2008, 09:20 AM   #5
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I would say, 63, that you have been very lucky. My 35 years experience with 2 stroke, intake driven,diaphram fuel pumps has shown me that they are not designed to move air. Granted, we do not have a substantive, quantitative measurement of how much air is in the line, but I think we can agree that it will not get better, only worse, and with air in the pump, the diaphram will stretch beyond it's designed limits and will begin not delivering the required amount of fuel to keep the carbs full ( a lean condition). This increase in movement will fatigue the material and it will fail. The quality of fuel blended these days also has a detrimental effect on all rubber parts in the fuel system which will hasten their demise.

I would also point out that if that small, plastic bowl vent tube gets clogged or melted shut from engine heat, your air bubbles have no way to exit the carb. They will displace the fuel, dropping the level in the bowl and cause a lean condition. I'm just saying that I would not risk this kind of failure with my equipment. I've seen way too many powerhead meltdowns because of fuel pump failure and at rpms far below 8000.

Gotta love this forum...the exchange of opinions and experience from all angles is unbeatable!
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Old 02-19-2008, 10:11 AM   #6
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I dont know I kinda agree with 63. Air bubbles are not an issue I have 4 sleds they all have air bubbles in the fuel line ABOVE the fuel pump next to the tank, and I have no leaks in any of the fuel lines.

IMO, there is no way to stop air from entering the line that runs from the tank to the fuel pump.

I could be wrong, but this is why I believe this:

Say you have a 1/4 tank of gas left, therefore 3/4 of you tank is filled with air. When your riding that 1/4 tank of gas is going to get sloshed around and you will suck some of the air in the tank into the fuel line.

Why it doesnt matter is becuase the fuel pump sits lower than the tank and as 63 stated air bubbles in the line will automatically go to the highest point in the line never reaching the fuel pump. I think you could consider it a matter of physics. Air in water/fuel/liquid will always travel upward.

If air is getting into the lines between the fuel pump and carbs, you probably then have a leak.

my .02 cents
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Old 02-19-2008, 11:37 AM   #7
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Perhaps I have been lucky. However, in miles ridden on these machines which easily exceeds 75,000, I think I probably would have seen some evidence of a problem with air bubbles in the lines since they are quite common. As a matter of fact, I also suspect I would have seen problems with the pumps themselves in that time. Yet the simple Mikuni diaphragm pump is one of the most reliable working parts on the machine - regardless whether it is placed high on the airbox, or mounted low on the tunnel. And while I suspect that blow-by pressures from worn or inadequately warmed-up engines places a far greater stress on that diaphragm than an air bubble or two in the lines, even my old, presumably worn 488 fan engine with only a ring change to its name in over 22,000 miles still shows no sign of fuel pump problems - and it has the pump mounted above the carbs and the tank, high on the airbox where air can accumulate if it will. So I can't buy the concept of air causing problems.

As for carb vents closing, since they are there to equalize pressures in the bowl with atmospheric pressures and allow engine pressure differentials to pull fuel at specific rates and mixtures, closing them might as easily cause a flooded condition as a lean one, either of which is going to give noticable running problems. Any time I've had a carb starve on a snowmachine - and it happens not infrequently- the cause is most generally a problem involving ice in the system somewhere. While a slightly lean condition is an engine failure hazard, a very lean condition: starved, does not allow the engine to run well enough to overheat IME.
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Old 02-19-2008, 01:44 PM   #8
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Well, here's what works for me...in four boats and four sleds I always keep the tanks as full as possible with 90 octane and stabilizer to avoid moisture absorption and phase separation, even in the off season. I rebuild the pumps every other season and all 2 stroke carbs every season. I have no air in any lines, I never get below third a tank, and I have to believe it is the reason that I have no starting problems, no bogging and no lack of top end. Our sledding season in Wi, USA is too short for me not to eliminate as many of the preventable failures as I possibly can. For me it is time and money well spent to ensure trouble free enjoyment of snow and water sport
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Old 02-19-2008, 02:16 PM   #9
63November
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I agree those are all good ways to prevent problems, especially when machines are stored or used at very cold temps. I also believe they are even more important when machines don't see as much use and perhaps not on a regular basis. Our machines tend to get run every day or nearly so during the 5-7 month seasons we have. I suspect we see different kinds of problems on a regular basis than do folks whose machines get used less frequently. In that regard, ice inside the pumps is probably the most frequent pump issue we see - and that usually results from the impulse side on machines that don't get adequately warmed up. It's a condensation problem caused by water vapor blow-by during warm-up combustion.
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Old 02-19-2008, 02:37 PM   #10
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ive got bubbles. ususally the feed line TO the pump is letting air in. check for cracks and loose hoses after and inside the gas tank.
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