Fuel pump problem

by Jake Dud in 2 Stroke Technical Questions 9 years ago

Hi guys,

Could you please assist where is the problem in my fuel lines?

Briefly, I got tired of a rubber starting pump (bulb), due to the fact that each year I needed to replace it twice. I've found it extremity unreliable.
Therefore I have installed the electric pump (0,2bar, 3PSI) in series with pulse one. The idea is to apply the electric pump for starting the engine and then let it off.

Please refer to the schematic attached.

After taxi'ing on the airfield I've noticed that:
a) engine start is smooth
b) after turning off the pump the engine went off
c) With the pump on I can't get over 4k rpm

Could you please advise why is that happening?

A few things to consider:
- 8mm (5/16'') hoses before Mikuni. Does it matter if after Mikuni it goes to 6mm (1/4'')?
- the mount place for the electric pump. Higher than fuel tank, needs to pull (instead of push) fuel upwards
- the overall height between the tank and the carb - 2ft / 0,6m
- is the electric pump produce significant amount of flow resistance when turned off?

Please share your knowledge as I'd really like to avoid such troubles in the air.

Cheers
Jake

pump_schematic.png (You do not have access to download this file.)

Re: Fuel pump problem

by kevin kirkendall » 9 years ago

kevin kirkendall

0 22 0

Jake,

The pump could be causing a restriction. I would remove it, hook a hose to it and see how it flows with a fuel source (could use a funnel) about 2 feet above the pump. It should flow like a weak urine stream. Also check that the hose connections are tight, to make sure your system isn't sucking air.

The check valve in the pump could be restricting flow, so could the flow transducer and the filter (if its dirty). Have you checked the flow out of the pump (right before the engine pump) with it on and a hose running into a container? it should be a much stronger stream. Also check the voltage at the pump while it is running, it should be over 11 volts. A bad ground can also cause a weak pump, but that won't show up during the voltage check. Voltage/ground problems can be due to bad connections or undersize wiring.

Is the vacuum line that powers your pump tight and in good shape?

What was your max RPM before you installed the facet pump? Did you change anything else?

Do you have EGT monitoring on your cylinders? Do they go high right before you go full throttle or at full throttle?

Ideally I think you would want the facet pump mounted below the fuel source so its gravity fed. Autogas can vaporize when a vacuum is applied at high ambient temperatures. With the engine running, tied down and at 3500 rpm (be careful by the prop) do you see any bubbles going through the filter (if its clear) you shouldn't. That means its vaporizing from high vacuum due to restriction or sucking air somewhere before the filter. Do you use Tygothane clear fuel lines? That also makes it easier to spot bubbles.

I prefer to have a filter mounted after the facet pump, since I have found small particles given off by the pump due to wear.

Those plastic clear filters can have a ridge on the barbs from the molding process that can allow air to be sucked in. If it does, use a knife blade and scrape the ridge down to where it is smooth with the rest of the barb.

If all this checks out I would take the engine mounted pump apart to check for blockage. Be careful with the diaphragm and don't use any chemicals on it. Also check the flow through the transducer, you should be able to blow through it easily. I would also check the carb bowls for debris. A new facet pump may have released something that is restricting your flow downstream.

Be very careful about running your engine lean due to a fuel problem. 2 Cycles will not tolerate a lean and hot burn.

Kevin K.
Sonex Jabiru 3300

Thank you said by: Jake Dud

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Re: Fuel pump problem

by Jake Dud » 9 years ago

Jake Dud

1 2 0

Thanks Kevin,

I'll try to implement your solutions and share my thoughts after.

Regards
J

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Re: Fuel pump problem

by Jake Dud » 9 years ago

Jake Dud

1 2 0

Got the reason. Freaking unbelievable :sick:

I have disassembled everything and replaced with fresh parts - and still got the problem unresolved.

After a long investigation, the reason I've found was neither the electric/pulse pump nor the 8mm (5/16'') hoses caused that.

It was due to these clever slim pipes on carbs, which are responsible for removing too much mixture from the float chamber. The holes on these hoses were glued by old mixture (were too slim probably). The effect- flooded engine during high RPM
After even removing the hoses , violà, everything works as intended.

Thanks for ideas,

Cheers,
J

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Fuel pump problem

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