Full power fuel starvation

[frank marriott]

Has anyone experienced this:

 

Fuel starvation at full power with only engine pump working. Switch on electric and all comes good.

 

Installed new fuel pump no improvement - starvation at slightly lower rpm.

 

Type 2 pump with the 74mm pushrod.

 

Checked main jet - clean with no obstructions.

 

With normal operations with electric pump on, the only other thing in play is the mechanical pump, at least to my knowledge??

 

 

[Bob Llewellyn]

Has anyone experienced this:Fuel starvation at full power with only engine pump working. Switch on electric and all comes good.

 

Installed new fuel pump no improvement - starvation at slightly lower rpm.

 

Type 2 pump with the 74mm pushrod.

 

Checked main jet - clean with no obstructions.

 

With normal operations with electric pump on, the only other thing in play is the mechanical pump, at least to my knowledge??

Speaking generally - I don't have a Jabiru - the engine should have been shown to develop full power with either pump out of action; so is there a fuel wasp's nest in the line, or a collapsing inner or something?

 

 

[frank marriott]

Bob,

 

Both pumps are in series and use the one feed. I have run out of ideas.

 

 

[Camel]

My advice is change fuel filter. The mechanical pump has to suck fuel where the electric pump pushes fuel through filter. I have seen filters causing problems like this in cars that suck through filter. The Z15 filter is a paper element and I would change it very regular no mater whether it looks clean.

 

 

[Bob Llewellyn]

Bob,Both pumps are in series and use the one feed. I have run out of ideas.

Yes, the idea is that the valves on either pump will allow fuel to pass through even if that pump ain't active.

Fuel hose is normally made with an outer, a robust string-reinforced body, and a liner. I have had happen to me twice (once car, once U/L); and heard of several times; the liner comes loose from the body. Now, if the liner between the engine pump suction and the elec pump outlet did this, with elec not running the suction partly closes the fuel line; and the elec pump pressurises it back to full size. To test this, get a length of line - the cheap clear not-really-fuel-resistant stuff is ideal - and run it through the cab, or on the ground outside, from the elec pump outlet to the engine pump inlet. Then ground run the engine. If it works fine under test, you've got yourself a good spare engine pump! - and the pleasant job of replacing the std fuel hose with something of at least equal quality... (as far as I know, NO lined fuel line is immune from this problem, ocassionally)

 

 

[Bob Llewellyn]

My advice is change fuel filter. The mechanical pump has to suck fuel where the electric pump pushes fuel through filter. I have seen filters causing problems like this in cars that suck through filter. The Z15 filter is a paper element and I would change it very regular no mater whether it looks clean.

If the filter is between the pumps, definitely try that first.

 

 

[Camel]

Actually the filter is before electric pump too but is very close to it ,the filter is between the header tank and electric pump. I still would change filter first no matter what.

 

 

[Bruce Robbins]

Frank,

 

We have also had a faulty electric pump cause a similar problem. The two pumps are in series, and Jabiru do not plumb in a bypass line. In this case it was Boost pump off - ops normal, pump on - engine faltered.

 

Replacing the electric pump solved the problem.

 

 

[frank marriott]

The electric pump provides 60 ltrs/hr as it should.

 

It is a straight swap to a new engine. No problems with supply to old engine. Filter fairly new and does not obstruct the 60ltrs/hr electric (tested flow rate)

 

Runs normal on engine pump up to just below full power. All indications ( to me) was it must have been a faulty engine pump even though it was new - replaced with another new one but problem continues. Runs perfectly, full static revs and climb out with electric on. First discovered the problem on test flight when the electric was turned off.

 

 

[RKW]

Could it be that the fuel pump is not getting sufficient lift from the cam lobe. Is the cam lobe worn or damaged?

 

 

[frank marriott]

RKW,

 

new engine including pushrod to the cam.

 

 

[frank marriott]

Thanks for the input. I was just hoping someone may have struck the problem before. I guess I'll have to wait until Monday and see what the factory has to say?

 

 

[Bob Llewellyn]

Thanks for the input. I was just hoping someone may have struck the problem before. I guess I'll have to wait until Monday and see what the factory has to say?

Worth asking - but unless I missed something, a collapsing inner would still fit the bill?

 

 

[frank marriott]

Yes, will look into that as well.

 

Would be unlucky for that to happen right on changeover, but not impossible.

 

I "think" the line is clear tube to the firewall and then rubber hose but will have to check that as well.

 

Any suggestions are good as I have run into a brick wall.

 

 

[Bob Llewellyn]

Yes, will look into that as well. Would be unlucky for that to happen right on changeover, but not impossible.

I "think" the line is clear tube to the firewall and then rubber hose but will have to check that as well.

 

Any suggestions are good as I have run into a brick wall.

Brick walls are not good for flight! Ummm - it couldn't be vapour lock, could it? If you mix fuels, and as they age, you can get vapour lock surprisingly easily...

 

 

[cherk]

Yes, will look into that as well. Would be unlucky for that to happen right on changeover, but not impossible.

I "think" the line is clear tube to the firewall and then rubber hose but will have to check that as well.

 

Any suggestions are good as I have run into a brick wall.

not something ...like carby float level ??

 

 

[Bob Llewellyn]

not something ...like carby float level ??

I've had the little rubber seal on a float valve needle disintegrate, causing blockage and yuk, but that was on an ooooold carby... and I'd think you'd notice the rich behaviour if the float sunk?

 

 

[frank marriott]

No fuel mix - straight avgas

 

Don't think float level, bowl full when removed.

 

 

[Bob Llewellyn]

No fuel mix - straight avgasDon't think float level, bowl full when removed.

I'd put 2 bob on it being s fuel line liner problem, because I can't imagine any other possibility to match the symptoms

 

 

[SDQDI]

not something ...like carby float level ??

I'm not overly mechanically minded but if you have just changed the engine and the problem started seems that the existing fuel line shouldn't be the problem unless something got into the line during the changeover, I'd be more inclined to suspect the new bits put on.

 

 

[turboplanner]

Might pay to try to blow though the electric pump when it's turned off, could be a valve is sticking shut. Also check polarity of electric fuel pump wires, could be current holding the solenoid.

 

 

[frank marriott]

No fuel pours through the electric pump when turned off just by gravity

 

 

[turboplanner]

OK in summary:

 

Engine starves when the fuel pump is switched off

 

• New mechanical pump fitted
   
   
  
• Both pumps in series with one feed
   
   
  
• Fuel filter upstream of both pumps - does not restrict 60 lt/min electric pump flow
   
   
  
• Possible inner fuel hose collapse
   
   
  

 

 

Next I would try bypassing the electric pump and filter

 

Then bypassing the filter only

 

If the electric pump in the off position is not blocking or partially blocking the line, then my thoughts are:

 

(a) line collapse which Bob suggested, perhaps sucked in on itself by mechanical pump, then expands again when the engine stops

 

(b) a batch of dirty fuel has left residue on the filter/gauze inlets.

 

 

[JabSP6]

Frank,

 

Just something else to think about. There were 2 different length pushrods for the jabiru supplied mechanical fuel pumps depending on which type of pump you use. If you have the shorter pushrod in with the pump that requires the longer pushrod then you might have the problems you are describing. Especially with the system performing ok prior to the engine swap. The new motor may have mistakenly been fitted with the wrong pushrod and you swapping pumps making no difference would be right if the pushrods is wrong. Jabiru can give you the dimensions of the correct pushrods for the pump you have.

 

Might be worth looking at if the fuel hose inner liner isn't the issue.

 

Safe flying

 

Andrew

 

 

[frank marriott]

JabSP6

 

Type 2 fuel pump with the 74mm pushrod fitted (type 1 has 72mm)