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Postby Chappy » Wed Apr 26, 2006 9:55 pm


Glad to hear you and your Lazair are OK! I might be crazy, but I rebuild my fuel pumps (and/or carb diaphragms) every 2 years now. I've found that the diaphragms start to get a little stiff after only a couple years, and that causes the pumps to start loosing their ability to work at 100%. With the fuel so low in most Lazairs, the pumps have to do a lot of work. I had the same situation on my Avenger (they are now called Hurricanes). The main difference was that it had a separate fuel pump that fed two carbs from a low mounted tank. I don't know what your setup is for pumping fuel, but I would sure look into it if it's 20 years old!

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Postby uscgairdale » Thu Apr 27, 2006 6:45 am

Assuming that you're using some Mikuni fuel pumps, a rebuild of them would be a good start. I think that it will set you back about 10 bucks or so each, maybe a bit more. It also doesn't take long....maybe a beer for each rebuild. They're a good pump, especially for the distance/head requirements for a Lazair, after all the smaller pumper carbs on 185's have done really well over the years.

I am going off of my memory here, but I remember the manual for my Pterodactyl talking about something like a 12 or 15" maximum distance for the impulse line on the fuel pump. Where are your pumps located? That won't be an issue if your pumps are mounted just in front of the reed valve assembly because it should be just a few inches or so, but that could be an issue if they're mounted elsewhere.

The only other thing that I can think of right now is a clogged fuel filter, especially is your fuel is old. I figure that you would have found that though.

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Postby JPXman » Thu Apr 27, 2006 3:19 pm

oh there are lots of variables at play for this problem. The engines never suffered this problem until the day of (ran about 5 hours before), because the day before we swapped out old fuel line around the carb and installed new impulse line (3" long) using automotive vacuum line.

So, we changed:
- fuel line
- vacuum line
- hose clamps
- new gas + oil

so something changed enough to create this problem. The more i've been thinking about it, the new vacuum line didn't quite stay round around the typical JPX impulse line install, it was just a tiny bit kinked (its a really tight bend though, not much getting around a small kink). So, i've got about 12 fuel pumps I can try :) I will be swapping some of those, changing vacuum lines, etc etc. lots to do today!

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Postby JPXman » Sun Apr 30, 2006 12:24 am

the plot thickens!

ok, shannon you can now sleep easier, i have removed the primer bulbs :) I installed all new fuel line, and hose clamped the joints around the fuel filter (i don't/can't use in tank filters with my setup). Installed a brand new mikuni fuel pump.

Start the engine up, idles fine, mid-range fine, max power - hey cool its running. I set it at full power, clamp the throttle so it won't move and wander off to clean some stuff. Sure enough, 5 minutes later, the engine just dies altogether.

1 pull later its running again.... idles fine, mid range fine, full power fine. 4 minutes later BAM dead - one cough and dies. gas in all the lines, not fuel starvation.... now i'm really scratching my head. i fire up the other engine and run it at full power for 10 minutes and no problems, but also no fuel bubbles in the line from the pump to the carb... this gets me thinking. i fire up the bad engine, and only at full power, there are little bubbles between the pump and the carb, that get worse as the engine runs longer. sure enough, the bubbles get pretty persistent, and you can even see a little vortex of bubbles getting sucked down into the fuel bowl... then the engine dies.

So, my attention is now turning to the engine and that port that drives the diaphram. perhaps the reed valve is not up to the task at full throttle? this is new territory for me, any advice would be appreciated. if the pump wasn't pumping as fast as needed, would it output bubbles instead of gas? this is the second and brand new pump and the engine dies the same way, so perhaps the engine isn't pumping the pump hard enough?

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Postby ola » Sun Apr 30, 2006 3:15 am

when it dies,is there a spark? when the condenser get warm it use to give up.
try to change all ,pumps,hoses to the other engine and try it.

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Postby flyalaz » Sun Apr 30, 2006 3:31 am

Hey Tyler,

What kind of filters do you have in the tank? If they are the plastic body with paper type element, it's possible that this is responsible for fuel starvation. I used those and I had similar problems with bubbles forming and the motor dying out. The only reason the motor kept running was because I pumped fuel into the carbs with the primer bulbs, and even there, the vacuum was strong enough to collapse the bulbs and force me to be very careful how much and fast I opened the throttle because I could have run out of "squeeze" in the bulbs to keep it running.
Those are all being scrapped out in favour of filters and valves as suggested by Shannon in the Rotax rebuild.

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Postby JPXman » Sun Apr 30, 2006 1:20 pm

excellent ideas - i will try a new capacitor for sure, and I will cross the fuel feed lines from the right engine to the left and vice versa and see what happens...

for the capacitor though, i can restart the engine 5 seconds after it quits from full power, and have the engine back at full RPM in 10 seconds and it will run for another 4 or 5 minutes...

I am also going to remove the filter and put in a straight-thru for testing, and feed the fuel pump direct from my red gas can I fill up at the service station.

hopefully with all of these options i'll figure something out


PS: daffy also suggested the crank seals. if they were a bit leaky would that problem surface as intermittent engine failure and bubbles in the fuel line?
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Postby Guest » Mon May 01, 2006 12:03 am

do you fly with 20 year old crank seals ?.


Postby russell » Mon May 01, 2006 6:45 am

Tyler, I don' know anything about the engines you fly but I do know for a FACT that if a fuel line is resting against an object that is vibrating a lot, small air bubbles wil be generated at that very spot.

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Postby Guest » Mon May 01, 2006 9:27 am

Leaky seals can make it hard for the carb to pull suction. Bubbles may just be vapors or air leak in lines. Check entire fuel system carefully.


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