Makuni fuel pump vapor lock

[Randers]

I was flying my single seater Airframes Unlimited USA1b two weeks ago. Perfect day - low winds ground level. About 65 degrees at the surface. I ascended to 1200 feet and flew around our area for about 45 mintues. As I was making my landing approach, I noticed that as I passed 600 feet AGL it got very noticeably cooler. Some sort of thermal inversion - hotter higher up. Anyway, on landing I gave it a lot of throttle at the end to check my descent - landed and switched off the engine. I packed the chute to taxi over to my trailer, but the engine wouldn't start. I had to push it all the way by hand. When I got home I discovered that I couldn't hand pump (squeeze bulb) past the Makuni fuel pump. I took the cover off and it sucked air - replaced everything and I could prime the carb ok and it ran fine. Needless to say I've got replacement parts on order for the fuel pump - new gaskets, etc. Anyone have any similar experience? The fuel pump is NOT located on the engine. What would cause this to happen? Was the brief full throttle on landing a possible cause? Randers in North Central Kansas

[kilowatt]

Randers,

Hey, Brother, where you flying from? I grew up in Kansas.

Kilowatt

[kilowatt]

I forgot to answer your post....

I personally don't think the temp changes, based on the altitudes you fly at, affected your pump's ability to function properly although you must keep carb icing in your mind. Temperature inversions at the altitudes we fly are pretty common in Kansas and, in my opinion, are really neat to experience "first hand".

The Mikuni mechanical fuel pump used on most PPC applications is relatively simple, consisting of a couple of rubber valves and plastic diaphragms.

I'm not sure what model pump that you have, but if AFU uses the same pump that I've seen on 90% of PPC's, you can be rest assured that it's very reliable; mainly due to its simplicity. In a little over 100 hours flying, I've never had a problem with my pump.

If I may...when was the last time you rebuilt your pump? I believe it is recommended you rebuild the pump once a year. Do you have a fuel filter cut in-line ahead of the pump? Is your pulse line in good condition? If not, Have you replaced the pulse line recently? Have you applied excessive pressure to the pump by over priming and damaged one of the diaphragms?

Your primer, depending on your application, may well be affected by the pump since it operates on the principle of flexing diaphragms to operate. These diaphragms are operated by the pulse line from the engine. One of the valves in the pump may not allow fuel to pass, depending on the vacuum or pressure created by the engine which exists in in pulse line. You may have a blown diaphragm, too. In my opinion, yes, the valves/diaphragms could be affected by your engine so much so that your primer was also affected.

You'll know if there's a problem when you crack the pump open.

Rebuilding your pump is a piece of cake (at least if it's the same one I have).

Kilowatt

[mignazito]

Sounds like a stuck checkvalve to me. It's gotta be real hot ot get vaporlock.

[Randers]

Kilowatt & Mignazito: I'm flying out of Concordia, KS. Thanks for the info, looks like refurbishing the fuel pump (Makuni DF44-211) is the way to go.... Parts should be here by Saturday. I've only had the PPC for a bit over a year, so am a newbie. Pulse line is fine - so a stuck checkvalve sounds like the problem. Appreciate the responses.

[woodyking]

Did you take a hose loose from the carbs before you decided it was not flowing through the pump? If the carbs were full, you would not be able to add more. If you had air in the lines near the pump, that could be from a leak ahead of the pump. When you removed the cover, the suction of air could be from gravity pulling the fuel back down the fuel line going upwards to it. It is normal to have air or vapor in this area when it stops running, just due to the warming of the fuel from engine heat, like a gas jug sitting in the sun will expand bigtime. I'd guess that the engine may have just been flooded, this will happen sometimes if you lift the front wheel high to turn the machine around for instance. A rough bump at the end of your taxi could do it also. Good luck, look closely at the little ridges where the flapper valves seat to be sure there is no oxidation to cause a loss of seal. I had one single Mikuni pump that did this, maybe from a drop of water that sat there for a long time, don't know, but a small section was corroded away, and when it broke loose, down I went. Woody

[indoruwet]

I couldn't hand pump (squeeze bulb) past the Makuni fuel pump.
If the carbs were full, you would not be able to add more.
Just wondering ....
Why is there a *squeeze bulb before the pump ?

I assumed that the squeeze bulb was another way of primering the carbs.
In that case, is the primer circuit not separate from the fuel supply ?