SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Nov 07, 2014, 09:35 AM
Registered User
Canada, NB, Fredericton
Joined May 2002
941 Posts
I haven't mounted/run my GT9 yet, but I'm surprised that the pump unit is that sensitive to heat and vibration causing vapor bubbles. Piston port induction engines with carbs mounted onto the side of the cylinder were pretty common, and AFAIK didn't suffer from the carb being heated and vibrated by the cylinder right next to it.
Perhaps the bubble issue is less related to the gasoline vaporizing, and more caused by thermal expansion of the metal parts making the gasket fit worse???

D.
darkith is offline Find More Posts by darkith
Reply With Quote
Sign up now
to remove ads between posts
Old Nov 07, 2014, 10:15 AM
Registered User
earlwb's Avatar
USA, TX, Grapevine
Joined Dec 2008
13,373 Posts
That is quite possible. But yes I have been off and on studying it too. I think Walbro did some rather sophisticated R&D when they developed the carburetor. When NGH separated the throttle from the pump and regulator, they apparently missed something in the design. It is probably some amazingly subtle things inside that didn't get done because the designer didn't understand what they were trying to copy and didn't include those things.
earlwb is offline Find More Posts by earlwb
RCG Plus Member
Latest blog entry: My new Gaui X5 Rc Helicopter
Reply With Quote
Old Nov 07, 2014, 01:16 PM
Radiodel-hooooo
Chophop's Avatar
Pleasant Valley Modelport
Joined Sep 2006
8,893 Posts
I have been thinking for a few years that a micro BLDC turbine type electric fuel pump with an adjustable relief valve would be nice. If I weren't busy trying to make a (better) stove again then I could have done it. A BLDC system from those micro planes might be a start.
Chophop is online now Find More Posts by Chophop
Reply With Quote
Old Nov 07, 2014, 02:06 PM
Registered User
earlwb's Avatar
USA, TX, Grapevine
Joined Dec 2008
13,373 Posts
That could be pretty neat. A little electric fuel pump with a regulator.

My theory about the NGH pump/regulator unit was that there is high pressure fuel on one side of the regulator valve and low pressure on the other side. This is where the fuel pump fills the regulator chamber with fuel and the diaphragm expands letting the valve close from spring pressure. Now on the low pressure side we have the engine's carburetor drawing fuel which helps reduce the fuel pressure on that side of the valve. Now since gasoline vaporizes more readily than glow fuel does, it tends to form gas bubbles. Unfortunately gas bubbles cause a lean engine run situation.

Normally with a regular Walbro carburetor, you don't see this problem as the distance from the regulator valve to the fuel passages and orifices or jets is so short. Thus the gas bubbles don't have time to form up and aggregate. Same thing for the glow carbs too. The main needle valve is right there in the carb in the spray bar.

You can see the effect with the remote needle valves on model engines too. When it is summer time you can start to see bubbles forming in the remote needle valve unit on the low pressure side leading to the engine. It tends to happen mostly in the summer when it is hot though. The hot fuel tends to vaporize more easily of course. There were some engines that had the remote needle valve integrated into the backplate and those engines couldn't be used in the summer down here in the south, unless you separated the remote needle unit from the backplate where it was getting heated up good.

Anyway, my thought was the pump cover isn't putting enough pressure on the gasket to seal it well in some cases. Sometimes if you have it you can use a real Walbro pump cover, and usually you can tell it is putting more pressure on the gasket seal around the edges. Or you can shave a few thousandths of a inch off the center post to let it put more pressure on the gasket. That stops a leak of fuel under pressure and also helps prevent air from getting in.

Now as to the bubbles forming and coming out of the regulator side of the unit. There must be something going on with the holes sizes, angles for the valve or something that tends to cause it. But then maybe I am way off in left field over thinking it.

I did get mine to work well enough to fly planes with though. But when I idle a engine, I can still see some bubbles forming and coming out of the pump/regulator. But at higher throttle settings or full throttle the bubbles don't form or if there are some bubbles they are real tiny and don't pose a problem.

Now there is the pesky issue of getting air out of the pump/regulator for the first flight of the day. I have no solution other than to run the engine and hold the plane at various attitudes and angles to work out the air bubbles. Sometimes it kills the engine and you have to restart it. But not draining the fuel out of the engine and pump/regulator helps a lot. I just stop the engine, drain the fuel tank and plug the vent. That keeps the pump/regulator air free until the next day I go flying.
earlwb is offline Find More Posts by earlwb
RCG Plus Member
Last edited by earlwb; Nov 07, 2014 at 02:08 PM. Reason: add more info
Reply With Quote
Old Nov 07, 2014, 03:18 PM
Registered User
Canada, NB, Fredericton
Joined May 2002
941 Posts
--speculation--
I believe most (all?) Walbro carbs have a very fine mesh in the high speed needle feed reservoir (under the welch plug, not the coarse intake filter), which supposedly serves to trap/break up bubbles by the capillary action drawing fuel to the mesh, and holding back large bubbles through surface tension.

Perhaps this is what the NGH pump/carb is missing, and the resultant bubbles are caused by a variable combination of:
-leaking gaskets (thermal deformation of case, fuel degradation, poor tolerance)
-gasoline vaporizing (negative pressure in fuel reservoir if drawn exceeds pump/float needle supply delivery, excess heating of pump)
-low regulator reservoir supply (mis-adjusted regulator needle, soft metal of needle valve arm bending, wrong spring)
-air leaking through pump and/or regulator membrane (poor quality and/or fuel degradation depending on local blends)
and without the fine screen to break up the bubbles, the engine suffers from these bubbles of gas/air vapor.
--end speculation--


Anyways, there could be multiple reasons for the source of the bubbles, and that might explain why different people end up finding a different fix that eliminates/reduces the bubbles for them. But, the overal source of the problem might be whatever fine detail NGH missed in the fuel system.

I should mount up my GT9 and give it a spin someday...but winter is approaching rapidly.

D.
darkith is offline Find More Posts by darkith
Reply With Quote
Old Nov 07, 2014, 04:57 PM
Registered User
earlwb's Avatar
USA, TX, Grapevine
Joined Dec 2008
13,373 Posts
Now that is a interesting thought. I'll have to see what I have for a fuel filter and put that in between the pump regulator and the carburetor and see what happens. That might be it.
Thanks for the idea or thought.
earlwb is offline Find More Posts by earlwb
RCG Plus Member
Latest blog entry: My new Gaui X5 Rc Helicopter
Reply With Quote
Old Nov 07, 2014, 08:06 PM
Radiodel-hooooo
Chophop's Avatar
Pleasant Valley Modelport
Joined Sep 2006
8,893 Posts
I studied cavitation and it takes a LOT of vacuum to make much of a bubble but makes lots of them. Most dissipate but the ones left behind can join up. Still nothing to what I see from engine vibration. Smaller sized tubing helped me. I did observe the bubble with 1/8 inch line but not much at all with this pink-orange 3/32 stuff.

The gas shoots quickly up to the carb on dry starts with the 3/32 stuff also. It's not as permanent as Tygon but it's only $1/foot. I see it hardens and can crack after a year or two.
Summing it up, it looks like oversized fuel line can cause it also.
This stiff stuff might be a good idea for pulse line too.


Looks good to me
--speculation--
I believe most (all?) Walbro carbs have a very fine mesh in the high speed needle feed reservoir (under the welch plug, not the coarse intake filter), which supposedly serves to trap/break up bubbles by the capillary action drawing fuel to the mesh, and holding back large bubbles through surface tension.
Chophop is online now Find More Posts by Chophop
Reply With Quote
Old Nov 08, 2014, 08:18 PM
Registered User
Canada, NB, Fredericton
Joined May 2002
941 Posts
Quote:
Originally Posted by earlwb View Post
Now that is a interesting thought. I'll have to see what I have for a fuel filter and put that in between the pump regulator and the carburetor and see what happens. That might be it.
Thanks for the idea or thought.
It's worth a shot. I imagine it would have to be fairly fine mesh though...I don't think I have anything that fine on hand.

I'm assuming it still allows the vapor/air to progress through, just probably at a more moderate rate and mixed with more fuel, so it doesn't cause a big lean event.
Probably won't help if the pump is supplying an insane amount of bubbles, so reducing those to a reasonable level is still probably required...

D.
darkith is offline Find More Posts by darkith
Reply With Quote
Old Nov 09, 2014, 05:24 AM
wdy
Registered User
United States, NC, Spruce Pine
Joined Aug 2012
125 Posts
NGH GT9 9cc Gas Engine With Rcexl CDI Ignition

I have a new in box for sale if anyone would want one

NGH GT9 9cc Gas Engine With Rcexl CDI Ignition

http://www.rcgroups.com/forums/showthread.php?t=2281428

Thank you
wdy is online now Find More Posts by wdy
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
For Sale Prop Chart and Thread finder for any Glow or Gas Engine unclegeorge Aircraft - Fuel - Engines and Accessories (FS/W) 2 Dec 10, 2011 12:31 PM
For Sale Prop Chart and Thread finder for any Glow or Gas Engine unclegeorge Aircraft - Fuel - Engines and Accessories (FS/W) 2 Oct 14, 2011 12:44 PM
For Sale Prop Chart and Thread finder for any Glow or Gas Engine unclegeorge Aircraft - General - Miscellaneous (FS/W) 0 Oct 07, 2011 11:46 AM
For Sale Prop Chart and Thread finder for any Glow or Gas Engine unclegeorge Aircraft - General - Miscellaneous (FS/W) 3 Sep 24, 2011 12:25 AM
Sold NGH GT17cc gas engine NIB ibcinyourc.com Aircraft - Fuel - Engines and Accessories (FS/W) 4 Jun 30, 2011 11:35 AM