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Old Aug 05, 2012, 09:49 PM
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Testing out the NGH 9cc gas engine Walbro Carb modification

I had modified my NGH 9cc engine to use a Walbro carburetor. After getting it all setup I do realize I had several other ways I could have done it. But I wasn't sure what the effects would be or how successful it might be either. So I sort of took a more simple approach to it. This carb I used is a WT-298-1 carb and it has a 7.14mm intake venturi in it, but the carb has no choke, so you have a harder time warming up the engine before it runs good like that. There are several other carb models that ought to work fine as well, and also some carbs with about a 8mm venturi size that might maybe work better too, but I haven't tried one myself.

All in all after getting the carb dialed in some, the engine was running pretty good, much better than how it behaved with the OEM two piece carb and pump/regulator. The Walbro carb exhibited no vibration sensitivity and was reasonably linear in its throttle opening from low to high speed. Now if you rapidly chopped the throttle from WOT to low, the engine had a tendency to flood out and die, but if you more gradually went to low throttle it allowed the regulator to work better and the engine would idle fine then. The engine temperatures I measured were about what measured with the stock unmodified NGH 9cc engine. I don't think it was running much more hot in temperature as the measurements were around the same at 376 degrees on the ground in a static engine run scenario. I assume the engine would run more cool up in the air flying around as the prop would unload more for the engine. The rest of the engine was running more more cool the farther down or way from the head you get to around 150 degrees at the bottom of the crankcase. Now this is at WOT on the ground in a static load situation though. The engine should be much more cool in the air flying around.

I did my test runs using a Zinger 11x7 prop as it was already reamed out to fit a crankshaft that was the same size as the Fox 1/2 inch crankshaft extender I used. I also used regular pump gasoline with about a 15:1 fuel to oil ratio and Amsoil brand 2 stroke oil.
The engine tends to spit fuel a lot and the main needle is near the top opening on the carb. I didn't have long enough screws to try a intake stack that I have, but I try it out next time as soon as I get some slightly longer screws. The carburetor did heat up some but not bad, a little above the ambient outside temperature.

First I fabricated a right angle water coolant fitting that was for a boat and converted it into a Walbro carb pulse pressure fitting. I then drilled and tapped the pump housing to use the pressure fitting. I think it would have been better to copy the same method they used on the MVVS engines, where they drilled a hole to match up with the carb pressure hole in the carb base. But I wasn't certain I had enough room to do it on the adapter that Gary Cee made for me. Thank you very much Gary Cee I really appreciate it too.









Another modification I did was I drilled tapped the backplate to use a larger crankcase pressure fitting (8-32 thread) in order to have the fuel tubing stay on better and also a larger hole for stronger crankcase pressure pulses going to the carb pump.


Then I reassembled the engine and mounted the carb.






A Fox 1/2 inch prop extender in 1/4-28 size fits just abut perfectly and it leaves plenty of room to clear the Walbro carburetor too.


And here it is, the engine is running and after dialing it in and experimenting with different pump diaphragms I found that it worked Ok with the more stiff blue diaphragm and also the white teflon diaphragm too. The engine needles are set a little more in using the teflon pump diaphragm. At first it took me a while before I got the engine to run as the carb doesn't have a choke on it, so I had to get the engine running and use a "smart finger" choke (carefully I might add) to keep it running long enough to let me warm up the engine and carb and get it dialed in on the needle settings too. At first I had to drip a couple three drops of fuel into the carb at idle (the throttle plate is almost closed all the way too) and run it until it ran the fuel off, and tweak the low speed needle and repeat, until I could get it to idle. Then I had to patiently wait and get the engine all heated up so it would run. From that point it was a simple matter to get it running good and have it perform well.





The fuel tank is set low and back some from the engine and I ran a long fuel line as well. The engine running wasn't affected by me moving the fuel tank or engine angle up or down. It stayed running OK without needing to tweak the needle settings. So the carb was pumping fuel all the time so the engine could hold a good needle setting without leaning out or running more rich.


The engine performance wasn't bad, the Zinger 11x7 prop tends to load the engine more than a Master Airscrew prop. But at the time I already had a Zinger prop reamed out to fit the Fox shaft extender and I didn't want to ream out a new prop for it yet, at least until I had run the engine some. The engine test stand throttle lever is a little too coarse for running the engine at a low idle speed, but I could get it down to about 2,500 rpms intermittently. That should be much easier with a servo controlled throttle on the engine. I also think that the OS AX55 Powerbox muffler restricts the engine a little more than the Mac's One piece AX 55 muffler too, as it seems to be down about 500 rpms in comparison with my other engine tests but that could also be the Zinger prop versus the Master Airscrew prop too.


I had leaned the engine out a little more and measured a peak head temperature at the base of the spark plug of about 388 degrees Farenheit. But when I richened it back up a little more, the head temperature dropped down to about 376 degrees F. The back of the cylinder head was measuring around 50 to 60 degrees less too. The tops of the cylinder fins were around 310 degrees F.





I had also tried out both the blue plastic pump diaphragm and the white teflon diaphragm and both worked Ok with my engine setup. The blue diaphragm did seem to run a little more weakly as I needed to turn the needles in a little more with the teflon diaphragm.

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Old Aug 05, 2012, 10:04 PM
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I ran out of time today in my engine testing as I wanted to try a old style Fox butterfly valve carb on the engine without a pump to see what happens and then with a Cline regulator and crankcase pressure venting. I also wanted to try a Perry pump out too. As well as running the Cline regulator and crankcase pressure venting with the stock OEM NGH carb too. But that is something for the next time to test.

Anyway this engine was running pretty good so I am inclined to swap it out with the other engine in my Ugly Stick and flight test it the next time I take it out to the flying field. Then I'll use the second engine to make these other tests with it.
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Old Aug 09, 2012, 01:45 PM
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Since the engine does spit quite a bit of raw fuel out of the carb when it is running. I thought I might try a intake stack on it out to see how that might work out. This example might be a little too big though it would work well. Anyway it is something to think about. It does block or obstruct the air flow more, so it is hard to say if it might cause problems or not.







A better version might be this short stack with a built in slide choke. It has a much lower profile and sits lower plus it has a choke lever too. I ordered one, so I'll try it out for size when it comes in.
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Old Aug 12, 2012, 10:52 AM
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I mounted the engine on my Ugly Stick. But when I ran the engine, it wasn't running well. It is like the fuel pump is weak and it isn't pumping the fuel good like it did on the test stand. So I am going to replace the base gasket, check the fuel tank lines and work forward to the carb, and check the pump diaphragm and fuel regulator out to see if there is something causing it to have fuel draw problems. I have to run the engine super rich to get it to run WOT with the nose of the plane pointing up. The external fuel bulb isn't sucking fuel through the engine either, which is a troublesome sign.







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Old Aug 13, 2012, 05:09 AM
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Hi earlwb,

On the test stand the clunk line is connected to rear nipple (pump side) of the carb, on the aircraft it is connected to the front nipple (regulator side), effectively bypassing both pump and regulator. Put clunk line on the rear nipple (with red cap on top picture of previous post), then it should work as on test stand.

Best regards
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Old Aug 13, 2012, 06:20 AM
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Quote:
Originally Posted by jmaderoy View Post
Hi earlwb,

On the test stand the clunk line is connected to rear nipple (pump side) of the carb, on the aircraft it is connected to the front nipple (regulator side), effectively bypassing both pump and regulator. Put clunk line on the rear nipple (with red cap on top picture of previous post), then it should work as on test stand.

Best regards
That was a good observation. I had been trying to get it to work right for quite a while and I even rebuilt the carb too. I cannot believe I did that either. Thank you very much for noticing that too.
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Old Aug 13, 2012, 09:44 PM
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I plugged the fuel lines in like they were supposed to go and the engine started running like it should run. Here is a short video clip after I had pretty much dialed it back in. So I'll charge the batteries back up and see if I can get to go fly it once again real soon.

Running NGH 9cc gas engine with a Walbro Carb on my airplane (2 min 10 sec)
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Old Aug 15, 2012, 07:52 AM
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I got to go out and fly the plane for five flights yesterday before our club meeting at the flying field. The engine performed well and ran great with the Walbro carburetor on it. The engine didn't need to be adjusted extra rich to compensate for poor fuel draw as the fuel tank emptied, so it ran just as good at the end of the flight as it did at the beginning. It was hot that day at about 105 degrees F. We also had a relatively brisk 15 mph crosswind breeze. The new grass on our runway has grown in really wel albeit it is getting thick and makes it harder for a plane to takeoff. Maybe I need some "grass" wheels on the plane.









Here is a short video of me flying the NGH engine with the Walbro carb on it. At least up until the video camera ran out of memory.

Flying the NGH 9cc engine with Walbro carb conversion (5 min 43 sec)
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Old Nov 26, 2012, 02:36 PM
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hello were you get the piece to mount walbro carb to,,,,thanks
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Old Aug 12, 2013, 09:21 AM
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Sorry for the late response. I don't notice your post until now. The Walbro carb adapter is made using a lathe. In this case a fellow forum member and friend made mine for me. He uses the forum name of Gary Cee here.
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