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Old Aug 10, 2012, 07:16 AM
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Canada, MB, Winnipeg
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Originally Posted by flypaper 2 View Post
Yeah, that's the one. Back then, I worked for a marina that had the Skidoo dealership. One of the salesmen raced the 245 RV , a 440 and 640 triple Blizzard and I kept them in racing form. After he was through with it at the end of the racing year, I bought it from him. The good old days

Gord.
The craziest thing we ever did was put a 775 blizzard motor into a little Elan for grass drags. This got my step brother a trip to the hospital with a broken arm on the first pass. Sled was demolished. The crazy good ole days

Ray
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Old Aug 10, 2012, 08:01 AM
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flypaper 2's Avatar
Kingston, Canada
Joined Jun 2004
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One of the guys that worked for me got a hold of a 640 with the centre con rod blown out and cut the bottom of the crankcase out of it. Just a slot in the bottom . He'd give the open bearings a shot of oil with an oil can and go out for a cruise running on the two outer cyls. Was no slouch even on two cyls. Lasted about a week.
Back to our previous program. Sorry guys.

Gord.
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Old Aug 10, 2012, 08:11 AM
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USA, TX, Grapevine
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Originally Posted by Nav_Aids View Post
Boy Earl you sure got a lot of testing in mind. I think the 3 needle Fox carb with muffler pressure will work the best. Earl how much counter balance did you have to add to the Stik to get the CG right with the Walbro up front?

Ray
Aww come-on the carb doesn't weigh that much.

None, it seems to still be about right. It went forward a very slight amount, maybe less than 1/4 inch. I am usually not a big stickler on getting the CG perfect on sport planes as long as it isn't tail heavy. But with a 3D plane it can be a PITA having to move the CG a little bit and then test fly it, move the CG and test fly, etc, then oops too far land fast!

Yeah I hope to find another three needle carb as I hate to use the one on my Fox blue head .60 engine as it is running really good on the Tiger .60 it is mounted in. It isn't going to hurt it, it is just re-adjusting the carb after I am done messing with it. I tend to leave the carbs alone after getting them dialed in and once in a while I have to tweak the main needle a little when the weather changes.
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Old Aug 13, 2012, 06:56 AM
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I didn't get very far this weekend with the engine running right on the plane. I messed with it for several hours Friday, Saturday and Sunday too. But someone later was kind enough to notice and tell me that I had the fuel line and the external fuel bulb hooked up to the carb backwards. Makes one humble sometimes when they do something stupid. I am glad I didn't stick my hand in the prop or something. Amazingly enough I almost got it to run Ok like that too. The needles were all the way out though. As soon as I get done feeling like a doofus, I'll try it again.
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Old Aug 13, 2012, 09:48 PM
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After being somewhat dyslexic over the weekend, I plugged the fuel lines in correctly and the engine started running like it should, and it was a straightforward process getting it all dialed back in on the needle adjustments.

Here is a short video clip of me messing around with it after getting it fairly well dialed in once again. I'll charge up the batteries and see if I can slip away from work a little early and fly it this week sometime.

Running NGH 9cc gas engine with a Walbro Carb on my airplane (2 min 10 sec)
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Old Aug 13, 2012, 10:15 PM
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Australia, NSW, Ashby
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Damn that looks good Earl, going to have to fire up the lathe
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Old Aug 14, 2012, 04:51 AM
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Oman, Masqat
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earl, are you planning to sell those carb kits? i would love to get my engine working properly, what pump are you using?
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Old Aug 14, 2012, 06:18 AM
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Thanks.

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Originally Posted by Boostoman View Post
earl, are you planning to sell those carb kits? i would love to get my engine working properly, what pump are you using?
I am currently running a Walbro WT-298 carburetor on the engine, it has the fuel pump and regulator built in. The carb has a 7.14mm venturi size. A Walbro WT-583 or a Walbro WT-629 and several others in that size range would likely work OK too. I have the white color teflon pump diaphram installed on the carb too. I did try the regular black rubber pump diaphragm but I somehow ruptured it or something when I had the fuel lines reversed. The plastic blue color diaphragm seemed to work OK when I test ran it on the test stand.

I didn't make the Walbro carb adapter, one of our forum members Gary Cee made it for me. You'll need to ask him about it. Thank you very much Gary Cee, I really appreciate it. You can PM him from his blog page http://www.rcgroups.com/forums/member.php?u=310840

I put the pics of it all over in my blog here:
http://www.rcgroups.com/forums/showthread.php?t=1706168
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Old Aug 14, 2012, 06:34 AM
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Australia, NSW, Ashby
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Gary Cee, would you mind putting up the specs for the adaptor, also the material you used fabricate it with. Just tapping into a known resource rather than re inventing the wheel as I am keen to put a walbro carb on mine Probably use a WT 526 which is an 8mm venturi but will keep options open at present.
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Old Aug 14, 2012, 06:39 AM
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Originally Posted by Capt. Quirk View Post
Earl, what diameter is the throttle bore? Also, you may get more consistent
tests results if you removed the primer and just choke it with your finger.

Based on your test results, the fuel spitting up might be fuel from the primer, or
the venturi might be too small, or the spring rate might be too little (not enough
pop-off pressure). Including the rich throttle response after chopping the throttle.

Another cause could be too much intake timing if it was spitting fuel with the
original carb.

If you have a carb with a larger diameter venturi, it may be easier to narrow down
the best set-up, because it is easy to make a venturi smaller.

There are 2 types of diaphragm fuel pumps, 2 stroke, and 4 stroke. 4 stroke
pumps incorporate a spring on one side of the diaphragm, just like the original
NGH carb did when it had 2 plates.
Bill M.
The WT-298 carb has a 7.14mm venturi in it. The carb doesn't have a choke built into it. But the carb has the high speed jet located about 1/4 inch from the top, couple that with the more aggressive port timing on the engine, and it'll spit fuel pretty good. It did spit fuel good with the original OEM carb setup too. So some sort of a intake stack or velocity stack works pretty good. My intake stack is a little large though. But that happened to be the one sold by Great Planes so I didn't have to make it. But I think a stack made smaller, more suited to the carb size would work better and I don't think it has to be so tall. I sorta like the primer bulb on the carbs. I need to whip up a mounting plate for the thing though.
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Old Aug 15, 2012, 07:58 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.

The temperatures were hot enough for the gasoline to vapor lock inside the hand crank fuel pump, if you left the gasoline container sit in the sun and get heated up. So I had to keep the container in the shade, albeit it wasn't much more cool in the shade though. But the Walbro carb didn't seem to be bothered by it at all. Without a choke on the carb, you have to warm up the engine good before you try going full throttle, or flying it. I ran the engine today without the external fuel primer bulb, but I will likely have it mounted on the plane later. I haven't decided if I really need it or not yet.

One thing is the engine would sometimes stall out or die if I cut the throttle after running it for a long time. I haven't decided it it was the heat causing the engine to run more hot, or whether I should have run it a touch more rich on the high speed or not. The carb does have a flat spot or dead spot where the carb switches from the low speed jet to the high speed jet. If you let the throttle sit on that dead spot the engine will die out too. But so far I have seen this effect on all of the Walbro carb engines I have messed with. Usually you richen up the low speed more and it moderates that effect, but you can't completely eliminate it. Since the Walbro carbs were not really designed for RC airplane use, but for utility engines there is not much one can do, unless they can come up with a mid range jet mod of some kind.

So now that the Walbro Carb conversion worked out really well. It is on to how long will the engine last before it gets worn out or something breaks.










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 Aug 15, 2012, 10:33 AM
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Istanbul, Turkey
Joined Aug 2004
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Good news Earl,happy flyings.
May I remind you to check/readjust the timing with a tach. to stay safe side for conrod.
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Old Aug 15, 2012, 10:53 AM
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United States, MI, Marysville
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Originally Posted by turk1 View Post
Good news Earl,happy flyings.
May I remind you to check/readjust the timing with a tach. to stay safe side for conrod.


Excellent tip !
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Old Aug 15, 2012, 11:31 AM
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Riverside Calif.
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Nice Job Earl,

The engine sounded great, did your peak rpm increase?

You started the engine with your hand, were you able to do that before?



Quote:
Originally Posted by earlwb View Post

One thing is the engine would sometimes stall out or die if I cut the throttle after running it for a long time. I haven't decided it it was the heat causing the engine to run more hot, or whether I should have run it a touch more rich on the high speed or not. The carb does have a flat spot or dead spot where the carb switches from the low speed jet to the high speed jet. If you let the throttle sit on that dead spot the engine will die out too. But so far I have seen this effect on all of the Walbro carb engines I have messed with. Usually you richen up the low speed more and it moderates that effect, but you can't completely eliminate it. Since the Walbro carbs were not really designed for RC airplane use, but for utility engines there is not much one can do, unless they can come up with a mid range jet mod of some kind.

Originally you mentioned it was running rich when you chopped the throttle, what fixed that characteristic?
Finding the answer for the engine running lean now depends on the changes you made up to this point.


The "dead spot" your referring to exists on nearly every carburetor type, they incorporate all kinds of strategies
to overcome that rpm range where manifold vacuum is rapidly decreasing and the negative pressure inside
the venturi has not dropped enough to draw fuel out of the main jet.

There isn't enough air speed or velocity flowing through the carb. Because of Bernoulli's venturi effect the
pressure isn't low enough to increase the rate the needle and seat opens and closes.

Another way to look at this is by visualizing a float level. You must consider gravities effect on the fuel inside
the float diaphragm because the carb is mounted vertically. At this particular rpm range, the fuel level is only
high enough to supply the low speed circuit,and when the venturi pressure drops enough the fuel level raises
to the height of the main jet circuit, and this occurs because the rate that the needle and seat opens and closes
increases too. If the carb was mounted horizontally, with the diaphragm on the top, the lean spot may be considerably
less noticeable. Because of gravity, the fuel would cover the entire float chamber surface, and therefore cover the
main circuit too.


Overlapping the 2 circuits is the most obvious. Many high performance carbs (are usually a poor designed modification)
require significantly more fuel on the low speed circuit adjustment, and a proportionally leaner high speed adjustment.
The total fuel delivered to the engine is the same, but this method makes more HP.

*** But this technique is also used to get the most performance from an engine, when using a high pop-off pressure. ***
Another example of this method, is used on Mikuni Super BN carbs. If you have a 65gram spring (low pop-off psi),
75 low jet and a 125 main jet, the total of the 2 jets is 200. Any combo of jets (with the proper spring rate) will supply the
same amount of fuel.

For example: Increase the low speed jet to 120, use a 115 gram spring (hi pop-off psi) and an 80 main jet. The same
amount of fuel is still being delivered, but from different circuits.


Another method is to drill the the upper most low speed bypass hole. This is a great mod for applications that
need better throttle response and acceleration.


Another method is to reduce the pop-off pressure, preferably by using a spring with a lighter spring rate.
(But cutting, or crushing the spring, raising the lever position is the quickest and easiest methods) You
should always test the pop-off pressure with the carb fully assembled after you make a change.


One other thought. That carb has the potential of dividing up the low speed operation and the
starting - idle operation. It would require a 3rd external needle added to adjust the starting and
idling circuit, and of course another hole drilled into the carb.

Fun Stuff



Bill M.
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Old Aug 15, 2012, 11:42 AM
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USA, TX, Grapevine
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Thanks all.

Capt Quirk,
Yes I could fire it up by hand although the electric starter is easier.
The peak RPMs stayed about the same, but I don't have to richen up the engine a lot to compensate for the poor fuel draw at WOT. The OEM carb copied the way the Saito carb worked. But the Saito carb is at the rear of the Saito engine right next to the fuel tank not in front.

Turk1,
Yes that is a good idea. But since the drive washer can't slip on the crankshaft, they machined in a couple of flats, the only problem would be getting it on 180 degrees off or having the hall effect sensor slip some where it is screwed on. So we only have a small amount of timing adjustment that we could make without drilling new holes for the sensor.



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