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Old Jul 14, 2012, 11:04 PM
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USA, TX, Grapevine
Joined Dec 2008
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Tinkering with a SV 17cc RC gas engine

Today I got to test run my poor SV 17cc engine which has some problems. One problem was it ran way too rich and the needle valves were essentially useless at dialing it in. But when I examined it more closely I found that it had a Walbro WT694 carburetor on it, which has a 12.7mm venturi in it. So I don't know if someone had swapped out the carb earlier or that was the carb that came with it. But in anycase I think the intake bore of 12.7mm is way too large for a 17cc engine. I think that a carb with a 9mm bore would be better for the engine.

So I rummaged around in my parts boxes and found a Walbro WT520 carb, which has a 9.53mm venturi bore in it. But when I tried the carb on the engine, it would barely pump any fuel at all. So when I checked it has a stupid blue plastic pump diaphragm on it. So I visited out local small engine repair shop and bought a rebuild kit and put in a black rubber pump diaphragm instead. Now when I test ran the engine the carb started working like it should work. Now the engine had a more linear throttle response and the high speed needle would actually adjust the mixture too. The previous big bore carb was at full throttle way before the carb throttle was at full open. But now with the smaller bore carb the engine was at full throttle when the throttle plate was fully open.

So now the performance was a lot better as I got almost s 1,000 RPM improvement over what it was doing slobbering rich with the oversize carb on it.
With a 13x6 Master Airscrew propeller I tached it at about 9,800 RPMs full throttle. With a 14x6 Master Airscrew prop I tached it at a little over 8900 RPMs. Now granted using my Model engine power calculator spreadsheet is only showing the engine developing a little over 1 hp means it is not going to be a powerhouse of a engine. But it is useable for certain planes or applications. The engine develops power about the same as the old baffled piston .60 engines we used to use, such as the Veco .61, K&B .61, Fox Eagle I, Fox Blue Head .60, HB .61 and so on. So choosing a airplane from that era that these .60 engines came from would have the size about right then. Of course maybe some of the more modern .50 size plane we have today might work OK too.

The engine isn't a powerhouse as it uses the classic piston port induction, no reed valves. It uses the piston skirt to control the intake port into the crankcase. The port timing is also mild. One could reverse the ignition timing and have the engine run in reverse equally as well as it does forward. But that capability also causes it to lose power though.

The Walbro WT520 carb may not be the best carb to use. But it did have what I thought was the more proper inside bore size for the intake venturi being it is a 9.53mm sized venturi. But there are many carbs with intake venturis around that same size too. One of the other carbs may be a better choice too. The carb does not have a choke on it, but it does use a remote primer bulb though. Except you need to use a third fuel line into the fuel tank as the overflow will travel through the remote primer bulb too. So you don't want to dribble fuel on the plane as the engine is running. Also without a choke you could have problems on cooler days getting it to start, warm up and run, before you can go full throttle and fly it. With a large prop, using a "smart thumb" might be dangerous to your fingers. But I am thinking about bolting on a slide choke setup I took off another trimmer engine a while back. This is a simple sheet metal affair that lets you slide a piece of metal across the intake to serve as a choke. Also I needed to drill and tap a hole in the pump cover as the engine uses a external crankcase pressure line to furnish the pulses for the pulse pump in the carb. Note that I also put on a machined metal throttle arm onto the carb too. This needed me to cut off part of the idle speed adjustment bracket, which isn't a big deal if you use the throttle trim or end point adjustments for idle speed.

So anyway, now I am motivated to make a exhaust tip baffle to insert into that huge gaping exhaust outlet on the muffler. The engine is just too loud like that with it open. Then I'll have to decide on a plane to mount it on too. So it might be a while before I use the engine on something.

SV 17cc engine test run

SV 17cc engine with 13x6 Master Airscrew Propeller on it.

SV 17cc engine with 14x6 Master Airscrew Propeller on it:

Here is a video clip of me trying out the throttle on the WT520 carb used on the engine.
Running a SV 17cc engine after replacing the carb with a WT520 using a 13x6 prop (1 min 23 sec)
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Last edited by earlwb; Jul 15, 2012 at 07:59 AM.
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