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earlwb's blog
Archive for July, 2012
Posted by earlwb | Jul 14, 2012 @ 11:04 PM | 8,407 Views
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.
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Posted by earlwb | Jul 06, 2012 @ 09:21 PM | 8,165 Views
One of our forum members "Gary Cee" was kind enough to make me a couple of Fox carburetor adapters for some other brand engines. I think he did a really excellent job as the adapter fit perfectly too. Thank you very much Gary Cee, I really appreciate it a lot too. This allows me to use the really good Fox carb on a engine that doesn't quite have as good of a carburetor on it. In this example it is a adapter for a ASP .61 engine. The OEM carb on the engine actually works OK, but the bore is too large. Thus you have to let the engine turn higher RPMs (smaller props) and use a more restrictive muffler, that provides extra back pressure for the fuel tank, in order to get good fuel draw out of the engine. Quite a few people ran afoul of the problem of a carb bore being a little too large for the engine when they used a Pitts muffler that had a lot less muffler back pressure for the pressurizing the fuel tank. They usually had to block off one of the exhaust outlets to increase the muffler pressure for the fuel tank, but even then it may have been somewhat marginal. But by using a carburetor with a little smaller bore to it, I can get good fuel draw for the engine, and it is much less sensitive to fuel tank location and also to needing extra muffler pressure. So I have more versatility and better all around performance with the engine. Besides the Fox carburetors are really good carburetors and well worth using on engines other than Fox engines too. The cost for a Fox carburetor isn't all that bad and quite attractive cost-wise when you compare the cost to some of the other brand name carbs as well. The flange design of the carb does work really well, in that it prevents warps from causing the carb to bind when you attach the carb using clamps or screws. But the flange makes it more difficult to adapt the carb to other brand engines though.









Comparing the larger bore of the ASP carb to the smaller bore of the Fox carb.