I was trying different carburetors on a Ryobi 30cc and was having high speed problems. I ran the engine on a test stand with nothing on the carburetor input. I noticed fuel coming out of the carburetor bore at high speed. It turns out that the prop wash over the carburetor was drawing fuel out (this is an engine with no reed valve and the carb and exhaust on the cylinder sides). I put a cup over the carb to block the direct airflow from the prop, and the problems went away. The original carburetor filter assembly would not fit on the new carb, so I made a baffle to keep the prop wash away from the carb input. The engine runs perfectly now, idle through full throttle. The carb has a 13.47mm bore, much larger than the original. These large bore carbs generally cannot be made to idle well, but now it is fine. I am sure most of you already know to keep the carburetor entry shielded from the airflow, but this is the first time I have had anything but the Ryobi 31cc with the rear mounted carb, so this was new to me. I even tried making a funnel to direct air into the carb as a poor man's super charger, but this made things much worse.

The bottom line is that you need to use the original carburetor shroud with the filter, or make up something to keep the airflow away from the carburetor. There should be static air at the carb input, just like on the original weed eater. Let the engine alone cause the airflow through the carburetor. I hope this helps for some of you (like me) that have limited experience with these engines.

This will solve the problems you encountered: http://www.horizonhobby.com/Products...=ZEN848ES08300

Trying to "supercharge" the engine with the slipstream just doesn't really work. Even if/when you got it figured out to balance the pressures and get good carburetor performance, the gain is only around 1% power at common model airplane speeds. A lot of the gains you see on cars with hood scoops actually come from feeding the engine cooler air, not the ram effect. If I remember correctly, you gain about 1% power for every 10 degree reduction in air intake temperature from whatever your baseline is...

AV8TOR

If I remember correctly, the Ryobi 30 is made more like the old Homelite 30?

My experience with Homelite 30's and I currently have two of them is that with running a 18 X 8 prop, a 11.11 mm carb is about all you should have on one.

The gain beyond using a 9.52mm carb is only about 150 rpm.

Larger than the 11.11 mm carb is prone to increased sensitivity of the surrounding air flow and pressure at higher throttle settings.

To even run a 12.7 mm carb, you need increased timing and a smaller prop to increase the rpm to take advantage of the larger carb.

The 13.47 mm carb is the size that are on my BME 50 and Brison 3.2 engines.

If you make your own velocity stack how should the inlet be situated? Cut at an angle and facing into the airstream, or the angle shielding the inlet from the airstream? Or, should it be cut off at 90-degrees. It seems to me that you would get a pitot effect that would actually suck instead of force mixture down the throat with a 90-degree cut. Does a trumpet shape actually do anything?

I have notticed that piston ported engines spit a little mixture back out. I don't see any cure to that, other than a reed valve design. Would a really long velocity stack, or intake manifold, do any good on the spitting fuel issue or are there consequences that would negate the thoereitical gain in effeciency?

The aluminum ones from Tower are cut at an angle. You can try them turned different directions. The base plate is just a clamp device helb by the carb screws

I have three of these and they all worked facing the wind.

The trumpet shape improves the air flow to reduce the turbulance of the incoming antake air.

The intake venturi or stack tends to "collect" the fuel spitback and allow it to be sucked back in to the carb on the next intake. So, the fuel economy is increased and raw fuel on the airframe is reduced.

Regards, Richard

Yep...

And you really will notice the improved fuel economy and less fuel spit back over the airframe.

Apparently the reeds in a reed valve engine don't completely stop reversion, because I have seen a very noticeable benefit in running a stack even on reed valve engines. A theory on that might be that it really isn't reversion that is occurring on reed valve engines, but the slipstream actually sucking fuel out of the carb itself. In any case, velocity stacks are a good idea...

In some cases/installations, you can't get the carb to go rich enough without a stack on it.

AV8TOR

I ended up just using the plastic cap from a can of spray paint. I cut it to fit the carburetor and put a piece of open cell foam in the large opening. This worked well, any fuel that happened to back out of the carb was sucked back in. Having a filter is an added advantage for me, since I fly out of an old dusty cow pasture.