RhesusMinus

I converted a Stihl Fs36 weedeater (same as Ryobi 31cc) some years ago, and I have just started to play with it. I have done a lot of modificatios to it, to find some more power. I did some bench runs, and I got 8600 with MA classic 18x8 and 7600 with MA classic 18x10. I quite happy with the result, but there is something that I cant figure out. The engine is fourstroking beetween approx 50-70% of throttle, after a sec or two it pulls hard to reach max rpm(starting from idle and full throttle). I have read som threads and it seems like this is common with this engine, any ideas how to solve it?. I have a bigger carb 11mm venturi, my first thought was that the carb was to big, but it doesnt seem so. The engine can idle all day long at 2000 rpm. It is not that important that I get a very smooth throttle, since I´m going too use it on an airboat, but I like it to be 100%. Any ideas?

captinjohn

Try a carb off another engine that has a rotor barrel type of carb. Some are very smooth through the entire RPM range. Capt,n

shaunmichaeljohnson

i also run gasser airboats but why is it important to get a proper 2-stroke transision as long as it dosent stall nad has good top end???
my boat 4 strokes a bit on the throttle up, i have a 28cc....er...."talon tools" engine with 11.1mm carb.

are you involved with any RC airboat forums other than on RCU?
shaun

RhesusMinus

Thanks, I have several carbs to try, will do some tests to morrow.

As I say in my post, there is not that important to have a smooth transition from idle to full throttle, but I would like to make it as smooth as possible. I will probably never use it bellow half throttle on my boat, except when launching.
It will never stall, neither at idle or full throttle, only hesitates a bit when going from idle to full throttle. I am quite happy with the topend so far. I am more into "normal" gas boating, it is my first airboat, so I dont know of other forums yet, but I am open for suggestions.

shaunmichaeljohnson

have you tryed fideling with the low end needel valve on the carb if it has one?
i would lean out the low end a little and see what happens
shaun

captinjohn

You can also try different props. The load at 60 % throttle may not be right. Just like a chain saw ...they will smooth out more in more of a cut. Capt,n

RhesusMinus

I have tried almost every carb settings, if I lean the L needle more, it will lean out on idle, richer will cause the engine to load up, so I think I am very close with the setting I have. I have only tried 18x8 and 18x10, the behaviour is almost identical. Could a posible cause be reed flutter at some rpms, I guess I know more after I have tried an another carb tomorrow.

RhesusMinus

I tried a new carb yesterday, WA 106 with 28/64 venturi, worked better, much smoother transition from idle to full. The other carb had 27/64 venturi. The reed broke after a while, so I have to make a new valve, but othervise I´m quite happy with the engine.

captinjohn

Sounds good! Let us know how that new reed works out. Thanks Capt,n

RhesusMinus

The new reed works very well. I used 0.15mm shim metal, it seems much stronger than the sheet metal from the floppy that I used first. I made the moussecan pipe a couple of centimeters longer, and the power increased even further. 7840 with MA 18x10 and 8900 with MA 18x8 as the highest readings, the transition from idle to full throttle is pretty smooth , idle is about 2100+-100, so I am very satisfied at the moment. I just hope it will stay toghether for a while.

BB Modss

the WYK-33 (rotary valve) is a magical carb.. I run them on my honda genset,pit bike,all my marine motors,and cant find a flaw, the low side needle CAN be adjusted, but not very far,im working on a fix for this.