Sizing OS carbs to model four stroke engine

[amoretti]

Hello, this is my first post so be gentle!

I wonder if anyone knows about what size OS carb would be appropriate for a 30cc Wall single cylinder engine? I have built two carbs for it over the years and neither perform as well as I would like. Since the engine is to be a test mule for magnetos I want to eliminate the carb variable. I chose OS carbs because they seem to be the most common.
Second where is the best place to buy the OS carb??

TIA, John in Wyoming

 

[gbritnell]

A short story for you. When I was building my Holt engine I built the carb that was included in the drawing set. Long story short it didn't work! What now? I had some model airplane carbs in my parts box but which one to use? First off, the air velocities of a slow running 4 stroke engine don't match those of a fast running airplane engine, 2 stoke or 4 stroke, so the venturi size doesn't need to be anywhere near as large. Well then you say, how large? I can't give you the answer directly. My Holt has a 1.00 bore and 1.25 stroke and I built a carb similar to a model airplane carb that has a main bore of .312 with a rotating barrel bore (venturi) of .25. My engine runs quite well with it. Could it run better, maybe, but I don't feel like making a handfull of rotating barrels to experiment. My suggestion is to start off smaller than you think and work up. I have an expensive O.S. carb with lazer cut mid range slots, 2 adjustment needles and a mid-range adjusting screw. I have never been able to get it adjusted properly, not so much because of the venturi size but because the needles are made for alcohol/nitro which an engine uses at about 2:1 compared to gasoline. If you need any other information about my setup please ask.
gbritnell

 

[Rustkolector]

Most of the RC carbs have relatively large bore venturi's for the 4 stroke low to medium speed engines we often build. OS does make a few with close to a .180" bore which should be more than adequate. Check Tower Hobbies for info. They show venturi sizes on most of their carbs. Start small and go larger if you need higher speed performance.
Jeff

 

[mu38&Bg#]

If you are not putting a load on the crankshaft or your engine a very small carb will be sufficient. A 30CC engine at full power at ~5000RPM should be fine with a 7mm carb. If it runs faster a larger carb is needed like a model airplane engine at 8000 RPM like the OS FS-200 which has a ~9.2mm carb.

If you are loading the engine you need to match the carb to the fuel. Glow carbs needles are designed for glow fuel. I have used glow carbs with other fuels and usually you can adjust the high speed (WOT) just fine, and usually the low end for a decent idle and transition. Very often if you are running gasoline the midrange becomes rich because the metering needle taper is not appropriate. Some carbs are worse than others. If you are not using glow fuel stay away from carbs meant for helicopters, cars, or boats. Those are designed to handle a good amount of nitromethane and have to flow even more fuel so the situation gets worse with gasoline.

 

[hudmut]

Well i have not long just done a 25cc engine and am running a copy 7.5mm carb witch are a lot cheaper about 1/4 the price and you can get them from a company called just engines there is loads to pick from as in size and fitting but the body machines very well and work with all types of fuel hope this helps you