WELL I GOT MY DECATHLON OUT YESTERDAY FOR THE FIRST TIME. ENGINE RAN GREAT ON THE GROUND. I TAXIIED UP AND DOWN THE RUNWAY SEVERAL TIMES AND IT RAN FLAWLESSLY, I SPED UP THEN CUT THE THROTTLE WITH NO PROBLEMS AT ALL. PROBLEMS STARTED ONCE IT GOT INTO THE AIR AND UP TO SPEED. THE ENGINE WOULD FOUR STROKE.. AS THEY CALL IT. I BROUGHT IT DOWN AND DID SOME MINOR ADJUSTMENTS TO THE CARB WITH THE SAME RESULT IN THE AIR. I BELIEVE WHATS HAPPENING IS THE CARB INTAKE IS LOCATED OUTSIDE OF THE COWL, I THINK THE AIR GOING OVER IT IS SUCKING OUT THE OXYGEN MAKING IT RUN RICH IN THE AIR CAUSING THE 4 STROKING. IT HAS SOMETHING LIKE A VELOCITY STACK ON IT BUT ITS REALLY JUST A STRAIGHT TUBE OFF THE ORIGINAL CARB INLET. WOULD IT HELP IF I CUT ONE SIDE DOWN AND FACED IT INTO THE WIND SO AS TO CUP THE AIR FORCING IT INTO THE CARB NOT ALLOWING THE CARB TO RUN RICH? OR DOES ANYONE HAVE ANY OTHER IDEAS ON WHAT I CAN DO TO FIX THE PROBLEM? THANKS

Your thoughts are most likely correct. Cut the tube off at an angle. Start out with the cut down side facing the front but you may have to turn it some one way or the other to finally achieve your goal.

If you continue to have trouble, two cycle gas engines when unloaded will often break into 4 cycle. You could try leaning your mixture some and/or changing prop size.

Good luck,

Jim

The velocity stack may help, but I never have seen much difference, other than keeping the carb from spitting fuel.

If the diaphragm vent hole is into the air stream. Solder a piece of brass tube over the hole. You may have to put a new hole in the plate (center) and seal the other one off.

Attach a piece of fuel tubing to the brass tube, and put the end through the firewall and into the fuse, so it will have undisturbed air.

This picture was not taken for this purpose, but you can see the tube in it.

how big of an engine is that? mine is a 40cc. the vent hole on it is probably about 1/2 inch. which is much bigger than what i see on your setup. am i talking about the same thing here? if not please tell me what im looking for..

after further looking into my carb opening it appears to be around 3/8 inch wide. the brass tube thing could be a possibilty but wont look too pretty being as it will be seen outside of the cowl.. the fuel line will also have to be of a larger type.. i was wondering though if maybe some sort or foam filter over the carb opening might work? wouldnt it keep the air from causing a suction inside the carb? i know it will also restrict airflow too but just throwing around a few ideas.

The vent hole is in the square steel plate cover beside the needle valves. it has a hole in it about 3/32 in. diameter.

hmm must be under the metal plate where the larger opening is.. i will take the plate off and look. so if i make that hole youre saying that if i line it into the fuse that it should take care of that problem??

well i took the carb apart quick.. i have a walbro carb but i cant find any vent at all around it other than where the velcity stack goes.. any suggestions?

The vent is a small hole in the steel plate that is on the side of the carb like was said previously.

Jim

I put a small plastic shield over this vent get it out of the prop blast. Some people run tubing from this vent hole into the fuselage like the previous picture shows. The vent hole is under the plastic shield in this picture.

Pointing the velocity into the prop blast is only going to cause you more problems. If anything face it away.

Wings is correct, in most cases you want the velocity tube to be angled to the rear, you do not want ram air into the carb. If the tube is withing the cowl, you need to experiment as to which way is best as the internal circulation will effect it. As to your problems where it first worked fine then failed after taxie tests, maybe the small screen in the carb is plugged up. This is the little fine mesh screen under the metal plate held on with the big screen right in the center. If the plane sat unused for awhile, this could have collecte a bit of trash. Remove it and scrub it off in fresh gasoline or alcohol then replace it. You might also need a carb kit (available at most lawn mower repair shops) which includes new flapper valves, screen etc.

I have a half dozen gas engines and they all run 200 to 300 rpm better with the velocity tube angle into the wind rather than away. There's no difference between running on the ground and flying except some unloading of the prop.

Good luck,

Jim

guess i have some experimenting to do then

WELL I HAD THE DECATHLON OUT AGAIN TODAY FINALLY.. I MADE A SMALL HOLE IN THE FRONT OF MY VELOCITY STACK FACED INTO THE WIND. RUNS FLAWLESSLY NOW. DOES NOT 4 STROKE AT ALL NOW THROUGHOUT THE WHOLE THROTTLE RANGE. I MUST SAY THIS 40CC ENGINE I HAVE IS A REAL BEAST COMING OUT OF A CHAIN SAW. IT HAS ELECTRONIC IGNITION AND IM USING A 20X8 PROP. WITH A PLANE THATS 16-17 POUNDS IT HAULS IT AROUND WITH AUTHORITY. ACCORDING TO THE THRUST HP PROGRAM IM GETTING 24 POUNDS OF THRUST AT 3.6 HORSEPOWER. NOW IF I CAN FIGURE OUT MY GLITCH PROBLEM IVE BEEN HAVING DURING FLIGHT EVERYTHING WILL BE FINE .. THANKS FOR ALL THE INPUT.. AT LEAST MY ENGINE TROUBLES ARE BEHIND ME..