fuel coming out of intake

If I run my OS52 4 stroke at full throttle I feel some fuel being blown back on the intake side of the mount. It is mounted horizontal. I assume it is raw fuel coming out of the intake. Is the intake valve open too long or something wrong with the seat. Seems to run ok from idle to about 75 percent throttle. Weak springs..?? out of adjustment. I am going to adjust the valves and see what it does. Jim

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jim breeeyar
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I take it that these engines have a crankcase vent line. You don't have it plugged, do you?

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Does the engine run properly at 100% throttle? By that I mean are there any tuning problems or running problems other than the symptoms you describe?

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Paul McIntosh


I have quite a collection of 4 cycles and a few 2 cycles. I have seen this caused by eight different conditions that I can remember. There are probably more, but they haven't bitten me yet.

1) Using muffler pressure forced too much fuel to the carb and the mixture actually got richer as the rpm increases. If this is coupled with #3 below, the extra fuel is sucked out of the carb. I restricted the pressure line.

2) The spray of fuel is actually coming from the fuel tube at the carb nipple. The muffler pressure caused the feed line between the tank and the nipple on the carb to expand slightly under pressure and fuel leaked between the inside of the fuel tube and the outside of the carb nipple. No effect on the running of the engine, but lots of fuel on the firewall. I installed a double loop, wire spring clip on the tubing at the nipple. I use clamps on the tank fittings and this probably saved me from fuel in the fuselage. This is the last time I used the thin-wall tubing supplied with some ARFs.

3) The mounting position of the engine and the configuration (no cowl) caused air to flow from prop blast to flow across the top of the carb and suck some of the fuel from the throat. This showed up as a wet firewall and limited RPM because of a "lean" condition. I used some plastic tubing from Home Depot (plumbing dept) that was a press fit over the intake of the carb and used my heat gun to form the plastic tube into a "ram". This made the high speed needle difficult to adjust. I used some twisted ss safety wire to clamp it on the carb lip. It came off once in the air and the engine went supper rich quickly.

4) Leaks at the carb to crankcase mount (2C) and leaks in the intake manifold joints (4c). The little "O" ring between the manifold pipe and the carb body is easy to damage if you take the assy apart.

5) Fuel leaked around a old (worn from vibration) needle valve in the needle valve assy. I replaced the needle valve, but the inside of the assy was also worn and the leak continued. I had to replace the entire assembly.

6) The crankcase vent was partially blocked. The crankcase pressure at the bottom of the intake stroke must have caused "reverse blow-by" and prevented the full shot of fuel from entering the cylinder. It took a while to find this one - I checked the crankcase vent because I was not getting above about 70% rpm. Cleaned the vent and both problems went away.

7) The intake valve was not seating because of some carbon.

8) On both a horizontal and inverted mount O.S. FS .91 - the engine ran fine at idle and high rpm. But, I couldn't seem to get a real clean transition. There was always extra fuel from residual pressure in the flex-tank when dropping to idle quickly and when shutting down. The fuel would "pool" in the manifold just enough to cause the problem. I replaced the "flex" tank with a rigid tank and this reduced the problem.

Well, there are eight things to check. Good luck.


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jim breeeyar

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