OS 91 FX inverted mounted problems
 

Hi Bax,

I am using a 91 FX in a 90 size patternship in which the engine is mounted inverted.I am using FAI Fuel (20 % castor oil and 80 % Methnol Fuel) with #8 OS plug. The only modification I have made to the engine is this that I have removed the silencer baffle to increase power.

The problems associated with the engine are:

It will never start in inverted position so I have to tilt the aircraft upright to start it each time although ample fuel is supplied to the carb------Why doesn`t it start inverted


When the aircraft is airbourne and upright the engine runs well, however when I do inverted flight ( i.e. engine comes in upright position) the engine leans out and picks about approx 1000-1200 rpm more than when it was running in inverted position and continues to run leaned for about 2-3 minutes even if I revert back to upright flight after that the rpm drops to the original upright flight rpm.

Best Regards,

Khizer

RE: OS 91 FX inverted mounted problems
 

Your starting difficulties are very typical of inverted-mounted engines. The glow plug is at the bottom of the setup, so it can accumulate liquid fuel, which doesn't burn well at all. You have to carefully make sure that the engine doesn't get too 'wet' when you're starting it.

If the engine changes the mixture setting when you roll to inverted flight, the location of the fuel tank is an issue. If the engine goes lean when you go inverted, it usually means that the tank's too high. When you roll inverted, it now becomes too low and the engine leans. It could also mean that you have to work on the airflow patterns in the cowl. Inverted flight may cause higher pressurees in the cowl, whcih would make the carburetor work better. Again, you have to do some detective work to find the answer.

RE: OS 91 FX inverted mounted problems
 

Bax,

I just forgot to mention that for better throttle response I am using OS 7L carb instead of 91 FX`s original carb because the acceleration and throttle response is better with 7L. This problem started when I switched to 7L. I have not yet adjusted the mixuture and it is at factory default settings. Could it be a mixture problem?

The reason for which I changed to 7L is this that my 61 SF has lightning fast acceleration and the FX series Carb is not as responsive as 7L. The throat and venturi dias are the same on both carbs.

Thanks

RE: OS 91 FX inverted mounted problems
 

You have to get the carburetor properly-adjusted first, and then see how things go. We can't assure you that the #7L carb will do the job for you.