I wonder if you can gain access to the input of the high-speed needle and bypass the pump entirely. ONly real application I can think of for a pumped engine is in an airframe where the fuel tank isn't able to be placed where it wants to be.

Hello David. I am having a similar problem with my OS FS-91 pump engine. It was somewhat bad in the beginning but I have managed to get it somewhat sorted but not totally. When it initially appeared, only by accident I noticed pooling at the pump. I ended up replacing the pump. Next I noticed fuel was seeping out from the cover seam of the regulator. I ended up getting a new regulator cover. Now the engine runs but it seems that it is starving for fuel at high end. The peak RPM tends to vary and it does not quite reach where it used to be. I am now wondering whether the regulator might need to be replaced. Unfortunately OS does not sell the regulator parts. One will have to buy the whole carb and I am not ready to do that. Anyway I am quite curious to learn what you find out with this engine. Matt

@@ahdofu Those pumped engines don't seem to have the best reputation? I keep hearing about them being problematic, especially after being not run for a long time. Dried out membranes, leaky gaskets and valves and so on. Doesn't necessarily needs to mean anything, can also be user error all the time.

It isn't a matter of giving up on it or not. It is clearly sub par for an OS engine and I have other things to spend my time on. I prefer to show engines that actually work out of the box as they should, not baby an exorbitantly expensive door stop. I guess when you are one of the few engine manufacturers left, you can produce garbage, jack up the prices for the engineering effort, and then sit back an wait for people to buy your engines. Nice plan OS. I do not recall seeing any stock pumped engines offered from Saito....

@@dmrcflyr2 I keep hearing that, qualitywise, OS peaked in the 70s and 80s and afterwards started to do decline somewhat again. What do you think about this statement? I have only a really small collection (despite my username, but I'm on it!) and not that much experience.

Stay tuned another run WAS done. It might be a few weeks before it gets posted however.

Excellent information! Thank you.

I have run this engine a few more times now also. I too found that 100% throttle is not good. The throttle needs to be opened about 75% max to reach peak RPM. So what the hell sort of design is that?

@@dmrcflyr2 yeah it’s weird and annoying. I didn’t expect such a failure from OS. I’m thinking to change the back plate and remove the pump and also replace the regulated carb with a simple non-regulated carb. Still searching in part numbers to see if parts from OS200s/OS120surpass can be used. Cheers for the update David👍🏽.