Ive read about the OS .91 fx Having some problems. Does anyone know if relocating the needle valve corrected any of the problems Ive read about or are they still a tempermantal engine. I apologize if this was asked before. I did a search and didnt come across anything so I thought I would ask. Thanks

Get the new Saito 125 .... It is a better engine . OS quality has been going down for years [:'(]

When I first got my (older) OS 91 FX it ran well but after some time, it developed erratic running. That is when I noticed the bubbles in the line between the needle valve and carb. I replaced the backplate, needle valve assembly and bracket (that holds the needle valve assembly to the backplate) and this seems to have solved the problem I was having; no more bubbles and the engine now runs well again.

Here is a thread that discusses the bubbling issue and may help: http://www.rcuniverse.com/forum/m_86.../tm.htm#861128

Hope this helps

Edited to add: I have since gotten another OS 91FX with the newer backplate/bracket/needlevalve arrangement and have not had problems with it.

on any of the engines that have a removable needle valve i allways remove the valve and remount it to the fire wall the added vibration allways causes the valves to leak air into the system as well as fuel out of the system. This happens to a lot of engines not just os i think os had a great Idea making the valves removable

OK, thanks for the link. It looks like the the relocated needle solved many probles along with having sufficient cooling to the engine. I only asked because I bought a 91fx on ebay that had the older valve but it appears the threads stripped out of the needle valve assembly. Tower sells the backplate with the needle valve attached and they sell the new backplate with the needle valve relocated and just wanted to be sure which one I should get. Now I know to go with the relocation. Does anyone see a problem with me cutting off the needle valve attachment point on the old backplate and just screwing on the new valve? I figure thats the only difference between the plates.

This will be my first OS engine and the reason why I went with OS is because everyone in my club runs OS without problems. So, I hope this one works good for me. As long as it is better than the MDS I have been dealing with for a few years I should be good.

After a lot of problems and deadsticks(even though I did remove the needle assembly from the backplate) , I mounted an mvvs carb and my 91 fx suddenly became great. I have been 3-d flying it a lot after the carb change. It runs great, transitions great and do not suddenly quit for no reason anymore. The mvvs carb is also quite cheap to buy.

I ran my OS91 FX for less than one tank on the ground, adjusted low and top end, and then it was ready to fly. What a great engine reliable, powerfull and very quiet[sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif]. It is now turning a APC 12x14 @ 9000rpm on one of my sport "speed planes". I am pretty sure you are gonna like it.

On another forum posting someone suggested using a magnum carb with the needle on the carb. I installed a mag 91 xls carb on mine - works good.

The original needle valve assembly was a cast-on part like on the .25/.46/.50FX. Apparently, heat, especially on the .91, ran up the aluminum stem and vaporized the fuel in the needle valve area, causing the bubbles and erratic running after 5 minutes of flight. The later ones have the same assemble as on the OS .65LA and the .46AX. This has a brass insert with a black plastic housing and the same needle. A sheet metal bracket attaches it to the rear cover. This one works well.

It used to be that Hobby Services would give you one of the new needle valve assemblies. Probably if you bought it used off the web, it has been run so lean that the cylinder or ring may be warped and it will never run well without some parts replacement.

I had one of the original .91FXs and had the problem. I bought a K&B .61 remote needle for $15.95 and the engine ran great. Try an Ultra Thrust or Jett tuned muffler for a thousand more rpm.