hey fellas,
i can't seem to get my os 91fx to run now. it's on a vectorflight extra 300 66" and the first two flights resulted in dead sticks. it would start just fine, idle well, and throttle up with no problem, but it would not run at more than half throttle for more than 30 seconds so i pulled the tank and discovered a pin hole in the feed line right at the tank. i then cut an inch off the feed line to eliminate the pin hole and put everything back together. now it will start when i prime it but only run for a few seconds and die. i have the needle valve out two turns from fully closed and set the mixture screw to 1 1/4 out. i did turn the mixture in and out 1/2 turn each way but it didn't help. there is a pitts muffler on it but i don't know how that could affect it.

i really apreciate it if anyone could point me in the right direction!

scotty

I took a quick look at the .91FX manual. The suggested starting point for the mixture control needle is 2 1/2 turns out. Here's the text:
"Close the throttle rotor gradually from the fully opened position until it is just fully closed. (Do not turn further.) Then, screw in the Mixture Control Screw until it stops. Now unscrew the Mixture Control Screw as indicated below (2 1/2 turns for the .91FX) . See if this helps.

Scotty,
Try opening the mixture control screw in 1/4 turn increments or like Chuck said, close it fully, open it 2 1/2 turns, and then start closing it until you reach a good setting. At 1 1/4 turns open, unless you're using a pump, it may be too lean.

its most likely a mixture problem because the pin whole made a difference in fuel pressure.

Hello Scotty:

I have the similar problem with the OS 91FX on my UCD60. I found the dead stick was always due to the engine overheating. There are separate reasons and it can be corrected.
1. Cooling, make sure there is more area in the outlet of the cowling than the inlet to allow sufficient air flow.
2. Let the engine run a bit richer on the ground. It will lean out in the air. This will avoid overheating.
3. If you use an auto filler, make sure there is no leak or the plunger is fully released after fueling. This can cause the engine to run really lean on high throttle. Try bypassing the auto filler.
4. Check the head gasket screws to be sure that they are fully seated.
5. Check your propeller pitch. It runs well on 15-4W or 14-6, because the engine likes high rpm.
6. The OS engines require a long time to break in fully (may be several gallons). You may need to let it run rich for a while and have patience with it.


Hope these suggestions help. It did on mine.

IDM

After you try all that stuff, let me know with a "PM" and I'll help you fix it.

Idmep is on the right track. My .91FX, and my friends' are both set up with Pitts mufflers, and had problems with deadsticks. In both cases, we found that running with 10% cured the problem, while the engine would die in ther air, after a few minutes, on 15%.

We're both running ancient Cermark Sukhois, cooling airflow is not an issue, and we're both experienced with low- and high-end needle adjustment. We tried props from 13-8 to 15-6; I use the 14-6, my friend uses the 15-4. The big difference came when switching to 10%. On 10%, they are reliable.

I'll also suggest you make your needle adjustments with a half tank of fuel.

Good luck,
Dave Olson

Pitts mufflers can also be the problem, as they have two outlets they create less pressure inside the muffler and hence less fuel tank pressure.. most guys i know of running them get something with a flat end (just a peice of solid 6mm rod) and belt one outlet with a hammer to squash closed one of the outlets, to still have the look but the same pressure as a normal single outlet muffler. Also check ya glow plug, i had a similar problem in my OS 46ax and was cured after changing the plug and retuning it.

The slimline large volume pitts for the 91fx has pipes with inserts with smaller holes - my 91 ran hot until I dropped from 15 to 5 percent, since its broken in it is good with 5 or 10 percent cool power.

skypiratescotty,

Using your fingers, try to rotate the fuel nipple (banjo fitting) on the remote needle valve on the back of the engine. If you can move it with your fingers it is too loose and will cause erratic and lean running. Torque the need valve body (not the needle valve) tightly against the fitting until you can no longer move it with your finger tips. That little fix took me from rotten disappointment to total satisfaction with OS engines.

I'm running two OS 91FX's, one with a Bisson Pitts muffler and one with the stock muffler and they both run reliably.

I have two OS 91FX's and two 160 FX's and two of these four engines came from the factory with loose fuel inlet nipple fittings.

thanks for all the help guys. the first thing i did was put in a new glow plug but it still won't run. i'm using 10% cool power and have a 14x6 prop on it. it seems like i have to really flood it to get it to pop off and then it dies in like 3 seconds. the needle valve is turned out to 2 1/2 turns and i've been turning on the mixture screw on the left side of the carb. i think that should be out 1 1/4 turns but don't know for sure.
i've got a tt 46pro and an os61fx that i tuned with no problems but this one sure is baffling.
again, i really appreciate all your help!

scotty

oh, and the igniter is definately charged.

double check that you didnt cross the pressure and suction lines when you fixed your pin hole leak - does your eng have the new plastic needle housing or the one with the banjo fitting on the back plate?

Go back and read what Piper Chuck said. You have your "mixture screw" (otherwise known as the idle needle) too far in. That is cutting off the fuel at low throttle. Set it at 2 1/2 turns out, using the technique outlined in Chucks post and try again.

Jim

UPDATE!!!!
not only was there a pin hole but the fuel dot was bad and the mixture screw was way too far in. it'll run now but no more than half throttle and still bubbles in the feed line so it has to be the fuel dot ( i hope). anyway, i'm not going to take any chances and just replace the dot, lines and tank. that way i'll be starting fresh.

it's just wierd because i spent so much time breaking it in and it always pops right off and runs strong.

scotty