os91 & Pitts mixtue problems

[Ben Holt]

I've just about fimished my Funtana 100 and today I ran up the engine for the first time (an OS.91 FX with a BCM Pitts style exhaust).

Following the manufacturers instructions I set the needle to 5 turns out, It started second flick. I then proceded with the 'break in' by running at full throttle at the 5 turn setting for one minute.

The instructions then said to lean out the mixture until the exhaust note changed from 4 stroke to 2 stroke for 10 seconds. This is where the trouble began, It didn't want to change note however much I closed down the needle, only begining to slow down at around 3 turns out. I was starting to think that it already sounded like it was running '2 stroke' so I wound it back out bit by bit until I was 10 turns out, with little effect on revs or exhaust note.

After refilling with fuel, and being worried about running too lean on the 'break in' I started this time at 6 out and then found the revs increasing a little around 5 out.

I've read that there could be a problem with the pitts exhaust not making enough back pressure to go to the tank, therefore making it lean??

Please help, I dont want to damage my new engine by setting the needle wrongly!

[Mike Bell]

Ben,
Are you sure there aren't any kinks in the fuel lines? I would expect the engine to die completely at 10 turns or at least be spitting fuel all over, so I suspect a fuel flow issue. If all is OK the change from 5 turn slobbering rich to 2-3 turn 2 stroke should be really obvious.
I've used Pitts type silencers before with no strange effects.

[Ben Holt]

Hi Mike,
No kinks in the system, The needle actually came out at 11 turns and the engine kept on running without fuel leaking out! So i'd agree that there's a fuel supply problem.
I've read on a few different forums that blocking up one of the two exit pipes can cure similar faults by creating more fuel tank pressure. Have you ever heard this?

Does anyone know of any other causes for these symptons?

[001]

Try running it on the ground without the pressure line attached, if the fuel tank is in the correct position and pipes are clear, at least you have a starting point. How much smoke is being emitted?

[Ben Holt]

There is quite a bit of smoke at 5-6+ turns, but not a lot more at 10 turns.
Its like the needle is having very little effect on the mixture.
If I run it without the pressure line on, surely it will be even more lean?

[001]

Hello Ben, sounds like a major air leak. Are there bubbles in the fuel pipes (from the tank to the needle valve and between the needle valve and the carb? Try replacing all fuel lines, check the carb. and engine backplate for leaks.
If you run it without the pressure line it will be leaner, but if the engine cannot be tuned with the main mixture control set at around 2-3 turns out (I have read the instructions),no matter what silencer is fitted, then there must be another problem. Try making a test stand & try it on that with new fuel pipes (and the OS silencer if available)..

[Ben Holt]

Hi Richard, What you say certainly makes sense. I'm not sure about the bubbles, I will run it tomorrow with the cowling off and try your suggestions. I'm using a two pipe tank with one of those plug in fueling valves in line to the remote needle valve. I guess that could also be drawing in air. does it matter where the fueling valve is fitted, ie how high? its level with the top of the cowl.

[001]

Position of fueling valve should not make any difference, position of fuel tank is important. Try by-passing the fuelling valve (it sounds like it may be the problem). If the engine is new, the glowplug is OK and the fuel tank is not high or low, and all the fuel lines are not blocked and not leaking there really should be no reason why the engine is not 100% let me know how you get on.Richard

[Aslan]

Ben....Have you tried the original muffler.I always run my engines in on the bench first.I also do a simple air leak test on the tanks before I install them.

[Ben Holt]

Hi Richard, I've been busy today! I re-checked all the fuel lines, fueling valve and tank for leaks, blockages or kinks independantly and together as a system, none were found. I then rechecked the position of the fuel tank (which can only really be in the position designed by Hanger9) which is approx 10mm from the top of the tank to the needle valve as recomended by the OS instructions.
Having checked all this I ran the engine again with the cowl off and had the same symptons (no air bubbles in the lines).
So, I decided to try what I'd read about on a couple of American forums, To block off one of the two exhaust exit pipes to create more back pressure for the tank feed. The size of the original pipe exhaust has the same Internal Diameter as one of the Pitts pipes. I blocked off the pipe nearest the engine with a piece of dowel of the appropriate size. I started the engine at 6 turns out as I had before and immediatly noticed that the engine was clearly 4 stroking and would barely rev. I leaned it out to 4 turns to let it rev then decreaced slowly until it started 2 stroking at around 3 turns out!! All appears to be fine now with the needle valve being much more sensitive to adjustments.
Many thanks for everyones help, anyone else heard of similar problems?