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OS 91 FX very slow to idle down

Old 09-18-2003, 08:31 AM
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Default OS 91 FX very slow to idle down

I have a 1 year old .91FX which exhibits two strange characteristics:
1) when I chop the throttle after running full out, the engine continues to run at high RPM (>4000) for 5-20 seconds before winding down to idle speed (2000 RPM). The longer I run at full throttle, the longer it takes to wind down.

2) the engine performance seems to be very insensititve to the needle valve position. I can turn the needle valve a full turn in or out and only see a couple of hundred RPM drop.

The engine will spin an APC 15x6 at 9700 RPM on 15% Omega. I have also tried an APC 13x8 - same behaviour just higher RPM
The muffler is a BCM pitts-style muffler, which supplies the tank pressure.

I have tried wrapping the needle valve with teflon tape, thinking that perhaps there was a poor seal (lots of posts about this issue!), but it didn't make any difference

Any thoughts?
Old 09-19-2003, 02:10 PM
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Default RE: OS 91 FX very slow to idle down

Usually, if an engine is very slow to idle down, the indication is that the idle mixture is too rich. This is usually due to a bad idle mixture control setting.

If the needle's rather insensitive, it usually means that fuel pressure's a bit low. You'll need to see what the cause could be. Insufficient tank pressure at high throttle can be a problem, and so can kinked fuel lines or too-small fuel lines.

Basically, you may need to go over the entire fuel system to see if there's a difficulty with fuel feed and pressure to the tank. If all else fails, put the engine on a test stand with everything in the open to work with it and debug.
Old 09-21-2003, 03:55 AM
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Default RE: OS 91 FX very slow to idle down

[8| Does this connect? need to know how long one can expect fuel to last. I have a standard 70% Methanol, 20% castor and 10% nitro mix which is 2 years old (haven't flown much for ages) and my trusty Max 90 (4 Stroke) - note not a 91 - is really running sick whereas before it was a dream and would idle, rev etc perfectly.

I am thinking of changing to synthetics as the castor really glugs up if the engine is left idle for an longer than a month. In this cas I have been told to use just 5% castor and the remainder being 5% nitro (they really are against it down here in Oz), 10% syntheticand the remaining 80% methanol.

What do you think?

cheers Badger1

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