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OS25LA setting up idle

Too rich at the bottom end

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winchweight10/07/2008 14:57:00
2516 forum posts
67 photos

Hi guys.

I am trying to set up 2 x OS25LAs on my Transall, to get them to idle consistently. I am using Coolpower 10% and I am trying to set them up on an engine test stand. The problems are:

1) Identifying the idle mixture screw - I can't work it out. There's a screw which appears to allow air into a vent on the front of the carb, and screwing it in reduces the air, out increases it. But neither way is effective. Pinching the fuel line at idle causes it to accelerate before eventually quitting. At WOT pinching causes immediate quitting.

2) Identifying the carb throttle travel screw. Same again, can't work out which one stops the throttle closing fully.

Engine Doctor10/07/2008 22:06:00
2328 forum posts
28 photos

Hi Shaun The slow running mixture screw is the screw at the front of the carb and screws in and out horizontaly.It adjust the slow running mixture by either restricting or allowing more air into the carb when the throttle is closed. If you look in the slow running air intake you will see the end ofthe slow running mixture screw .turn it until it about half way across the hole.The throttle stop screw screws into the top of the carb . turn this screw until the carb  can be completely closed  (no air gap) . Open the throttle enough to start the engine with the throttle trim on your TX and start your engine . Adjust the fuel mixture for correct running at full throttle . When you ar happy with the full throttle setting close the throttle and alow to idle for a few seconds. now open the throttle fully . If your engine coughs and splutters and speeds up slowly it is too rich and the slow running mixture screw needs  turning out . Only make small adjustments ,say 1/8th of a turn  and retry. If you open the throttle and the motor dies and hunts for fuel then the idle mixture needs richening (srewing in) Adjust until you get a reliable idle and good pick up.Hope this helps .


winchweight10/07/2008 23:35:00
2516 forum posts
67 photos

Thanks ED. I downloaded the OS instructions and have had a go pretty much as you describe. I'm trying to get them to idle at similar RPMs, so I've painstakingly set them up as identically as I can. Tomorrow I'll try the engine runs again, this time in the model, as I didn't realise I needed the throttle servo to control the carb movement and maintain the idle position.

Thanks for your help, I'll let you know how it goes.

PS it was suggested that I try a pair of OS F Type 4st plugs instead of my usual OS No8. What do you think?

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