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HomeAircraftHelicopterHIROBOOther › Fuel mixture setting ????
08-04-2018 11:27 AM  4 months agoPost 1
Hirobo aus


Queensland, Australia

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Hi all. I'm running an OS 50SX-H (good old engine) in my SDX 50. I recently changed my fuel mixture from 25% nitro to a 20% nitro mix. In Australia at the moment, it's almost cheaper to by a new BMW than it is to buy 5 litres of nitro. Anyway...…, I have the needle valve dialled out a full two turns from fully closed. When she's airborne, the engine seems as though it's bogging down a little, but when the main tank gets to about half full, the engine speeds up to a nice speed, and the rotor RPM increases to a nice stable speed. Have I got the engine running too rich, or do I need to up my nitro percentage back to 25% ??????? Any hints or tips would be great. Cheers - Adam

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08-04-2018 01:17 PM  4 months agoPost 2

rrKey Veteran

Meridian, Mississippi

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Is there any chance you've got a pinhole in your clunk line? That will cause a motor to seemingly inexplicably and excessively lean out 1/2 through a tank. As long as the hole is submerged in fuel it doesn't matter. As soon as the fuel level drops enough to expose the hole, air is injected in the fuel supply.

One of the things we like about nitro is that the needles becomes less sensitive with the addition of nitromethane. For instance...If the acceptable operating range for an engine on 30% nitro is +/- a quarter turn, it will be a click or two on 0%. The less nitro, the more sensitive the engine becomes.

Also, the more nitro, the more you have to open the needles. By reducing the nitro, as you have, I would expect to have to lean the motor some.

Just a few random thoughts...


"Well, nothing bad can happen now."

08-04-2018 01:34 PM  4 months agoPost 3


The Villages, Florida

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Definitely follow up on Bill’s thoughts and replace the fuel lines .....
Also, I found that using a header tank gave me more consistent runs from full to empty.....

Been there, done that and old enough to know better.....

08-04-2018 11:59 PM  4 months agoPost 4
Hirobo aus


Queensland, Australia

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Thanks for the tips gents. Funnily enough, I have changed the fuel lines for the main and the header tank, and it still does the same thing. I might try leaning it out to about 1 1/2 turns from fully closed and see what happens. If no joy after that, I might look at bumping up the nitro content in my fuel mix. Cheers. - Adam

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10-01-2018 11:18 PM  75 days agoPost 5

rrKey Veteran

Morgan Hill, CA. USA

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This is an old thread, but I wanted to add a couple of things:
* 2 turns on the needle is the recommended start point in the OS Engines 50SX-H manual. Not the finial setting for the needle.
You should not just set it to 2 turns and never adjust it again.
Using the original 25% nitro, my guess is that you should have ended up with the needle set to something much less than 2 turns.
And the setting will change 1 or 2 clicks based on the weather (day to day and hour to hour).

* Less nitro in the mix will need less fuel.
Whatever the optimal setting was for 25% nitro would need to be 3 to 4 clicks leaner with 20% nitro.
You can run 0% nitro if you like. Just make sure you tune the engine for 0% nitro

Read the "Adjustment" section (page 15).
It spells out pretty clearly what you should be doing.

This question should have been posted in the Engines Plugs Mufflers Fuel section. It really isn't related to Hirobo

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