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Adjusting the low speed?

Old 06-05-2011, 04:50 AM
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abufletcher
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Default Adjusting the low speed?

A got the new carb unit for my RCV91cd and took the model out to the field for some tune-ups and test flights. I've been told (by the people at RCV) that the factory setting for the low-speed screw (in the center of the throttle arm) is fine. But to get a smooth idle (very smooth at about 2400rpm) I had to turn this screw (clockwise) about a full turn in 1/4 turn steps. But while this solved the idle problem, the engine seemed to be running lean at the full-throttle setting, even though I was only getting around 8500 (less than the 9000 I've had in the past).

So my real question is this: What is the low speed screw actually doing? I'm assuming it adjusts the gap in the spray bar, but don't know in what way. And would making this adjustment have the effect of leaning out the engine at the top end? Normally the RCV91cd puts out a LOT of oil during a healthy run. But today it was running hot and with little oil even with the needle valve opened out about 2-3 turns (which is usually a LOT for this engine). I did two flights. After the first the engine was hot...and hot for quite a while. I opened up the needle valve a bit for the second flight and heat wasn't as much of an issue, but I wasn't getting as power from the engine either. It's not bad, but also not ideal. I'm glad I'm learning the ins and outs of this engine before putting it in my Albatos C1.

Anyone have any ideas or suggestions?
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Old 06-05-2011, 05:47 AM
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ceecrb1
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Default RE: Adjusting the low speed?

the very simple explanation is its doing the fuel to air mixture for the lower half of the throttle positions..

Altering this will effect the idle stability and the smoothness of transition from low to high rpms.

A basic way to tune an engine is just simply, set the high rpms first with a tacho... get highest rpm then richen to add just to remove a few rpm so it does not lean out and overheat.

Next just start lowering the throttle, if you find it becomes unstable, stop lowering then open or close the low speed needle till its better... then continue lowering the gas.... repeat..

now you should be able to give it sudden stabs at the gas and it should neither hesitate or stop..
If it hesitates, you can lean it a little.. but I mean just a TOUCH of the screw at a time.
If it stops, it is too lean so open.. but again, a touch at a time.
It is ok for it to hesitate just a TINY bit.. again this will keep it rich and help prevent overheating.

Once you have a good transition, re-check the high end.. just incase.

This method will give you the best reliability in the air... other methods will give you better strength etc etc but this has always worked for me and it keeps things simple..... plus the "theory" works in all motor types, from my os55ax to my 3d gasser.
(before people mis-quote me I KNOW it is not the same..... but i'm trying to simplify things).

Every change you make must be as small as you can make them and once the engine has warmed up... also dont try to rush it.
make a change... wait a few seconds, try things out a little THEN make another change... Just give it a few seconds longer than you think it needs.. just to be sure you are testing the change... and not the moment of transition between was was there and what you last changed... ie give it time for the changes to take effect.
dont go wild with the screwdriver.. you will just chase yourself.


I think it would be very hard for any manufacturer to be able to tell you vie email or phone how many turns the needle will need.. it depends on everything, prop type and size, fuel (mix, age and brand), weather, cone or no cone, diameter of fuel tubing, if in a closed cowl or not etc etc etc.
Old 06-05-2011, 05:56 AM
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abufletcher
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Default RE: Adjusting the low speed?

Great explanation! Thanks and saludos desde Japon! By the way, I did first set the high end (and was getting around 8800 which is fine with this engine) but it started running unreliably at about 3800. So I started making adjustments to the low setting screw in steps as you suggest. I didn't however readjust the high end each time. But when I did get a good idle I adjusted the high end but couldn't get anything above 8500 and even at that the engine was running too lean.

Anyway, I'll try following your method next time out.
Old 06-05-2011, 06:23 AM
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nine o nine
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Default RE: Adjusting the low speed?

I'd suggest other possible solutions. You obviously know how to tune your engines. Any change from what has been normal on any engine suggests to me a speck of dirt in the fuel metering system or old fuel which has lost some volativity. If your fuel is fresh and of the proper oil and nitro content then there might be dirt in the system. Start from the fuel tank and go forward until you find the problem. A tiny pinhole in the fuel line can cause the problem too. Flush the carb and I'll bet you'll be in business. Oh...change the plug too. Mitch
Old 06-05-2011, 06:35 AM
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Default RE: Adjusting the low speed?

Remember if you adjust the bottom it affects the top and vise versa. What you are looking for is the best balance between the two. Every adjustment with the lower screw make sure either the top has not changed or readjust it. If you start loosing performance you have gone too far. The top is more broad so unless you make sweeping needle changes the bottom is usually OK.
Old 06-05-2011, 09:39 AM
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Default RE: Adjusting the low speed?

Some carbs are very sensitive to the low speed setting. When I first ran my ASP 4.0 Radial it could not be made to run rich. Checking the manual showed the low needle to be in the recommended range and after installing larger fuel lines and fuel nipples trying to get more fuel into it I started fooling with the low speed needle and it now runs like a top with the low needle about two and a half turns out from factory recomendation, Yes the carb was removed and cleaned and checked for gritters or manufacturing dross and was super clean. It just plain doesnt work the way they said it was supposed to. If you're getting lean runs at the factory setting then its not right. You've already done some experimenting and found out a few things, keep at it till it runs cool enough with good power and you get the best idle you can. They are all similar but what worked for ceecrb1 may only give you grief. Its a shame those engines dont seem to perform well overall and thier reputation is less than Sterling cause there are so many places they just plain fit better. Also you know what Allan went through with his SSW. Ck everything for possible air leaks that could be contributing.
Doc

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