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RCV 91CD full throttle problem

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Old 03-31-2009, 12:38 AM
  #1
pauser98
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Default RCV 91CD full throttle problem

I have a RCV 91CD engine with the following problem.

This engine almost always starts at first kick, idles at 2,300 with 14x6 Master Airscrew prop and Omega 15%.

Throttle advance from idle to 3/4 throttle is smooth and rpm reaches 8,900 at this point. But from 3/4 throttle
to full throttle, the engine splutters and rpm drops to 5,500 and remains there. The high end needle is
set up 1 1/2 open. Closing it by 1/4 will not get the rpm up. Neither does opening it by 1 to 2 turns.

I notice 8,900 rpm is the RCV specifications for this engine and prop combination.


Chris


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Old 03-31-2009, 12:25 PM
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Default RE: RCV 91CD full throttle problem

Hi, I think the pitch of your prop is wrong. since being a 2/1 gear, your pitch should be around 12. check the prop instructions for prop size and rpm. let me know what you find out. good luck.
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Old 03-31-2009, 02:44 PM
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Default RE: RCV 91CD full throttle problem

I think you are confusing the 91CD with the 90sp. The CD range do not have gearing and operate on props of the same size as other 91 four strokes. A 14 x 7 or 8 is a pretty standard prop size for a 91 FS.
With the sp series it was pointed out to me on another forum that the engine will peak just prior to the barrel being pushed fully open, and I have found this to be the case during the running in process of my 120sp. I had the same symptoms as you where I could get full engine revs with a little movement left on the barrel, then if I pushed the barrel to It's limit the revs would drop off quite considerably, and the engine would splutter a bit. I don't know why It's like this, but now I know I'll set the throttle up so the barrel doesnt get pushed to it's stop. Incidentally, I noticed the revs were peaking when the hole in the barrel was showing clean edges all around, the edge of the hole starts to just get covered if the barrel is pushed to It's limits. I guess there is just too much travel on the barrel on my SP. I feel that if I tried to set max revs with the barrel at It's stop I would be in danger of a very lean and damaging run on what I would think was not full throttle by pulling the stick back a bit but hitting that propper fully open area (If that makes sense?) I will set the servo travel so the barrel never goes past showing a clean hole.

Walts.
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Old 03-31-2009, 02:47 PM
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Default RE: RCV 91CD full throttle problem

Quote:
ORIGINAL: pauser98

I have a RCV 91CD engine with the following problem.

This engine almost always starts at first kick, idles at 2,300 with 14x6 Master Airscrew prop and Omega 15%.

Throttle advance from idle to 3/4 throttle is smooth and rpm reaches 8,900 at this point. But from 3/4 throttle
to full throttle, the engine splutters and rpm drops to 5,500 and remains there. The high end needle is
set up 1 1/2 open. Closing it by 1/4 will not get the rpm up. Neither does opening it by 1 to 2 turns.

I notice 8,900 rpm is the RCV specifications for this engine and prop combination.


Chris


Hi Chris,
The CD series uses standard props so your prop selection is right on. Try leaning the high speed needle until the enginse smooths out at peak RPM. Our records show that 1 turn is about right but it does vary somewhat, about 1/8 turn either way in practice. To check the setting, briefly pinch the fuel line and look for an approx 200 RPM increase before it quits to be sure you are not too lean. Follow the owners manual instructions from there. If that does not work, contact me directly for more assistance. BJ


Best Regards, Bill Jensen

BJ's Model Engine Service*, 51 Hillside Dr., Beacon Falls, CT 06403
phone: 203-888-4819, email: wbilljensen@cs.com
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Old 03-31-2009, 03:29 PM
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Default RE: RCV 91CD full throttle problem

Hi Chris,

I remember one mistake I made when installing the throttle linkage (on the SP series, however): The barrel in the carb has some axial play. When the barrel rotates it moves out and in a few milimeters. A spring inside pushes the barrel outwards. What happend to my engine after throttle linkage installation was that the stiff rod pushed against the barrel axially, against the force of the spring. At certain rpms (in my case at the low end) the barrel vibrated in and out thus changing mixture uncontrollable.

If the CD series have the same habit as the SPs and reach max. rpm just before the barrel rotates fully open I believe Walts posting is number 1 solution for your problem.
If the problem persists, I suggest to check the throttle linkage


regards
Andi
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Old 04-01-2009, 02:11 AM
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Default RE: RCV 91CD full throttle problem

In my case, at peak rpm, there is still about 2mm to go before the barrel is fully opened.

Now I set the engine servo to travel to this limit, otherwise during take-off and if I fully opened the throttle, the engine spluttered and the take-off is aborted just before lift-off.

I did disconnected the whole carb assembly and blow directly through the inlet opening while slowing opening the throttle. I noticed that the air flow increased progressively as throttle is opened, and just beyond the 2mm-to-fully-open mark, there is a "whoosh" and air rushed freely through. Maybe this causes the splutter as the air mixture becomes too lean?

I will remove the crowl and do a pinch test again as Bill suggested.

Thanks,

Chris
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Old 04-04-2009, 03:42 AM
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Default RE: RCV 91CD full throttle problem

I think I have found the problem .... it's the O-ring of the the high end needle. I changed the O-ring
and the problem is gone. Now the engine reached 9000 rpm at full throttle with 14x6 prop and 15%
Omega.

Thanks guys for your help.

Chris
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Old 03-28-2010, 11:38 AM
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Default RE: RCV 91CD full throttle problem

Chris,

What model are you using this engine in? I am thinking of getting one for a 11.5 lb scale plane.

Mike
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Old 06-11-2010, 08:40 AM
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Default RE: RCV 91CD full throttle problem

Mike,

Sorry, I was off-line for a while. I am using the RCV-91CD in a Top-flite Spitfire now. Empty weight about 3.8kg without the fuel. I use an OS flexible extension tube for the muffler and a 14x8 master airscrew.

Attached is a picture.

Chris
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