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HomeRC & Power✈️Aircraft🚁HelicopterEngines Plugs Mufflers Fuel › Carburetor Rotor Guide Slot - Damaged From Clogged Mid Needle Port.
02-07-2018 08:41 PM  18 months ago
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Kernersville, NC USA

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Carburetor Rotor Guide Slot - Damaged From Clogged Mid Needle Port.

I was having some trouble with an OS .91SZ.
When I first started flying it in a Raptor 90SE, the mid-range needle was tuned good, the high speed and idle was good as well (with minor adjustments here and there for weather changes) but I started flying hard for quite awhile and I really kept the engine on the high speed needle without giving it a break. All seemed good.
Then one day when I was in the mood for some mild flying, I noticed that the mid-range needle was extremely lean.
The needle was still at about 1-1/4 turns out as it was originally so I decided to pull the needle valve assembly out to clean it and the port going to the rotor mid-range groove using a can of compressed air, I didn't remove the rotor, I found that the port for the mid-range needle was completely clogged for some reason, I always use a good filter at the inlet nipple, the idle and high speed was still clear, so I'm not sure how the mid-range needle port got so clogged up, I'm guessing that it was a clump of thick oil since I never give the mid needle a chance to work much at all, that makes sense cause the oil probably was blown through the hole in the rotor to clear it.

After cleaning it I set the mid needle back to about 1-1/4 turn out and it seemed good but still not quite right, I can usually tell by the sound and the power level so I started trying to fine tune it again.

It would be just right one flight, then lean the next, I'd richen it up by 1 or 2 clicks, then the next flight it was rich again, some flights it was too rich one minute, then just right the next minute.
I was getting concerned about it so I broke out my trusty digital microscope, I inspected the glow plug first and immediately noticed tiny flecks of aluminum dust in the glow plug and I freaked out! What is grinding in the engine? was what I was thinking.

I tore the engine apart and couldn't find anything wrong with it so I decided to tear the carb down and when I pulled the carb rotor out, it was covered with aluminum dust mixed with oil, it was glittering.
After using the microscope again, I noticed that the slot in the rotor that the guide screw rides in was badly worn at the mid-range point by about .016" on each side of the slot, that's a total of .032" play and with the rotor out by .016", that's over a 1/3 of the mid needle groove and hole blocked so it was now making sense that I couldn't get it tuned.

For those that don't know, the idle and high speed fuel supply comes from the end of the rotor (inside of the mixture control assembly where you adjust idle) and the mid-range needle is what lubricates the rotor, so, without any oil on the rotor, the guide screw started grinding on the slot in the rotor causing it to wear bigly (That's a Trump Word ).
I had to do a thorough cleaning job including the bearings which were removed to be on the safe side, fortunately nothing in the engine was damaged but I did have to buy a new rotor for the carb.
It didn't start injecting the aluminum dust into the engine until I unclogged to mid-range needle port, the fresh oil supply on the rotor was flushing it through the engine.

Just remember that on carbs with a mid-range needle, that's the only thing that lubricates the carb rotor.

Checkout the attached photos of the damage to the rotor.
These photos were taken with my digital microscope. That microscope has been a real asset for my helicopters and many other things I have including making sure the head of a tick is out when tick season is high around here

Also, I have attached a photo of the inside of my one way bearing in the clutch, it was squealing when I first hit the starter and it would spin with the motor since it wasn't fully disengaging, I could feel it trying to spin when I pulled my stater off, turns out there was a bunch of crud built up under the rollers and between the rollers and the little springs in the cage. That's the last time I'm using grease I think. Of course this was after about 20 gallons of fuel.
I think from now on I'll use something like ATF or synthetic oil.
I was surprised to find out that the cage is made out of plastic. Some are made out of stainless steel but most RC OWBs are made out of plastic it seems (which I guess is fine really).
Some say not to use oil because it will get slung out but with the cup around the cage, the oil would just get plastered to the outside portion of the cup when it's spinning, then come back to the rollers when sitting still (if you don't use too much oil that is.
I posted this photo in case some of you would like a closer look at these things and.......because I could .
OWBs engage when the starter shaft presses on the rollers, then the rollers are riding on the springs to freewheel when disengaged (I think I got that right).

BTW, be careful if you use air to blow out whatever you use to clean the OWB, I did that and one of the rollers and a spring blew out!
Try finding those tiny things on a carpeted mat under your feet.

Anyway, I hope this helps someone!

Have a great day and fly safe!!


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HomeRC & Power✈️Aircraft🚁HelicopterEngines Plugs Mufflers Fuel › Carburetor Rotor Guide Slot - Damaged From Clogged Mid Needle Port.
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