stunthanger.com
Engine basics => Engine set up tips => Topic started by: frank mccune on September 23, 2016, 05:15:35 PM
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Hello all:
I have been having a problem with a couple stunt engines that refuse to run anything but two cycle. I guess that I can just learn to live with a rich 2 cycle. The one engine is an older O.S. .40S. It was designed to be a stunt engine and do the 4-2-4 thing.
It has ben suggested that the engines are running too cold to be able to break 4-2-4. A larger prop was suggested. At present, I am running an 11x6 on both of these .40 engines. I do not have the ground clearance for a 12" prop. Would a smaller venturi be the answer?
Suggestions or comments?
Tia,
Frank McCune
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Plug? Mebbe it's too cold?
What size venturi and spray bar?
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Hello all:
I have been having a problem with a couple stunt engines that refuse to run anything but two cycle. I guess that I can just learn to live with a rich 2 cycle. The one engine is an older O.S. .40S. It was designed to be a stunt engine and do the 4-2-4 thing.
It has ben suggested that the engines are running too cold to be able to break 4-2-4. A larger prop was suggested. At present, I am running an 11x6 on both of these .40 engines. I do not have the ground clearance for a 12" prop. Would a smaller venturi be the answer?
Suggestions or comments?
Tia,
Frank McCune
OK I got one.....How was your OKRA???? LOAD 11.5x4.8 Bolly CF
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Hello all:
I have been having a problem with a couple stunt engines that refuse to run anything but two cycle. I guess that I can just learn to live with a rich 2 cycle. The one engine is an older O.S. .40S. It was designed to be a stunt engine and do the 4-2-4 thing.
It has ben suggested that the engines are running too cold to be able to break 4-2-4. A larger prop was suggested. At present, I am running an 11x6 on both of these .40 engines. I do not have the ground clearance for a 12" prop. Would a smaller venturi be the answer?
Suggestions or comments?
Tia,
Frank McCune
What makes you think it is running too cold?
More info would be helpful
Randy
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Hello all:
I have been having a problem with a couple stunt engines that refuse to run anything but two cycle. I guess that I can just learn to live with a rich 2 cycle. The one engine is an older O.S. .40S. It was designed to be a stunt engine and do the 4-2-4 thing.
It has ben suggested that the engines are running too cold to be able to break 4-2-4. A larger prop was suggested. At present, I am running an 11x6 on both of these .40 engines. I do not have the ground clearance for a 12" prop. Would a smaller venturi be the answer?
What exactly does it do when you richen the needle? Does it just stay in a 2-stroke? Or does it just quit at some point? The former is probably a venturi that is too large, the latter is probably the plug. In either case, a bigger prop won't help.
Brett
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Difficult to comment without more accurate information, but if your engine really runs cold, try reducing the air cooling and see what happens. You can do it for example by stretching O-rings between cooling fins (a nice&easy trick for those cold morning flights). But such is only meant for fine-tuning the thermic balance, it's not a solution if the thermal balance is badly off.
Uncontrollable leaning can also happend with bad tank position, if tank is too much outside of the point where fuel sprays out in venturi.
Try adding compression (by removing head shims), it may help too.
But I have found that when my engine runs cold, it goes rich as the temperature is not high enough for clean burning. Then, after being loaded in a maneuvre, it goes to a leaner mode for a short time when back in level flight.
L
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If you want to reduce cooling as an experiment, Lauri's idea would be a good way. Another method would be to put a shroud over the cylinder head fins. This can be pretty easily fabricated from 1/64" plywood and maybe some balsa. One member here had problems with his new-ish Brodak .40 stop cold at the top of the RWO. At my suggestion, he made a nice muff for the cylinder head to get more heat into the head and cured the problem for the short term. When his engine had more time on it, I'm sure it was no longer needed. The deal is that the part of the cylinder head that fits down into the cylinder (most, not all engine) can be too loose a fit. When it fits correctly, the head will stretch and stabilize the cylinder diameter at the top ("pinch") and keep it round. Those are important details.
I really don't understand designers of engines like the XLS Magnum/ASP engines and the ST G.51 going away from that (head sits entirely on top of the liner flange), except that it saves weight and material. Brass and chrome aren't as cheap as aluminum, especially if they can use a casting with much less machining required. These do run just fine, but I'd expect a different piston/cylinder fit...less taper toward the top. That also saves $, since it's easier to hone a straighter bore than it is getting the taper just right, plus the cylinder head then has no tight tolerance diameter requirement. Tight tolerances cost money, by increasing the scrap rate and/or increased machining operations, i.e., roughing cutter, tool change, finishing cutter, part check on the machine, etc. Ewwww. Reminds me of how I wasted my daylight hours prior to May 2010! y1 Steve
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I really don't understand designers of engines like the XLS Magnum/ASP engines and the ST G.51 going away from that (head sits entirely on top of the liner flange), except that it saves weight and material. Brass and chrome aren't as cheap as aluminum, especially if they can use a casting with much less machining required.
It's also to reduce a precision machining step. The part of the head that goes into the cylinder needs to be fit as well as the piston.
Brett
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It's also to reduce a precision machining step. The part of the head that goes into the cylinder needs to be fit as well as the piston.
Brett
Yeah, I said that. ;) Steve
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Hello all:
I have been having a problem with a couple stunt engines that refuse to run anything but two cycle. I guess that I can just learn to live with a rich 2 cycle. The one engine is an older O.S. .40S. It was designed to be a stunt engine and do the 4-2-4 thing.
It has ben suggested that the engines are running too cold to be able to break 4-2-4. A larger prop was suggested. At present, I am running an 11x6 on both of these .40 engines. I do not have the ground clearance for a 12" prop. Would a smaller venturi be the answer?
Suggestions or comments?
Tia,
Frank McCune
Drop the nitro content, and how is the piston to liner fit?