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Author Topic: K&B .45 Sportster  (Read 217 times)
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whiskers
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« on: April 06, 2019, 11:44:33 AM »

Hi folks,

Looking for a bit of advice re the above motor. I just picked one up that was advertised as brand new un-run. It's one of the older ones with the NVA on the carb. When I un-boxed it the piston (viewed through port and plug-hole) was completely new looking with no discolouration at all on skirt or crown or, indeed, any scuff marks. It had no oil staining as such and all I did was give it a check over and clean the carb of some jelly like oil residue. Clearly it was as advertised. Happy me! I did some research and popped to the local Emporium for some stuff. Fuel was Model Technics Duraglo 5% nitro and I added a good slug of castor oil to bring it up to about 22% oil content as per advice. Prop is 11x4 which is what I had lying around. Plug is what came with it and un-marked.

Now, I can get it started and running but It won't take more than 1/3rd throttle without slowing to a stop. It will go onto full throttle but slows to a stop quicker. It will only run acceptably with the idle mixture disc fully richened. It is well smokey so I don't think it's lean. There appears to be no air leaks and I have run it rich 31/2 turns out on NVA (won't really run acceptably richer than that) and leaned off to about 2 turns out,again runs bad and does seem too lean. It will idle well on the rich setting of the idle mixture disc and transfer from idle to speed is OK if i don't let it idle too long.

I tried a 12x6 APC prop which I think smoothed it out a bit, these motors are supposed to like BIG props. I may try a 10x6 and perhaps add a bit more oil to the fuel. Any other ideas?
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lincoln
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« Reply #1 on: April 06, 2019, 08:48:13 PM »

Our RC club used to pylon race with the Sportster 20. The one guy who could make them run right would put some kind of liquid gasket maker between the pieces of the engine and an 0-ring under the carburetor. In our case, the problem was usually that the engine leaned out as the run went on. I remember in one race I had to fly inverted to keep it going. When you're flying low, and you're a bit tense, flying inverted isn't necessarily a good idea! I don't know if your problems are from the same cause, but it's something to consider.

As I recall, the mufflers on the Sportster 20's were amazingly effective.

I'll admit I'm not an engine expert, so I don't know what NVA is.

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Konrad
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« Reply #2 on: April 06, 2019, 09:22:05 PM »

I thought these came with a remote Needle Valve Assembly.  These are fine sport engines a 10x6 is getting a bit on the small side. They need a bit of break in, so don't expect much until you have some more run time. Can you run her rich and at full throttle? If not then there is an issue with the NVA or carb. Can you post photos? Do use muffler pressure. At least until the engine is broken in.  Big plus on using 22% oil. I like my oil 25% live with 50% of this being castor. The glow plug is a K&B long reach.  

Lincoln's duscription is a. Classic example of a tank being too low relative to the discharge port of the NVA. 
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Cut it twice and it's still too short!
whiskers
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« Reply #3 on: April 07, 2019, 04:51:01 AM »

Thanks guys. The earlier engines had the NVA on the carb like mine has. By referring to NVA I mean the needle valve assembly. It certainly is a quiet engine! Sorry but no pics.

No, the engine won't hold rpm when rich at full throttle. At more than 1/3rd to 1/2  throttle the engine seems to load-up and slow to a stop. Closing the throttle to 1/3 open before it stops restores the RPM. This is the same throughout the range of NVA settings that the engine will run on (from too lean to too rich). I don't suspect any air leaks...all looks good and the 'O' ring seals are all in good order. No leaks from the head either.

I think it probably needs a lot more running. I read these engines often require a lengthy break-in process. Most info says not to use muffler pressure on these engines. I have an exhaust stub with a pressure nipple so I may try this...If they don't require pressure why do the makers provide exhaust stubs with and without a pressure nipple?!! So that's defo worth a try.

My engine has only gone through about 5 ozs of fuel so far. I will try a 10/6 prop, which is what is recommended on MECOA website for break-in, and more castor and see If I can get it to hold revs wide open after some more gentle runs.
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Konrad
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« Reply #4 on: April 07, 2019, 10:29:41 AM »

The engine should run with the throttle full open. If it really is loading up all you need to do is lean the NV to 1 to 1.5 turns open (this really is a pointless measurement, we are concerned with flow not thread engagement )

 If it only runs to 1/3 throttle this is an indication that the discharge port in the NVA in partially blocked (lean).  Remove the NVA and look for contamination in the spray bar.  Remove the NV and look for contamination in the NVA. Also look to see that the shape of the NV doesn't look bent.

Yes, most folks liked the midrange response with the tank vented to atmosphere. It is used as a tuning option. But right now you are just trying to get her to run rich and at full throttle.
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Cut it twice and it's still too short!
whiskers
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« Reply #5 on: April 07, 2019, 03:17:15 PM »

Thanks Konrad. I'll have another look at the carb later this week and give it a another good clean out. I'll report back...
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Konrad
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« Reply #6 on: April 07, 2019, 08:26:35 PM »

What I fear is going on is that the carb can't flow enough fuel at full throttle. So as the engine heats up it is slowly slowing down looking like  it is loading up. When it might actually be lean.

When it does this again try pinching the fuel line. If it goes from loaded up to high rpm to quit then you were too rich. Then you need to lean out the NVA. But if the engine keeps slowing down and quits then you are lean, and need to look for the fuel obstruction.

All the best,
Konrad
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Cut it twice and it's still too short!
whiskers
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« Reply #7 on: April 15, 2019, 04:01:09 AM »

Well I removed the carb (again) and had a really good look. I discovered the spray bar was slightly bent where the threaded part exits the 'mixture disc'. I have very carfully straightened this out and I can now get to nearly full throttle before the engine wants to slow and stop. Tried to do the pinch test on the fuel line but I can't get to it in time!

I will now try with the pressure nipple from the muffler connected. I'm slowly getting there I think.
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Konrad
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« Reply #8 on: April 15, 2019, 02:12:19 PM »

Well I removed the carb (again) and had a really good look. I discovered the spray bar was slightly bent where the threaded part exits the 'mixture disc'. I have very carfully straightened this out and I can now get to nearly full throttle before the engine wants to slow and stop. Tried to do the pinch test on the fuel line but I can't get to it in time!

I will now try with the pressure nipple from the muffler connected. I'm slowly getting there I think.
Can't get there in time"
Really sounds like a lean condition! I'm sure you have a fuel restriction in the carb or fuel line. And/or the sray bar is mis-aligned causing very poor fuel draw.
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Cut it twice and it's still too short!
whiskers
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« Reply #9 on: April 18, 2019, 03:01:17 AM »

Yes, that's what I now think. I'm going to totally strip out the carb and make sure everything is as it should be and replace all the 'o' rings etc. I will pay particular attention to the alignment of the spray bar so the idle mixture slot is correctly aligned (as best as I can).

This thing's fighting back but I refuse to give in!
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