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Post  Wiztom on Sat Jun 27, 2020 6:00 pm

Hello,

Having some issues with a Cox .051.
Engine has good compression and flips over like it should. Using SIG 35% fuel. Has new Glow plug that I can hear sizzling. I can see fuel in the line to the carb. Has a KK needle valve.
Prime small amount of fuel. Flip a couple of time. Hook up my dual 1.5V batteries in parallel. The engine will fire and run up with the prime and then stop. Continued flipping yields short starts and stops and eventually nothing. I have repeated at settings from 2 turns out to 4 turns out. No real difference.
If anyone can point me to where I need to look I would appreciate it. It would seem to be a fuel problem but again plenty of fuel to draw from in the inlet line.
I have other .51 that do not have this problem.

Thanks

Follow up:

Thank you for all of the very good suggestions. I have disassembled the needle valve and venturi, and I now think I understand the problem. I did not realize that there is no spray bar per se. The venturi pin hole has to be lined up perfectly with the pin hole in the needle valve assembly. I believe it was off center. I am not sure how to ever get the two perfectly aligned but I marked the venturi and the need valve assembly. I assume that you just have to slightly move it one way or the other to get optimum fuel flow? Not even sure that matters since its either right on or its not.
Will test tomorrow and see if this is the fix.

Regards


Last edited by Wiztom on Sun Jun 28, 2020 1:59 pm; edited 1 time in total (Reason for editing : Follow Up)
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Post  balogh on Sat Jun 27, 2020 6:17 pm

The venturi has 3 tiny radial holes to atomize the fuel flow when entering the venturi. You may want to blow these holes free from dense castor. The single hole on the needle valve body may also be clogged. And lastly there may be unwanted air seep along the needle valve threads into the carb that you may eliminate by pulling a silicon tube on the threaded nipple so that the other end acts like a seal lip on the beedle valve thread.
If all these do not help then check if the radial play of the crankshaft is too excessive in which case you need a new crankcase.
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Post  sosam117 on Sat Jun 27, 2020 8:45 pm

Wiztom wrote:Hello,

Having some issues with a Cox .051.
Engine has good compression and flips over like it should. Using SIG 35% fuel. Has new Glow plug that I can hear sizzling. I can see fuel in the line to the carb. Has a KK needle valve.
Prime small amount of fuel. Flip a couple of time. Hook up my dual 1.5V batteries in parallel. The engine will fire and run up with the prime and then stop. Continued flipping yields short starts and stops and eventually nothing. I have repeated at settings from 2 turns out to 4 turns out. No real difference.
If anyone can point me to where I need to look I would appreciate it. It would seem to be a fuel problem but again plenty of fuel to draw from in the inlet line.
I have other .51 that do not have this problem.

Thanks

If yo have a KK needle valve the black part that is on the needle valve (threaded area) It will crack because of age.
If it is cracked air will bleed in there.
You can get a replacement for that from Cox International.
They are the same seals that are used on the Sure Start engine needle valve assembly.

I think this is the link. Also, it is good to have extras when the do get old and crack.
https://coxengines.ca/cox-.049-.074-spray-bar-seal-10.html

I had the same problem with my KK needle valves last year, The crack is there but you really can't see it with the naked eye.
I like the KK needle valves ( too bad they are no longer made).
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Post  Oldenginerod on Sun Jun 28, 2020 12:06 am

Being fine thread, I would think the KK needle valve would need to be screwed out further than 4 turns for starting.
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Post  Iceberg on Sun Jun 28, 2020 12:25 am

Might the holes into the Venturi have some dirt in it? Maybe take out the Venturi and make sure all holes clean. Poke a needle through them. Sometimes that might stop the run?


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Post  sosam117 on Sun Jun 28, 2020 8:07 am

Oldenginerod wrote:Being fine thread, I would think the KK needle valve would need to be screwed out further than 4 turns for starting.

On my Tee Dee .049 with the KK needle valve assembly, 4 turns is where I would start.
One thing I try to see if having a problem with fuel draw is to cover the venturi, turn the engine over (by hand) and see in the vinyl fuel line "if" I can see fuel being pulled into the engine. You can't see that in the surgical tubing fuel line.

I do use surgical tubing for my planes when I fly them.
I use the clear vinyl fuel line to check out problems with fuel draw on the test stand.

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Post  Iceberg on Sun Jun 28, 2020 10:33 am

If the Venturi homes are clean and still no vaccum is the Venturi housing cracked or Venturi not tight and sealing?
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Post  coxaddict on Sun Jun 28, 2020 4:01 pm

If the engine can run in the opposite direction then you have a left hand crankshaft. You can remove the back plate and observe the orientation of the crank pin with the intake port facing up. If the crankpin is on the right side it's a left hand crank. left hand cranks are not common but is used for control line flying as it tends to torque the plane towards the outside of the flying circle. I like to run my reedies left handed using the proper propeller for this same reason.
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Post  happydad on Tue Jun 30, 2020 1:03 am

coxaddict wrote:If the engine can run in the opposite direction then you have a left hand crankshaft.  You can remove the back plate and observe the orientation of the crank pin with the intake port facing up.  If the crankpin is on the right side it's a left hand crank. left hand cranks are not common but is used for control line flying as it tends to torque the plane towards the outside of the flying circle. I like to run my reedies  left handed using the proper propeller for this same reason.  

I have extra used tee dee crankshafts. PM me. Need to know if there is a 2 on one of the crankcase mount arms. They should go in pairs. Tiny length. Search tee dee medallion crankshaft data.

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Post  Surfer_kris on Tue Jun 30, 2020 2:47 am

Wiztom wrote:
The venturi pin hole has to be lined up perfectly with the pin hole in the needle valve assembly. I believe it was off center. I am not sure how to ever get the two perfectly aligned but I marked the venturi and the need valve assembly.

They don't need to be aligned, there should be enough room for the fuel to flow around to all four pin holes.
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Post  Wiztom on Tue Jun 30, 2020 8:24 am

So there is clearance around the venturi that allows fuel flow. Very interesting. After cleaning the venturi and needle valve assembly and using compressed air, there was no difference in the problem. I ended up putting two different stock carb assemblies off of other .51's that I have. The second one made no difference. The third one, which is an older model ran it for several minutes. I could get it to lean out but I could not get a drop in rpm on the rich side. Still not sure what is going on.

Thanks for the knowledge.
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Post  happydad on Tue Jun 30, 2020 11:11 am

Wiztom wrote:So there is clearance around the venturi that allows fuel flow. Very interesting. After cleaning the venturi and needle valve assembly and using compressed air, there was no difference in the problem. I ended up putting two different stock carb assemblies off of other .51's that I have. The second one made no difference. The third one, which is an older model ran it for several minutes. I could get it to lean out but I could not get a drop in rpm on the rich side. Still not sure what is going on.

Thanks for the knowledge.

And don’t forget all other parts except the piston and cylinder are the same as the .049. It may just be an issue with the “carb assembly”.

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Post  Wiztom on Tue Jun 30, 2020 12:47 pm

I have ordered three new plastic carb bodies. I plan to change them out on three of my .051's. I understand that they deteriorate after a while and may leak air.
I have an refurbished anodized one I bought that I am going to try and break in. If it runs ok, I can compare the parts to see if there is anything I am visually missing.
I think its a K&K needle valve as well, or at least a knockoff.

Thanks
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Post  balogh on Tue Jun 30, 2020 3:40 pm

Sounds like an air leakage to me. If the replacement carbs did not make difference, I would check:

1 if the plastic carb body is cracked somewhere
2. If the collet nut is spanned sufficiently and the carb body sits and seals tight
3. If there is excessive radial play of the crankshaft indicating a worn crankcase that cannot seal along the shaft.
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Post  coxaddict on Tue Jun 30, 2020 4:34 pm

I have purchased used Tee Dee engines that had the venturi bore enlarged. These require bladder system to run as the engine will not draw fuel on its own. I use the shank end of a drill bit to check the size of bore.
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