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Need help troubleshooting a Cox .049 engine

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Need help troubleshooting a Cox .049 engine

Post  Cox International on Thu May 24, 2018 9:22 pm

A customer returned this engine because it would hardly run 2 minutes: https://coxengines.ca/cox-.049-engine-texaco-ii.html and we did the following:

Changed to an OEM (325) head (and tried with more than one head in case one leaked)
Changed all the gaskets
Changed the fuel pickup line
Made sure the NV / spraybar assembly sealed
Ensured that there was sufficient compression (strong)

The engine starts easily and runs for about 3.5 minutes at full RPM but constantly decreases and increases RPM unless one continually fiddles with the NV setting. Also, it will run well with about 4 turns of the NV and 2 turns; it is in between that full RPM cannot be easily achieved. Turning the NV in or our out one full turn often makes little difference.

After the engine runs out of fuel there is less compression (hot engine) but still seems to be enough although it can take 10-20 flips of the prop to restart it.

To note is that the play in the conrod / piston socket is fairly big (will reset at the office tomorrow)
The fuel I have at home is probably 7-8 years old (stale?)

Can any of the above lead to the symptoms?

Something wrong in the NV / spraybar / backplate assembly?
Looses compression when hot?



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Re: Need help troubleshooting a Cox .049 engine

Post  Mudhen on Thu May 24, 2018 10:00 pm

Compression. If the engine loses compression while hot, then it is a bad cylinder/piston fit. When cold, the engine will appear to have good compression, but the likely culprit is that the piston is not truly round. Instead, it is "cauliflower" shaped, (out of round.) To test this: When the engine is cold, turn the engine to TDC and hold. After a second or two slowly move the piston down from TDC. If it appears to suck air while the piston is moving down,(or you feel a mild resistance rather then a compressed air forcing the piston down,) then the piston is not round. (Assuming a sealed cylinder and glow head.)

Decreasing RPM. If after a minute or two of peak RPM the engine begins to slowly lose RPM, then it means the crankshaft/crankcase fit is too tight. (The heated metals are expanding causing increased friction and load.) Unfortunately there’s no way to test this when it’s cold. The solution is to run the engine at a mid-RPM setting for a minute, then reduce RPM to a low RPM value. Let the engine cool, then repeat. After a tank or two of this process, try running it at peak. If the problem continues, repeat the procedure. It will eventually break in and run normally.
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Re: Need help troubleshooting a Cox .049 engine

Post  Cox International on Fri May 25, 2018 1:43 pm

Mudhen wrote:Compression. If the engine loses compression while hot, then it is a bad cylinder/piston fit. When cold, the engine will appear to have good compression, but the likely culprit is that the piston is not truly round. Instead, it is "cauliflower" shaped, (out of round.) To test this: When the engine is cold, turn the engine to TDC and hold. After a second or two slowly move the piston down from TDC. If it appears to suck air while the piston is moving down,(or you feel a mild resistance rather then a compressed air forcing the piston down,) then the piston is not round. (Assuming a sealed cylinder and glow head.)

Decreasing RPM. If after a minute or two of peak RPM the engine begins to slowly lose RPM, then it means the crankshaft/crankcase fit is too tight. (The heated metals are expanding causing increased friction and load.) Unfortunately there’s no way to test this when it’s cold. The solution is to run the engine at a mid-RPM setting for a minute, then reduce RPM to a low RPM value. Let the engine cool, then repeat. After a tank or two of this process, try running it at peak. If the problem continues, repeat the procedure. It will eventually break in and run normally.

Thanks Mudhen Smile

The engine has very good compression and compressed air forces the piston down after 2 secs @ TDC. Having said this, we tried this on several brand new Surestart engines and the engines all lost compression within a second at TDC and sucked air when the piston was pulled down. We are assuming this rectifies itself during break-in as we have sold many 1,000 of engines with the "looser" tolerances of latter-year production engines.

The crank / crankcase assembly is on the tight side due to anodizing but, as you mention, breaking it in will resolve this.

What we did find this morning, however, was that the tip of the needle valve was bent. Replaced the NV and shall take it home tonight and run it again on the weekend.

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Re: Need help troubleshooting a Cox .049 engine

Post  fredvon4 on Fri May 25, 2018 2:30 pm

Sorry Bernie if this comes off as critical...BUT to evaluate a customer returned engine I would do my level best to use good fresh fuel

Not much to add to the response you already got but in track my many years now of playing with a LOT of your products...

I find sometimes the piston cylinder fit is not optimal......too easy I just swap around

I occasionally had a crank to Case mis match (too tight) just polished the crank

every tanked engine so far has been air tight and NVAs good to go out of the box

Bottom line is I think your products are very good for my use...... I have had NIB Black Widows...way back ...from Cox that were cantankerous until me and dad figured out what was wrong....tinkering with dinky engines can be tedious but real fun when the full on peaked out scream come to life....grin
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Re: Need help troubleshooting a Cox .049 engine

Post  Cox International on Fri May 25, 2018 2:38 pm

Completely agree with the fresh fuel thing but we are 500km away from the nearest city and shipping fuel through the mail or bus service is a big no-no. But then I would think that stale fuel would produce constant poor runs rather than the gradual decrease in RPM.

The customer bought the engine two years ago and said it ran fine in the past. Methinks the bent NV may be the culprit; shall see....
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Re: Need help troubleshooting a Cox .049 engine

Post  GallopingGhostler on Fri May 25, 2018 3:15 pm

If possibility of a leaking needle valve seat(?) Huh... removing spring and putting a small section of fuel line to seal the gap would help verify that. Smile

I'm sure the answer is out there and probably something very simple.
A substituting known good used parts might help eliminate or verify. Popcorn

If there is a reliable and trustworthy friend in town, may be let someone else play with it a little bit who has the time might be able to find the answer. Smoking
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Re: Need help troubleshooting a Cox .049 engine

Post  Cox International on Fri May 25, 2018 3:30 pm

We simply swapped out the complete backplate assembly Smile

Town of 12,000 .... probably a dozen modellers (almost all electric) and probably no one that even knows what a Cox engine look like lol.

I shall play around with engine this weekend.

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Re: Need help troubleshooting a Cox .049 engine

Post  Kim on Fri May 25, 2018 4:43 pm

It should be 'put on the road'!!!

"The Traveling Cranky Engine"
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Re: Need help troubleshooting a Cox .049 engine

Post  Admin on Fri May 25, 2018 4:44 pm

Kim wrote:It should be 'put on the road'!!!

"The Traveling Cranky Engine"

Those who can run the tank empty in one go wins a prize!

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Re: Need help troubleshooting a Cox .049 engine

Post  GallopingGhostler on Fri May 25, 2018 4:48 pm

Admin wrote:
Cox International wrote:Town of 12,000 .... probably a dozen modellers (almost all electric) and probably no one that even knows what a Cox engine look like lol.
Those who can run the tank empty in one go wins a prize!
Past month I'm still figuring out why my latest engine acquisition has no compression. lol!
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Re: Need help troubleshooting a Cox .049 engine

Post  Levent Suberk on Fri May 25, 2018 5:32 pm

May be dirt under reed, or reed is bent.
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Re: Need help troubleshooting a Cox .049 engine

Post  Cox International on Fri May 25, 2018 5:34 pm

Levent Suberk wrote:May be dirt under reed, or reed is bent.

Did check for that and it's a Mylar reed.

We'll see what happens over the weekend Laughing
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Re: Need help troubleshooting a Cox .049 engine

Post  Cox International on Sat May 26, 2018 8:13 pm

OK, the engine now runs fine with a runtime of 3.5 min @ full RPM (13,500 - 14,000 w/ 6x3 prop and 25% Nitro, OEM # 325 glow head). However, what is interesting is that, when the head is changed to this version: https://coxengines.ca/cox-.049-glow-plug-adapter-texaco.html runtime is between 2 and 2.25 minutes; whereas RPM is the same as with OEM glow head.

Fuel tank is completely empty, confirmed by the 8cc of fuel in the syringe I used. I don't get it lol!

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Re: Need help troubleshooting a Cox .049 engine

Post  Oldenginerod on Sat May 26, 2018 8:30 pm

Cox International wrote:OK, the engine now runs fine with a runtime of 3.5 min @ full RPM (13,500 - 14,000 w/ 6x3 prop and 25% Nitro, OEM # 325 glow head). However, what is interesting is that, when the head is changed to this version: https://coxengines.ca/cox-.049-glow-plug-adapter-texaco.html runtime is between 2 and 2.25 minutes; whereas RPM is the same as with OEM glow head.

Fuel tank is completely empty, confirmed by the 8cc of fuel in the syringe I used. I don't get it lol!

So did swapping out the backplate fix the stability problems? Are we just dealing with an engine that's not fuel-efficient?
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Re: Need help troubleshooting a Cox .049 engine

Post  Levent Suberk on Sun May 27, 2018 3:55 am

You changed bent needle valve?
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Re: Need help troubleshooting a Cox .049 engine

Post  fit90 on Sun May 27, 2018 7:56 am

Cox International wrote:OK, the engine now runs fine with a runtime of 3.5 min @ full RPM (13,500 - 14,000 w/ 6x3 prop and 25% Nitro, OEM # 325 glow head). However, what is interesting is that, when the head is changed to this version: https://coxengines.ca/cox-.049-glow-plug-adapter-texaco.html runtime is between 2 and 2.25 minutes; whereas RPM is the same as with OEM glow head.

Fuel tank is completely empty, confirmed by the 8cc of fuel in the syringe I used. I don't get it lol!


I have had similar experiences with parts built Texaco engines. I have found that with much larger props the engine will run better with the hot insert plug. But, the run time is noticeably shorter. I still don't have any idea why. I am using Sig Champion 25% and either a Cox 7x3.5 or Cox 8" (I don't remember the pitch) prop.
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Re: Need help troubleshooting a Cox .049 engine

Post  getback on Sun May 27, 2018 8:22 am

Bernie , how many hd. gaskets did you use ? If it up'ed the compression then i would think it would use more fuel , if switched to a smaller prop it will turn up more and bee more fuel efficient turning more rpms . It is possible the engine is calling for more fuel because of the load and the higher comp. Memorial Day
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Re: Need help troubleshooting a Cox .049 engine

Post  Cox International on Sun May 27, 2018 12:25 pm

I swapped out the complete backplate, including the bent needle and the engine now runs fine.

First used an OEM head with one gasket and then the 5-fin conversion w/ hot insert and three gaskets (because the 5-fin conversions induce higher compression).

What I don't understand is why the fuel does not last as long when the same prop is used and the same RPM is achieved. Regardless of if the compression is higher or not, if the RPM remains the same, should fuel consumption not remain the same?

But if that is how it works, would decreasing the compression (adding more gaskets) decrease fuel consumption?

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Re: Need help troubleshooting a Cox .049 engine

Post  Surfer_kris on Sun May 27, 2018 2:00 pm

Cox International wrote:

But if that is how it works, would decreasing the compression (adding more gaskets) decrease fuel consumption?


Yes, I think that you need to decrease the compression further in order to have the same run-time.

The compression ratio and the amount of fuel can both be thought of as a way to alter the ignition point. With a higher compression ratio the engine will fire earlier, a richer needle setting will counteract that. So if/when you decrease the compression ratio and the needle will have to come in a little in order to have the same timing, and the engine will thus run longer on a given tank volume.

To add to this there is also the heat-range of the plug, a hotter plug will make the ignition occur earlier and it will thus need a richer fuel setting to counter-act that. So it is possible that the inserts not only provide a higher compression ratio but also a hotter plug, both these things will have to counteracted by a richer needle setting.
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Re: Need help troubleshooting a Cox .049 engine

Post  Cribbs74 on Sun May 27, 2018 3:37 pm

What Kris said is correct, a simpler way to put it is you made the engine less efficient by using the 5 fin head. With the stock head is was able to put out the same power while using less fuel to do it.

So, best bet is to go back to the stock head or keep adding shims to what you have now.
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Re: Need help troubleshooting a Cox .049 engine

Post  coxaddict on Sun May 27, 2018 6:58 pm

I saw on You Tube where someone made a clear cylinder head for a glow engine.  You could see the fuel coming out of suspension sloughing around in the combustion chamber and not burning. Probably due to the acrylic cylinder head not keeping the fuel hot enough to remain atomized.  Maybe the same thing is happening with the 5 fin clamp.
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Re: Need help troubleshooting a Cox .049 engine

Post  Cox International on Mon May 28, 2018 10:09 am

Thanks for all the replies Smile

This engine was designed to be run with an 8" prop and at low RPM and we have reports of customers getting runtimes in excess of ten minutes with our engines (they made some mods). I was running it a full-tilt with a 6x3 prop.

Will send the engine back to the customer and explain the compression thing.

Thanks again for the support  Hand Shake
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Re: Need help troubleshooting a Cox .049 engine

Post  GallopingGhostler on Mon May 28, 2018 10:47 am

Cribbs74 wrote:What Kris said is correct, a simpler way to put it is you made the engine less efficient by using the 5 fin head. With the stock head is was able to put out the same power while using less fuel to do it.

So, best bet is to go back to the stock head or keep adding shims to what you have now.

You're right, when turning a smaller prop, then it is no different than any other Cox reedie. That 5 finned head would be best put to use when turning a larger diameter prop like a low pitched 8". The exploding fuel-air charge is doing more work to push that larger more resistant to the air stream prop, increasing tendency to overheat with the standard glow head.

Plus, a Texaco aircraft clipping along at say, 25 mph churning an 8" prop has less wind past the head versus a trimmer smaller aircraft flying at 35 mph or higher with a standard prop.

What impresses me is that these little Cox reedies are torquey buggers, still producing work at lower rpms. One doesn't need to be at the peak rpm for them to produce good work. Very Happy
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