The disappointing OK Cub

My guesses:
1- dirt or debris in fuel line, spray bar hole clogged, not allowing full fuel flow, or the tip of the NV is broke off inside the spray bar?
2- the rust on the splines of the prop driver makes me want to think there is rust or castor goo causing friction on the case to crankshaft mating surfaces between the drive plate and intake opening.
3-cylinder to head seal leaks. Do you have another Cub to swap heads or glow plugs with?
Clear our the passageways for the fuel, and air, then make sure you’ve got a good hot glow plug. gotta be one of three things choking the RPM’s ....but you know that!

These little engines can't really run at such low speed. Was it maybe bouncing back and forth, not making it through TDC?

IIRC Bob's tach is times 10 display so I assume 2,500 rpm

and I would be wrong

I have never had any of those Cub metal tank mounts, but I would suspect the tank screws are leaking fuel into the engine crankcase in some manner. If fuel were being drawn from the spray bar/venturi surely the needle valve would be effective?

A similar problem was caused by the red tank mounts on a Cub 049A I worked with. A tank mount screw had perhaps been screwed into the crankcase without the tank, or too long a screw used, as the screw hole had been punched through into the crankcase. This allowed fuel from the tank to leak slowly along the case screw channel and directly into the engine. The engine was difficult to start, and would run very rich when it did start, but shortly flood to a stop!

Bill

Thanks guys, yes, great compression. Except for the rusty prop screw and alignment peg it looks new and never ran so I did not disassemble it other than checking the needle valve.

Times like this is wish I used the movie option on my camera and had a youtube account. Almost signed up last week but they ask to many questions.

Maybe my 250 rpm's was a bit low, but it was running and did so for about a minute and a video would confirm this. It was eager to start with lots of good feeling pop, you know what that is like but when running it would not change rpm's through the whole NV range. Curious. First time I have ever seen this.

Dude, Why struggle? Use it as a doorstop and go with Cox power! We all know it works!!!! Just teasing,Mark

Mark Diedrichs wrote:Dude, Why struggle? Use it as a doorstop and go with Cox power! We all know it works!!!! Just teasing,Mark

It's the challenge Mark. Read my Cox misadventure...….

I hear you Bro, I once had a weedwacker that I wouldn't let beat me!!! All about the challenge, Mark

i go with number 2-3 anyway stopped up NVA can bee just a tiny piece of crap messing with you bad !! Been there done that !

Getback is prolly right, though there are a number of things that a hard core Cub guy can do with these engines. Nostalgia F/F guys like to hop them up and get some pretty impressive results in terms of power to weight ratios.

http://www.flyfreeflight.com/Site/OK_Cub_Rework.html

The design itself is sound as it is based largely on Ray Arden's .099. The problems arise from inconsistent QC practices and metallurgy.

I got a load of 4 or 5 a long time ago when there was a sale. It said they were for display or collection. I guess that is all they are good for. Two would not run at all. I sold the one before I knew and felt bad about it. They were all new. After reading lately, I have heard there is a bad seal at the needle valve where it screws into the housing. Maybe check over the needle valve, blow though and see if it opens up enough after a few turns or whatever. I should go over mine too. The one that ran seemed to run well enough to pull a plane.

smooth_bill wrote:..... I would suspect the tank screws are leaking fuel into the engine crankcase in some manner.

These tanks don't have any screws into the case. Only mounted on external lugs. The screw-in backplate could possibly leak, drawing fuel straight into the case. Bob, just try again using an external tank and leave the mounted tank empty.
Also, use a small prop. These don't like to lug.
Some of the Cubs didn't have the port in the crank ground out properly. Is this the first model .049 (pre-"A"/"B") or is it an .074? These tanks only fit those two models.
Honestly, I've never had much success with the first model .049 even when everything seems ok. .049B was a much better engine.

rsv1cox wrote:
Maybe my 250 rpm's was a bit low, but it was running and did so for about a minute and a video would confirm this.  It was eager to start with lots of good feeling pop, you know what that is like but when running it would not change rpm's through the whole NV range.  Curious.  First time I have ever seen this.  

Sounds like it might just have been rocking back and forth...

What prop-size did you use?

Thanks all for your responses.

For now I have retired it to a display stand but intend to remove the tank and see what's up.  I also need to run some Remoil spray through the NVA and blow it out with compressed air and try again.  Just got fed up yesterday with balky engines after a rewarding day with the Testors/McCoys the day before.

I do like this engines design, the only one I have with that style attached tank.  Although the top mount hole is nearly impossible to get to.





A couple of years ago I sold on ebay 40-50 engines mostly Cubs, Wens, and McCoys.    The extent of my Cub collection now.  I usually keep the propeller with the engine as received.  I know I ran one of the .099's just to see what they were like but I don't think I ran any of the others.  I know I never ran the diesels.



Oh yes Chris, I think the prop used was about a 6/3.

i WOULD CLEAN OUT THE SPRAYBAR AND MAKE SURE THE DISCHARGE HOLES ARE NOT PLUGGED.  MAKE SURE THE SCREW IN REAR COVER IS SNUG.  I GOT 5,800 TO 13,800 RPM ON MY USED CUB.  MY TEST RESULTS ARE ATTACHED.  THE SHORT STROKE 049B IS WAY FASTER THAN THE LONG STROKE CUB.

One day I realized I had an HKG plug in my Cub instead of an Arden.  While I was putting the Arden in, it occurred to me it was a long, not a short plug as is common in .049s.  But as the Cub was first, there was no long plug to put in it.  So I sorted out some props and plugs and went out to run them.  I used Cox 25% nitro fuel.  You can see the Arden plug worked noticeably better than the OK Long or Fox long plugs.

I also ran an .049B on a couple of the props.  It was at least a couple of thousand RPM stronger than the Cub on any of the props.

If it has a good piston to cylinder seal and a good glow plug, it should run.

I still thinking that it didn't actually turn over, only rocked back and forth?
This can easily happen if the compression ratio is too high. Perhaps try with less nitro and a different prop.

The needle looks very blunt or perhaps even too short? You can check the operation by blowing through a fuel line and listen to the "hiss". There shouldn't be any air coming through when the needle is closed, and if you open it until you hear a slight "hiss" you have a good (enough) starting point for the needle setting.

If it has a good piston to cylinder seal and a good glow plug, it should run.

I still thinking that it didn't actually turn over, only rocked back and forth?
This can easily happen if the compression ratio is too high. Perhaps try with less nitro and a different prop.

The needle looks very blunt or perhaps even too short? You can check the operation by blowing through a fuel line and listen to the "hiss". There shouldn't be any air coming through when the needle is closed, and if you open it until you hear a slight "hiss" you have a good (enough) starting point for the needle setting.

If it has a good piston to cylinder seal and a good glow plug, it should run.

I still thinking that it didn't actually turn over, only rocked back and forth?
This can easily happen if the compression ratio is too high. Perhaps try with less nitro and a different prop.

The needle looks very blunt or perhaps even too short? You can check the operation by blowing through a fuel line and listen to the "hiss". There shouldn't be any air coming through when the needle is closed, and if you open it until you hear a slight "hiss" you have a good (enough) starting point for the needle setting.

Geez Kris, I don’t mean to nag, but you’re beginning to sound like my ex-wife!!  

Yes Honey, I heard ya the first time.....

Marleysky wrote:Geez Kris, I don’t mean to nag, but you’re beginning to sound like my ex-wife!!  

Which part is that, the too short needle or the rocking back and forth...

Sick, sick, sick is all I can say.

ticomareado wrote:Sick, sick, sick is all I can say.

But funny!  

Thanks Bill, I will bring out a few of those Cubs and swap needles and glow plugs around when I try it again.

Anybody got a Cub just like this one? Picture?

Bob