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Post  Tobaiies Wed Apr 20, 2022 3:49 pm

Hi Guys,

I have a little problem with one of my Cox pee wee's.
I can start the engine without a problem at about 3 turns at the needle calve. But if I close the needle valve, the engine stops, if I open it further it starts to rev up. why is this the case?
In addition, it stutters firmly at lower speeds.

I cleaned the engine and changed all the gaskets and the fuel hose. I use a new needle from Cox International.
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Post  davidll1984 Wed Apr 20, 2022 5:02 pm

I tink it discribe a air leak mabe As I can not be there to see but possible that even with the needle unscrew at max the engine does not change speed keep screaming as hell mabe the spi To large ?
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Post  Ken Cook Wed Apr 20, 2022 5:03 pm

Hopefully, there's not a piece of a broken needle from a existing needle at the bottom of the seat. You need to thoroughly insure the hole is open and free. This entails cleaning out the tiny hole at the bottom of the seat and the hole perpendicular where the o-ring on the tank back seats.
I would take the engine apart and insure the reed valve is working properly. This means hooking up a syringe to the venturi stack where the o-ring seats. You ABSOLUTELY shouldn't be able to withdraw the syringe. If you can, the reed isn't seating properly. Take it out, clean it, flip it over, or get another. Just because it's new, doesn't mean it has to work. Try the syringe test again. The syringe should be able to blow air through, just not suck any air.

Reassemble the engine, screw the needle valve down until it seats, don't overtighten it. Using the syringe, plug the fuel nipple with a finger or a capped plug and use the syringe with a piece of fuel tubing on the opposing nipple. Pressurize your tank and listen for air leaks. Technically, no air should be escaping and the syringe should fill the tank with air and not be able to be fully depressed. The syringe should actually rebound back at your fingers. If you hear air internally, it could be leaking from multiple areas. Needle valve seat on the top of the tank, screws that retain the backplate on the rear, the tank to tank back connection, the tank to crankcase connection. You need to address any of these areas, You can lightly sand the rear of the crankcase using 400 grit paper and a little oil on a piece of glass to lap the case flat. If need be, new gasket at the case to tank, Screws are a bit more problematic, I like to just replace them as new ones can sometimes remedy a leaky screw.
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Post  davidll1984 Wed Apr 20, 2022 5:05 pm

addition, it stutters firmly at lower speeds ok ....???but did it spit oil wit unburn fuel ???
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Post  Tobaiies Thu Apr 21, 2022 2:56 am

Thank you for the detailed help!
I'll try it tonight.
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Post  Tobaiies Thu Apr 21, 2022 3:00 am

davidll1984 wrote:I tink it discribe a air leak mabe As I can not be there to see but possible that even with the needle unscrew at max the engine does not change speed keep screaming as hell mabe the spi To large ?

Right, theoretically I could turn the needle all the way out, or all the way in and the motor would still run.
The fuel burns up nicely, leaving only the castor oil.
What is the spi? 😅
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Post  706jim Thu Apr 21, 2022 10:23 am

Sub piston induction. When the piston is at the top of its stroke, you can see a little space between it and the exhaust port. Air gets in there during operation.
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Post  Tobaiies Thu Apr 21, 2022 12:45 pm

Ken Cook wrote:              Hopefully, there's not a piece of a broken needle from a existing needle at the bottom of the seat. You need to thoroughly insure the hole is open and free. This entails cleaning out the tiny hole at the bottom of the seat and the hole perpendicular where the o-ring on the tank back seats.
           I would take the engine apart and insure the reed valve is working properly. This means hooking up a syringe to the venturi stack where the o-ring seats. You ABSOLUTELY shouldn't be able to withdraw the syringe. If you can, the reed isn't seating properly. Take it out, clean it, flip it over, or get another. Just because it's new, doesn't mean it has to work. Try the syringe test again. The syringe should be able to blow air through, just not suck any air.

           Reassemble the engine, screw the needle valve down until it seats, don't overtighten it. Using the syringe, plug the fuel nipple with a finger or a capped plug and use the syringe with a piece of fuel tubing on the opposing nipple. Pressurize your tank and listen for air leaks. Technically, no air should be escaping and the syringe should fill the tank with air and not be able to be fully depressed. The syringe should actually rebound back at your fingers. If you hear air internally, it could be leaking from multiple areas. Needle valve seat on the top of the tank, screws that retain the backplate on the rear, the tank to tank back connection, the tank to crankcase connection. You need to address any of these areas, You can lightly sand the rear of the crankcase using 400 grit paper and a little oil on a piece of glass to lap the case flat. If need be, new gasket at the case to tank, Screws are a bit more problematic, I like to just replace them as new ones can sometimes remedy a leaky screw.

Everything in the engine looks and works fine.
The tank was not completely tight at first, some air escaped from one of the rear screws. I was able to tighten them all a little more and the tank was sealed.

So I tested the engine, but found that the problem was not solved.
I was able to start the engine at about 5 1/4 revs when I started to close it died after about a rev.

Can this now be due to the SPI? How do I know it's too big? and how could i fix it?Cox pee wee will not set properly. 358ac511
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Post  Cribbs74 Thu Apr 21, 2022 2:48 pm

The Pee Wee is finicky. Have you done what Ken suggested yet? That will fix your problem.

All my Pee Wee engines run excellent because I listened to his advice.
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Post  Tobaiies Thu Apr 21, 2022 3:25 pm

Cribbs74 wrote:The Pee Wee is finicky. Have you done what Ken suggested yet? That will fix your problem.

All my Pee Wee engines run excellent because I listened to his advice.

Yes, everything as he described, so I'm surprised it didn't get better.
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Post  Ken Cook Thu Apr 21, 2022 6:46 pm

You need to define what you mean by rev. Is this a reference to the amount of turns you have the needle valve out? In addition, the SPI is irrelevant in my opinion. I currently have six Pee Wee engines currently in use. While I don't use the tank, I use a remote tank, SPI is present and I rarely do a ball socket reset. I don't make a habit of taking these apart unless they're not working at all.  The engine is designed to have SPI and what I see in your pic appears completely normal compared to mine looking at it visually.

             SPI can increase if the ball socket becomes stretched. Resetting the ball socket will tighten up the play. If your unfamiliar how to do this, you can very easily destroy the piston. If you have the piston out of the engine, hold the piston and push and pull on the rod. You should have little to no play. However, if you reset the piston which will require the special tool, your still going to have the gap under the piston because this is how the engine was designed. Typically, too much SPI causes initial starting to be problematic.

          I have to ask, are all the internal serviceable parts new? If not, your fighting a uphill battle. The copper beryllium star reed that was stock in most Pee Wee engines is so fragile that looking at it can ruin it. Have you removed the fuel line off of the barb on the backplate? Using heat to remove it is helpful and important in my opinion, it will prevent the old white metal barb from breaking as this is very common on the Pee Wee backplate. Push a pin through the hole in the metal backplate and flush the hole with fuel or alcohol to insure it's totally free. Remove the needle valve and from the top down flush with force and if needed, put a thin pin or wire down the hole to clear out any debris or gook.

        If the reed has  even the slightest crease, dent, corner bent,  it's ruined. The other is the circlip, the circlip holding the reed in ISN'T uni directional, even though it can be installed both ways there's only ONE correct way. The reed needs to rotate under the reed clip.  The very last bend of the reed clip ( straight legged portion ) is bent upwards. This portions needs to be opposed to the reed and not against it squashing it and preventing it from moving. Therefore, once the reed is installed, take a X-acto knife and carefully push on one of the corners of the reed to insure it rotates under the reed.

              Then, do the suck and blow test as I described using the syringe. You need to do this even if the reed is new. The reed is the heartbeat of the engine and even the slightest leak will cause problems and problems exactly as your describing whereas the engine will run but not be responsive to needle adjustments. It runs rough and sometimes just quits.  You also need to insure your needle valve isn't leaking.

          Do as I said above in my first post, screw the needle down until seated, place a drop  of oil on the needle valve threads where it enters the backplate. Pressurize the tank and look for bubbles around the threads.
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Post  rdw777 Fri Apr 22, 2022 6:08 am

Hello Tobaiis, Good for you on trying to get your PeeWee to run. From your description of symptoms it still sounds like an air leak or reed problem. It’s trying to compensate getting enough fuel to match how much air is coming in and it’s way out of design spec so you get erratic running. Just because everything looks good inside the engine doesn’t mean that works good. The engine looks very nice on the outside btw, Probably very little run time.

Think thru each one of Ken’s test recommendations slowly and carefully and perform them. If you have trouble with any one item bring it up again. The only thing I can add to tank sealing: If you have trouble with the screw heads leaking you can pull some fiber from a cotton Q-Tip, Form into a little thread and use it for a gasket under the head. A little castor oil from the pharmacy helps hold it together while you install. The seal between the tank bowl and tank back is critical, I use a piece of cotton sewing thread to go into the groove in the tank bowl, Castor oil holds it in place during assembly. Also, Did you replace the tiny black O-ring gasket it the center of the bowl on the tube?

A couple of other basic things outside of air leak: We assume your fuel is fresh and has a decent nitro content, Around 25% works good on engines this size. Is the glow head new?…Just because it glows brightly doesn’t mean it will work properly, A new one at least takes one problem out of the equation..,, Does the engine behave differently while running with glow plug clip attached as opposed to removed?

Keep trying and let us know what you find out. Don’t worry about SPI at this point, It’s set at the factory for best running characteristics. Good luck!
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Post  Tobaiies Fri Apr 22, 2022 11:19 am

The problem was the reed clip. it was the wrong way as described.
I turned it around and changed the reed. (but didn't changed the small O-ring.)
Now the motor runs fine and is very accurate to adjust.

Nice to learn something new again!

I use fresh fuel with 35% Nitro, 20% castor oil and 45% methanol. The clow head is brand new.

The engine runs best with the needle valve about five turns out. Does this differ between different PeeWees because of different needle valves?


Last edited by Tobaiies on Fri Apr 22, 2022 11:24 am; edited 1 time in total
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Post  706jim Fri Apr 22, 2022 11:21 am

No two engines will run with the same needle settings. About 2 1/2-3 turns is most common for this engine
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Post  GallopingGhostler Fri Apr 22, 2022 12:25 pm

706jim wrote:No two engines will run with the same needle settings. About 2 1/2-3 turns is most common for this engine

Yup, an understatement. I have found this observation most interesting in my personal experiences. This .049 Babe Bee probably from the latter 1960's after clean up, a field find by my son from an open field in Gallup, NM back in the mid 1990's, required the needle to be opened a full 4 turns to run properly. This was odd, because as instructions through the years, start from about 2.5 to 3.5 turns open (also threading made a difference, course versus fine, etc.), then close to peak around 2.5 to 1.75 turns, then tweak a quarter to half turn rich for flight. (Leans once in the air.)

Cox pee wee will not set properly. Restor11

None of my other engines required the needle to be opened 4 turns for peak. Huh...
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Post  getback Sat Apr 23, 2022 6:49 am

I would think needing more turn out = more fuel , either a blockage in the fuel flow or engine has alot of mile on it WORN that it needing more fuel to fill the gap .
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Post  GallopingGhostler Sat Apr 23, 2022 8:12 pm

getback wrote:I would think needing more turn out = more fuel , either a blockage in the fuel flow or engine has alot of mile on it WORN that it needing more fuel to fill the gap .
Maybe if I stop using the lube loaded fuels for my vintage Testor's McCoy .35 Red Heads & K&B Green Heads might close that needle another turn. Wink
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