TD .051 300-stroking!

Hi Ken -
It's a Cox 5x4 black prop, stock Cox head, my mix: Omega 23.5%N/22%(50/50 castor/synth) fresh, Cox red tank mount on the bench, no cracks in the body, jets clear, don't know glowhead age (I got it from Ron C a couple months ago) but it appears to be nearly unrun.
It does turn higher rpm when opening the needle beyond 4 1/2 turns.

Terry opening the needle is telling me that the engine is struggling to draw fuel. The red tank mount has a insert on the rear of it which is glued in. There's a possible chance that it could be leaking air. A pressure test under water could reveal this. I'm not a big fan of those mounts. I would also place a small piece of squarely cut tubing over the needle valve if it's not already in place. Everything else you mentioned seems to be good. Do you have another venturi to compare the one your running with? Sometimes they have been opened up to run on crank or bladder pressure. You could choke down the venturi by squeezing a a small piece of tubing down the throat of the venturi and run it again, just don't block the jets with the tubing. Many times the TD was offered with several head shims from the factory in the late 80's. They came with 3 in place and sometimes they had a additional one in the box. Not that this would change your running issues, but it may offer more rpm's if they were removed if still in place.

Thanks for the suggestions, Ken. I'll check those out and try a standard tank.
Results to follow.

And it turns out to be something simple - glow clip leads were shorting intermittently.
Fixed that and it runs 18.8K on the above fuel/prop w/2 shims.
I don't know how that stacks up against most 1/2A TDs but it's a good bit more than my BWs.

wha-tah-hey wrote:And it turns out to be something simple - glow clip leads were shorting intermittently.
Fixed that and it runs 18.8K on the above fuel/prop w/2 shims.
I don't know how that stacks up against most 1/2A TDs but it's a good bit more than my BWs.

Good deal Terry. A bad glow-clip gave me fits a couple weeks ago. I was trying to bench-run a couple of rebuilt reed-valve engines.. and couldn't fathom why either engine wouldn't start past "sputtering". It turned-out that one of the wires in the clip I was using was literally hanging by a thread. Swapping to a different/better clip made all the difference.

Glad that you figured it out!

As much as I love most everything Cox, the one product I just hate to use is the Cox glow plug clip. I use the aftermarket clip from Bernie on .049 and .020 engines, and I use a Holland Hornet glow plug clip for .010 engines. The Cox clips always seem to have poor connection on the glow head after they get corroded and fuel soaked. Some day I'm gonna have to try soaking a clip in vinegar to see if it cleans up the contact area and makes them work better. I've tried burnishing them, but I still don't like the contact that I get. I'm sure it's just me. But there you go.

The Sour Mark

I am glad you have finally solved the issue regarding the clip.I second to all opinions here regarding poor fuel draw. One more point of air leakage maybe along the crankshaft. Some TD-s abused on unbalanced props will have their ckankcase fit along the shaft loosened up, thus making the start up and run of the engine very cumbersome. The problem aggrevates if the engine is hot because when heated, the aluminum crankcase expands radially a tad more than the steel shaft and the air leakage along the shaft thus increases. (Actually this can be felt with your fingers when you try to move the shaft end radially when the engine is hot. I once bought a TD on ebay that had this problem and once I swapped the crankcase with a less used one, the engine woke up and run like all good TD-s.)

The symptom apart from poor run is fuel dripping at the prop drive plate when the engine runs.

My TD049/051 beasts run typically at 20+k on the bench with my favorite 5x4 black nylon prop sold by Bernie (https://coxengines.ca/cox-.049-propeller-5-x-4.html) , the fuel is 60/20/20 (all castor). They unload around 23k in the air....I audiotached one of my vids on youtube with my veteran (250h+ hours runtime) TD 051 powering the R/C Toucan.

Air leak was definitely a part of the problem.
While the engine has almost no run time on it, not so the tank mount.
I found both aluminum tubes to be loose and examination showed multiple cracks in the plastic material.
Time to retire it to display use only.

wha-tah-hey wrote:Air leak was definitely a part of the problem.
While the engine has almost no run time on it, not so the tank mount.
I found both aluminum tubes to be loose and examination showed multiple cracks in the plastic material.
Time to retire it to display use only.

Have you thought about making a mount so you can run the engine some more? I made a mount that you could try. I used a piece of 1 x 2 scrap-wood.. and bored a 3/4" hole thru on-center, near one end. Then I used a saw to bisect that hole.



The rat-tail file was used to cut a relief for fitting an OK Cub.. so you could skip that step.



Keep in mind that the screws will be biting into "end-grain".. so long screws would be better. This is just a "quickie-mount".. and using harder wood would be much better than my fir example. Position the engine and mark the locations using an awl for drilling pilot-holes for the four screws. This mount can be clamped in a vise or you can drill two small holes for attaching with wood-screws to a post outdoors. An L-bracket attached to the back can be used to fit a tank-platform.

Great Idea, Roddie, but I'm ahead of you.
I glued maple strips to cheap panelling, drilled for mounting bolts (still have to get some), RB'd a tank and it's good to go.

I need to balance the prop but the new setup helps a lot.
It needles correctly now and it picked up ~300 rpm to 19.1k on my 23.5N fuel/Cox 5x4.
More nitro'd no doubt raise the rpm but I'm not chasing max performance.

Thats a Good looking engine Terry , good to hear yer getting things together and good runs

Thanks, Eric.
I went to only 1 shim and picked up a good bit more rpm on the various props.
I couldn't get the Cox 5x4 to balance so ordered some from Bernie, but a wildfire has him shut down so guess my TD testing is on hold for a while.

In my experience TD049/051 engines overheat with only 1 shim, with 20% nitro in the fuel. My best runs have been achieved with 2 or sometimes 3 shims. Even with 2 or 3 shims the cylinder wall, the top of the piston and the glow head shows rapid precipitation of varnish, but - other than more frequent need for devarnishing - the engine runs happily.