I have been running a TH .75 for about 2 years now and it hasn't given me much more than a headache! I have it now installed in a BH Matrix but everytime I run it for over 3 minutes, it shuts down. Both times I've been able to get it back okay, with only the first deadstick ripping out the right wheel pant and axle.

I am running it at 11k with an APC 12x7 prop. When I tested on the ground, it also would run about 2 minutes sounding healthy and then aferward it would loose some power.

I guess it may be overheating but I don't think I have too few holes. I mean some of the engines run purely in cowl. Right now I have maybe a 1 in to .7 out.. Could this not be enough?

From what I remember, You should have at least 3 times the exit area as intake, so if you have a 2 inch intake, you should have a 6inch exit. It also sounds a little like you might be running it *lean*. 2 things you can try. First, pull the cowl and fly without it. If the problem goes away, you know it is an overheating problem. 2nd, richen it up. maybe 1 full turn out. Try that and see what happens. Let us know.
C2K
(edited word in stars)

it's possible that maybe it needs a new glow plug,it may not be able to handle the fuel and gets snuffed out? I installed a new plug in my chopper, hovered it a few times the day before went out the next day did a few hovers fired it up for the third time that day set the idle up before take off and it would all of sudden fall flat, installed a new plug and wolah!.. it was fine...the matrix has plenty of air intake that isnt your problem.

I think canopy has a good suggestion. Only thing is that I have to wait until the weekend to try it!!! And if it doesn't work, I have nothing to fly!!! :-( I'm on the verge of just ditching the evil engine and buying another!

Glow plug is fine since it fires up again without changing anything.

But we'll see... thanks for your suggestions.

Is the engine inverted? Where is your fuel tank in relation to your carb? I have TH 75 in a bashed Four-Star clone biplane. It just has cheeks, no cowl. This engine is a power house for me, and has never given me trouble. It is mounted upright, fuel tank centered with the carb. I always leave the ground a tad rich, and let it lean out in the air. You may be leaving the ground too lean, and it leans out even more in flight.

From my limited experience with TH 75 it like to be a little on a rich side.. The only problem I have so far (knock... knock on wood ) is that it is actually do not want to stop when you shut it down... And as far as I know this is a "known problem" with somewhat "air leaking" carb. Good luck! Let us know how it works out for you.

Deadeye, pretty plane you have. I have my engine cowled with the carb in line with the tank. I fly about 1/4 to 1/3 turn out of peak. I've also had some leaning problems once in the past so I do run her a little rich. Now I have tried to run her richer without too much success. I make sure that a little fuel is coming out of the exhaust.

The last flight I had with this engine was when I leaned her out too much and she faltered as she turned away from the runway (after flying 3-4 minutes). After that landing, I've not used the engine since. Now when I fire it up, there is some clacking sounds, like metal to metal. This sound is less audible after it warms up but is still there. I'm wondering if the last flight I had I had leaned the engine so much I bent the rod or caused other internal damage. Any bad experiences like that!!!??!

I'm not familiar with that particular plane, but if you have less exit than entry (if I read your post right 1 to 0.7) then you could definitely have a cooling issue. I think flying it without the cowl is the trick to helping to ID the problem.

If you don't get it to work, then box it up and mail it to me and I'll throw it out for you....

Duke

Dukester - you know, I know that there is suppose to be a 1 to 2 inlet to exhaust ratio but I rarely see that. I mean I see so many Corsairs, Jugs flying without any exhaust outlet other than for the muffler. Is it absolutely necessary for adequate airflow?

By the way, when I took apart the engine, I realized the liner may have been off alignment. Meaning that all the ports were off up to 30%. If that was the case, would that potentially be the culprit? Otherwise everything was tip top.

In the first place, try flying the plane with the cowl off (recheck the balance first). That will validate any overheating problem.

I've heard all sorts of formulas for inlet/outlet ratios. The most valid one comes from Andy Lennon who recommends 1:1.4 inlet to outlet. To keep his engines cool, he baffles the inside of the cowl so that all the incoming air cools the engine and doesn't become trapped in 'dead-air' pockets. All the airflow is used. The baffling is done with lightweight stuff so as not to weigh down the front of the plane.

Just my $.02

Bob

N1EDM - thanks for the word of advice. I will block the left inlet. That should give me about 1 to 1.4... Yippee!!!

Fourstroke, I think you're missing the boat. If you only have 1 square inch of inlet area, taking away half of it is not going to improve the situation. That is not nearly enough inlet. Your engine is overheating and you need to give your engine more cooling air.

I know this from personal experience. I installed a .75 in a fly baby. I drilled some holes in the front for cooling, but tried for the most part to preseve the cowl. I cooked the engine good on the first flight and it died after about 3 minutes. I deadsticked in immediately and could still feel the heat when I got to it. I cut about 2 square inches of inlet and about 3 square inches of outlet. The inlet location is in front of the cylinder and my Pitts style muffler (side mount). This provides plenty of cooling right where I need it. I haven't had any further problems. Luckily it doesn't appear to have ruined the engine, but I'm sure it cut the lifespan. Cooking engines is NOT good for them.

thanks ksechler - i don't think I have too little inlet at this time. take a look. If I close off one of its inlets (naturally the one away from the cylinder head, I still think there should be enough breathing space. What do you think? I actually have taken out the chunk right in front of the carb as well.

Fourstroke is giving good advice for your plane.

If I can add my own $.02, I wouldn't take away ANY inlet space. I'm not sure that you have enough now. I wouldn't plug any holes at

all. Just have more outlet area....

Bob

It's gotten worse. I opened it up today and I think I've found another problem... the clunk fell off again!!! Third time!!! I think me and this plane just isn't meant to be!!! I'm going to put a tie wrap around it this time :-( I'll keep you guys informed! Cheers.

As a general rule (even with full scale aircraft) you should have at least 1-1/2 times the outlet area as you do inlet area. Having more outlet area than inlet creates a vaccuum that will actually suck more air into the cowling and past the engine thereby cooling it better. Having more than this amount of outlet area won't necessarily create more vaccuum depending on the design of the cowl. Inline cowlings like say a P-51 Mustang cowl will not cool any better by having two or three times the outlet area whereas a radial cowl like on a Corsair tend to work better with as much outlet area as they can get. Most scale models of inline or tubine engined aircraft need to have more cooling inlet area than what would be scale because the full sized birds have radiators and intercoolers to keep them cool whereas most scale models have air cooled engines that need larger inlet area to promote better airflow and cooling. Try enlarging the inlet area and the outlet area and I would be willing to bet money that it will help to solve your problem if not correct it completely!

Also check for air bubbles in your fuel line and make sure that the clunk in your tank isn't too close to the back of the fuel tank. I Always test run my engines without the cowling and make sure that everything checks out before installing the cowl. That way you will know if the engine doesn't run properly that there is a problem with the way the cowling restricts cooling or fuel flow to the engine.

Good luck!

Heavy Metal - The inlet/outlet radio was no problem. had at least a one to two ratio on the cowl. Recently I switched to using 30% cool power and it has helped to alleviate deadsticks. I'm not sure why that would be but at 15% nitro, the engine wouldn't run worth crap. I think some of it is the prop I put on it. The APC 12 x 7 just may have been too much prop with 15%.

Fourstroke,
Things like having the clunk fall off, etc., happen to all of us. Don't be disillusioned. We all go through it. At least you found the problem, found out why it did what it did, and you fixed it. It'll never happen on your next plane.

Another thing about the cowl... is there a lot of 'dead air' space in there? Sometimes that can be filled with balsa or foam so that the air gets channeled around the engine and not into pockets inside the cowl. You just have to re-check your CG after you do any work on the nose.

I'm sure, in the end, the problem will turn out to have a very simple solution...

Bob

N1EDM - thanks for the words of advice and encouragement. i have broken in over 10 engines and flown over a dozen so i'm okay in terms of knowing what to look for. naturally having had so many planes doesn't really mean that you are any better at tuning an engine than someone who is just starting, but... i think i am okay. i just think my 75 was a dud. or rather is a dud... with the 30%, it runs okay... but no where enough power for its gas consumption.

It would be interesting to see what would happen if you added extra castor oil to your fuel. I usually top off a fresh gallon of store bought fuel with 6 to 8 ozs of castor. I have heard extremely good and extremely bad reports about the .75, after 2 years of struggling with this engine, even bench run problems, there must either be a random air leak or run away heat condition that causes the liner to grow out of control. Castor can solve both problems to a certain degree.

I agree with combatpigg. Extra castor will make the engine run cooler & will actually restore a bit of lost compression. I had an OS .46 FX with a peeled liner that wouldn't keep running on standard fuel, but I boosted the total lubricant content to >25% with drugstore castor & the FX would stay lit --it wasn't like new, but it was usable.

30 percent nitro is too much nitro for this engine. I believe you mentioned hearing metal noises when flying - that's pre-ignition from running too much nitro and running too lean. If this engine doesn't run well using 10 to 15% (with a 12x7 prop), then you have cooked the engine. One of the worst combinations you can run into is runnning a high nitro with too lean a mixture and a hot plug. Certain death for an engine.

ED STEFAN - i couldn't agree more... the engine is not WORTH putting 30% in!!! lol... i'm going to sell the lump and hope to buy some more 30% for my O.S., YS and Saito engines :-D lovely! even if i can only get a tank, it's better than trouble!