Almost lost the Twinstar a few months back. The engines are TT .36's. One died and the other started boggin down. They had quite a bit of time on them flying off grass. Since then I've taken the engines apart and cleaned them in a sonic bath paying particular attention to the carbs. All back together now and have run them in on the test stand. Now for my problem. One tops out 2k rpm higher than the other. When matched pretty close at the top and idle, they are nowhere close in the mid range. They both run independent great (excellent transition), just not together. They used to run perfectly together. Any ideas. I'm planning on putting them in a TF DC-3 eventually.
Edwin

Ed, two possibilities come to mind.

First make sure there's no air leak around the carb bases. Or around the clamping screws.

Second is a trick I learned from Johnny Brodbeck, lo these many years ago. Uneven head screw torque.

Mount the slower turning of the two engines on a stand, and while running at peak loosen one of the head screws, and tighten it to just a little more than snug. If you have a drop in rpm with the first, go to the opposite side and do that one. Work your way around to all of them, what Johnny pointed out was distortion of the cylinder from uneven tightening.

Since your engines were fine before tear down I think one or the other of these is your problem. Assuming you didn't get the pistons and sleeves mismatched.

A twin flies better, a real go-getter.

Bill

I neglected to mention they were not fine before tear down. When the engines where new on up to about 30 or 40 hours run time. I'm just guessing thats about the time one started not peaking out the same as the other. I DID notice that the cylinder head on one had a couple of head bolts that were not as tight as the others. Your head experiment makes since to me. I'll give it a try.
Edwin

Edwin:

With 30-40 hours you probably need new piston/sleeve assemblies.

You might want to investigate the "Bru-Line" air filters, catch a lot of dust, engine lasts longer. They are small, easy to install/maintain, and cheap.

The R/C car boys have good air filters available, but the really effective ones are big and ugly.

I'm using Bru-line, pleased with them. One of my twins has a pair of Magnum 46 XL engines, five years old with ??? hours on them, Bru-line from new, they are still like new.

Try the head screws, might help.

Let me know.

The TT Engines at 30 hours, have probably lost all their powers.

Bill.

Thanks,
I was afraid that would be the case. I was hoping to get about 80 or so hours out of those engines. At this time its kinda difficult getting TT parts. Been waiting 3 weeks on minor stuff. Think I might as well look for some different engines for the DC-3 when I get to it. Too bad, these engines were real screamers and were matched perfectly up to this point. Just loved watching that TwinStar go verticle on takeoff (so did the young guys in the club). It was highly modified to take the stresses. <big G>
Edwin

Edwin:

Talk to the car boys. Even with their good air filters they replace P/L sets commonly with only 10-15 hours, and that's for the "on-Road" cars. The dirty ones might last 4 or 5 using the inexpensive engines. And that is where the difference is most noted: the inexpensive engines usually scream, but the expensive ones scream a lot longer.

But don't trash the TT engines, you'll find P/L sets for them, they'll scream again.

TT's will go, K&B's more so.

Bill.

Before you do too much engine work, make sure you have matched props. Test multiple props on the same engine and match the two with the closest peak RPM.

If your engine out-of-sync is constant in nature and difficult to remedy, you can actually use mismatched props to correct for this. Again, test multiple props on the same engine, and then use two props with dissimilar peak RPMs and put them on the two engines appropriately.

Regards,
George

Mulligan:

George, that's a good thought regarding the props causing out of sync. But, using mis-matched props isn't a good idea - when you have rpm sync you have mis-matched thrust, and that's another problem.

If your twin always goes to one side,
there's something else yet to be tried.

Bill.

I hate to openly disagree with a poster but 20-30 hours on a TT (or any other engine as a matter of fact) is not a reason for a piston and cylinder liner change. However, a lean running engine (not run 300-400 rpm shy of max lean rpm) may experience such a shortned life.

Besides checking for airleaks and possible fuel line problems, the first thing I'd do is install new glow plugs in your engines-OS A3. It is not unusual for a glow plug that is going bad to cause the top end to drop off a bit.

Also, if you cleaned the engines real good (sonic bath) it is possible this caused your problem. Let me explain that I am assuming you burn a fuel that has some castor in it. It is not unusual for castor deposits to form in the engine that actually provide or improve engine compression. I had a perfectly good engine that ran fine that I "killed" when I cleaned it in antifreeze. After the cleaning the engine had no compression. The gastor gunk built up in the piston/cylinder assembly actually provided most of the compression seal. I wish I had never cleaned it!

If you suspect engine compression, here is a neat little tip that can add a little extra life to you engine (actually two different methods of doing this) without having to buy a new liner.

Method 1: Take a (very) small pipe cutter and cut/notch a 360 degree groove just under the top of the piston.

Method 2: Remove piston and using a hammer and a punch, punch the piston from underneat. (lay piston upside down on a wooden work bench). Install piston in cylinder, squirt a little honing oil and rotate piston to ensure nice fit. Reinstall all components.

Good luck and please let us know if you solved the problem.

Bill, you're absolutely right- one should not use mis-matched props intentionally. I didn't think that through before opening my yap

George, total absense of perspiration - I've done a lot worse.

Homebrewer, disagree. If the engines are used in a marginally dusty environment you can grind the pistons away in a pretty short time. You will note Edwin is in central Texas. Dusty air is a way of life. That's why I suggested the intake filters.

Regards piston refit: I've used the pipe cutter trick, never found it lasted too long. The whack with a punch is a new one. I have heard of "Cooking" the piston to make it "Grow," don't know how hot or how long. Seems I've seen it in one forum or another, but don't remember where. I have always replaced them when the rest of the engine was still good. AND, I prefer ringed engines except for flat out speed.

Whether new or rebuilt, make your twin go full tilt.

Bill.

Something could be said for linkage adjusting also. I saw a guy (a while back) tinker with his throttle linkage until he got the faster of the two slowed down slightly and sinc the two. I've never tried it, but it made sense to me.


Just my $.02 hope this helps

TE, you're right. All the way down to servo arm and throttle lever angles. Everything has to be set properly to get up and down synchronization, that's one of the many reasons I like to use the electronic synchronizers. There is always the slight difference from one engine to the next, set everything as exactly as you can, the engine computer does the rest.

Twins synced to a sweet song, Man I can fly all day long.

Bill.

The engines were running way out of sync before I tore them down and cleaned them. That was why I did that. I've swaped glow plugs and tried 2 new ones. I've been using tower cheapies. They've worked for me for many many years in the 2 strokers with no problems, so I dont want to hear about tower plugs being crap. You wont convince me. I've also tried mccoys (dont remember the number). Only tried OS plugs once about 10 years ago and didnt see any advantage to using them. Made no difference in engine performance. Cost didnt justify them and I can honestly say I've only burned out one tower plug in 10 years. All other suspected plug problems turned out to be something else. I cant say without a doubt, but I dont believe the engines have been run lean. I'm not exactly a novice at this. Both engines run great separately. They just dont seem to be a good pair now. My last resort was to adjust the linkage to see if I could get them closer. I am going to do that.
Edwin

Sir,
If I could add one more idea.
The post I wrote in rcwarbirds to use the computer radio to "Map" engines together is too long to post here, but you might see if it can help you at http://www.rcwarbirds.com/htdocs/dcf...rumID8/17.html
This uses the mixing process to match one engine to the other without synchronizers. Just a thought, and hope it helps.
Twinman

Twinman,
I've been hearing about that for a little while. It IS an interesting idea and will investigate it a little later.
Edwin

Edwin,
Hope the idea helps with your problem. It does bother me ( And you) that the same engines are 2000 rpm apart. That really sounds like a real problem in one. 4-500 maybe, but 2000, something else is wrong.
The other advantage of using two channels, for the throttles, is that on channel can be on a rotary knob and makes idle matching easy..Just don't hit that knob!!! Been there done that!!
Good Luck,
Twinman

Just read an engine article in RC Report that indicated the life of a 2 stroke was 15 hours, somewhat more if broken-in properly. Could that be correct?

Life of any mechanical device is based upon stress, and maintenance. Run a ringed engine slightly rich and no lean runs, it will have a long life. Run it hot and lean a few times and it is toast quick.....less than one hour. Forget after run oil in humid climate, I am in Houston, and you have bearing problems. Do a poor break in such as too rich on an ABC engine.....less life.
You will get a bunch of opinions on engine life hot potato. For me, I get pretty long life from engines...exact hours, I am not sure I can tell you that, but over 15 I can assure you.
If the engines both have the same compression by flipping over with both engines fairly dry and one gets a lot more if oil is added, it is probably worn out. In most cases a rebuild is cheaper than a new engine.
I will go get off my sqeaky soap box and let the next guy take this hot potato.
Good Luck,
Twinman

MikeJdad:

In racing use TBO can be as little as two minutes. Check the top fuel drag boys - a tear down after each run. The Reno racers probably have a TBO in the range of one or two hours max. And that Rolls Merlin had a rated combat life of only 200 hours.

The only model engines I've seen with a rated TBO were the early Norvels, on the instruction sheet they said six hours. And the last Fox I flew lasted less than one hour - the crank broke. But that was in a racer, so it doesn't count for sport use.

Engine life can be greatly extended by using intake air filters, my filter of choice is the "Bru-Line," available from Tower and others. If you want a really efficient air filter check the ones the car boys use. But they are big and ugly, might tend to block cooling air to the cylinder on an airplane, I've never tried one, the Bru-Lines do me very well.

I have a pair of Magnum 46 engines on a twin, they are five years old, can't be told from new, Bru-Line since new. An eight year old Diablo still has its original K&B 61, Bru-Line from new.

A recent picture of that Magnum powered twin is on page 96 of the Feb 2003 Model Airplane News.

But my old trainer is on its third engine. The new engine has an air filter, the first two did not.

My oldest plane that has not had an engine change (without accessory filters) is seven years old, twin AMD engines - these are the ones that Norvel sells as AME - they have an inlet screen as stock. But the whole airplane is probably less than two hours TT - well within the rated six hours.

Regardless, 15 hours is around 100 flights for most of us, and for most of us the planes just don't last that long. The last plane I destroyed (dumb stupid foolish error on my part) had less than 30 minutes on it. Airplane and engine four months old.

Or consider the average homeowner and his power lawn mower. He buys a new one every couple years, and throws the old one away with maybe 10 hours total time.

So actually 15 hours TBO isn't too bad, considering.

But the more expensive engines, with higher grade materials, can be made to run into the hundreds of hours without problems. Stay with at least 20% oil, always at least part castor oil, (yes, even on four stroke engines) keep them clean and not lean, and don't try for peak RPM.

Enjoy.

Twin engines old and slow? New P/L sets will make them go!

Bill.

Bill, I'm building a P38 with Saito 91's. I am interested in your recommendation on a synchronizer. I am planning to run an electronic glow driver on each engine, plus a synchronizer....just don't know which one yet.

email me please
vince