RappyPilot

Hi all

I have an old Thunder Tiger 91 Four Stroke that a friend gave me. It's seems to be be quite old as it has one of those sliding choke levers and just looks old. The engine is inverted in a Hangar-9 Spitfire.

For quite some time I really battled to get a consistent idle from the engine. From fiddling with the low end to checking the valve gap, cleaning it inside out. I have tried it all. Eventually I managed to get it to idle slow enough that my plane does not creep forward when it's on the runway ready to take off. But still not as good of an idle as I get on my other four bangers.

I noticed that once I have had a flight and have come in to land that I can trim the idle down by about 10 clicks and then she idles fairly well. When it's cold I realy have to crank up the throttle trim again. The engine also turns over relatively easy with seemingly less compression than my other four strokes. And quite oddly is much easier to start when warm that when cold. A friend of mine has a Hangr-9 Miss America with an OS91FS and he just seems to get oodles more power out of that engine (No no no, lets not start a OS vs TT debate here)

Is it possible that the engine needs a new piston ring?

Marcel

MinnFlyer

My Feedback: (4)

It sounds like you know 4-strokes pretty well, AND it sounds like you tried a few good experiments with it. From the way you describe it, I would say that replacing the ring is a bonifide possibility.

earlwb

A new ring may be certainly possible in this case. But usually one does a compression check on the engine cylinder. If the compression is low, you squirt in a little oil and check again. If the compression goes up, then you need a ring, if the compression doesn't go up, then it is the valves need work.
They do make compression gauges for glow engines too.
But one can get a ring and install it and see what happens, the ring does cost less than a compression gauge and you can get the same results as to whether it works or not.

blw

My Feedback: (3)

I would first make sure the low speed needle is set as lean as possible and still give good throttle response. I would check compression like Earl said. Be sure to open the throttle some when you check it.

Campgems

I've got to agree with BLW on the likely culprit being the carb adjustment. It sounds like an over rich low end. You can't just adjust the low end though and be done with it, you need to go through both adjustments a couple times to get things right.

One other issue with inverted and semi inverted engines is that they collect a lot of oil in the rocker cover. When you first start them, it has to be heated up and go back into a mist state than can be circulated and burnt off. The richer the engine runs, the more oil will accumulate. I had a similar issue with an O.S 52 that was mounted in a Phoenix Fun Star. The design is that the engine is mounted a 7:30 looking at the prop. It smoked and sputtered and required a very long warm up then ran just great, never a dead stick. Idecided to check the valves at the field one day and when Itook the rocker cover off, there must have been a half tablespoon of oil in it. I thought of putting a drain nipple on the rocker, but I killed the plane and the problem went away for now.

You might try storing the plane with the engine upright and see how it behaves that next trip to the field.

Last point, comparing compression between makes is not very productive. I've a couple Magnum engines and they have almost no compression compared to a like size OS. The Saitos I have seem to have more than the O.S engines. They all feel different. They all run well.

Don

Sport_Pilot

Starts better warm than cold.  Actually this was the case with my inverted TT 91 till it was well broken in.  I mean like a few gallons, not a few tanks.  This engine like Saito takes a long break in.  The fact that it has collected oil and dirt over the years making it look old doesn't mean it is actually broken in.

Give it some time.  Also consider going to much lower oil content.  Four strokes really don't need 18% oil because some of it is retained in the crankcase. Inverted four strokes run better with about 14-15% oil. 

RappyPilot

Hi all

Thanks for all the opinions Could be low end, although I spent 3 weekends fiddling with it, and then every guy at the field with a screw driver had a try untill I was so confused about where the needle should that I turned the needle in completely, blew into the carb with a piece of fuel tubing attached to the nipple and slowly opened the low end utill I could feel air going through and opened it another half a turn ... I will tinker a bit more this weekend. It does spew out quite a bit of smoke when it trasitions from idle to WOT but does not splutter or die, so I guess I could lean it out a bit more.

Good point on the age of the engine, I have no idea how old that engine is so, it could be 10 years old but only have been run 10 times.

Campgems

Marcel, I looked in the manual and I can't find a "factory starting point" for the needles. Checking my engine, a used one that I've never ran, and I found the high speed needle in a bag. I"m going to guess that the idle mix is about right. The head of the screw is about 3mm in from the outer part of the barrel that the throttle arm mounts on. So, here is what I would do.

Start with the idle mix about 2.5 mm in from flush with the outside of the barrel. Start with the high speed open three turns. That should get the engine running. Isuggest starting here rather than where the engine is now at as needles are a balancing act and is hard to know really where you are starting from.

You mentioned that it was inverted, I suggest you mount it on a test stand until you get it running very well then mount it in the plane and just fine tune it there.

Fire up the engine and let it warm up a bit and then go full throttle. Lean out the high speed for max RPMthen go rich 300 to 400. You need a tack to find this point. Now, set the throttle to the slowest speed that will keep the engine running. Leave the throttle there and very slowly lean the idle mix to get maximum RPMat that throttle setting. Note, if the needle is stiff as the one in my TT 91, you may need to hold the throttle arm with your hand while turning the idle mix. When you have found the best high speed setting, then go back to the WOT again and re adjust the top end. Go back again to the slowest idle you can and keep it running and again without changing the throttle adjust for max RPM. At this point, you should be able to idle at around 2200RPM. If it is higher, go back to the high speed and adjust it again and then back and fine tune the idle some more. It may take as many as four trips to get a balanced adjustment.

Now you have to fine tune the idle mix for transition. Run the engine up full throttle for a few seconds then go to you idle. Let it idle for 20 seconds or more then as quick as you can go to full throttle. If the engine slowly gains RPM, or stumbles a bit before gaining rpm, the idle is too rich and needs to be leaned a little more. Go 1/16 turn at a time until the engine jumps to speed when you hit the throttle. If the engine just dies when you jam the throttle, it is to lean. Adjust out 1/16 turn at a time until it jumps. It should respond very quickly to throttle when adjusted right, and with the right prop.

Once you invert the engine, you will need to fine tune both needles. A glow driver may be needed also depending on the engine. The common problem is idle issues as it is easy to flood the glow plug when the engine is inverted. If the carb is adjusted to peak on both ends, it should be OKwithout an on board glow driver.

If you don't have the manual, send me a PMand I'll send you a copy.

Last, the puff of smoke when you hit the throttle is good. You are not far off from where you are at now.You may get away with fine tuning from where you are at now, but Ipersonally would start from scratch and go from there.

Don

Don

jaav

I have one in a P47 inverted with OB glow.
The engine was is a SPAD before hand and was run over propped which lead to detonation, It destroyed the liner and piston.
I fitted an new liner and piston kit, Prior to the rebuild this engine had done a lot of time and never dead sticked or failed to start.
Now its always hard to start (only start it with the plane inverted) but once started its perfect. It still has very low compression after 2 liters. Its is slowly getting better with running.
When its hot she will start 1st time.
What would be the compression on one of these engines, My other TT and Mags have a lot of comp.

Campgems

I decided to get my TT 91 out of mothballs and see how it ran and what the needle settings were. I got everyting setup on the test stand for this morning then discovered that Ididn't have a prop spinner to fit that didn't have a bent back plate. I did discover something though.After pulling the plug and pulling the barrel out of the carb and soaking things with some after run oil. Idiscovered that there is very little compression on it also. Then the light blub went off, my two TTtwo strokes feel almost like there isn't a piston, the lowest static compression I've ever seen, this 91 feels the same. I was making a comment about it to one of the guys at the field this morning and his comment was that all the TT engines he has seen had what seemed as little to no compression, but all ran well. I know my 46 two stroke is a great running engnine once I got it tuned in. I have some spinners on order and they should be here by mid week and I'll get a sense of how this engine is going to run and will post the results.

Don

RappyPilot

Hi all

Well, another weekend of flying and testing.

Fiddled with a the low and high end quite a bit and managed to get it a bit better, but still the same problem, bad idle when and hard to start when cold.

But I must say, went for a flight, landed and the idle was super sweet, 1800RPM, I consistently managed to that RPM when the engine is warmed up, still baffles me why it does not want to idle below 2900RPM when its cold.

Maybe I should just let the engine warm up nicely before taxying to the runway and live with the fact that it is quirky eninge.

Anyway, a few pics of her flying.

estradajae

You may want to check the cam timing as well?? not likely to be the problem, but who knows.

Campgems

Cam timing usually just affects the top end or the low end, and does't change after it warms up. That engine's RPMrange is 1800 to 11,000, so as long as you are close to those numbers, the cam timing is OK.

Still sounds like a fuel issue, mix not the actual fuel. Marcel, is there any surging at mid range, in the air?

Very nice looking plane I might add

Don

RappyPilot

Hi

Midrange and top end are 100% perfect. Idles 100% perfect if the engine is warmed up after a flight. Just no idle when cold. I was also thinking it could be fuel. I am using Morgans Cool Power 15% Maybe I should try a different type of fuel. Maybe a castor/synth blend of some sort. I can see and smell, that the previous owner used fuel containing castor oil.

earlwb

Maybe there isn't anything wrong with the engine. My car and motorcycle engines won't idle either, until they warm up good.
I have some other glow engines and they aren't all that good for idling until they warm up good too.
But I do have some engines that idle fairly well when you first start up the engines cold.

Hummm.
One other thought is when you first start the engine, the fuel tank is full of fuel, but at the end of the flight the fuel tank is low on fuel. Thus if the engine is idling well at the end of the flight, but not at the beginning, then maybe it is running too rich at the beginning. Id the carb is set up well, then it starts to look more like a fuel tank location issue in relation to the carburator. You could try only putting in a small amount of fuel and start the engine cold and see how it behaves with a low fuel level inside the fuel tank.

But then I see this a lot on other engines as well. I even have a couple that do that as well. But with a ARF plane you tend to be limited in how good of a fuel tank location you can get.