Minch

My dad decided to buy a fourstroke some months back to put on his new Flair Swallow, we have tried everything to try and get this fourstroke to run properly but it keeps throwing props, even in flight.

First we ran the engine in with about 1/2 to 3/4 of a gallon of fuel, then we tried to tune the engine, every time we went a notch above 1/2 throttle it would throw the prop. We tried setting the low end by blowing through the fuel inlet and adjusting the low end needle until some air was just coming through. We then eventually got it to run at full throttle then after a minute or so it would throw the prop. We then checked the valve timing and found the exhaust valve timing to be out, this was redone, the engine seemed to run better and it was throttling and idling great on the ground. As soon as the plane took off we got no more than a circuit out of it before it would throw the prop. We tried retuning it and thought we had it right, so we went and flew it tonight. As before the engine quit after about 1/2 a circuit and the plane suffered a broken but repairable fuzz. Can anyone suggest anything else to try before we put a hammer thru the fourstroke!!!!!!!

bdtsr

My Feedback: (5)

Your engine is too lean on the high end. This causes it to backfire and throw the prop. Turn the high speed needle out at least one full turn, start the engine, bring it to full power and readjust the high end to peak rpm then richen it up to drop the rpm about 200 to 400 or so. Repeatedly running it lean will damage the engine.

Checklst

Although the cam timing could be off my Bet is it's running to LEAN both of my TT 4 strokes are 3 full turns out on the high speed needle turning 8900rpm. I did lean on out for max rpm several month ago and it did the same as yours above 1/2 she went lean and spit her prop. Richer her up and you will be fine.

pikebishop

also how much nitro are you running and what kind of glowplug? how many rpm below max are you setting it for.

MrMotor

It sounds like you have the 91 and have not installed the rest of the venturi (choke assembly). The 91 will always run too lean without the choke assembly installed.

Homebrewer

My Feedback: (8)

Its probably a combination of being too lean and NOT BROKEN IN YET. The TT 4 strokes take a little longer than the average engine for them to break in.

Minch

We are now using 10% nitro but we initially tried 5% nitro as a few of our club members have this particular engine running on 5%nitro, inverted and without any onboard glow.We are currently using as OS type F plug but we have also tried Enya no.3 and Firepower F7. As for it being lean, the top end needle was approx 2.75 to 3 turns out, we tuned it for max rpm@8900 - 9000rpm and then richened it a notch or two before commiting it to the air. Also someone mentioned about the choke, this was fitted initially but then removed, maybe worth a try refitting it. My father is anxious to know what is wrong with this engine so we will probably mount it on a test stand and have a play.

The plane we crashed last night with the TT91 FS is now back in one piece but with a two stroke motor, a reliable ST 90.

bdtsr

My Feedback: (5)

One or two clicks may not be enough. You need to drop the rpm 200 to 400 off of peak on the rich side. One or two clicks most likely won't even drop the rpm by 100 or so.

Sport_Pilot

The choke assembly for this engine is a venturi which improves fuel draw. If you run this engine without it and without a pump it will run fine on the ground and initially in the air. As the fuel runs out it will eventually run too lean and quit or throw a prop. If you install the choke assembly you will find that this is the best performing non supercharged .91 engine made! Provided you have run at least a gallon of fuel through it to break it in properly.

pikebishop

defenitely put the choke back on. it really needs the venturi effect to run properly. I think you will be pleased with this engine. Oh yeah , check all bolts for tightness after run in and don't overtighten the rocker cover as it strips easily. also make sure you put some teflon tape or high temp permatex on the exhaust threads so they don't loosen

ajvpb

My Feedback: (3)

Definetly too lean. Never be afraid to make it overly rich. Too rich is ALWAYS better than too lean. Tune it to max rpm, then open until you can hear an obvious rpm drop. It gives it a good amount of fuel, and since they lean out in the air you need it to be rich on the ground. That is what is happening to you, it is borderline on the ground and in the air it leans out and then your done. I always run my engines rich, whether they are new or 3 years old.

AnthonyH

My Feedback: (16)

I agree with it being too lean. I just broke mine in and after 4 tanks it was outperforming a OS .91 with same fuel, prop and day.

You have to use the choke as already stated. This is a strong engine and takes a little more to get it broken in.

Minch

Thanks for all the advice, we are going to mount the engine on a test stand over the next few weeks and give it another try.

HarryC

My Feedback: (1)

I have the TT91 mounted inverted and have had all the problems you mention, it actually threw the prop when the plane was in the pits and seeing that meat slicer fly off across the runway was rather sobering.

All the solutions given in the posts above are correct. I initially removed the choke mechanism due to the close fit to the mount, the engine ran okay-ish but was never entirely happy. I modified the mount, refitted the choke and that went part of the way to solving it.

The reason my TT91 threw the prop was simple - too lean when throttling up. Four-strokes will backfire if lean. You can't tune them by ear like a 2-stroke. You must use a tachometer, adjust to peak rpm then richen off at least 300rpm. My TT54 threw a prop in flight once when it leaned off during the flight. Do check the needle friction device is in place so it can't rotate in flight.

You mentioned that you found the exhaust valve timing was wrong. Exhaust and inlet valve timing is in a fixed relationship on the cam, so it is impossible for the inlet to be correct and the exhaust to be wrong. They are either both right or both wrong. When I got my TT91 it would not run above 5k rpm. Eventually we discovered that the cam had been fitted 45 degrees off true and the timing was miles out. I have heard of other TTs with this factory fitted problem so double check the timing. This is actually very easy to do. Remove the rocker cover and glow plug. Drop the long end of an allen key down the plug hole so it falls and rises with the piston, then you can very easily see when the piston is at top dead centre - if TT can misassemble engines why rely on the timing mark they made on the prop driver? As you turn the engine, at every second rev at tdc the exhaust valve will be closing and the inlet valve will be opening. They should be equally open and closed, so the rocker arms will be parallel to one another when you look at them from the side when the piston is at tdc. If the timing cam is even just one tooth out, the rocker arms will be very clearly going at different angles to one another at tdc.

I suspect you meant that the exhaust valve gap was wrong, these gaps change quickly during running in, then settle down. Keep an eye on the valve gaps every 10 flights for the first 30 flights or so.

My TT91 now runs totally reliably. After the initial disappointment, once the factory problems and user errors were sorted it has proved to be a reliable, powerful, and enjoyable engine. I use an OS F plug, Graupner 14x7 prop, and various 5% to 15% nitro fuel, pure synthetic oil.

Harry

Spaceclam

i assume that your engine came with two nuts to go on the propeller, not one like a four stroke. i don't know about the backfiring, but it shouldn't throw props. tighten the first nut down really tight, and when you tighten the second one down, tighten it against the first nut, not the prop. that will keep it from coming off.

Jazzy

Even with the double-locking-safety-nut a four-stroke can and will throw a prop when backfiring if the engine is not set up properly. I know from experience.
If you hear a nasty clattering noise just before the prop becomes it's own flying/spinning projectile it is too lean or your cam gear is off.

If it is inverted-good luck!

Spaceclam

my four stroke backrifed once or twice, and nothing ever happened.

Jazzy

Spaceclam, if your prop did not loosen or come off after backfiring consider yourself lucky.
While almost all common four stroke engines exhibit similarities, they are not the same. The TT four strokes, particularly the 91, have a higher tendancy to arrive from the factory with the cam gear off one tooth than most other brands. It has been documented here on RCU and owning one myself, I can attest to it.
This engine does not seem to tolerate a lean high speed needle. Even slightly lean will produce the affects described by Minch.

Minch, If it did run well with 1/2-3/4 gallon of fuel through it I would say it is definately a fuel problem. Check your fuel lines for leaks (cracks, gaps, splits, etc.) and try running it a bit richer. I had a cracked fuel line (previously unnoticed) and I trashed a spinner and two props before I found the problem. Placing a piece of fuel tubing over the needle valve will help seal it against the throttle barrel and eliminate any leaks there.
As mentioned, the choke assembly is a necessity with this engine.
What prop are you using?

herc75

About the only thing I can say for sure without examining the engine myself is that the starting point needle settings in the manual are waaay to lean so hopefully you aren't even near there. Double-Triple the recommended to start with.

I have run TT 91's for many years and now own 3 total...2 running and one on standby. I can actually start to hear mine begin to pop very quietly (not really backfires, not sure what it is...guessing the start of preignition) as they are starting to lean out....when I don't hear that noise anymore when holding it in a vertical position, then its good (then richen it up another couple clicks. Can't remember the last time I had a deadstick tuning this way....however, it does take some time

All have run WITHOUT the choke installed from day one. I ridgidly apply centering my tanks on the center of the carb line and as close as possible...not sure how much that helps, but its what I was taught.

Check your gaps, check your cam...

Nick

proptop

My Feedback: (8)

Hey herc, ever had any problems w/ the rocker (lash adj.) screws?

herc75

Heh, man did those things get a cussing for about the first year or two I owned mine. Definitely a three handed job in setting those things and geting them tightened down correctly.

Once you get them set in the spring after rechecking...CAREFULLY place 1 small drop of Thread lock on them about them the rest of the season.

Coat everything...gears...cams...lifters...valves...you name it in Rislone once in the spring and fall and run dry...and I haven't had a problem one.

I run Cool Power 4C glow and OS F Plug in mine. I have heard many times of fuel making all the difference in this engine. I have recommended trying this fuel more than once to people complaining about reliability in running and it has made a world of difference....

I may have been lucky, but many years ago when i bought my first one, I was very worried it would "suck". However, since then I feel they basically suffer from just bad perception. While many say they aren't the greatest engine for a first 4c, I believe the opposite they are 1/2 price of many others so if you trash it you are only out 170 and not 340. Additionally, they are very rugged engines (except for where the cylinder head attaches to the muffler)..don't ask.

Thanks,
Nick

Sport_Pilot

(except for where the cylinder head attaches to the muffler)..don't ask.

That seems to be a problem on most 4C engines!

proptop

My Feedback: (8)

The reason I ask is because my exh. screw broke half way thru the nut! Wasn't over torqued either, just fractured just above the rocker. Now I guess I'll see what kind of service I get from Ace, 'cuz I e-mailed 'em 2 days ago, and no reply yet...

herc75

When My muffler snapped off...on a maiden voyage I actually had a composite engine mount snap. The engine (twisting around) smacked into the fuse and snapped the muffler off at the header. Of course, then it went back up and ate the middle leading edge of the wing before dieing.

I actually landed this model dead center in the runway with the engine dangling from the golden rod...was really amazing.

Anyways, when I ordered my cylinder head I got it from here.

http://ehobbies.com/rc-parts-depot-t...arts-list.html

The service was great as the owner actually emailed me a couple times just to let me know it was on back order. But that he'd airmail it for free. They have many more parts and you will probably have to call them as I didn't see what you are looking for on their web page, and can order for you (albeit probably through Ace anyways). Sorry to hear about the nut...could you find an appropriate size set screw to substitute?

Maybe you could even cut an appropriate size threaded rod to size and camber it with a dremel?

Nick

bishop stortford

I have had two new TT 91's (one replaced the other) and one replacement carb. Neither engine would run properly and a replacement carb. was supplied by the shop. The secondary needle was seen to have a step turned into it (on two of the carbs.) The step had a different profile on each needle. I could never get these engines to run properly (I have over 20 years experience of model 4 stroke engines).The step in the needle moves towards the the main jet and blocks it completely as the throttle is closed. It can never be tuned with this happening. I eventually tried an ASP 80 4 stroke carb on one of the engines and it improved things slightly. But I returned the TT91 under guarantee and eventually obtained a refund. The only 4 strokes that I will use now are O.S., Enya, and Saito. Laser engines have a great reputation but I have never owned one. The best advice I ever had was 'Buy cheap. buy twice'. How appropriate this is in the world of R/C airplanes!