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Thunder Tiger .91FS throwing props.

Problem with a TT four stroke .91 throwing props at high RPM

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Graham W28/05/2012 12:00:28
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Hi all, I have an irritating problem with my TT .91FS, the engine was fine when last used about 4 years ago before standing it up and storing it in it's box, it was treated to a bathing in after run beforehand. The engine was fitted to a new model and taken to the flying field, on run up I could not set the mixture as it was way too lean, so I richened the mixture by about two turns, on opening the throttle it revved up and spat off the prop nuts washer and prop. So the bits were collected and put back on and I tried again. I richened the mixture once more as a lean mixture will cause the issue, but instead of curing it it made it worse, this time losing the locking nut and washer, the needle is now 3-4 turns open. The engine idles lovely. On getting home an engine guru in the club stripped it down and checked everthing out and re-assembled it making sure the timing was correct. He ran the engine up and guess what?... it did the same thing, he tried various mixture settings al to no avail, finally a new plug seemed to cure it so it was returned to me, whereupon it was duly fitted back in the model and taken to the flying field. Sure enough i ran fine, setting the mixture rich I flew the model, however, part way through the flight it became lean and started to "pink" so I landed and richened the mixture one complete turn and took off again. After two circuits the engine made a horrible noise and threw off the prop, I managed to get it down without incident and saw that both the nuts had gone, the prop finally coming off when the model was stationary. I am at my wits end, what on earth could be causing this problem? The engine did seem to be very hot, so pre-ignition of the fuel seems to be the most likely cause, but why?

Edited By Graham W on 28/05/2012 12:01:04

Steve Hargreaves - Moderator28/05/2012 12:08:49
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Sounds very odd Graham......first the obvious...you are tightening the prop nuts up sufficently & in the right order I assume? You can really apply some torque here & probably need to to ensure things are secure.

4 strokes throwing props is usually down to pre-ignition as you say....usually caused by a lean mixture which is what we seem to have here. Opening the needle "one full turn" should take it from lean to very rich & this doesn't seem to be the case here. This leads me to suspect a blockage in the carb or fuel plumbing somewhere. I would suggest you strip down the carb looking in particular for congealed caster oil or similar in the needle valve.

Other than that what fuel/plug/prop are you using?

We had lots of fun discussing a TT54 that was reluctant to keep its prop in place just here....

Graham W28/05/2012 18:10:16
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Fuel is 10% nitro 15% synthetic + 2% castor oil, plug is an OS F and the prop is a 15X6 K series Master Airscrew. The prop nuts were done up really tight. What I can't understand is the richer you made the mixture the worse it got, it never actually ran rich, it would spit off the prop first.

Engine Doctor29/05/2012 09:47:00
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Hello Graham . I had an Enya 60 FS that had very similar problems some time ago . I tried every thing the supliers suggested including , plugs (various FS and Enya No 3) , fuel (diferent oil and nitro content) etc. I eventually got it to run very swetly by fitting a Taylor short reach idle bar plug. The short reach lowered the compression ratio a little and stopped the pre ignition .Fuel was 5% nitro synthetic . That treated the symptom and made it a very usable engine but not the cause . The compression was too high ,possibly a maching error ? You could try fitting an alloy shim cut from litho plate and annealed prior to fitting . you would have to readjust the tappets once fitted .For the cost of a plug its worth a try. **LINK** Link to Flair who supply Long and short reach idle bar plugs.

Edited By Engine Doctor on 29/05/2012 09:47:55

Steve Hargreaves - Moderator29/05/2012 10:35:50
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My LMS stocks the Taylor range of plugs so might be worth a try....see here

This sounds very odd.....fuel, plug & prop all seem fine to me. You say the motor was fine before it was stored so something has changed between then & now.....you say you filled the engine with after run oil before storage (a good thing). Where does the crankcase breather exit on this engine? Into the inlet manifold or does it just discharge to air? Might there still be some oil in there thats getting into the combustion chamber & causing a "mild hydraulic lock" (horrible description but hopefully you get my meaning...teeth 2) & this is causing the engine to throw the prop?

Other than that I'm still confused by this opening the needle one full turn......IMHO that would take a slightly lean engine to quite rich which would be evident by lots of smoke & reduced revs....if it doesn't then I still think there may be a blockage in there somewhere.....or possibly an airleak....

Graham W29/05/2012 13:29:19
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Thanks for the input guys, the crankcase breather was free flowing into air and I modded the inlet manifold to plumb it through the engine, but to answer your next question it did the same with the breather connected to the inlet and off the inlet. I have just purchased a pair of head gaskets for it, and another set of nuts, which I will try one gasket at a time to see if lowering the compression will do any good. As regards the plug, it seems odd that a new plug straight out of the packet will change it and then after it has run a while it does it again, and in the air, the prop has unloaded and there is sufficient airflow to the engine, it's fitted in a Hanger 9 Thunderbolt so there isn't an enclosed cowl issue. The only part that hasn't been stripped down and inspected is the carb, maybe I should do that next.

Steve Hargreaves - Moderator29/05/2012 13:35:31
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OK...be interesting to hear the results.....thumbs up

Incidentally I assume you plug the breather connection to the inlet manifold when the breather is disconnected yes...?

I wonder if a lower nitro fuel might help...10% certainly isn'y high but I wonder if 5% or even zero nitro might help (clutching at straws now!!...dont know)

Oldbaldfella29/05/2012 14:24:03
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I use 5% on my SC52 and have never had issues. As said, tightening the nuts up enough is always tricky.......dare I give it a little bit more?

Martin Harris29/05/2012 14:34:47
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I've held back from posting until now as I didn't have anything much to add above what has already been said on this thread and the one Steve referenced.

However, it does seem that the engine is going lean during operation on some occasions - i.e. it can be running perfectly for some time and then go off tune.

This suggests something is wrong with the fuel feed and as it appears to be going lean, I'd suspect an intermettent blockage - given the storage time, possibly congealed oil or a foreign body in the carburettor? Given the jet sizes involved it doesn't need to be anything very large either...

I often get engines running properly for myself and clubmates with the aid of the bicycle pump that I use for air retracts. Open the needle valve fully, throttle wide open, pump air smartly into the fuel nipple (throttle closed then repeat with it open) - remove needle completely and pump again throttle closed and open. Double check the needle for cleanliness and reassemble.

I'm assuming the tank and tubing have been carefully checked for foreign objects or silicon flaking from the internal tube walls?

Just make sure no-one is peering at the carb when you do this - fuel can be expelled quite forcibly and it isn't a good thing to get near the eyes!  Best to wear eye protection!

Edited By Martin Harris on 29/05/2012 14:38:55

Graham W31/05/2012 09:37:38
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It's beginning to look like the carb may be the culprit, as the engine also behaved badly on the test bench too, so that eliminates the model plumbing. I shall strip the carb down and give it a thorough clean and blow out using an airline and compressor, I can't blow that hard..... I still have the head gasket ploy to fall back on.

Ian Skeldon 102/06/2012 23:10:16
108 forum posts

Hi There Graham,

A little trick used many times when engines have a lot of compression, is to fit a second copper washer to the glow plug, this in effect increases the capacity slightly whilst also decreases the compression, it possibly will also produce a slightly later timing of the firing and thus combined it should stop the problem (assuming the carb is ok?)

HTH

Ian

Keith Miles 208/08/2020 00:07:12
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Reviving an old thread...

I came back to the hobby about 10 years ago with no prior experience with four strokes. I now have four running in models and three in boxes, as yet unused

Of the four that I have been flying, none has yet to even show any signs of throwing a prop. These include a Laser 120, OS 81 and 52 and well used ASP80.

One of the unused boxed engines is the above TT91.

Two club members are currently experiencing the prop throwing phenomenon with their TT91 engines. One was bought from new and has recently developed the problem for no immediately apparent reason. The other, I think, was bought second hand and might have had the fault when the chap bought it. Not sure.

The former engine is owned by a very experienced RC flyer and retired engineer. The second is owned by a retired vehicle mechanic. Both are currently head scratching.

I believe that in both cases, the problem is occurring on the ground and on start up but can confirm that in due course.

My TT91 was intended for use in a still boxed ARTF Mustang but based on the recent experience of my two club colleagues and that of others as reported in this forum, I am wondering if I might be better off leaving my TT91 in its box or using it as an ornament!

I can’t help feeling that there might have been something intrinsically wrong with the design of this particular engine and it’s making me nervous!

Surely, a properly designed four stroke, properly used, should not be an unpredictable assassin?

Edited By Keith Miles 2 on 08/08/2020 00:15:32

Jon - Laser Engines08/08/2020 01:04:45
5621 forum posts
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I have come across 2 TT 4 strokes that were timed wrong from the factory and always ran lean. As i have only ever had dealings with about 5 of their engines its not a good record.

That said, a friend has a TT91 and its a really nice engine

Keith Miles 208/08/2020 01:48:01
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Posted by Jon - Laser Engines on 08/08/2020 01:04:45:

I have come across 2 TT 4 strokes that were timed wrong from the factory and always ran lean. As i have only ever had dealings with about 5 of their engines its not a good record.

That said, a friend has a TT91 and its a really nice engine

Thanks Jon. One of the owners I mentioned (my brother in law, actually) intended to check the timing, as a matter of course but is currently away on holiday up’t north. He is currently puzzled as to why the fault has suddenly developed. It was, apparently fine previously as seems to have been the case with a previous contributor.

So, at least one nice example out there?

(So far?)

Perhaps a positive note to go to bed on?

Ta!

smiley

P.S. I presume the aforementioned ones ran lean until the timing was corrected, if so, that’s possibly three positives, assuming that the alignment marks were in the correct position and just misaligned on assembly and that they ceased throwing props thereafter!

smiley

Edited By Keith Miles 2 on 08/08/2020 01:57:30

Jon - Laser Engines08/08/2020 10:20:09
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The two i came across with incorrect cams were TT75's.

One was new and a customer returned it to me the day after he bought it from the model shop i was working at during that time. In order to reduce the time the customer had to wait i took it home and ran it that evening, found the problem, and fixed it.

The 2nd was a club mates engine. Bought 2nd hand but its previous owner had, allegedly, only run it once or twice from new before selling it and it had never been apart.

After fighting with it for some time i was asked for my opinion and noticed something was wrong when i went to hand start it and the prop didnt fall in the right place. I stripped it down on the bench, put it right, and its very happy owner flew it for the rest of the day after we popped it back in the model.

I was also given an Enya 53 to run in once and that had incorrect timing as well as the punch marks on the inlet cam was in the wrong place. It was dead obvious to me as it didnt run right, and the timing and cam profile enya use is very similar to a laser so it stuck out like a sore thumb when i took the cam cover off. I have also known ASP engines to have their timing marks in the wrong place making timing them a little tricky. 

Edited By Jon - Laser Engines on 08/08/2020 10:22:49

Keith Miles 208/08/2020 11:39:30
464 forum posts
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Jon,

Thanks for that.

Seems, indeed, that we can’t take certain matters for granted and would I be correct to assume that, if timing marks are wrong, it would not be obvious to anyone lacking the necessary experience and difficult to check, let alone correct, if you also lack the confidence to take an engine apart and rebuild it?

That said, even with my very limited knowledge, the only thing that I can think of that might cause a previously sound (or seemingly so) engine to start continually throwing props, provided nothing else has significantly changed, would be a loss of grip somewhere between the prop and the prop driver which might have been masking any timing issue.

And I would have thought that, over time, compression tends to decrease although perhaps not enough to overcome gradual loss of grip on the prop.

All of the past comments regarding manually reducing the CR, making shims and making prop washers out of shoe leather etc. etc. whilst interesting is, from my perspective, a little disconcerting for at least two reasons!

Martin Harris08/08/2020 11:50:35
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I haven't owned a TT91 but the ASP/SC versions have been well behaved. My feeling is that the TT versions may be slightly tuned - perhaps a higher compression ratio? A pupil had one of these engines in a large trainer which ran perfectly well but was very prone to throwing props during starting. The nuts were definitely tight and correctly assembled - these engines have a slightly different locking system to the OS and other clones where the split compression nut fits before the compressing one - but the engine would kick the prop loose every time.

We developed a technique where he would apply the starter and once the engine started to spin, I would energise the glow and it was never a problem after that. This suggests to me that the ignition timing is effectively over advanced - which is either caused by too much compression, too much nitromethane or too hot a plug.

I'm often asked how tight to do up prop nuts and my standard reply (assuming they are using a standard sized spanner) is to tighten them until your eyes start to bulge. I've yet to strip one of these threads on either a crankshaft or nut and I don't hold back! I tighten the first nut on 4 strokes and then while holding the prop, tighten the lock nut without a spanner on the first one which avoids any possibility of reducing the clamping pressure on the propeller. Use both spanners together when disassembling though.

It appears that this latest problem report is similar to the situation above but in the case of prop throwing in the air, it might be caused by carbon build up in the cylinder head causing detonation - especially in an older engine that has been run on castor. Another factor is lightweight propellers which can lack flywheel effect and be more prone to loosening - but not normally a problem with Master Airscrew "cudgels"!

 

Edited By Martin Harris on 08/08/2020 11:52:06

Jon - Laser Engines08/08/2020 15:23:58
5621 forum posts
271 photos
Posted by Martin Harris on 08/08/2020 11:50:35:

We developed a technique where he would apply the starter and once the engine started to spin, I would energise the glow and it was never a problem after that. This suggests to me that the ignition timing is effectively over advanced - which is either caused by too much compression, too much nitromethane or too hot a plug.

Edited By Martin Harris on 08/08/2020 11:52:06

Its also possible glow voltage was too high. Some guys wind up the glow power until the the poor thing is like a supernova!

Martin Harris08/08/2020 18:58:52
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Nothing wrong with a Supernova Jon - my Priory Models one flies beautifully...

Funnily enough, the thought crossed my mind after posting but I was just getting ready to go to the field and didn't edit it. I suppose I could have claimed the hot plug reference covered it! blush

I always use a glow stick myself.  Saves on replacement glowplugs when the power panel electronics fail...

Edited By Martin Harris on 08/08/2020 19:00:58

Keith Miles 209/08/2020 17:42:35
464 forum posts
6 photos

I have been doing some further extensive research on this subject since my recent post and have now read a significant number of past threads (with some familiar names in the contributions!) both in this forum and others on the prop-throwing phenomenon.

Many thanks to all, past and present, for expanding my knowledge and improving my understanding!

There seems to be nothing conclusive, at present, to suggest that the TT91 might be especially prone to this phenomenon despite one or two comments that it might run a relatively high compression ratio, so maybe it’s just a matter of “RTFM” and treating it with the usual standard care and applying the usual safety precautions including, perhaps, a regular check on the tightness of all prop nut(s)!

Most occasional problems with engines might simply be annoying but this one is potentially downright dangerous and one would hope that manufacturer’s would have done their best to minimise the likelihood of it happening, especially when operated in accordance with their instructions and without having to “home brew” a solution or adopt a starting technique which requires two pairs of hands, one behind the prop, ready to power up the glo plug and one in front to spin the engine up first! This method was also suggested to me recently by a very experienced fellow club member and whilst I’m sure it might work, and I would be happy to try it if required, it’s not exactly convenient and would still seem to be merely, perhaps, masking a problem.

It will be interesting to discover, in due course, and when my two fellow club members find time to investigate, what is causing their TT91s to misbehave and whether or not an appropriate fix for either, or both, is achieved. Maybe I’ll get to post the outcome in this or another thread for those few IC fans still remaining!

In my case, I am unlikely to be putting the Mustang together for quite a while as I currently plan to do some work on existing models first and my next project is likely to be the assembling of a Seagull Extra ARTF for which I also bought a yet to be run Saito 62B.

I just hope that I live long enough!

😊

Edited By Keith Miles 2 on 09/08/2020 17:58:08

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