vwalton

I've got a brand new Thunder Tiger .91 FS that I bought on eBay and I've got a LOT of questions about it. Please, anybody, answer whichever or how many of the questions you have time for.

Quick background: I put on a wooden 12x6 and started the engine and the first time it started right up. The second time it (in retrospect) backfired and kicked the prop loose. But after that it seemed to have NO compression hand-turning it, so I figured, uh-oh, I broke something. I took the top end apart and it looked fine, so I put it back together and although while turning the prop over manually it seems to have very little or no compression, it started right up.

So I broke the engine in by running 2 tanks through alternating long rich and short lean periods, gradually increasing the lean and decreasing the rich. BTW I do NOT have the choke assembly on, but will put that on and see if it solves any of my issues. So, my issues, in order of importance:
[ul][*] I did not get a manual with it. I have downloaded manuals for OS and Magnum .91 FS's and they're very similar (to each other), but of course I don't know how similar they would be to the TT because I can't find a download for the TT. Anyone have one I can beg, borrow, steal, or download?[*] The engine runs WOT at around 10,500 but is VERY LOUD. I was at the field with 4 or 5 other 4S flyers, and this thing was off the scale in terms of loudness. The sound is also of a different quality: brack-brack-brack-brack rather than brup-brup-brup-brup, if you get my drift. Is an issue with the muffler - is it known to be crappy? Or might this really loud, brackety-brackety sound indicate something else wrong?[*] The engine vibrates like nothing I've ever seen before. The clunk bounces around in the tank so bad that at about 1/3 full I start to get bubbles in the line. I'm afraid it'll shake itself right off the firewall. I've checked the prop balance (without a prop balancer - I put it on a razor blade and it seems the balance point is right in the middle of the hole). Any ideas?[*] It won't idle. AT ALL. I'm hoping someone will point me to a manual, so I can get the low-end mixture default settings, and perhaps the choke asbly will help. I've also found on this forum the valve timing and clearance settings, so will check that. Any other suggestions?
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I don't want to tire you with my other smaller issues (like NO valve cover gasket - I'll throw some permatex in there) until I get these major ones resolved. I'm confident that this engine will work eventually, but it sure is a challenge for a newbie. Thanks in advance for any advice / help.

TRYING HARD NOT TO DESPAIR

w8ye

My Feedback: (16)

Lets start off by putting a 14-6 on it. The prop is way too small.

vwalton

I will certainly try a larger prop but I believe I've found a more pressing problem. I feel like an idiot. In taking it apart to check the valve clearance (WAY fat BTW) I took off the head and discovered a noticable lack of a head gasket. I mean, I'm kind of naive about 4-stroke engines, but ...

Moral of the story: never buy an engine on eBay - not even alleged "NIB." The money I saved is long gone in wasted time and frustration.

Unless TT .91 FS engines don't have head gaskets??

XJet

I don't know if they have head gaskets (I've never had the head off mine) but w8ye is right about using a bigger prop.

You can use the recommended 11x10 to run them in (because you're not going to run at full throttle for extended periods) but I find a 14x6 is great for sport flying and a 13x8 is just about right for warbirds (pulling 10,400 RPMs on the ground when peaked but richened out to 9,950 for flying.

These engines also seem to like the Master Airscrew K-series props more than the lighter APCs

Although my TT91FS is slightly louder than my Saitos, it's still a very quiet motor -- to the extent that it simply doesn't sound loud enough to be scale in my P51. For that reason I've just ordered a Perry pump and will be fitting a straight pipe to the exhaust.

If yours sounds too harsh it might be running too lean and detonating -- or you might even be getting valve-bounce with that tiny prop.

w8ye

My Feedback: (16)

The engine originally had a head gasket. As a repair part, the head gasket is included in gasket set PN132.

However, I do not see the lack of a head gasket causing you any problem at this time.

I've found other four strokes with the head gasket having been removed. I ran one for a full flying season only to discover that someone prior had removed the head gasket. The engine ran good.

Enjoy,

Jim

pikebishop

put the choke assembly on it aids in fuel draw and the engine will not run well without it.

vwalton

Great responses - thank you very much! What I'm going to try is this: I've ordered the gasket set but will put the engine back together today with no head gasket for the time being per w8ye's post. I'll set the valve clearance to .03-.1 mm (.001-.003 in.) at TDC per another post I found here. I'll check the timing (again, with the help of another post) and put on the choke assembly. I have no access to a 14x6 prop but can get one by Tuesday. Anything else you gurus recommend I check while I have it taken apart?

Sport_Pilot

The small prop was partailly responsible for the vibration, but also sometimes they vibrate more when not broken in. Though thats mostly ABC and lapped iron engines.

carrellh

I wouldn't bother with the choke assembly. It has absolutely no effect on the way the engine runs. It only exists to block the air intake to improve fuel draw when priming the engine. If you use an electric starter the engine will prime easily and fire right up.

As you've noticed, you get totally opposite opinions about the same thing. I'll state that I'm no 'engine guru' but if you look at the choke assembly it makes sense that there's no way it can impat the way the engine runs.

vwalton

Yeah that's what I was thinking - it extends the mouth of the carb about 3/8" but what good that does I have no idea.

Today I reset the valve backlash and timing - timing was off by a tooth - and started it again. It starts pretty well and idles a little better, but kicks and shakes real bad at idle (backfiring?). Also it doesn't sound much sweeter (maybe a little) and still vibrates pretty good at full throttle. I'm going to postpone further testing until I can get a 14x6 on it. Thanks everyone for your interest and help - please stay tuned, because I don't think this is over by any means...

w8ye

My Feedback: (16)

The missing head gasket could make the engine more prone to backfiring

A lean mixture will also cause back firing.

There is a mist of air/fuel that stands out from the basic carb body on the TT four stroke. This get blown around at times and the fuel is wasted. This deal about the mist sometimes getting blown away and sometimes not causes erratic and unsteady running in some installations. In some installations, it makes no difference about the choke body but in the ones where it does effect the engine, the choke body seems to contain this fuel mist better and makes the engine run more economical and steady.

Enjoy,

Jim

old propnut

By all means install the choke assembly, theTT91FS needs the chock assembly to have enough fuel draw. Without it the engine will run lean and be very touchy on the needle valve setting. The best prop for this engine is a Master Airscrew 14/6,After the engine is well broken in you will like it.

Sport_Pilot

I second what old propnut said. On other four strokes the choke assembly is just that, plus a velocity stack. But on the TT it is a venturi that is slightly smaller than the carb throat. That means it acts to help draw fuel even when the throttle is wide open. You may be able to get it to run well on the ground. But you will have a dead stick after using about a 1/4 of a tank of fuel.

ridergary

My Feedback: (75)

You know the head gasket or shim is probably there, it's just they are hard to see if your not familiar with what they look like. When you look at the head they can appear as part of the head itself as they are a shiny aluminum shim that usually sticks to the head. Maybe check closer. Definately go with the bigger prop as recomended and once you get the low end needle set some of the vibrations will smooth out.....Gary

vwalton

Ridergary, you were absolutely right. I looked closer - a LOT closer - and the gasket was inside the head. If you don't know what you're looking for, you'll never see it - I was expecting gasket material, not metal. And the valve cover doesn't have a gasket apparently. I flattened it with some 320-grit sandpaper on a piece of glass and it isn't leaking anymore.

Things are looking a lot better. I think my major problem was the valve timing and backlash. I also had a bent pushrod and a too-small prop that incidentally had a pretty good nick in it that I didn't think about - I've since been reading about aerodynamic balance on props and I think that might have had a lot to do with the vibration. I put on the choke assembly and a new 14x6 prop and she runs MUCH better. Thanks to all of you for helping out - I think this one is going to have a happy ending.

Thanks again.

Jazzy

VWalton,
For break-in definately use a smaller dia. and higher pitch prop. My TT91 wouldn't run well at all with a 14X6 until I ran a bunch of fuel through it on the test stand with a 12X8 (maybe an 11X somethin'). I ran it incredibly rich at almost WOT. Slowly leaned it over the next few tanks and started working on the low end. Still a bit tight but runs great now with a 13X8. I believe my HSN is around 5 1/4 turns out.
Before I did a thorough break-in it spat more props, threw more propnuts and trashed more spinners than any engine I ever saw! (It didn't like the tank position with it's inverted instalation).
With over 1.5 gal of fuel through it the compression is showing no signs of letting up. It is almost too much (cold)!
Do a lengthy, really rich break-in for the TT 91 and you'll be happier. Yeah, the choke assembly works wonders for this engine.
Keep an eye on the valve clearances for the first few weeks of flying. At least till it runs really reliably through the throttle range.

Good Luck
Jeff