i have a thunder tiger .91 four stroke. I just bought it used. still looks new. Im running omega 15% and a fox miracle plug and a 15x6 k series prop. first issue is. it does not start very easy, seems like its struggling to get enough fuel, it back fires sometimes spins the prop off. 2nd issue when it does start, it just sounds like it is struggling to keep running and when i pull the glow starter off it drops more rpm than i think it should. and when you rev it up i cant even get full throttle and its just like it runs out of fuel. ive had the needles adjusted at every different point there is and it doesnt matter i cant get it to run right no matter what i do. Does anyone know the correct valve adjustment setting. i have a saito .91 and it runs great and this thunder tiger just sounds terrible compared my saito. any tips would be greatly appreciated. im going to mess around with it some more tomorrow and ill post what i come up with. thanks

It does sound like it is too lean of a needle setting and too much prop. On my 91's I use 13x8, 14x6,15x4 props and 15% nitro fuel. Your idea of adjusting the valves should help.

Here is a link to the manual.
http://www.thundertiger4u.com/pdf/f_91s.pdf
Page 9 has the valve adjustment information.

Make sure your engine has the choke-adaptor/venturi fitted.

If this isn't fitted then it will never run properly and wont' draw fuel well at all.

Some people think this part is optional but it isn't.

Not true. Mine runs fine without choke adapter and always has. The requirement for the choke adapter is a myth.

Denis

I would check and adjust the valves.

Then I would treat the engine as a new one and follow the break-in procedure from the manual.
Your fuel is fine
11x10 prop
OS "F" plug - my personal preference

If you've never run it *with* the choke adapter, how do you know it doesn't run a lot better with it? :-)

I'd try richening up the low speed needle and starting it with a smaller prop like a 13x8 or something.

It runs perfect without it. No room for improvement.

Actually, I find that the choke is really useful on my TT91FS in a P51 Mustang because I don't run a muffler. Without the choke I'd never get the damned thing started because I'd never get it wet enough. With a muffler you can just put your finger of the exhaust and turn the motor over a few times so that the pressure pushes the fuel to the carby but without a muffler that just won't work.

I finally got it running good today. the valves were way out of adjustment. one was tight and one was at about .020, i adjusted them to .004 and it runs great and starts easy without the choke. i just put my finger over the exhaust flip the prop 4 or 5 times hook up the glow starter and hit it with the starter and it starts right off. Its running full throttle at about 9,500 rpm with the 15x6 prop and idles at about 2200 rpm. I know the prop im using is quite large but i have the engine in a bud nosen trainer and the fuse is about 8 inches wide so i have to have a prop large enough to get around the sides. Also thanks for the instruction manual it came in quite handy.

I sure agree with you now. Without a muffler you do need a choke to start and get it warmed up a little before going full dependent on venturi to deliver fuel.[8D]

Denis

I would normally say 14x6 is the correct prop for a 91 four stroke but if your engine is doing 9500 rpm with a 15x6 the only word that comes to mind is 'impressive'.

The TT91FS is a surprisingly powerful engine every bit a match for my Saito 100 with some props.

also agree

I haven't read the other replys but IMO my experiance is that you need less nitro and/or a colder plug with a 15-6 prop. I would try either 10% nitro or a K&B 4C plug, if that doesn't work try both. If that doesn't work try 5% nitro. That engine much prefers a 14-6 but I have run a 15-6 without issues.

I think they replaced the carb with a smaller one on later models, they may not need the choke adapter. However it will still operate as a velocity stack and save fuel.

An extra half inch on each side isn't getting you much. Actually you would be suprised how effective a prop is when the back wash is hitting a surface behind the prop. Yes it hurts its performanc, but probably not as much as you would think.

Actually, my TT91FS is a real fuel-miser.

My in my P51D I had a problem because the fuel tank was way too high which caused problems trying to get a decent idle with the TT91FS.

The only way I could lower the tank was to use a smaller one - so I fitted an 8oz tank I had laying about.

To my surprise, this still gives me all the flight time I could ever want, despite most of the flight being full-throttle low-passes :-)

It uses way less than my Saito 82, even though the Saito *is* tuned properly with a nice lean bottom-end and only 200RPMs rich of peak at the top (and yes, I have a velocity stack on the 82 too).

Quote:

ORIGINAL: chashint

Here is a link to the manual.
http://www.thundertiger4u.com/pdf/f_91s.pdf
Page 9 has the valve adjustment information.

Charlie,


There is one thing on page 9 I cannot subscribe to and this goes for all four-stroke engines...

It reads to turn the prop to a location where "both lifters/followers are 'somewhere' on the cam's base-circle"...
The machining accuracy of the cams may be such, that the base-circle is not perfectly concentric with the camshaft...

Each valve must be regarded as an individual adjustment item; and that means it must be adjusted 'alone', with the very middle of its cam's base-circle on its lifter/follower, without any regard to the position of any other valve...


This is very easy to find...

Select one valve.
Look at the rocker of that valve and rotate the prop find its 'point of maximum lift'.
From there, rotate the prop exactly one complete turn, 360ΒΊ (this will rotate the camshaft exactly 180ΒΊ; one-half of a turn, to the point opposite the cam's peak) and adjust that valve's lash only.
Repeat this procedure for the other valve.

Also the adjustment should target the minimum value (to maximize total lift and duration)...
The go/no-go procedure described is just too crude and could result in a power loss and more valve-train noise/wear.

Or the cams may be such that the base circle is perfectly concentric with the camshaft, thus the manufactures recomendation.

Apparently the instructions in the manual were adequate, he adjusted the valves and is very satisified with the engine's performance.
While I understand that some people are not satisified with "good enough" I have not had any issues with the go / no-go method. In fact I am a huge believer in "if it ain't broke don't fix it" so I never even check the valves unless one of the engines starts to run poorly. At that time I will work on it, otherwise it is fly em and oil them if they are going to sit for a while. All of my engines run great and I hardly ever have to fiddle with one of them so I am a happy camper.

Quote:

ORIGINAL: chashint

Apparently the instructions in the manual were adequate, he adjusted the valves and is very satisified with the engine's performance.
While I understand that some people are not satisfied with "good enough" I have not had any issues with the go / no-go method. In fact I am a huge believer in "if it ain't broke don't fix it" so I never even check the valves unless one of the engines starts to run poorly. At that time I will work on it, otherwise it is fly em and oil them if they are going to sit for a while. All of my engines run great and I hardly ever have to fiddle with one of them so I am a happy camper.

Charlie,


I have seen cases, with several engine makes; even full-size, where a cam's base-circle was not concentric with the camshaft...

Since my method, in a single-cylinder, four-stroke glow engine, would barely take a couple of seconds longer, to achieve results you can be more sure of, I find it is worth the trouble.


And as to performing valve adjustments, or for that matter, checking the valve-lash; I do it only when necessary, not just for the fun...

I have no argument with you, but I think the manual should tell you the better procedure; not the 'easier' one...

I fail to see how your method is better if the camshaft is concentric as it apparently is. Your method is only better if the camshaft is not concentric or unknown.

Quote:

ORIGINAL: Sport_Pilot

I fail to see how your method is better if the camshaft is concentric as it apparently is. Your method is only better if the camshaft is not concentric, or unknown.

Hugh,


This is why my method IS better...

Any camshaft can either have base-circles that are concentric (within tolerances), or a bit off...



If my method is used, the possibility of getting it right is independent of the fact...

It will be right, whether the base-circles are, or are not concentric.


Using the procedure stated in the manual will get it right, only in case the base-circles are concentric with the camshaft...
If the specific camshaft does not have concentric base-circles, the adjustment will be WRONG with this procedure!


Since when you receive an engine, you don't normally disassemble it to ascertain the base-circles' concentricity, it must be regarded as an unknown.

MY method of adjusting the valves will then get the right results regardless.