I got a second hand TT91 4C from a pal, he changed the piston, sleeve and bearings. Also, got the valves etc adjusted. He is good at such things. Today I tried to run in the engine. It had problems.

1. Engine idles when the carb is HALF open (2,700 RPM);
2. Engine peaks at 6,800 RPM with an APC 15x4W; and
3. Engine peaks at 6,200 RPM with an APC 13x6.

Running Cool Power 15% Heli fuel, OS F plug (new). I am lost as to what the problem is, it will start easily but why the above problems? I need some help ... thanks!!!

It sounds like the cam timing is off, that thing is not running strong. []

FBD.

Jim

Also on the Thunder Tiger 91, there is a outer choke assembly on the carb that people tend to want to remove. The engine will not run correctly with this piece removed.

Enjoy,

Jim

Thanks guys, will look into the timing. Oh man ... I removed it the choke unit, I will install that again. You guys are great.

Just a question, not about the top end only but the carb is open 50% just to idle. What could be the cause of that? Might it be that the low end is too lean? But the transition is smooth.

It is hard for us to tell in a situation like this. Tell us more about how it is running as the throttle is closed t othis 50% position?

Enjoy,

Jim

It idles smoothly at 50% throttle, I mean smooth and the transition is smooth too. I just opened the cam gear cover and all I see is an oval knob that operates the exhaust valve only. I see some gear behind that oval knob. Small punch mark do I look for? Is it the gear behind the oval knob? Thanks.

Okay ... I see punch mark ... hehehe ... its dark, the mark is at the bottom when piston is TDC. One question ... which TCD, combusiton or exhaust? Does it matter?

I noticed that punch mark is at the bottom only just as the exhaust valve is about to close AFTER the exhaust cycle. Means when mark is at the bottom the oval knob has just passed the push rod, its not directly below the push rod.

Put the piston all the way up. One time when the piston is all the way up, both valves should be open. If this is the case, turn the crankshaft one full turn until the piston is all the way to the top of the cylinder again. This time the dot on the side of the gear just beside the oval cam should be straight down.

When the engine is at idle, does the engine run slow enough for you?

What top speed does the engine run now?

Yep, choke ass. must be on carb.

Being that the engine is newly rebuilt it probably needs broken in again. Not as extensive a break-in as the first go 'round but run in nonetheless. Run it really rich for a handful of tanks then start tuning/tweaking the needles. HS needle setting may end up being 4.5 to 5.25 turns out - further for break-in. This is not uncommon. Use a piece of fuel tubing over the point where the needle enters the spraybar to help prevent airleaks.

I set my cam gear with the notch on the drive washer at 12:00 and the piston at TDC compression stroke. Instructions say to insert cam when piston at TDC compression with dimple at the top ("...mark must point straight up... ". (NOT angled toward the pushrods.)

My TT91 does not like 14 or 15 inch props but does like to spin up to around 11k. If you have a 13X8 try it first for initial run-in. (Any 11-13 in prop with high pitch - 8 or above - will do nicely.)
It doesn't sound like you have the destructions - right?

Good Luck,
Jeff

w8ye - at 50% throttle it idles at about 2,700 RPM or so. The plane will not run off on the ground.

Jazzy - I did run it in, was running a little rich, can see smoke but was running it at about say 3,500 RPM only ... I did that for a tank, lasted about 30 mins. This dimple thing is now confusing me. All I can say is that during the TDC of the compression stroke the punch/dimple is OPPOSITE the pushrods. That to me is 6 at the o'clock, 12 o'clock would be right at where the push rods are.

Am gonna try again tomorrow and then see how she goes. Thanks guys!!!

Here is an image of the manual.

The cam retarded one tooth will give a good idle, and a low top end. Advanced one toth will give hard starting, the top end about the same as correct timing, and a very rough idle.

With retarded valve timing it usually takes a higher throttle setting for idle too.

All in all, I'd tend to agree with Dave and Jim that the cam is off time.

Bill.

w8ye

I realize that this is only a sample space of 1, but my TT91FS runs just fine without that choke assembly.

Denis

Surprisingly, it doesn't matter if the dimple on the cam is 12 O-Clock or 6 O-Clock just so it is one or the other when the piston is all the way up. The cam only travels half the speed of the crankshaft. One time the piston is at the top the dimple will be at 6 and the next time it will be a 12.

Enjoy,

Jim

W8ye,
You're absolutely right. In this position though, if you don't turn the crank to rotate the cam lobes the pushrods and rockers might be a bugger to assemble under pressure.
(I never did it this way...)

I always assemble with the cam followers on the low part of the lobe. But I've seen people put them together the hard way too but that's just not me.

Enjoy,

Jim

Jazzy - truly appreciate your help!

w8ye - I was trying to figure that out! Hahahahaa ... I took and look and I said how the heck do I get the punch mark in at the bottom and seeing the lobe pushing against the push rods!

Something doesn't sound right. It may simply be the carb arm is out of adjustment as he is saying it idles at half throttle and doesn't rev up where it should at full throttle. Could we be so lucky that it only a linkage problem instead of an engine problem?

Thanks for all the help ... found the problem. Was a loose screw on the rocker arm assembly, hence both arms were not able to open the valves properly. Thanks a lot!!! She runs nice now ... will idle with the proper carb setting and turns my APC 15x4W at about 9,000 RPM, I am happy, lovely for my WACO 60

That's great that you have the engine running correctly.

I was wrong about the cam timing being wrong but while you were looking you found the real problem.

Enjoy,

Jim

tIANci,

Glad to hear you got her runnin'!!

You do realize that you have to remove the rocker arms to be able to remove the cam don't you?
With the pushrods under pressure it cannot be done.
I usually remove the entire head. That way I can clean the valves and combustion chamber.
This does not affect the valve adjustment.

Good Luck,
Jeff

Jazzy ... yes I realised that ... such a pain! Anyways, thanks for the advice, at least I did learn a lot. The wonders of the internet!

The TT .91 four stroke will run nicely on the bench without the choke. But, unless you are running a pump, in the air it will lean out at about 3/4 tank and deadstick. It may also run without the choke if you do not open the throttle fully.