I am going to break in a TT91 4-stroke and it states you need a tachometer so the RPM's dont get over 5k. Do I really need a tach or can I just let the engine run rich and slow to break it in and not have to use a tach.

If your ears are really good and can tell the RPM of a four stroke, then you don't really need a tach. However, sometimes it can be difficult to tell what RPM a four stroke is running at just by listening to it. A tach is not that expensive.

How is your ear to the tune of 5 grand? Run it rich like you mentioned and you shouldn't have a problem. As long as it is good 'n rich it would probably be OK even at 7K+.
Use the recommended prop for break-in and run it close to WOT (wide open throttle) using your high speed needle to tone DOWN the RPM. If you have some extra castor oil on hand, add a little to your first few tanks. Say 1/2-3/4oz to 16oz of fuel. It is a good idea to pre lube the whole engine with oil (10-30 is fine) especially the valve train. Takes a long time for the lube to get there under normal conditions. Remove your rocker cover and oil up things in there and inject some into the crank breather also. You can leave it off if you like for break-in just make sure you replace it the correct way when the time comes. Some fella's take the rear cover off before any of this and rinse out anything that may have been left from machining? I use kerosene for this and it is a good idea to do it. You don't want to use a solvent base cleaner as it will eat up the o-rings. 99% of the people don't do this, so the choice is your's?

I run the engine for about 5 min. Shut it down and let it cool. Repeat that for the first tank. Second tank run it dry. Third tank I start to lean things out a bit at WOT using the high speed and giving it brief high RPM settings. You can usually tell if things are still too tight?
I Broke-in many of 4 c's without a tach and never had a problem. Pre lube and don't overheat by running it lean and you will be fine. Good Luck.

Happy Fly'n....

I have the Powermaster 15%nitro and 18% castor/synth fuel is it wise to add 4oz of Longs Drugs 100% U.S.P. Castor oil (Laxative) to the gallon to get the oil content up to 20% or is that fuel okey at 18% for Break-in.
Also I have a Two Blade 14x6 prop and a 12x8 three blade prop which is better for breaking the TT91 in. Or should I buy a 13x6 for break in.

Baz,

No, I think we have two different types of Castor Oil here. May be I am thinking of Cod Liver Oil? The Castor Oil used in Glow Fuel is Very Poisonous! Something you don't want to ingest.
The fuel you have listed should be fine unless the instructions state other wise? The 14x6 prop. should do fine. If you have some 10% Nitro 20% Oil (2Cycle), that would be better. Just pre- lube the engine (as stated earlier) and run it good and rich. Take your time with the break-in and you will be well rewarded after.

I have never used Power Master Fuel. It is not available in Canada as far as I know? I Know some of My American Fly Buddies don't and won't use it. They refer to it as "Rust Master".
It is a blended fuel and you might want to post a thread in the engine section regarding using it for break-in. You will get all kinds of answers I'm sure, so you will have to define fact from fiction there.

Happy Fly'n....

There are many, many people running PM fuels here in Central Texas where humidity is a high year round. I haven't seen or heard of rust problems yet.

John

Just Sarcasm John... just sarcasm....
I have never used it, so I can't comment on it or Knock it for that matter. I don't believe everything I hear, but I do listen well.

Happy Fly'n....

I'm with ya!

Personally, I like PM fuel. It's good stuff, runs well and is very consistent. But, when I order fuel, I order from Magnum. To me, it just runs better

John

Basil, I have a TT .91 FS on the bench right now. I started breaking it in last year, and everything went great up to about 8000 rpm. Over that speed I would hear a crackling noise, and when I would try to restart it for the next 5 minute run, it would be hard to start, or not start at all. I would take it back, check the valve settings, and find the lock-nuts on the valves were loose and the clearances way too loose. I would re-set the valves, tighten the nuts and go back to the field for another session. I went through three cycles of this difficulty, and then ran a search on this engine here on RCU. I came up with a report from one of the moderators that this was a common problem with the TT .91 FS. His recommendation was to take off the valve cover before starting any break-in, loosen any screws or nuts under the valve cover, set the clearances, then re-set everything with loc-tite. Other than that, it's a great engine. I am just waiting to get a couple other projects flying before I give my TT an alcohol bath, since I don't know if loctite will stiffen up well when applied to an oily surface.

I have been breaking in my TT91 1 quart at a time in the back yard. I've used around 1/2 gallon of the Powermaster up. I went ahead and used it when one guy E-mailed me that that was the only fuel he could get his TT91 to run good on when he was breaking his in. The engine starts with just a flick of the finger. They say before starting to press in the choke and turn the prop clockwise to draw fuel. I found that it's better to press in the choke while trying to start the engine. Starts very quickly that way. I'm getting great compression from the engine. Noticably after the first tank. I checked the clearances of the valves and there both staying put all tight on the nuts. Maybe the Thunder Tiger Quality controll is taking care of it's loose nuts!!
One waga waga is when I take the prop off along with the spacer to where the bearing is showing the main shaft will move back and fourth with a small amount of play. See if yours has this play. Of course when the prop goes on and the nuts are tightened the play will be gone because the shaft is pulled forward.

I use Powermaster fuels myself, and no problems so far. I use Powermaster YS 20/20 in my YS four stroke engines and Powermaster Mean & Green in my OS four stroke engines. They all seem to run best on either Powermaster or Wildcat fuels.

Get a tach...they aren't that expensive...unless you buy a TNC.

The primary reason for break-in on a four stoker is to begin seating the ring properly...it won't be fully seated until the engine has seen ~3-5 gallons of fuel. An 8 ounce tank is what I like to use for these runs. The first couple runs need to be slobbery rich (5K or less)...even so rich the glow ignitor needs to stay on to keep it running. After that I gradually lean the engine, ~500 RPM at a time, and run it through two tanks at full throttle at each new setting, letting it cool 10-15 minutes between tanks, until I reach the full lean setting after going through a gallon plus of fuel altogether. It takes a little time to do this, but the results make it well worth it!

IMO, the props of choice for this engine are an APC 14x8 or 15x6; plug is an OS 'F'; fuel...10-15% nitro/20% lube.

The plane I am buing requires a smaller prop. Can I use a 12x8 prop on the TT91 four stroke to reduce the prop diameter distance to the ground. I heard garman makes one but master airscrew only makes a 12x6. Would the 12x8 be good for a Cessna and the 12x6 be good for a P51D. Also what size 3-blade
spinner would I get for this size prop a 21/2" or 3". And what size sapinner would I get for my present 14x6 2-blade prop.

Others typically run their four stroke engines faster than I like to run them...I like to stay 9-9.4K on the ground as it will unload in the air up to ~10K...fast enough for all those moving parts IMO. The 12x8/12x6 will let the engine rev somewhat more than my ideal, so a larger prop is in order IMO. See if you can get a 3 blade Master Airscrew 13x8, hopefully that will give you the ground clearance you need.

In as far as spinners, I don't care for them...I prefer to use Dubro spinner nuts. So, I don't have a lot of practical information as to the good ones or bad ones to choose.

I will order a Tach they have a Glo Bee that does 2-3-4 blade props for $28 at Tower. When I was out flying today it didn't find it easy to richen up the engine so it will unload. I was too rich and got a dead stick but it was a pretty landing anyway. If you do this by ear do you just open the needle valve untill you hear a noticable richening or do you just open it 1/2 or 1/4 turn after the highest revs.
This will probably be harder on the TT91four stroke.