From using my 2-stroke nitro plane and rc cars i feel i have a pretty good handle on tuning two stroke engines. Are there any major differences in tuning 4-stroke engines? Would i already be competent enough to tune one? I recenly broke the older os 60fp on my Goldberg Tiger 60, and i found an os .91 fs 4-stroke for sale for $150. I saw on tower hobbies that they still sell the surpass II and III. Do they still sell parts for the .91fs? Is this a good price for the motor? Thanks for all your help.

There is not a MAJOR difference between 2 and 4 strokes. Read the manual that comes with any engine you buy and you can't go wrong. Yes you can get parts for a .91fs and no 150 is not a good price if you compare it to these auction's. http://shop.ebay.com/i.html?_kw=os&_...ts_Accessories

I quit dealing with tower a long time ago.

OS 91 4st one of my favorites I have 5 of em, 150$ for a older fs not a great deal, you can often pick up a older surpass for 120-150$ of of ebay that will give you more power for your buck, some even new in the box.
the surpass II style of engine is by far the cleanest of the surpass engines since it recurculates the breather oil into the intake manifold, where as with the older surpass has a case vent that a small piece of tubing vents it to the bottom of the cowl or fuse.
tuning is relitively easy, with either the older sp or spII the II being the newer and simplest but there both pretty strait forward, low end screw in the barrel and a high speed needle on the other side.
after buying a couple of the older engines which ran quite well, the carbs were a little sluggish in the tuning dept. and needed cleaning and in one case new rubber seals for better tuning qualities.
they don't call em surpass engines for nothing they have 25-30% more power than the older fs engines and parts are still available finding replacement parts for the older OS engines can be really hit and miss somtimes going to ebay for "parts engines" to canibalize for needed parts.

The major difference is in properly setting the high speed needle. On a 2 stroke it can be done by ear most of the time as we can hear the difference in pitch as the engines speeds up or slows down. Not so with a 4 stroke. To properly set the high speed needle a tachometer is needed.

You seem to be very knowledgeable about these engines, what is the advantage of the "pump" on the newer versions of the surpass engines? Is it worth the extra money if I bought them new?

I do not have experiance with a "pump" engine so I asked that same question at my flying club.  The guys told me that when a pump works, it is great.  But when there are problems with them they are a royal pain and difficult to correct.  My local club members recommended against buying a pumped engine.  Their opinion was "line up the carb with the fuel tank correctly and you do not need a pump". 

I usually prefer the pumpers but for the only reason being I can place the fuel tank any place I like. I usually install them over the CG of the plane and don't care about them being lined up with the carb or close to the fire wall. I have a couple of the .91s without the pump and they are outstanding engines, tank location is the only difference in the set up. A pumper has a regulator so when in the air they aren't supposed to go rich or lean, most the time this is true. Like your club members stated, the pumpers are great until you run into a pump or regulator problem, I'm having a problem with one of my 1.20 pumpres now and it is really a pain trying to sort it out so there is a lot of truth to that. The pumps are a one piece unit and can not be rebuilt. If you have a diaphragm problem in the regulator OS will not just sell you a new one, they have you send the complete carb into them for service. I just did that, cost a bunch for that $2.00 part to be installed. If I am buying a new four stroke I have been buying YS, better prices and much better engines. I can buy every part ever needed for them from there service center and it dosen't cost an arm and a leg like with an OS.

How did you break your OS 60?

As someone said, you cannot really rely on the exhaust note of a 4 stroke to know when you're at the right needle setting.

The best way to go is a tach. I've personally never had one...but I've stayed away from the leaner side of the needle.

Pumpers are great for the reasons already stated. Set the engine for maximum performance and forget about nose up tests, etc... the pump keeps things going.

Most Gasoline engines have a built in fuel pump as a point of reference.

As stated rebuilding the pump adds a small degree of complexity, and some additional cost... plus it is one more thing that can go wrong ( hint: FUEL FILTERS help keep debris out of the pump ).

However I really like them.

You can't rebuild an OS pump. Even if you were able to get it apart there are no parts for it to do a rebuild. OS will not just sell you a regulator diaphragm so you can't rebuild that either. As much as I like my OS engines this is one of the reasons I quit buying them new. I do pick up used ones and rebuild them though, pumps or no pumps. I have never paid more then $100.00 for one, that's my top price.

Greybeard...thanks for the confirmation.  And I understand why opjose likes his pumpers.  If you  have had success with something and it continues to work well for you, why would'nt you like it? 
When I buy I look at price as well as quaility.  Since I have decided on the Magnum .91 4s. a pump is not an option anyway.  Less than 200 bucks and I have read many posting from flyers who love their Magnums.  I have yet to read anything that says the suck.  Like all machines, a proper installation and break in is critical.  Correct me if I am wrong but in a Tiger 60 taildragger I should install my fuel tank so that the centerline of the tank is level with the carb of the Magnum.  True statment??? 

They suck. When they are on sale they only run $150.00 so they sell very well. It's a love hate thing with Magnum, I just don't care for them but others find them a great engine for the price. My own choices are YS, OS then Saito in that order. Magnum isn't even on my list. In two strokes I like the SK line of engines but I have only used there .91 so know nothing about there other sizes. True statement and it goes for all engine and tank set up unless it's inverted or a pumper. Inverted I mount them just a bit lower. Some engines draw fuel a bit better then others but it's a pretty good rule of thumb to follow. Keep in mind this is just my thinking on the Magnums and what I think is just my own view. I have a lot of friends that love them and feel they are the best thing sense micorwave pop corn. If you get a good one then you too will think they are great, get a bad one and you will feel like I do about them. I have never gotten a bad one from the big three.

Only for some people.

I know MANY modelers (myself included) who never use a tach to set our 4-strokes

I don't disagree as I do it without a Tach at times myself. But for someone just starting into 4 strokes a Tach will help immensely in properly setting the high speed needle.

Ok Grey Beard, ya got me.  Living proff that if you hang around long enough you will see and hear everything. 

That being said, I was planning on mounting my Magnum Cylinder head down.  There is an old thread in the Kit Building forum on a Tiger 60.  That fella built a cowl around his engine and inverted it as well.  It looked so clean.  Lemme see if I have this right.

An inverted 4s would be mounted with the carb slightly lower than the fuel tank centerline.  Correct?  </p>

can you give me a link to that old thread? I would really like to see the pics of that Tiger 60!

I have several 4 strokes you can't hear the top rpm and can't hear them drop a few hundred. I don't always use a tach but I can dial an engine in a lot better with one. Steve, you want the tank center line just a touch lower, about an 1/8 or 3/16 then the needle valve or spray bar if you go inverted. Not a big bunch. I use foam on my tank compartment so I can add and subtract it if needed to adjust the tank. If the fuel is siphoningit usually tells you there may be a tank location problem. Some engines don't care as much as others. There is no real fixed rule, it's just my starting point for any inverted engine that isn't pumped. Other people will have other ideas on the subject, I try not to etch anything into stone.

I have some 29 Saito four strokes and 6 OS four strokes

I don't use a tach. Wiggle the needle some, you can hear the rpm change just like on a two stroke.

HEAR! Well there's the difference, you still have your hearing!

Well I cannot hear what my wife says but I can still hear a lot of low frequency things

My wife says I hear what I want to.

When I was young, the sound of the high voltage oscillator in the TV about killed my ears. Thankfully I no longer hear it.

I have two magnum 70 4 strokes, great motors, and 2 YS 4 strokes, much greater motor. I feel you get what you pay for with magnum, not a bad motor, just not the best motor. Never had too much trouble with my magnums.

Ahhh, the sounds of the "flyback".....now, I have a continual sound like that in my ears, but it's a result of the 18 weeks of chemo I had 10 yrs ago!

Years of running Dad's open exhaust free flights in the basement as a kid and a life time of gunfire have taken it's toll on my hearing so sometimes I even use a Tach with my 2 strokes. I can still hear the wife if necessary!

My 26 year old son had Chemo and Radiation for 12 weeks this past years. He says the only after effect is that chocolate tastes weird. Glad to hear your at the 10 year mark Bax, thats a good milestone.

Our hearing loss and the continious ringing in our ears for a lot of us has more to do with age than noise exposure.

But I'm guilty of running model engines in the basement in the 50's myself.

AnAlternateEgo, Ive never tried this before but here we go....


http://www.rcuniverse.com/forum/m_4997091/tm.htm

That should be the link to the Tiger 60 build thread.  Right now it is on the first page of the Kit Building forum.