OS Max25

I have acquired couple of older Max25s : one is the standard (I presume) c/line motor with what appears to be the small venturi : other is the r/c version. Are they in all respects the same save for the r/c carb (to be replaced) ? I am intending to use them in a profile style model as an advanced stunt trainer. Is the motor ok in standard form with the standard venturi/needle plus muffler? Any advice on mods plus initial prop selection and tank setup etc?.
Cheers

>I have acquired couple of older Max25s : one is the standard
>(I presume) c/line motor with what appears to be the small
>venturi : other is the r/c version. Are they in all
>respects the same save for the r/c carb (to be replaced) ? I
>am intending to use them in a profile style model as an
>advanced stunt trainer. Is the motor ok in standard form
>with the standard venturi/needle plus muffler? Any advice on
>mods plus initial prop selection and tank setup etc?.
>Cheers

Yes, the engines are the same. And they are mild engines quite suited to our CL flying style. Although not having the power of a modern schnurle, they also do not require any modifications (other than the venturi) to make them sing. Oh, and I have venturis available for these if you need one.

Thanks for the advice : I would like a venturi and are tongue mufflers available? Do you have any advice on prop choice and tank setup? Model is likely to be based on a profile design similar to the "Middle" of Igor Burger so fairly up to date and will build it light.
Cheers
Chris

>Thanks for the advice : I would like a venturi and are
>tongue mufflers available? Do you have any advice on prop
>choice and tank setup? Model is likely to be based on a
>profile design similar to the "Middle" of Igor Burger so
>fairly up to date and will build it light.
>Cheers
>Chris

I have had tongue mufflers, may still have. I can get them otherwise. I have experience with the 2 version of this, not the 25, but the 25 should be a better engine (same size, weight, 25% more power.) I would suggest starting with a 9 x 6 prop and then experiment from there. Different manufacturers are going to make a difference.

As for tank size, use a uniflow wedge of about 2 1/2 to 3 ounces. Just set it up as you would with any other engine.

Send me an e-mail for whatever you want. I would rather handle things that way.

Had one of the sweet little engines at one time. Sold it in a weak moment to someone who wanted it worse than me.
I guess I would think of it much as a 25 sized 35S at least as to era and construction. Pulled a fairly light Ringmaster pretty well unmuffled. My particular engine did not really want to run consistently in a breaking mode-liked to two cycle mostly and backing the needle off resulted in inconsistency as I recall.
You will want to run a fuel with a relatively high castor oil content. Say 25% or which at least a half is castor and more would probably be better. Powermaster 10/22 with 5 or 6 oz of castor added should work pretty well.
Have fun!
Jim

>Had one of the sweet little engines at one time. Sold it in
>a weak moment to someone who wanted it worse than me.
>I guess I would think of it much as a 25 sized 35S at least
>as to era and construction. Pulled a fairly light Ringmaster
>pretty well unmuffled. My particular engine did not really
>want to run consistently in a breaking mode-liked to two
>cycle mostly and backing the needle off resulted in
>inconsistency as I recall.
>You will want to run a fuel with a relatively high castor
>oil content. Say 25% or which at least a half is castor and
>more would probably be better. Powermaster 10/22 with 5 or 6
>oz of castor added should work pretty well.
>Have fun!
>Jim

Thanks, lots of useful info in there : fuel wise, don't know if Powermaster is available in the uk but should be able to get something equivalent mixed up. Any thoughts on a type of plug as a start point? It seems these old engines will be a good economical intro to learning the pattern ....
Cheers

The OS Max 25 looks like the little brother to the OS 35 Stunt. At about 5.4 oz, it is more than 1 oz lighter than the OS 25 FP. The FP will have more power and probably more fuel economy. However, the Max 25 is a really nice, smooth running engine. Being an ounce lighter than the FP makes it attractive for a moderate sized stunt model. I have campaigned in Old Time Stunt with a Cosby designed Pagan (Aeromodeller 1951 Annual) with the OS 25 Max 25 for the past 6 years with some success in the event at VSC and the Nats.

The Pagan has about 360 sq in and weighes 25 oz. I have used a variety of props, but it likes the APC props around a 9-5. I have not even tried to get the things to do a 4-2-4 break. I run with a pretty fast 2 cycle (11,000 rpm or more) and have not used any kind of muffler. Line length has been from 58 to 62 feet. 2.5 oz of fuel gets through the OTS pattern with time to spare. I use a Super Tiger needle valve instead of the OS.

The fuel I use is basically the Power Master GMA mix, 10% nitro, 22% oil with a 50/50 mix of castor and synthetic oil. In hot weather and at our altitude here in Tucson, the engine likes 15% nitro better than 10%. At sea level and cooler weather, it will run just fine on 10%. I have well over 200 flights on each of two of these engines with no problem (except for the one that was flown into the pavement).

The engine will have adequate power for a stunt/sport model with 350 to 400 sq in and with a reasonable wing loading.