It is no doubt a nice airplane but the wing loading and type of airplane would be above my skill level. I would want 1000 sq inches and would expect the all up weight to be at least 2 pounds above the advertised weight.

Made a muffler adaptor plate for MB290 and ST mufflers and will test sometime tomorrow.

Bill

MB 290 25cc prop data.

16x8 MA 6500 ---- Box stock except for spark arrestor screen.

16x8 MA 6700/6800 ---- Remove exhaust deflector and packing in muffler.

16x8 MA 7300 ---- ST 2000 muffler.

It appears that most any exhaust change improves RPM considerably.

ST sure does make ugly mufflers. However the muffler to engine adaptor should work with a tuned pipe.

Bill

What is the weight of this beast ready to fly?

How much could you trim off if you remover everything you could?
Including coil mounts and all.

If it runs good on glow fuel or gas/glow fuel mix...I will remove all metal and spare parts off. And then weigh it. Until then...I do not know. Thanks Capt,n P.S. Maybe some engine guru will know!!!!!

I'm still shaving metal off mine.. everything is getting cut off. I did come up with a prop adaptor using the stock parts that mount the blower impeller and a few washers. Should work fine.

How do you guys go about modifying the exhuast duration? I don't know much about modding two strokes, I assume you are opening up the top of the port to allow it to exhaust longer? what is the best way to go about doing this, and how much should I remove? Its a cheap engine and all, but I would prefer not to destroy it. until I crash, that is.

car54: are you going to run it with glow plug.....and take spark ignition off from engine? Thanks Capt,n

Have you run the engine and have you decided on a muffler. What exhaust duration are you designing for? The two issues are sort of interrelated.

Bill

Many years ago the motorcycle racers would remove material from the piston to modify exhaust duration. After looking it appears that 1mm could be removed between the top ring land and the top of the piston. Over the years my degree wheel has disappeared. Will make another one to check the difference in duration.

Flipflop reported the exhaust timing to be 122* in post 44. Improvised a test and it appears that removing 1mm would gain about 7* duration.

Whatever you decide you should test before and after so you will know when you went too far.

I got a chance to talk to the Pontoon boat builder today. We talked about building exhausts and selling them. A plane jane exhaust for a 25cc Mac or similar engine would be $16 plus shipping. Now on a more complex exhaust, the price may be around $25- $30 or more. We intend to cut the price of a $99 exhaust to about 1/2 of that or $50. This guys welding is top notch. Will be working more on this when we get back from vacation. May even have a webb-site someday. With converted engines. Best Regards Capt,n Added... Forgot to say....to keep price down , the buyer would have to in some cases supply us with the exact dimensions he wants the exhaust to be.

I am going to keep it as a gas engine with spark ignition. Its going on an ultra stick 120 with a tuned pipe. The whole purpose behind this for me is to fly with a small tank of cheap gas.. I'm tired of burning thru a gallon of glow every weekend with my saito .72 and OS .61FX

Heard of GASOLINE/GLOWFUEL with glow plug conversion????

Captn’John,

I am ready for two mufflers.

There might be a method to make the mufflers somewhat universal fitting several engines and we should talk. ST and MB290 are very close and flipflop reports that the Evolution and G-26 are also close. One muffler might be made to fit five engines. I will also find out the spacing and dimensions of the Moki.

Most of us can take material off and drill holes but don’t have welding facilities. Often I have simply wanted a rectangular box closed on one end with outlet tubes on the other end.

Bill

Bills: Getting ready for vacation. I would like some input on how many and how long you guys want the outlet stacks. Length may not matter. Buyer could cut off to length he wants. I need to know tube diameter and how many outlet tubes. Usually 1 or 2. Plan on making some Pitts style also, with a round can. Thanks Capt,n

Is a exhaust like this .....be the preferred style?

I will build mufflers for any engine conversions you guys need for 25.00 - 35.00 dollars shipped lower 48 other places may require additional charges , I am starting to make one for the mac in this thread . let me know if and what you would like.

For what it is worth.

The little muffler that came with the MB290 engine is very effective at noise reduction. The outlet slit is less than .080� x .900� which is roughly equivalent to one 3/16 ID tube.

It is probably a useless fact but I was curious.

The number of tubes and the size is most likely a cosmetic decision. Whatever looks good is OK.

Bill

Gotta leave tomorrow. How far do you want tubes to go inside the box chamber? 1/4 or 1/2 inch....any tests done on the effect. Need input for that. May not be critical at all though. Capt,n P.S. Know of any good Hobby shops along I65 South or I75 going North.

Captn’John,

The exhaust gas travel in the muffler appears to be about 4 1/2� from entrance to exit. However our replacement mufflers are not very effective as mufflers and also not critical. I would suggest the simplest approach possible. It is almost impossible to design mufflers that are effective without a considerable amount of test data.

Enjoy you trip.

Bill

Additional information was found at:
http://www.rcuniverse.com/forum/m_1079866/tm.htm
in post 17. The information indicates that exhaust volume “must be at least 400cc.� The discussion is about MVVS 1.6 gas engine but probably applies to other similar size gas engines. The information seems authoritative. Sixteen times the engine displacement will be a large muffler and about the size of a 13.5oz Coke bottle. The muffler supplied with the engine will hold about 2.6 oz or about 76cc or three times the engine volume. The ST 2000 muffler volume is about 5oz.

ST 2000 muffler
16x8 MA 7300

Ran the engine with open exhaust and a pipe. Got less RPM and burned fingers. The standard exhaust duration is probably not long enough to be effective with a pipe.
16x8 MA 7350 with open exhaust.
16x8 MA 7100 pipe at approximately 27� length (90* header bend to rear of pipe)

Ok guys I have the template made for the muffler for the mac 25cc , How many want one I am going to make some up this weekend.

I need some input on this. I found two weedeater blowers 1500i 25cc. At Northern. Since they are blowers the carbs are mounted on the side not rear. I own a Great Planes Giant stix 80" Do you guys think this engine will be a good match for this plane?
The weedeater blowers are $55 each. I will get the conversion kit from Wackerengines for this motor.

Thanks
John

I have some mufflers for the 25cc made they are in the market place 25.00 shipped

The mufflers look good. Do you happen to remember the volume?

Bill

4.5 ci if I figured it right.

Ken...are the pins in the photo holding spacers in place? So muffler will not crush when you tighten the bolts. Thanks Capt,n

yes I use ends of welding rods,aluminum just to make sure the spacers stay till you are ready to put the bolts in.

Hey Converts!!! I found yet another source for the Mac MB290. Go to a Tractor supply co. They have had them for some time. Up on shelf...none on display? I cannot understand why they do not display at least one????? Capt,n OH! Price $89[:-]