I bet that is why jets are shaped like a needle in front!!!!! Just think of the performance lost because the front of the plane is not shaped good. They push to much air!!! Capt.n

OK Capt’n I am ready to purchase another muffler for experimentation.

Bill

Sneaking up on performance.
5-9-06 test - 8000 RPM MA ‘K’ series –11# 9oz thrust

Hey Capt. you are right about lost performance because of the shape of the plane and another thing i see is most of the planes being used with the conversion engines are very high drag. You can get better perormance out of the conversion engine with a low drag plane.
Ken

It may not work on all planes....but sometimes the use of a big spinner makes for more speed. Nice too if you need nose weight. Capt,n

Hey BillS.
Where do you order your engines from?
I checked local stores with no luck.I wanna order one off internet.Or is it possible to order just an engine from the manufacturer?

Tractor Supply co and Menards have the MB290. The Menards version is model YMB290. It has the Yardman name on it. There may be other places also that have them. I found a engine parts view. Its under YMb290. I will try and post a link to that soon. Capt,n

Capt’n steered me to a Home Depot store that had six on close out and I purchased four. Georgia in the office was very helpful. The UPC number will probably be necessary for Georgia to look up remaining stock. The UPC number and store phone number is somewhere in this thread.

Hope they still have some left. Good luck.

Bill

Up-date on the Mb290 engine. This engine has a 2 year warrenty. I had a throttle shaft break on one of mine. I called the toll free number in manual. They told me to take it to the nearest dealer. Took it there and found out if the cost for repair exceeds 75% of the cost of unit, you will need to take it back where you bought it for exchange for a new one...or a brand of equal cost. Kinda strange. They say the parts are not to available either. I will see what happens next. Capt,n P.S. Glad I have 2 of them.

Experimented some more with a stock MB 290 engine. Thought everyone might like to know that Ken’s muffler is good for 1100 RPM on a stock engine with no other changes.

Started with a stock engine.
16 x 8 MA K series prop – 6700 RPM

Muffler only change but did turn the carb screws
16 x 8 MA K series prop – 7800 RPM

The other engine with modified exhaust duration turns 8200 RPM. When the engine cools I will modify the duration on the the stock engine. After having looked at several other 26cc engines I might be a little more aggressive with the port width on this one. Based on how the Hi speed needle adjusts I suspect we are running out of carburetion at 8200.

Bill

Bills: Sounds like you are getting that Mb290 going pretty good. Of the 2 I have, one has a broken trrottle shaft as you read. I dicovered I can controll the RPM by the other end of the carb shaft that has a plastic/nylon part that will also work to controll RPM. It stiil runs great. I can get my full money back, but have not decided too. I want to keep one going as a leaf blower. Too find another may be hard??? I wonder if anyone is using one of theses engines in a plane yet. How about that GUYS????? Capt,n

Capt’n the one you use in the airplane will need different throttle linkage anyway and you can always connect the servo linkage to either end of the shaft.

Bill

Capt’n finished cutting and tested the second cylinder. Left exhaust duration on the second cylinder at 150* and there appears to be about 150 RPM difference. In other words 5* less seems to cost 150 RPM but is not considered significant. Other factors such as run time, temperature or humidity or needle setting could have contributed to the difference. May change my mind as testing progresses. Continue to believe California lean carburetion is currently the limiting factor but not willing to buy a bunch of carburetors for experimentation.

With Dremel tool and a tile-cutting burr the cylinder cuts like butter. It is a delicate but easy task. Didn’t touch any other ports. Matter of fact the transfer ports seem strange in that they are flat at the top, which is unusual and not understood.

Bill

Bills: Great news on that tile-cutting burr, that is what I need. Where did you get it? What do you think. Maybe just making the exhaust port wider and leave the timing alone would be best. What happens if you cut the botttom of exhaust port to get more flow? On that throttle....the shaft broke and now the control knob and shaft has to be held in. If you take it out the air mixture goes way off. I am going to take that one back for full refund. The last dealer I took it too cannot get parts. MTD will pay him 1/2 hour for paper work. Bottom line.....NO PARTS for the MB290 0r YM290= Yard Man.

Lowes

A very bad idea. The exhaust duration on a stock engine is 122*.

Don’t think any gain will be seen but the engine could become a boat anchor.

With a 3/16� brass rod and a Swiss mill ******* file I could probably make a shaft.


Bill

Bills: Have you determined the best exhaust timing? And if you use that timing....is making the exhaust port any wider going to help? I am returning that bad shaft engine because it has another problem also. If I was to fix it...I would just go to a little bigger carb or take a shaft out of another dead carb.

No I will be testing for months.

Maybe but piston and ring life is somewhat shortened with wider ports.

The shaft is too simple to agonize over. Make, steal or improvise something.


Bill

Just to try and help out with some new info....

I took my stock muffler, a hair over 8 oz. Slit it in half at the bent over flange, and seperated all the internals out..lots of junk in there..I cleaned everything and jb welded the two outer can halves back together , with the two standoffs, or tubes that keep the seal tight at the ehaust once installed..I didn't skimp on the jb weld..and I came out at just UNDER four ounces on stock muffler...man the original thing was heavy! Now I am no expert on whether the jb weld will hold, or what the rpm change will be. But I thought someone might be interested.. Unfortunately I am not sure when I will get this motor on a stand and propped..so I can't guarantee any running info in the near future..

hope this gets the ideas started though, anxious to hear some flight reports ..mine MIGHT be going on a 62 inch 3d profile...

I am definitely interested in your findings. Please keep us posted.

The original muffler appeared to be cross flow designed. Do you have any pictures of the inside?

Bill

Actually after reviewing what I did.. I may have used the original piece that fits the ehaust port, and an internal piece of the mufflr as the outboard section..I believe the screened part was the original outer piece? Sorry didn't take to many notes during the operation!

anyhow, here are a few pics (hopefully) i think my motor in this configuration weighs 3 pounds 10 ounce....

Hi Bill!!! What is your duration intake timing???? I think that the intake timing coupled with the exhaust timing plays a big part on the increase of rpm's, another thing, how big is the carb's venturi I would scrape some aluminum to make the diameter a little bigger.
Bill Pomplun A.K.A. Aero Nut.

Bill,

For some reason I did not check the intake timing but I will check it and report.

The biggest obstruction in the carburetor throat is a huge fat head on the screw that holds the choke plate.

Bill

Bills: Some guys do away with the choke plate, and add a velocity stack. You can choke it with your thumb or finger. Less restriction! Best Regards Capt,n

The choke is a slide and does not restrict the opening. Incidentally the engine turns same RPM with or without air cleaner. Despite conventional wisdom I am always reluctant to remove functional parts unless a solid reason exists or performance gains can be measured. No need to make starting more complicated.

Bill

Intake duration is 140*. The venturi is 10.5mm.

Bill

Has anyone worked out the throttle linkage on the MB 290 blower engine? Could you post pictures?

The spade tabs on the coil can be used for positive engine kill.

Bill