Hey guys, I have a brison 3.2 in a zero I am flying. This engine is great and really hauls the mail. My question is that when I tune it on the ground, it has an awesome transition from idle to WOT. When I am flying at anything less than 3/4 throttle, it seems to burble or sputter. Transition is still good while in the air and i can blip the engine from idle to higher settings to control decent on landings for example. This effect does not seem effect the running of the engine and when throttling back up it readily responds. I just dont want to get into a dead stick situation while flying.

Any help would be appreciated.

This condition is normal and you will be unlikely to get away from it completely.

You could try some different size props. They are $25 each? This will not completely do away with the burble but might possibly lessen it a little bit?

As you already fear, the important thing is that the engine doesn't quit.

Try a velocity stack on it.

BubbasanJohn,
w8ye is correct in that the important thing is that the engine doesnt quit .
The 3.2 like many other engines will have a mid range burble and the best you can do is to tune most of it out . You need to find the sweet spot between transition and a clean mid range .
1) Be sure ignition timing is set at 28 to 30 degrees BTDC (throttle should be wide open if you have the mechanical advance)
2)Warm the engine up by flying a 5 to 10 minute flight , Land
3)Throttle the engine up to full throttle , set high needle for max RPM and no more ,do not peak and add any extra needle , Max RPM only !!!!!!
4)Throttle the engine up to the point where it runs the roughest in mid range throttle.
5)Making only very small needle movements ,adjust the Low needle in (leaning) until the mid range just begins to clear up.
6)Check throttle response , if the throttle response is clean with no hesitation stop moving Low needle
7)If throttle lags or hesitates richen Low needle only enough to regain good throttle transition.
Once Low needle is set for a good transition and a fairly clean mid range go to wide open throttle and set High needle for peak RPM again.
If after setting the carb by the steps above the engine is hard to start cold or hesitates when trying to throttle up when cold ,allow the engine to warm a little while when starting or richen the low needle only the width of a screw driver blade .

may be air preasure in the cowl is making your carb pump more fuel in then needed.try venting your carb diaphragm cover,the one with the small hole in it, to out side the cowl.this has work real well in the past for me when i had the same trouble.you can just make out in the one crapy picture the aluminum tube on the end of the fuel line,with the end of it pinch shut with two small hole drilled in it.it is tuck in just between the ignition and fuse,under the jip ties.

+1

You may have the carb intake too close to the cowl. Try flying it without the cowl to see what happens. Dan.

I knew this was the right Place for help!!!! Awesome!!!

CK1-Ok regarding the tuning, I have done the steps to tune the engine. Even to the point of dead sticki a couple times because too rich or lean. Currently, the engine just purrs at low idle and wants to rip the fire wall off at WOT on the ground. In the air I have had nothing but awesome performance dispute the burble/sputter. I beleive the manual advance in question is- the white ring with the push rod? I wOuld not know. How to effect change with timing.

closetguy, the cowl doesn't have a lot of outlet for air to pass thru. Not horrible. But a typical radial cowel installation. A little space on the sides by design and a 1/4 or slightly larger gap around the head as it protrudes a bit. Will try this too.

Stuka Larry I will try the stack. Does the end of the stack need to protrude past the cowl or is it ok inside?

W8ye, his was my initial thought. It is an expensive prospect for trial and error. Currently running a master airscrew 25 x 6/10. I like these props performance for the buck seems to be a nice balance (just bought a biela for my hellcat and felt a tad dirty by paying its price). It seems to do awesome for this engine and airframe set up. Make sense to prop up or down? Too much rpm change with the variable pitch prop on throttle transition?

Thanks again for your time.
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Bubba, this video explains the timing
http://www.youtube.com/watch?v=GBXFpxWg7vY

Adjust the pushrod from the carb to the timing ring to get it right. Lengthen it to increase the advance and make it shorter to reduce it. 28 degrees is right. Dan.

Oops I just noticed that you are using a 25 inch prop. Most guys use a 22 inch prop. My Brison 3.2s pur along at 6800RPM with 20 inch props. Dan.

All day Dan. I really appreciate the information.

Dan, i
Might have mistyped the prop size. I think it is a 20x6/10. Will check that.

The best prop I have found for speed on the CMP 50cc Zero using a Brison 3.2 is a Xoar 21x10 laminated . It provides the best engine overall engine /plane performance

now, sense you said you have a round cowl that is the problem,to much air and no were to go.run the vent line out the bottom of cowl,in between the landing gear.solder a brass tube to the diaphragm cover,at the other end of fuel tubing put a two inch piece of brass tube,pinch one end shut,drill two small holes one inch apart in tube.your troubles are now over.

Ok, will try that prop. <div>
</div><div>Will try the vent line solution,  couldn't I just open the bottom of the cowl to allow better air movement?</div>

How about a picture of the cowling front and bottom so we can see what you have .

I have the front area blocked off with a dummy radial with only an opening slightly smaller than the size of the cylinder to let air in . Under neath I have several openings to allow a negative pressure condition to allow the hot air to escape .

Bubba, do it the easy way. Just cut the cowl away. I go the field to fly, not to screw around with a cranky engine. I want plenty of cooling air and an exposed carb intake. Dan.

no sense in cutting everything up,when five minutes to vent the carb diaphragm cover will give you the same tuned carb on the ground as in the air.also it may not take care of the problem for sure.i had to do this mod.on a 50cc profile with the entire engine expose,but it did cure the air passing over the cover plate vent hole.
i never had to do this mod for any of my planes until i started to fly round cowl planes,it takes care of the problem.
i like it so much that i do it as a routine for every plane i set up,just in case.

just remember this is a piston ported engine, not a reed valve engine. they will all sound different, you need to remember what your weedeater and chain saws sound like when at part throttle and light load. no amount of tuning will get rid of it, just the nature of the beast, and they are! long live Brison's

Still working on pics of the cowling.

Rick, yes these brison engines are beasts. Unreal power and torque. I love it. I have had only one dead stick that was due to messing with needles when all was fine. What is the difference btw a piston ported engine and reed engines. I don't own a weed eater and my wife makes me buy electric chain saws so I won't be temped to remove the engines. Are zenoah piston ported too?

Appreciate all ideas and comments thus far! Thank you

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yes, Zenoah engines are piston ported, most chain saw engines are of the piston ported design, engines like Desert Aircraft, DL, 3W, are all reed valve engines, at the rear of the case there is a reed valve that opens and closes with engine case pulses to allow the fuel to enter the case, then be drawn up into the ports to the cylinder, on a piston ported engine, the fuel enters the cylinder from the carburator as the piston moves down it pulls fuel into the engine from the cylinder and then it flows down into the bottom of the case to be drawn up the ports to the top of the cylinder. I hope this makes sense to you.

Rick, ok. I'll admit I know little about the internal combustion engine. I do know I like them in my airplanes and automobiles. So I did a little looking on the web and now I have a few more questions if you don't mind to better understand my brison.

So from my reading, a piston ported engine is the simplest design 2 stroke? Can it have a reed valve or does it use a rotary valve? Or other valve type? I was assuming the zenoah was piston ported as it sounds alot like the Brison. Are their advantages between reed porting and piston porting?

Thanks for your time.<br type="_moz" />

A Piston ported engine is neither reed valve nor rotary valve

Yes, the Zenoahs are also piston ported.