I am so frustrated I am about to go back to glow.
This is a 17# yak with brison 3.2. I do not trust it when hot and humid. Example:

Yesterday morning it was in the mid 80's. First flight run I tweak the needles and it is purring like a kitten. Now it is only getting 6600 on top with a MSC 22x8 and a B&B muffler.(new bison on the way) In flight it was running great. Not a bobble or burp at any throttle setting or attitude. As the day went on it got into the 90's with high humidity. Now it losses 300 on top(down to 6300) and within the first couple min's of flight the mid range is really burpping and popping to the point it sounds like it will die. Never did and I sure did not push my luck.
I spent alot of time flying and land. Tweak the needles and do it again. Went both ways rich and lean made no changes in performance.
That makes me think it is getting too hot. Unfortunatley I do not have a temp gun or anyone esle in the club.
This is my second summer with gas and I seem to spend more time down than up. Everything I have learned about them is from here and trial and error. No one else around has gassers.

Bought this engine used last summer and sent it to Garry and Harvey. They told me it was 11 yrs old and had never been apart. They stripped it down and decarboned it, new bearing, timming ring and such and sent it back looking like new. I also sent the ign. to CH for a new plug wire and check up.
The carb is vented to the fuse. Lerned that last year when it kept dying on the Columbo extra. Too bad I did not learn till after it died on aproach 5' off and it pancaked causing some damage. Hence the Yak.

Here are some pics. If anyone has some idea's I sure would appreciate it. I dont have much more hair to pull out.

The air intake in your plane is much larger than the air outlet. It should be the other way around.
Put a round baffle inside the cowl, flush with the lower cowl cutout, leaving only the black section open, (where the engine is).

Hot humid air needs much leaner settings than cool dry air.
At 1/3rd throttle, set your idle needle just lean enough to break into clean non-fourstroking running. Set high needle for best rpm, then back off 100-200 rpm. When throttling up, the engine should accelerate crisply, then slowly climb to best rpm, without sagging.

I agree. The engine is fully cowled with an extremely large inlet area. There is no baffling to direct the air through the engine, but that same air can deflect easily completely around the engine, providing no cooling effect. The bulk of the muffler is also inside the cowl helping to retain heat. Don't blame the engine, it's trying to do what it's supposed to but getting beat to death by the installation.

Place a loose pile of stones on the ground. Direct a flow of water at the stones. Watch the water deflect the bulk of the flow around and not through that loose pile. Air is also a fluid and acts the same way.

Good timing with this post. I'm considering a 50cc Yak. With regards to the cooling and baffling, any advice on how - and with what type material to fabricate a baffle for a Yak cowl. Pics welcome if anyone has them.

Thanks - Jaketab

Well Pat, I got lazy and bought the side mount. Still had to put .25" spacers under the engine for it to fit. Did not gain any r's but all the guys at the feild said it sure sounded better. They could here themselves think.
I also cut another hole in the cowl where the pan is. Flew yesterday with temps in the high 80's and high humidity and it never stumbled. It felt smoother than ever like it can breathe.
Had a gentleman pm me saying he has been flying a 33% without the spinner. The other weekend he decided to put it on and within 5 min. of flight it went hot. Said he took the spinner off and flew it the rest of the day without over heating. Something to ponder.
The next test will be when it gets to the 90's again.

Thanks for the help all.

Thats what I was going to suggest....I think the big spinner is directing the air away from where the air should flow. Smooth Landings capt.n

Use a dummy radial engine and free the section in front of the gasser cylinder. Small guide plates from there alongside the cylinder work wonders.

I saw one setup that had one of the dummy cylinders in place just in front of the gas engine cylinder, and opened up spaces left and right from the dummy. This placed the (larger) gas engine cylinder in a very turbulent airflow, and it worked well. Air extraction was by using the NACA cowl gill flaps

Unless there is a problem with the engine itself I dont see a problem with cooling on this setup .
It's setup the same as my Yak w/ a Brison 3.2 thats been flying like this for over a year with no overheating concerns (in 90 + temps.).
Your engine is basicaly uncowled compared to most caps , extras, 540 etc w/ plenty of airflow around it and open on the bottom .Why would you need any better cooling?
I suggest looking at your carb , check the screen , be sure the spacer block and gaskets are intact etc...
The removal of the spinner will change several aspects within the cowl (airflow , engine load etc ..) and will change engine performance (slightly)
But i would still check for a more "root cause concern"
I have noticed a change in power and a definate increase in midrange gurgle caused by too much oil in the gas (i use amsoil 100:1 unless i run out
and had to use some stihl mixed at 32:1 . It reduced my power and rpms and started gurgling .
Just some Ideas.

I agree with most, block off the inlet except where the engine cylinder is. Make sure the outlet is 1.5- 2x the inlet.

the best material for a baffle is the carbon fiber laminated sheet . less than 1mm thick , very easy to use and lasts a long time . make a template from cardboard , repilcate it and u can first tack it with cayno , then run a bead with epoxy/ milled fiberglass mix. done

I have always heard that the typical rule of thumb is for gas you really should have 4x the outlet then the inlet.

Next time you are in your car roll down your driver's side window about 4-5 inches. fell that very little air comes in. . . while the window is down roll down the right rear window on the passanger's side the same distance as your driver's side window. then (while watching the road) inch the rear right window down and when you reach about 3x the distance you will actually feel the air move across you head to exit in the rear. . .same principle but different vehicle! I know this for a fact because if i smoke in my wife's car I use that method. . . TRUST ME IT WORKS! Im not in trouble other wise i would have new planes!!!!!


thats my1.5 cents

oh, you can get away with a 1 to 3, but 1 to 4 is recomended
for glow it is typically 1 to 2

thats the other .5 cents

The problem with the engine in the picture is that there is no path for airflow
the opening of course should be ONLY to the finned area of the engine

getting any path to create a low pressure area looks difficult -in the picture
the ol 1-3 rule is good for starters
but on round cowls getting low pressure aft the engine can be a bear - One of the new YAK designs actually uses large scale louvers at aft edgeof the cowl - a great idea
but if possible on other setups add a lip at the front edge of whatever outlet you can make - to create turbulance and hopefully low pressure in the cowl - this is how th air gets thru the fins .

The typical rule of thumb is at least 1.5 times the inlet, with the outlet placed at a low pressure zone. (info from NACA reports)
That slowly crept up to 2 - 3 - 4 times the inlet, whilst forgetting the low pressure outlet. Now I often see a large outlet that allows the firewall to act like an air scoop, to let air in, not out. With two openings struggling for priority to let air in, the cowl is a bad place for the engine to be in.

Don't forget that the performance loss and mixture changes you are seeing is not just from the engine getting physically hot, but from the hot air trapped in the cowl that is now sucked in your carb, effectively changing your mixture. Unlike a ZDZ & DA's rear carb, the Brison's carb is stuck over there on the top right side of the cowl where there is little cool air. You might also open up one of your louvers right behind the carb.