Try changing the length of the spigot or positioning it at another point around the venturi. I've found that feeding fuel in from the side can sometimes be really picky and cause this kind of problem.  Putting the spigot on the engine centerline may help.  The shape of the end of the spigot can affect how the fuel comes out too.  Burrs or an uneven shape can make the fuel form different size droplets as it comes out.   I suspect the turbulence caused by the regular through-the-venturi NVA breaks up the fuel droplets more uniformly.

Hi Marvin,

Have tried several props over the years mostly Rev-Ups in 5 and 6 pitch and the Thunder Tiger seems give better overhead and sq8 performance with a pretty good 4-2-4 run... The others just seemed kinda anemic. Although I did the cardinal sin of making two changes at once, venturi and prop, maybe I need to revisit the Rev-Up's. Actually would be better with a wood prop as far as CG goes the TT props are as heavy as APC's.
 
I'm running straight Power Master 10-22, hoping to be able to say with the same fuel I run in all my other two strokes. Would be a real PTA to have haul special fuel just for my classic airplane.

Phil,

The spigot is on the engine CL, It's set up like a normal ST sprinkler but with a small eyelet protruding into venturi. Same setup that worked great on my ST 46's. I can't change the needle location without rebuilding the cowl so kinda stuck with what it is.

As I said it runs great except for the little burp, would think if the fuel wasn't atomizing it would have other issues.

Not trying to discount anyones suggestions, just searching for the best solution without having to spend a week revamping something when I'm trying to get 4 airplanes ready for Brodaks in less than two weeks.

At Brodaks last week it ended up being fix the problem or not fly. It was so bad the engine was actually quiting, thankfully it never quit at the wrong time while I was sorting out the problem. I tried several different plugs including an OS-F, using two plug washers, replaced the tongue muffler with a tube muffler, no muffler pressure and everything I did seemed to only make the problem worse. It wouldn't even get off the ground with the 4 stroke plug, would quit as soon as it was launched.

The final fix was to reduce the oil content of the fuel to 20% and plug two holes in the tongue muffler. Joe Gilbert loaned me a couple tanks of the fuel he runs in his PA 65, 10% nitro 20% oil mostly synthetic and the Tigre ran like a watch. Plugging the muffler held a little more heat in and seemed to make it four stroke stronger. I also ran a full 12.5 X 5.5 Thunder Tiger prop instead of the one cut down to 12.

Will need to do a few test flights here to see if this setup will work as well in this Oklahoma heat as it did in PA. Thanks to those that offered suggestions....