I have a fairly new B 2.4,have about 30 flights with the needles set rich for break in,I leaned both out a little and then went to a lighter loaded prop(from 20X8 to a 20X6). The engine always had a very rich mid-range which I lived with, but with the 20X6 it started hesitating 3 times/second at high throttle,I tried adjusting the HS needle while at full throttle, it stopped the hesitation,then after going to idle and then back to high,it started hesitating again. After several tries I realized that just touching and holding the HS needle stopped it from hesitating. ( it sounds sort of like waa-waa-waa-waaa-waa)like its revving and the fuel/air supply is being interrupted at a high rate-approx. 2-3 times per second. A couple gasser guys helped me check out a few things like dirt in the screen,the diaphram gaskets,and I flushed out the HS needle port with air & fuel. With no results, I went back to the 20X8 and it ran normally. After more carb cleaning checks and trying the 20X6 again,I then went back to the 20X8 a second time and now it hesitates with that prop???
I run 64:1 with Belray synthetic. The gas guys said they never experienced that problem before,the only suggestion they had was to solder a brass tube to the regulator inlet,attach fuel tubing and route it to the carb opening next to the choke valve. All the above adjustments was without the cowl on.

Your condition is also news to me. BTW, there is absolutely no reason to be running a Brison engine rich for break in. Tune it to provde the best performance from the first run. It will take care of it's own break in if done that way.

In the meantime, look for leaks in the fuel supply plumbing and for loose screws an retainers on the carb. At the moment that's all that comes to mind. I don't believe that pressure equalizaition at the diaphram is the issue here, but an easy check would be to simply silicone an open ended cover over the diaphram hole that has the open end pointing to the rear of the plane and the closed end to the front. Do not block the hole, though.

Has anyone used a 20X6 on the 2.4 without any issues?

I have an identical problem and posted a thread 'Carb Trouble' to ask for help. Reading your description of the problem was like reading what I wrote in my own post.

I am thinking about rebuilding my carb. Scott/Brilleli sells a rebuild kit.

I am running a master airscrew 20x6 on my brison 2.4 and it runs and pulls great. I have it on a 14lb. Stinger and it will pull it almost out of sight. Iam turning it about 7400 R.P.M.s. Hope this helps. Wayne

Its good to know it runs OK with that prop,at first I thought the lower pitch caused the problem but after going back to the 20X8 a second time it started acting up. With the 20X6 I got the mid-range I wanted-mostly 2-stroking and clean transition,I did tach the high end @ 8400RPM (20X6)but didnt leave it there for more than a second or 2. It must be something in the carb thats wrong?

Mixmaster, You are getting a lot more R.P.M.s than I am with that prop. My brison is older that I got from here on rcuniverse. I replaced the ring with a frank bowman ring and mine is getting stronger as I seat the ring,but i dont think i will see your numbers. Wayne

Try leaning the low end a little and making the top end richer. Let us know how that works out. You midrange is largly controlled by the low needle. You should lose some of the high end rpm, but imo it's peaking too high anyway.

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There is nothing unique about a 20x6 prop that will cause your problem, other than the fact that your engine will rev higher and may require more fuel. Normally, with the needles peaked, this would not be true, but with the needles set to run rich, it is a possibility.

Engines will surge when the fuel tank venting system is compromised or the fuel pickup clunk is too close to the back of the tank. Another possibility is that the brass vent tube in the fuel tank is too close to the top of the tank, blocking off air from entering and then fuel from exiting the tank.

I agree with Silversurfer on the break-in issue. All you are doing by running a gas engine rich is prematurely coking up the interior of the engine. This practice should be left for glow engines, where it does make a contribution to the engine's break-in by helping remove excess heat via the expelled unburned glow fuel. Gasoline does not provide the cooling advantage of alcohol. However, on the plus side, gasoline is a lubricant of sorts, whereas alcohol in glow fuel is not.

My Brison 2.4 swings a Master Airscrew 20x10 at 7200 rpm and a Mejzlik 20x10 at 7000 rpm. For me, a 20x6 prop is too small.

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It has been quite a while since I had my Brison 2.4, but even to me a 20x6 did sound too light.

I have the 2.4 with a Bolly wood 20-8. Turns that at about 7300. Also had a Master Airscrew 20-8 liked it too, it turned a bit faster, about 7500. A Master Airscrew 20-6 would be fine I think, nothing wrong with revving that motor a little. I might even try it my self.


In the past people used 18-10s on that motor, and Brison would recommend it.

I deliberately under-proped it on the advice from Ultra -RC and seeing what other gasoline flyers(3-D) are doing in my area. This is done mainly for 3-D flying,I wouldnt run at full anyway since its way more power than the airframe needs. What I was trying to accomplish is getting the mid-range leaner so I have the throttle resolution needed to harrier & hover and allow the plane to fall in a hover. The engine ran fairly well with a 20X8 and Mej. 20X10,peaked at 7700RPM with those props but the mid-range is slobbering fat,nothing but 4-stroking. I did lean out the low end untill it surged,then richened a little with not much change in mid.

Mine has run great with the Zinger 20X8 and 18X12. Last week I broke my 18X12 and tried to use the Testor 18X12 and never was able to get the engine to run anywhere near correct with that prop. Went back to the Zingers and it tuned right up and ran smooth again??? All I can figure is the Testors are way lighter then the Zingers and the engine likes A heavy prop?? Guess that's why we are supposed to try different props and see what the engine/plane likes best. Still odd though.

Does anyone here have any experience with APC 20x8 prop?
I have a Brison 2.4 engine and will be trying this prop next weekend.
Henrique

I too think a 20x6 is to-to small for the Brison 2.4, I have a 20x8 on my MVVS 35cc,, try a 22x6 or 22x8 or 21x8, if you just have to run a 20 incher go up to 10 on the pitch..

Oh, if you try a Zinger they flex alot, you may have to go up to a 22x10 with a Zinger..

Jim

Problem solved,took the tank apart but didnt find anything wrong,then I soldered a brass tube to carb & routed silicon tubing to the fuse interior,that did the trick. I put the 20X6 back on and it ran good,will try a 19X8 next. The low end on this thing is taking so many turns in to lean it out its incredible, but as long as its 4 stroking in mid range and not quitting/surging, it still could go in more. My faith in Brison is starting to come back.

I lean my low end needle in till it starts to sag with the throttle advanced quickly. Then richen it up a little till you get smooth fast transition. Thats it, don't run it a little rich, there is no need. All you do is carbon things up, and fowl plugs. Run the high end up to max rpm, and back of just a touch. No need to run that needle rich either. Just make sure its not sagging on up-lines.

Mine has be flawless, 3 years old!