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K&B .35 75 series Empty K&B .35 75 series

Post  Eddy Thu Aug 10, 2023 11:49 am

Have a receintly built Sig Banchee with a K&B .35 - series 75 engine mounted. I chose the K&B engine as it is almost equal to the Fox Stunt .35 in power output, but having 3 more oz. extra wt. The issue I am having is engine speed up after launch. Fuel is 5% nitro and 28% oil at a 50-50 mix. I am using a 2 1/2 oz. standard wedge tank that is mounted 1/8 in. higher than engine center line through a filter. Running a Master Airscrew 10/5 prop and a standard glo plug. I have made sure the spray bar orifice hole is directly centered behind the spraybar. I have worked with launching at 10,000 rpm then 9,700, 9,500 and finally 9,300 rpm. All to no avail as the motor will speed up after launch and level off with 4.5 or .6 second laps, that is way too fast, as I want 5.2 or so second laps. The carb insert seems to be the correct one as it looks similar compared to the Fox stunt carb. Some what frustrated as I want to tame this good engine, just can't slow it down. Hoping Ken may chime in with his knowledge or any one else. One thing I have not tried yet is a uni-run tank.
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Post  GallopingGhostler Thu Aug 10, 2023 1:21 pm

Try changing props. What are you using, 10x6? Try 10x5. Also, what line length are you using? 60 feet? 50 feet? If shorter, I'd try longer lines. And, 4.5 second laps IMO is not bad timing (but that is me).

You can reverse the prop so it runs less efficiently backwards. That will also slow it down. I don't have a K&B Series 75, but it could be possible that these love a faster wet 2-cycle than 4-cycle when run rich.

Here, I've kept my lap speeds up a bit, but that is because I'm at 4,300 feet elevation and we almost always have winds exceeding 15 mph. (Calm days are rare - land is flat with absence of trees.) Line tension is essential.
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K&B .35 75 series Empty Re: K&B .35 75 series

Post  Ken Cook Thu Aug 10, 2023 3:56 pm

Eddy, your dealing with a powerful engine there. This particular version I found always questionable in terms of it's use. Because it wants to run fast. In general, most K&B's seem to be high revvers. This is the large trumpet venturi engine? While it does come with a insert, it seems large to me at least for stunt purpose. I will say your fuel choice seems correct. This is a Dykes ringed engine. While castor could calm it down a bit it will also varnish it up and quite fast. The Banshee is a fairly sizeable plane.

           One thing that I find concerning is the Banshee nose length. Sig also provided lite ply with many of these models which is also concerning if you used it. The reason I say this is because if the nose is resonating, it's causing the fuel to foam and it's making things run lean. But, if you feel there's no vibration issues, I wouldn't hesitate to try a uniflow tank. I feel you will get far better results in doing so. This can be accomplished in a hard tank or plastic. I have had success using both. Unfortunately, you will have to plumb the plastic which really can be a chore.

        I also believe and I would try a 11x5 prop if you have one. See if that knocks down the rpm's a bit. A uniflow tank can and sometimes will lean out on release. It usually settles down after a lap or two or at least until the last few laps where it might speed up a bit but not so much like a standard vented tank.


            You mention that your tank is 2 1/2 oz. That size tank is awfully small. Typically the tank for that size plane is nearly 4oz's or larger.  A standard wedge is 2" wide. This has the fuel feed line outboard. That can be problematic to certain engines due to fuel draw. Seeing that I'm already questioning the K&B's venturi size, combining this with a tank which utilizes it's pickup so far out, this could explain the speeding up on launch. The fuel load is now encountering centrifugal force creating a lean condition.
I would also try your current tank with one of the vents capped off during flight. This will assist fuel draw and prevent siphoning. You didn't mention if you were using fuel line as a extension on the filler pipes angled into the airstream. In the event you are, try capping it as I mentioned.


    The ideal situation for a large profile stunter is to bed the tank into the outboard doubler and sink it into the fuse. This brings the pickup pipe closer inline with the centerline of venturi.  If you have a uniflow tank, by all means give it a try.

         When I want to use a engine for stunt that has tendencies to run fast such as the OS FP .40, I go the head gasket route. I use lighter pitch props and I let the engine run where it wants to but at least the speed can be dealt with. It can be somewhat difficult to locate things such as head shims . Most K&B engines are interchangeable so these should be inexpensive and readily available.
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Post  Eddy Thu Aug 10, 2023 9:05 pm

Ken, Thank you for the informative reply. A couple things I didn't mention after the fact. The 2 1/2 oz tank is a Brodak tank with the fuel pick up coming from the middle bottom of the tank. It is lined up with the spray bar inlet, and only 3 in. of fuel line. I also have a legnth of popsicle stick, 1 in. long virtical at the very back of the fuel tank that measures 3/32 in thick to kick the back of the tank out to keep from fuel starvation while doing over head 8's. I am wondering now if that might contribute to the run problem by forcing too much fuel causing the speed up. I am looking at a Brodak medium wedge uniflow tank now and will try going to that soon with a 11x 5 in prop as well. If all this does not tame this engine I have several Fox .35 stunt engines that I did not want to use on this plane.
In looking at Peter Chinn's review of the Fox Stunt and the K&B series 75 engine, both seem some what matched:
Fox .35 stunt: c.r. = 8.5, wt. = 6.4 oz. .45 bhp @ 12,000 rpm
K&B .35 c.r. = 10.0, wt.= 9.1 oz. .50 bhp @14,000 rpm
I have tried to block off the fuel fill line on the tank, used fuel tubing on the over flow with a slight slant cut in the line facing into the air stream. No tubing on either fill and over flow at all, still the speed up. I just may give up and put the old Fox stunt on and start all over. I really do like the K&B engine, just wished I could figure it out. Eddie
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K&B .35 75 series Empty Re: K&B .35 75 series

Post  Ken Cook Fri Aug 11, 2023 4:26 am

I personally feel that while the Banshee was designed around the Fox .35, it seems a bit much for the plane. The reason I say this is due to the Fox being mounted profile. When upright or inverted in a full body plane, the Fox offers a bit more power. Mounted on a profile, you don't achieve the same run. You could also pad your K&B engine off of the beams using some 1/8" aluminum. While it's not much, it can help sometimes. While you do have a shim at the rear of the tank, the yaw of the plane due to leadout exits also impacts this. If the plane is hanging by the leadouts yawed out, the shim thickness might need to exponentially doubled.

I've mentioned Brodak tanks many times on here. You can have 10 of them and there's a chance all ten don't work properly. You could also have half of them good. I take them apart and insure that #1 they're 100% clean inside and that the pipes are soldered. I've had dozens of the pipes just fall out or the tank is filled with oxidation due to not being cleaned. Always air test with a 5oz. syringe and inflate it like your trying to pop it.

This may not be necessary at all but it has helped me several times. I 've wrapped bubble wrap around the tank 2-3 times around and secured it to the plane. It doesn't last forever but it can sometimes offer you a good needle setting on the ground. I would also at least try this which is to give your fuel jug a squirt or two of original formula Armor All. It must say original. While this seems a bit odd, it can work and it offers no ill impact on your fuel or glow plugs as some claim. It just prevents the fuel from foaming in the tank. A test in a clear jug shaking the fuel prior to adding will result in the fuel getting a head of bubbles. Once spritzed, shake away and you will see no bubbles whatsoever. None of the above could even be the problem, it's just simple solutions to rule out any possibilities of vibrations.

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