copelab

Ordered a jug of 5% and some Sig castor oil, plus some McCoy plugs an a 12-9 prop. How much castor should i add to the fuel do you think?
Bob

1QwkSport2.5r

If it's like typical fuel today, it's probably 16-18% oil and a mix of castor and synthetic. Bushing engines do better with castor due to its viscosity, I'd run 22-25% oil total. Do the math when you know how much oil is in the fuel and figure as an example; adding 6oz of oil to a gallon of fuel with 16% oil brings it up to about 19.7%. If it's 18% to start with, 6oz would bring it up to about 21.6%.

spaceworm

[QUOTE=1QwkSport2.5r;12096476]Yep. RC Airboat World forums went down for good. The Admin didn't have the time or money to keep her going. Sad, we airboaters lost a LOT of great build info/logs. ...
QUOTE]

Sorry to hear that RC Airboat World has passed.

My very first RC experience was with airboats on the Arkansas river in Wichita, Kansas in late '40s. I think they were HAM radio operators and the boats seemed pretty big. I Have no idea what size or type of engine they were fitted with, but I was blown away by their agility. Of course they were single channel, escapement "servo" rudder only, but man did they rip around. In mid 50's my first RC ever was a airboat powered by a Cox .049 and controlled by a friends borrowed TX to my home built Lorenz gas tube receiver, on 27MHz. Still have the engine and the RX. Good times.

I have a Dumas Big Swamp Boat with a K&B .40 on it. I am thinking of putting on my K&B .61 for better speed. Prop clearance may be an issue. What 3 blade prop would you suggest? Thanks.

Sincerely, Richard

1QwkSport2.5r

The Big Swamp Buggy is a bit of a tough one to judge because they're not the most stable as kit built. If converted to flat bottom and balanced well, they really scoot. A .61-.91 are good sized engines for the weight of the BSB, but the boat needs to be balanced just right to not blow over when you take her down from full power. We usually prop airboats smaller than airplanes, but a heavy BSB might need a bit more disc area and less pitch to get her moving. So for a K&B .61 that usually used an 11x7 or so, I'd probably try a 3-blade 11x5/6 or if you can fit a 12" prop, maybe a 12x5 or a cut down 12" to 11.5-11.75x5-6. You would need to test a few 2-blade props to find what runs best and drop 1-2" pitch for a 3-blade. Keep a tractor configuration of you can, much better prop choices available. Many of us use APC due to their rigidity and efficiency but MA props work well too - just go up 1" pitch for an MA.

What prop were you using on the .40? 10x6?

My Kamikaze flat bottom is 32"x18" at about 8lbs and with a SuperTigre S90 or Enya R120-4C is a rocket. My BSB is at least 10-12lbs, but I didn't build it. I'm actually going to be scraping it.

spaceworm

Thank you very much. I had not considered a .90, I may try my ASP .91. What prop do you recommend for the .91? I will have to check the prop on the .40. Running the ASP on low/no nitro will help reduce fuel costs. What balance point has worked for you? Thanks again.

Sincerely, Richard

1QwkSport2.5r

If you have a good running .61, I would try that first. The BSB is not a boat (as kit built as a tunnel hull) that lends itself to being overpowered too much because if it gets going too fast and too much air gets under it, it'll blow over as soon as you let off the throttle. A general rule of thumb for flat bottom boats is have the thrust line (prop) at about 1/3 hull length measured from the transom. If it blows over, move the engine back to put more force on the bow. If it porpoises, move the engine forward to lessen the bow pressure. Hydrofoils are balanced so the bow is heavy and hangs lower than the transom when holding the boat up by the prop. A tunnel hull like the BSB is a kind of hydrofoil in a way, so starting off with a heavier bow may not be a bad thing. If the bow digs in and plows or doesn't plane out well, move the engine forward to lighten the bow.

copelab

Changed the plug with a McCoy,added 6oz of castor to a jug of Omega 5% an installed a 12-9 prop.Went deadstick a couple of minutes into the first flight.Seemed very hot. Replaced prop with a 12-6 and played around with the needles.Next flight was ok but still not happy. I must say that when i started up and started to burn that castor oil,there wasn't a bug to be seen!
Bob

1QwkSport2.5r

The main needle was too lean.

copelab

Perhaps,I'll try again tomorrow if it is not too windy.
Bob

buzzard bait

I haven't followed this closely, but it certainly sounds like an engine that has not been fully broken in yet.

My Sportsters responded very well to the heat cycling method of break-in. Since they are not meehenite pistons, I'm not sure this makes any sense, but for whatever reason, I got good results from it. I ran them rich for 2 minutes, leaned out for a bit, stopped, and allowed to cool down; then repeat several times. I did this with a 20 and a 65. Both have excellent compression now, run well, and do not over-heat.

Jim

1QwkSport2.5r

It is best to treat the sportsters to the same break-in you'd do on a ringed engine. Sloppy 4-stroke rich for a couple tanks, then gradually lean them down a few hundred rpm at a time over the course of a couple hours at least. I don't and wouldn't ever try to do a flying break-in with any engine, but especially these. After a gallon on the bench, I'd run it in a model but at least 1,000rpm rich of peak rpm. It would probably barely fly the plane though. I ran mine on an airboat for a few gallons. My .65 engines weren't squeaky tight like some others have noted. My .28 is squeaky tight though.

copelab

Not much luck again today. I think I am now the club record holder for dead sticks. I can get one good flight and the next one will be crap.I feel it is close but I cannot hit the magic adjustment. Today I also tried a Bisson muffler and tank pressure an it also dead sticked on me.In all fairness,i did not do much of a readjust on the needle with that muffler. The engine also blows a lot of raw fuel out from somewhere.I have found an old Soundmaster muffler an I may try that before returning to the K&B muffler. You guys got anymore ideas? By the way,this is not a new engine an it should be broken in by now. I am starting to think a new carb may be in order but I am not sure it is worth it to spend the bucks.

1QwkSport2.5r

First question - does your engine (it's a .65, right? Have a :91 stamped in the backplate recess? Second question - why not readjust the needle when you changed mufflers? You should always readjust the needle for prop changes, fuel changes, and muffler changes. I make it standard practice to Richen the needle 1/2 turn before starting the engine for the first time since my last outing with it and readjust for peak mixture, then back off until a fuel line pinch gives a slight rise in rpm. Using a standard 300-400rpm rich of peak doesn't work for every engine. Some need to be richer than that. When you think you have the needle set where it should be, run it to WOT and briefly pinch the fuel line and note the sound of the engine. Immediate sag - too lean. No change - peak mixture, Richen it up a little. Slight rise in rpm, perfect setting. Large rise in rpm - too rich.

Last question - is the engine cowled?

The Sportster engines use a bushing for the crankshaft, no ball bearings. Because of this they will sometimes spray oil or fuel from the front of the engine. A small amount is normal. If it sprays a lot, either the crankcase is too worn to maintain a good seal (which can instigate lean conditions and dead sticks/flameouts). The early sportsters didn't have bronze bushings around the crank, just the high silicon aluminum crankcase itself. These early ones tend to be a little messier than the later bronze bushed versions. Using all or mostly castor oil in the fuel helps with this. I have a pair of early sportsters that made a big mess around the front of the engine from fuel spray. Going to all castor oil and higher oil content helped. The magic brew I use is 5% nitro 25% castor. Still a little messy, but much better than it was. My engines are target drone engines (they have the :91 stamp) and had bad carbs in them. The drone Carburetors are machined differently than normal sportster carbs because of their intended operation - WOT until shot down or crashed.

dennis

My Feedback: (90)

I haven't bothered to read all of the posts but for aircraft use having run lots of them over the years I'll offer the following things that have made running these engines painless.
They are plain bearing low RPM engines forget the instructions and prop it accordingly, also use the correct fuel.
They won't really do well on low oil fuels and were designed when high castor limits were in force. No matter what use at least 22% oil and it can be 50/50 mix of castor/synthetic. Use a hot plug and do not use anything but a long reach no idle bar plug.
Now the big new is no K&B Sportster is really happy with a 12/9 prop it is highly overpitched and working it's butt off. On the other hand it likes large diameter medium pitch props. Try a 13/6 ad if you have a sport plane with a long gear a 14/4 or 5 will also work.
Nitro is best left to 5% and contrary to your old hobby shop salesman 15% nitro hasn't been necessary in any sport R/C engine for the last 20 years. RPM is never over 10K with large props.it is a lugger not a revver.
Treat it like a large 4 cycle of the 1980's and it will perform perfectly.

Dennis

1QwkSport2.5r

Dennis - he was using the high pitched 12" diameter prop because of ground clearance. I had suggested a 13x6 as well which is what I use on my .65s (APC though).

copelab

I was being lazy by not adjusting the needle or maybe I was just fed up. It does not have 91 stamped on the back plate an it is not cowled. As a matter of fact it is installed on a 1987 vintage Ugly Stick which I am using to test a Dx8 tx/rx. Today I was also using a 12-6 prop.

1QwkSport2.5r

Then given this information, I think either your engine isn't as broke-in as you say it is or it's just being set too lean. These engines run like Swiss watches almost - if propped heavy, the right fuel is used, and you don't rev it to 14k.

dennis

My Feedback: (90)

The 91 on the back[plate does not signify that the engine is a 91. It denotes that it was made for the Russian Flogger foamy that the army used for target practice. In other words it was military production and probably surplus after the program ended.

Dennis

1QwkSport2.5r

I dont think anyone said there is/was a .91... I did say that if the engine has the ":91" stamped in the backplate that it would be a Target drone engine. Those engines had crummy carbs for RC use due to being machined for a special specification by the military.

I don't think K&B ever made a .91?

spaceworm

No, but they did make a "100". Had a built in mount and muffler combination. AIR See them at MECOA

1QwkSport2.5r

I knew that. Their lineup went from the .61 Twister to .65 Sportster to K&B 1.00. Most of their bigger engines are tuned for bigger props - the Twister .61 is timed comparably to the .65 Sportster.

copelab

Tonight I was able to replace the nose gear in the Ugly Stick and gain enough clearance to install a 13-6 prop.Forgot to mention this engine has about 20hrs on it. It should be broken in or worn out by now.It will be a few days before I get to try it again. I assume most of you guys do not use muffler pressure.I was running slightly over 10k with the 12-6 prop so it was not that lean.I have also re-installed the stock muffler.As it stands now,5% Omega fuel with 7oz. of castor added to it.A McCoy mc59 plug and an APC 13-6 prop.
Bob

1QwkSport2.5r

I've run mine with and without muffler pressure. I lost 60 (yes, sixty) rpm by using muffler pressure. I still use muffler pressure and the reason I do is it keeps the fuel mixture more consistent. It draws fuel fine, but it just seems to run better with pressure used.

My .65 turns a 12x6 APC at 11,010rpm or so and a 13x6 APC at 10,900rpm on 5/25 fuel. So if your engine is in good mechanical condition, you should see peak numbers around there. If you're setting your needle to say 10,300-10,400rpm or so, you should be okay; but I would use muffler pressure and see if that changes the engines behavior. I use the stock muffler with the baffle removed on mine. They actually run really well on the stock muffler and don't really lose any power because of the way the engine is designed. If one used a Pitts muffler, you'd likely need to plug an outlet tube.

copelab

Ok,muffler pressure it is.I'll try the Bisson and the Davis Soundmaster as they have pressure taps.
Bob

1QwkSport2.5r

If the Bisson is a Pitts, plug an outlet so it builds enough pressure. The factory K&B muffler is typically tapped in the extension between the exhaust stack and muffler itself. My early 1990s engines were already tapped, but my early .20 Sportster wasn't drilled and tapped. It's easy to drill and tap those fittings.