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K&B 7.5cc

Old 04-05-2020, 02:11 PM
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TimMillner
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Default K&B 7.5cc

What fuel mix should I use on this vintage K&B 7.5 outboard motor?
Old 04-05-2020, 06:20 PM
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the Wasp
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I had 2 K&B 45 Outboards, back then I ran 55% Nitro in one of them, the other I ran 20%.

checkout my link, it says up to 25% Nitro. but you can buy head shims to lower the compression to use a higher Nitro percentages.

if you are just having fun I would just run 20% or 25% Heli fuel with 20% oil !

the link says to use half & half Castor/Synthetic,. if you know Glow Engines well I don't see why straight Synthetic wont work. we use it in Helis and they are fan cooled not water cooled..

you should post/talk to the guys in the Boat Forums.

Jim
http://www.mecoa.com/kb/48/7500.htm

Last edited by the Wasp; 04-05-2020 at 06:23 PM.
Old 04-11-2020, 06:09 AM
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1QwkSport2.5r
 
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Full synthetic in marine and truck engines is bad news. It’ll work, but it will kill your engines if you run them hard with a lot of nitro. Half castor half synthetic is at best a minimum. I run 80/20 castor/synthetic blend in my marine engines.

I don’t care what works in helicopters or sport plane engines. Hard running truck and boat engines will undoubtedly run better and longer on fuel with more castor in it. They also benefit from medium or medium/cold glow plugs, too. Turbo plugs are ideal, but standards work too.
Old 04-11-2020, 05:43 PM
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Originally Posted by 1QwkSport2.5r View Post
Full synthetic in marine and truck engines is bad news. It’ll work, but it will kill your engines if you run them hard with a lot of nitro. Half castor half synthetic is at best a minimum. I run 80/20 castor/synthetic blend in my marine engines.

I don’t care what works in helicopters or sport plane engines. Hard running truck and boat engines will undoubtedly run better and longer on fuel with more castor in it. They also benefit from medium or medium/cold glow plugs, too. Turbo plugs are ideal, but standards work too.
you run 80/20 castor/synthetic with only 20% Nitro and no tuned pipe ?

Jim
Old 04-11-2020, 10:35 PM
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Originally Posted by the Wasp View Post
you run 80/20 castor/synthetic with only 20% Nitro and no tuned pipe ?

Jim
Most of my high rpm high strung engines get 20% nitro and depending on platform (air, surface, marine), get a different overall oil content. That oil content is either 70/30 or 80/20 castor/Klotz Techniplate respectively. I use tuned pipes or tuned mufflers on the engines timed aggressively enough to take advantage of said exhaust systems. All of my truck engines run tuned systems as do my Jett engines, as well as all of my modified engines.

After running a 50/50 oil blend in a ringed ST .90 on my airboat and having the ring rust to the liner overnight after running for a day changed my tune on oils in glow engines - especially ones used in boats. They need the moisture protection and synthetics miscible in methanol in general are BAD when it comes to post combustion chemistry in the presence of moisture and combustion byproducts. There is more to it than just being “RC Fuel”. Understanding what goes into the engine, what comes out of it, and what of that stays in the engine will make those willing to listen understand there’s more to it than meets the eye.

Castor oil might be gummy, sticky, and messy. But it’s Affinity for metal - and hot metal more specifically give it a huge edge over the generally accepted “good” synthetics.

My NOS K&B 3.5 marine will get a diet of 20% nitro and 22% oil - which will be 18% castor and 4% synthetic. I expect to still have cylinder pinch at the 2 gallon mark whereas with commercial fuels pinch is usually gone at a gallon or two. This has been my experience anyway.
Old 04-12-2020, 05:52 PM
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I don't remember what oil mix that was in that 55%, that was way beck, the last time I run glow boats was 91 as I remember, it was the K&B 45 Outboard on a 3-D model 42" Ruff Stuff V.. then I went to Gas.

thanks for the info.

Jim
Old 04-12-2020, 06:46 PM
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My buddy had a 4x4 Rc truck that ran like crap. He even bought the 33% nitro fuel. Couldn’t get a good setting with it to where it made good power and ran consistently for almost a gallon of fuel. I offered a squirt of my fuel I run in my Picco and Novarossi engines - 20% nitro 11% oil(8% castor 3% Klotz Techniplate). It was like a light switch - it went from a piece of junk running engine to a real sweetheart in (literally) a matter of about 15 seconds. I have done a lot of experiments with oil ratios and types in my highest strung engines - I haven’t tried a synthetic oil yet that is far and away a clear cut replacement for castor. I have a couple more brands/types I want to try out. For now, what I have been mixing has been perfect. I don’t need to burn 55% nitro to have fun though. I might try some 35%, but not much of it. Nitro is stupidly expensive right now.

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