K&B 7.5 Outboard - RC Groups
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Apr 03, 2012, 07:57 PM
.... But you can call me Dave
DepthsofHell's Avatar

K&B 7.5 Outboard

I just picked this sweetheart up in a trade, thanx bro! It is air cooled and has the gold head. I have never ran an outboard this big and I have many questions. I run O'Donnell 50% is that ok to run in this engine? Glow plug? Prop? High speed needle setting? Hull size? And I'm sure I will have more! Guess it is time for a scratch built tunnel. I'll probably need some help from you F1. I'm working on putting up pics but my computer is not cooperating.
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Apr 03, 2012, 09:06 PM
.... But you can call me Dave
DepthsofHell's Avatar

......... And Here They Are!

I don't intend on mounting it to the Mirage, that was just for the pics. Or should I ????
Apr 04, 2012, 12:11 AM
RC boat Rock Star
Cute, but don't hang it on the Mirage as it's just a little too small for that engine. I had one of those many years ago and the plastic just won't handle it.
O'Donnell 50% is fine but about the most that you'd want or need to go for it. Any higher and you're not really gaining anything plus the needle gets harder to set.
OK, run it somewhat on the rich side as the fuel itself helps to keep it cool. Where the high-speed needle needs to be depends on a lot of variables, including temperature, humidity and altitude. Usually the settings that K&B gives you are most of the time on the very rich side.
I love the McCoy MC-9's as I ran them in everything from .12's on up from 45 to 65% nitro. You might want to check out O'Donnell glow plugs also as I've heard nothing but good about them. I've got a card of them but haven't tried any out yet.
Scoring a hull is getting harder all the time as the popularity of any outboard over a .21 has quickly faded over the past few years as K&B is basically the only ones making them and they're not exactly pumping them out like the old days. HTB Boats is one of the few left making a decent hull for them and they're not cheap.
Oh yeah, props. Octura X- 447,448, 450 or 646/3's run very well on this motor. It will depend on what the boat likes. It might like a Prather 235 also if you can find one.
Apr 04, 2012, 03:12 PM
.... But you can call me Dave
DepthsofHell's Avatar
Excellent, thanx Ron! My mirage has been totally reconstructed from nose to transom, much like everyone else's. I know that hull couldn't handle it, it could barely handle the OS .21. I figured I couldn't find a hull, that's why I will probably build one from plans. What range am I looking at though, 32-36" is what I was thinking? I believe we have a Prather 235, I'll have to check. Thanx again for taking the time to write that up Ron, alot of good info in there.
Apr 04, 2012, 03:44 PM
Registered User
F1 madness's Avatar
Hey bud
sweet trade,ill help where i can but never had a 7.5 size OB its gonna be tricky but i certainly have ago and advise where i can
At a guess i would i would say 34" - 36" maybe slightly bigger depends on a lot of factors.normally i would buy a leg then design to suite it.
Ill do some research for you
Apr 04, 2012, 11:56 PM
RC boat Rock Star
No problem. I owned two 7.5 cc OB tunnels so I went through the learning curve.
My first one was an Aero-Tech 7.5/11 which was a barge weight-wise but ran surprisingly well and sort of drifted through the turns. The 2nd was a then new DPI, one of the first 6 out of the mold. Seeing how I had already had two of the 3.5's and loved those I was hoping that the big brother would be as nice. Unfortunately it had some nasty rockers at the rear of the sponsons that made setup a nightmare. If you got it right, it was an OK hull but if somebody messed with it at all then it was junk.
A shot of the DPI I have but none of the Aero-Tech.
Apr 28, 2012, 03:05 AM
.... But you can call me Dave
DepthsofHell's Avatar
So what engine am I working with here? I am having a hard time finding info on her. What model? Semi pro, pro, super omega ultra pro???? I'm just curious about horsepower and RPM range. It's my understanding that the K&B out boards are faster than the current OS, but are more temperamental.
Apr 28, 2012, 08:00 AM
RC boat Rock Star
This one is referred to as the gold head version which was their first air-cooled .45, a larger version of the 3.5 SS. There is also a red head which is the next step up in performance, better known as the Pro. Your's is most likely the Semi-Pro model as the red anodized head on those is also more square.
The K&B's aren't faster than the O.S.' as that's why O.S. is taking over the market. The latest version is faster yet with a few more mods done, the most noticable being a longer muffler. They aren't temperamental as they will take a lot of abuse and aren't that finicky on needle settings. O.S. doesn't make any other outboard than the .21 so there's not one to compare to the .45 that you have.
A little history lesson now. The SS and the air-cooled .45's have some throw-back to the old days of outboards that were made in the early years. The muffler cannisters were from the exhaust throttle versions from back in the day. It's more noticable on the 3.5's as all they did was plug the holes where the exhaust throttle butterfly was. When they redid the lower unit, it made me curious as to why they didn't modify that and further than they did as it no longer needs the exhaust system from the water-cooled versions so a couple of ounces could have ben shaved off there.
Apr 29, 2012, 11:04 PM
.... But you can call me Dave
DepthsofHell's Avatar
Great write up, thanx Ron! Appreciate the history lesson as well, I enjoy knowing the background on things. I wish OS would make a 7.5 outboard, I know I would buy one. Time to do some more research.........
Apr 29, 2012, 11:25 PM
RC boat Rock Star
It's too bad that there isn't an alternative to the K&B .45 and larger outboards as those sizes are now dying off as K&B is quitting making a lot of them.
When I ran K&B .21's they were a very cheap even at the time $115.00, a great place for a new boater then to have a low-cost place to start at racing or just running for fun. Get addicted to the .21 then you went up to the .45's and beyond.

For a short time there were a few alternatives but you still needed a K&B lower unit . Jim Irwin had an adapter kit so that a CMB could be mounted, I've seen a couple including Jim's own. John Otto make his own lowers and are very unique.
Apr 30, 2012, 09:05 AM
.... But you can call me Dave
DepthsofHell's Avatar
Well we don't have a Prather 235, shoot! I have been running GrimRacer props because my shop orders through Great Planes/Tower Hobbies. What ever I need off tower they order for me. When they order it's in my hands in two days and I don't pay shipping. Anyway I noticed the 42x55 is the equivalent to the Prather 230. Would this be a suitable prop? Or is it too small? I don't want to under prop this beauty!
Apr 30, 2012, 09:20 AM
RC boat Rock Star
Too small. Look for Octura X-series 447, 448, 450 (either 2- or 3-blade) or the 646/3.

I don't know if you've pulled the head off the motor or not yet but do yourself a favor and put a little mark on it somewhere so you know which way is up or down on it. I can't say for sure on the .45's but the .21's had a specially made head with "pie-cuts" in it. Terry Keeley designed then in the first place then K&B "borrowed" the design.
Last edited by Ron Olson; Apr 30, 2012 at 10:30 AM.
Apr 30, 2012, 11:38 AM
.... But you can call me Dave
DepthsofHell's Avatar
The grim prop was wishful thinking, never hurts to ask. Offshore Electrics here I come, I'm not finding any of those props on the ZippKits site. This outboard probably won't be hittin the water too soon, unless I can score a hull.
Apr 30, 2012, 12:07 PM
RC boat Rock Star
Good .21's a are spinning 42mm props now but it's mostly one of those tweener props, too big for some, too small for other engines.
Steve probably has in stock what you need.
Years ago Tower used to handle a lot of boat stuff, Dumas and Prather kits, Warehouse Hobbies RTR boats and Octura and Prather props. They pretty much got out of boats until their subsidiary got back into it with the AquaCraft's.
May 21, 2012, 02:18 PM
.... But you can call me Dave
DepthsofHell's Avatar
Okay, my friend stopped by a shop in Sacramento while he was down there. He picked up some fuel and grabbed a prop for me. Well, he grabbed me a M447, not an X447. After some research I found out the M series is the same as the X series, except the M series is detongued. Will this prop be suitable for this outboard?

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