K&B 7.5cc outboard - RC Groups
Thread Tools
Aug 26, 2009, 01:38 AM
Registered User
tmattusa's Avatar

K&B 7.5cc outboard


I am an airplane guy. A neighbor brought over a Prather Lap Cat II that he bought around "16 years ago" and asked me to get it running. He bought the boat at a hobby store and never did anything with it.

I spent a few hours on this boat making rechargable nicad packs etc. After getting that ready, I began on the engine. I didn't realize that the K&B 7.5 runs backwards! (Or maybe all marine motors?)

We took the boat to a neighbors pond and did get it running. My problem with this motor is that I cannot seem to get it to run at any speed below screaming! I do remember one of my first airplanes had a K&B with a Perry Carb that ran so-so. The throttle setting seems to have little to no effect on motor RPM.

Questions. Is there another adjusting screw for this motor other than the high speed needle valve? (Remember, this engine is around 16+ years old). I did notice in the center of the throttle arm there is a screw but I'm not thinking this is a setting screw for low to mid range(?) It seemed to seat and was not adjustable.

Also, does Perry still make replacement carbs for the K&B engines? I do remember many airplane fliers using Perry carbs on K&B's. Is that still the case?

In addition, I have to cover the carb with my finger to get enough fuel to draw each time that I try to start the motor. I feel that there is some sort of air leak at the carb.

Thanks in advance!

Sign up now
to remove ads between posts
Aug 26, 2009, 06:58 AM
Registered User
Putting your finger over the carb is perfectly normal with a marine engine. It should scream anyway, however if you have absolutely no throttle control you could buy a new K&B carb. It doesn't run reversed, if you look at it it's still going the same way as any other engine. The small screw in the middle of the throttle arm is the low end adjustment needle. Could you post some pics of it?
Aug 26, 2009, 10:10 AM
Registered User
tmattusa's Avatar
I sure could. In looking at a web site for K&B (which appears to have been bought out?) it looks like they still sell that very engine with little changes over the years except for a remote needle valve. You may have solved my problem as I recall that screw tightened all the way down which would make lean as heck. If that is the case, then it could probably run that way with the high end out quite a bit.

K&B says that the high end should be out 3/12 turns to start. I'm used to airplane engines that are about 2-2/12 turns out.

I think I'll tweak his engine a bit more before he puts out $$$ on a Perry carb!
Aug 26, 2009, 10:18 PM
Registered User
I hope you are not useing inexpensive sport model airplane fuel of 10 to 15 % nitro. Marine engines like 30% or more. This can cause a marine engine to stall constantly or not idel at all.
Aug 27, 2009, 10:57 AM
Registered User
tmattusa's Avatar
Actually I was. I do have 30% heli fuel-Wildcat, so I can try and run that AND adjust the lower end. I gave the boat back to the owner and had him check the low end set screw and he said it was "seated tight".

I just love taking on someone elses project! Uggghhhh!

Sunday it's back to the pond. As I said, I'm an airplane guy, not a boat guy!

Aug 27, 2009, 03:07 PM
Registered User
Originally Posted by cratfy1
I hope you are not useing inexpensive sport model airplane fuel of 10 to 15 % nitro. Marine engines like 30% or more. This can cause a marine engine to stall constantly or not idel at all.
I'm using 5% fuel in all my marine engines, including my CMB 90 and Novarossi 61 right down to Kyosho 16's. No problem.
Aug 31, 2009, 11:06 PM
Registered User
tmattusa's Avatar
Well, we went out last night and tried the boat again. This time, we used 30% nitro and turned the low end screw out one turn (I believe this was the factory setting per the web site). Same exact problems. The carb barrel floats around so darn much that it cannot possibly hold it's needle setting. Just too sloppy.

I told my buddy to buy a Perry carb for his K&B. I think his is old and warn out.

When it does open up and run (there is no throttle control what so ever), it really is fun to run around the lake with. Really cruises!
Sep 04, 2009, 09:50 PM
Prop Buster

Kb 7.5

Hi Tamattusa, I know what your going through I have been running 7.5's for quite a while. the stock carb sucks.THE engine also takes alot of break in.But it's a very good motor and it needs alot of TLC. You Must run @ least 30% nitro to get the stock carb set up so you have full throttel control is a pain in the butt.I think the stock carb has a .240 bore it's to small and the barrel hunts for for idle.I have had Geraghty Perfofmance RC set me up with a custom carb @ .375 bore no spray bar no idle adjust nothing just the barrel W/ a ext. needle valve.no more problems!!! JUST A WORD OF ADVICE:Take the boat put it in it's stand be sure the lower end is locked in 2.5-3.5 turns out hook up ext.water pump to the head a elec. fuel pump works good for cooling.Start the boat in the stand once it starts check cooling back out the valve run a few rich tanks then tweek it in.OH don't ever turn the prop by hand you will damage the flex drive.THis also needs to be kept lubed or it will distroy it's self. Get a manual for it.Contact Geraghty perforfance......geraghty@nelson-tel.net 715-926-6096 hope I could help..GOOD LUCK WALLY
Sep 06, 2009, 12:17 AM
RC boat Rock Star
Back to the beginning. OK, K&B and OS outboards run in the opposite direction that plane, car and other marine engines run. Thunder Tiger OB's rotate normally as they use a gear reduction in the lower unit.
The carb should be about 3-1/2 turns to 4 turns out. You must have one of the really old ones but K&B offers 3 other carbs that will bolt right on, two that do have low-speed needles. If it does have a low-speed needle it will be inside the throttle arm.
Their new site is www.mecoa.com .
Being a plane guy, your engines are always under load while you've got a prop on. With boats, we only get the load when in thew water so you have to set the carb slightly richer, some lag when you blip the throttle out of the water. Never run one wide open out of the water or he'll be parts shopping.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
For Sale K@B 7.5 Outboard roscottjr Boats (FS/W) 2 Jan 18, 2006 12:47 AM
FS: K&B 7.5cc DF-IC engine NIB with Turbax1 Fan rcjetpilot Aircraft - Fuel - Airplanes (FS/W) 6 Dec 27, 2004 08:48 PM
FS K&B 7.5cc parts mdttb Boats (FS/W) 3 Dec 18, 2004 01:02 PM
K&B 3.5cc outboard rcmatt455 Racing Boats - Internal Combustion 3 Sep 18, 2004 10:53 AM
Wanted: K&B 7.5CC patriot 11 Aircraft - Fuel - Airplanes (FS/W) 0 Oct 01, 2003 01:29 AM