TJK

My Feedback: (69)

After replacing the backplate bearings in our K&B 7.5 outboard, my son and I had a disappointing few days at the lake. We could not get the engine to run consistently. At the end of the first day of frustration, I discovered the front casing bolts wee all loose. I tightened the bolts with locktite and looked forward to the next days boating. The next 2 days brought no satisfaction. We could not get anything more than a semi-reliable idle, with a very touchy transition that usually resulted in a dead engine within 15-20 seconds.

I am new to boats, but not r/c engines. I'm very familiar with aircraft engines and their tuning, and I have experience with the K&B 45 in ducted fans. What I recall most of their history was a sub-par stock carb.

Anyone have any suggestions or insight? What fuel and plug works well in this application? Any suggestions for carb needle settings or replacement carbs?

One more basic boating question; where do you position the boat for starting? I initially started the engine out of the water and placed it in within 10-15 seconds after small carb adjustments, never going past idle. I finally placed my start/transport stand in the water for start and adjustment, and this eemed to work much better. I could take the time to adjust the carb without worrying about cooiling or overreving, and providing proper resistance to the drive train.

We'd love to get some runs that we can time in terms of minutes, and not have to use the kayak to retreive every attempt!

Thanks in advance for the help.

Tom

J232

I had a problem like that. For me it ended up being bearing airleaks or other airleaks, fuel and glowplug. I also replaced the carb parts (not the body) Runs nice now... Anyways it did what you are saying

TJK

My Feedback: (69)

A friend suggested 15 to 25% nitro fuel. I was using 5%. Any ideas?

Tom

hoggwild89

I run 25% nitro in my K&B 7.5 outboard. The specs say I can use up to %50 (Yeeha, git er dun!)

TJK

My Feedback: (69)

What plug are you using, and are you running the stock carb? Any problems, or does it run reliably for you with your setup?

Thanks,

Tom

Ron Olson

My Feedback: (1)

The K&B 3.5's and 7.5's love Nitro. I ran 40-50% in mine. Higher and the needle gets hard to set. Start the carb at around 3-1/2 turns out on the high side and even with the throttle arm on the low. Boats need to run richer than planes most of the time. Use boat fuel, not the airplane stuff, it has more oil in it.
Plugs. MC-9 or MC-59 work well. No idle bar plugs.
The stock carb isn't too bad, I like the older ones better than the newer ones. They have a history of the barrel falling out or breaking the barrel.

J232

Im using 30% heli fuel but I added a few oz of oil in the jug... next jug will be that xp 40% boat stuff

cneal

My Feedback: (4)

K&B motors are pretty reliable once you get them broke in you can run up to 50% nitro but you are better off with 25 or 30% they are easier to set a more reliable idle and transition. Also if you are mounting the motor on a adjustable motor mount (highly recommened) try raising your motor up about 1/4 " good luck and like the man said "GET R DONE"

Crabstick

Hi j232

Usually if the motor stops within about 30 sec's of getting on the water, it is fuel related. Check your entire fuel system for leaks, block one end and blow into it, i submerse mine under water, ( in the sink) and blow and look for bubbles. check you have sufficent fuel flow , by blowing into the pressure line and makes sure its coming out the other end, check your clunk in your tank is not caught around the wrong way.

richen the boat up a little, as the prop touches the water and loads up, the motor will lean out, if the needle is too lean it will cut the boat out.