7.5 Help!!

[JohnBoy]

I have a K&B 7.5 Pro aircooled outboard. Question:

Idle rich/lean disk on carb. Is it clockwise to lean it and counter-clockwise to richen it up at idle.

Just installed HTB pipe and motor runs great wide open. Midrange is a dead spot with lots of smoke from exhaust and if I wide open and let off quick it stalls. The motor will not idle for long without loading-up and stalling. Also it acts as if it's lean when it quits running but is running way fat as it takes some time to clear up and there is lots of smoke from the pipe when it's running.

I have checked the carb and fuel system for blockage but all looks good, I've tried different props and all kinds of settings on the needle and disk without changing the problem. Also the motor ran perfect the first 4 runs with the new pipe and then developed the problem so I don't think it's the pipe. The top end is 5 mph faster on radar even with the poor running at idle and midrange.

Any help greatly appreaciated,

John Smith

 

[Jerry Dunlap]

John,

I was not able to get consistent performance using the stock carb that came with the 7.5 Pro. Most of us running the 7.5 Pro in the Northwest have replaced the carb that came with the motor with the smaller carb that came with the series before the Pro - part #9445. If you do not have access to an older K&B carb, you can neck down the throat of stock 7.5 Pro carb using brass tubing. I inserted three pieces of brass tubing to narrow the throat. The #9445 7.5 carb has a throat bore of .470.

Turn the brass low speed disc clockwise to "lean" - you're on your own if you only have a digital clock.

Jerry Dunlap

 

[JohnBoy]

Jerry, What do you mean in your first post "If you have a digital clock your on your own"?

What I can gather from your post is that the 7.5 Pro carb air inlet is too big causing low vacuum to draw fuel through the needle and poor throttle responce. If this is correct how could the motor run good the first 4 runs then start to have this problem?

Has anyone had sucess with the 7.5 Pro carb and HTB pipe? Others have told me the stinger outlet size needs to be reduced for the pipe to work correctly with my combination.

I would like to try to make the Pro carb work as with my automotive background I know that larger venturi size will make more topend while loosing some bottom end and creating idle problems. i can deal with these problems but the motor is runningso poorly at midrange i think there may be something wrong other than just the carb air inlet size.

Thanks for your responce and please realize I'm not questioning your knowledge. I just like to ask as many questions as possible until I can make an educated guess and fix the problem myself.

Thanks, John S

 

[Jerry Dunlap]

John,

All I can go by is my experience running the 7.5 Pro with the stock muffler.

Hopefully, some one else can provide additional imput.

JD

 

[Slideblues]

Jerry, no reason to apoligize, I got it and it think everyone else did to. I edited out the rest.

John, I might suggest you follow JD's advice he is one of the best there is, Or as you said gather the info and fix it on your own.

Gene ;D

 

[JohnBoy]

Gene, If I have upset you or Jerry that was not my intentions. If you are the moderator or owner of this board please email me at [email protected]. I am new to this board and don't fully understand what's right or wrong with my post. I apologize if I've offended anyone. Again I am just trying to get information so I can fix this and if I knew what was wrong with my engine I would be out running it instead of asking for help on this board. Jerry I still don't understand the "I'm on my own if I have a digital clock" could you explain? What is a digital clock?

P.S. I am a newby to model boats so bear with me while I learn.

Thanks, John Smith

 

[Slideblues]

OK, take two:

John I am one of two moderators of this forum, the owner is Tom but he is offline at the moment.

The edit was my call, not JD's.

We try to keep things lite and have fun while letting the info flow. Thats why we joke around some.

The joke was:

If you only have a digital clock, how could you know what direction clockwise is? There are no hands.......

Gene ;D

 

[JohnBoy]

OK I'm a "DA". I thought it had something to do with clocking the boat to find out speed. I will try to be a lite as possible with my posts. Thanks for explaining the clock thing.

John S

 

[Slideblues]

Its Cool,

Everyone has a roll, so to speak with their writing, It takes a little while to get used to everyone's style,

Enjoy!!!!

Gene ;D

 

[JohnBoy]

Gene, Maybe I'm confused but you post states "Everyone has a roll on this board". I you saying mine is the "DA". Please clairify

John S

 

[Slideblues]

Nope, not at all, its not a smart or dumb thing, everyone expresses themself different, and it takes a while to get used to the way they do it, that's all,

Gene ;D

 

[JohnBoy]

Thanks

 

[not2bright]

why does midrange matter dont you only use full throttle when racing : :

 

[Propjockey]

...and when did you get style, Gener? ;D

Welcome aboard, John!

 

[Slideblues]

Hey!!!

Well I am kinda a cross between Grandpa Walton and Ted Nuggent

Gene ;D

Forgot ta mention, the large throat dia. while increasing volume, reduces the speed of incomming air across the spraybar and may result in a bog or flat spot in the powerband, thats why sometimes less is more.

Read that somewhere,

 

[Grimracer]

Gener

You also have a little Horshack from "Welcme back Cotter" in your posting style.. Did anybody ever tell you this before? ;D

Grim ;D

 

[Slideblues]

He,He,

Who me?????

Maybe a little Curly from The Three Stooges too......

Wop-Wop-Wop--Wop, Well Certainlly

Gene ;D

 

[rippinwater]

John,

Jerry is right about the carb. choice. You might also want to drop your head clearence down to .008 in. You will then be able to run the engine leaner and run in the higher RPM ranges, (where this engine produces most of its power) without dying.

Ya know.......You are breaking hearts all over this country by running this engine as a "mod engine". :'(

 

[JohnBoy]

Great information rippinwater, How do I measure the head clearance. The only way I know is to put some clay in the chamber then measure the crushed clay. Any other ways to measure head clearance. Also please explain why the Pro should not be run as a mod engine. I don't understand. Is there a better engine to run as a Mod?

Thanks, John S

 

[Jerry Dunlap]

John,

We measure deck clearance using a thin piece of soft solder. Shape the piece of solder like a "L" and with the glow plug removed slip it into the top of the engine. Turn the motor over several times, remove the solder, and measure the end of the solder that has been compressed.

In my opinion, there's no reason not to run the 7.5 Pro as a mod motor. One thing that some guys who do run the 7.5 Pro with a pipe have done is to convert the motor to water cooled. It is possible to purchase a water cooled head for the 7.5 Pro. Gary Preusse at G&M Models (630) 279-2451 might have that item.