'75 johnson 50 hp low speed problem

[medic2051]

This is a repost, can't find original on site.<br />First season with '75 50 hp. Motor has new power pack. She is running rough at low speed. Some times when you put her in gear and accelerate she quickly bogs down and stalls. This may happen several times in a row, then problem disappears.<br />Could this be a coil arcing out? What about low speed needles? Can you replace without removing carb?<br />Also, I want to replace the current prop with identical prop. How do i determine what pitch is there now?<br /><br />Thanks!!!

 

[Joe Reeves]

Re: '75 johnson 50 hp low speed problem

Medic.... If the prop numbers aren't imprinted on the prop outer hub between the blades, remove the prop nut and spacer The numbers should be on the face of the prop just in back of the washer.<br /><br />First check compression and spark. The compression should be approx 100+ psi and even on all cylinders. <br /><br />Next, with the s/plugs removed, check the spark. It should jump a 7/16" gap with a strong blue flame on all cylinders. (The s/plugs must be removed in order to obtain the highest rpm available).<br /><br />If those two items check out okay, the usual cause of bogging is fouled carburetors.

 

[medic2051]

Re: '75 johnson 50 hp low speed problem

Thanks for the reply!<br /><br />What about those low speed needles? I adjusted them once, and the motor improved. How do you know if they need replacement, and do you need to pull the carb?<br /><br />Thanks again

 

[Joe Reeves]

Re: '75 johnson 50 hp low speed problem

Unlikely you would need to remove the carbs unless they require cleaning. Also unlikely they would require replacing unless someone really screwed them in hard, damaging the tip bevel. Proper adjustment procedure follows:<br /><br />(Carburetor Adjustment - Single S/S Adjustable Needle Valve)<br /><br />Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.<br /><br />Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.<br /> <br />Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting. <br /><br />When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

 

[medic2051]

Re: '75 johnson 50 hp low speed problem

you are a God. i am not worthy. Thank you.