75 Johnson Dogs Out

[hkgustai]

I have a 1976 75 HP Johnson. It runs great at about 5500-5800 RPM/27MPH but every once in a while it'll bog down and won't go above 3000 RPM/10 MPH. <br /><br />At first I thought it might be starving for fuel, but when I put it in neutral and increase the idle, I can rev it all the way up. <br /><br />I can't reproduce the problem out of the water, and it's intermittent. One day it'll run fine for hours, and then the next day it won't. When it happens, it's right after I'm running wide open for about a minute after I put it in the water, but then it won't go above 3000 RPM, unless of course I put it in idle. <br /><br />Could it be something to do with the throttle cable/linkage slipping? What am I missing?

 

[wilde1j]

Re: 75 Johnson Dogs Out

Kevin, first of all, don't rev the engine beyond 2000 RPM, unless you itching for a new powerhead.<br /><br />Next time the RPM limit happens, see if the primer bulb is hard and if pumping it up restores max WOT RPM's. If it does, fuel problem (pickup strainer, engine mtd fuel filter, crap in cards, incorrect float level, or air leak most probable causes.) Also try spraying premix in each carb throat. If RPM increases, fuel problem isolated to one carb.<br /><br />If it's definitely not a fuel problem, you may find you're not running on all cylinders. At idle, pull off plug wires one at a time. RPM drop indicates dead cylinder.<br /><br />Throttle linkage unlikely cause, unless someone has messed with them recently.<br /><br />Let us know what you find.

 

[hkgustai]

Re: 75 Johnson Dogs Out

Thanks for the reply Jim, I'll definitely keep the rev's down! <br /><br />I've tried the primer bulb before and that didn't help. I've also switched tanks, but same result. I've checked out both tanks and they're clean with no debris. Could debris be floating in one of the bowls and occassionally block it up?<br /><br />Could a bad cylinder be intermittent? Because sometimes it runs great, and then I'll be cruising along and it just loses power and I can't get RPM's above 3000. Then the next day, no problems. <br /><br />I've never had it repeat out of the water with muffs on, but I'm assuming that's becuase the lack of back pressure. True?<br /><br />On another note: Should I be running at 5500 or 6000 when opened up? Don't want to cause more problems than I already have.