97 Johnson 90hp coughs at low RPMs

[dogclinic]

OK. 97 Johnson 90 v4 crossflow keeps couging at low rpms. I tightened and re-tightened all fuel connections (some seemed loose so I replaced zip ties with hose clamps). Still coughs. I pulled both carbs -- they were clean enough to eat off of, but I blasted all the jets out with carb cleaner anyway. Still coughs. Runs great at high RPMs -- I took it out for a full-speed cruise with no problems until I came back to idle speed. WTF is up?
I replaced the exhaust cover gasket and the thermostats over the winter -- could I have screwed something up there? I'm running 90% fresh fuel mixed with some treated stuff leftover from last year -- could fuel be the issue? Help!

 

[Bosunsmate]

Re: 97 Johnson 90hp coughs at low RPMs

Is it spitting back through the carbs?

 

[dogclinic]

Re: 97 Johnson 90hp coughs at low RPMs

Is it spitting back through the carbs?

Not that I've noticed, but the cover plate to access the carbs seemed a little bit wet with fuel

 

[Bosunsmate]

Re: 97 Johnson 90hp coughs at low RPMs

Try running it at low rpms and see if anything is running or blowing out the front of the carbs. I hold a bit of paper out the front of the carb throat, it should be kept on getting sucked in and not blown backwards
Could be a damaged inlet reed

 

[V153]

Re: 97 Johnson 90hp coughs at low RPMs

Could be a damaged inlet reed

Eh mebbe, but when a reed breaks it usually goes in to the cylinder(s) not out. And lemme tell ya it can really ruin yer day ...

Have you checked the condition of your sparkplugs, gap, plugwires etc. Might be something as simple as that.

I know the mantra is to use whatever plug recommended by the manufacturer, but I've used NGK Irridiums for as long as they've been available. With no ill effects. Run the BR7HIX.

Honestly believe they help the motor idle more evenly through our ever increasing no wake/manatee zones. Seem to reduce that occasional wheeze or sneeze at idle?

 

[dogclinic]

Re: 97 Johnson 90hp coughs at low RPMs

I'm hesitant to blame the carbs -- I've seen snotty carbs and these things were super clean. I've got the recommended spark plugs in and they're gapped correctly. Since I did pull and replace the exhaust cover, I may have contaminated the mounting surfaces for the coils -- could an improperly grounded coil cause the coughing?

 

[gordwa]

Re: 97 Johnson 90hp coughs at low RPMs

check all the vacuum hoses and make sure the check valves only let air in one way.take them out and blow thru them

 

[dogclinic]

Re: 97 Johnson 90hp coughs at low RPMs

Do I have vacuum hoses on a 97 Johnson?

 

[dogclinic]

Re: 97 Johnson 90hp coughs at low RPMs

Turned out to be a bad coil. Thanks for the effort, though!