Merc cough/sneeze while idling...what is it?

[JoeMan]

I've got a '95 Merc 60 OB which is truly running great. However...what I notice often while at no-wake speed, but especially while sitting and idling is that perhaps every 10 seconds or so, the motor jerks slightly as though it has a split-second "cough", and the rpm drops some at that moment, and then comes right back up. Maybe a better analogy for what it's doing would be a "sneeze". It's just a very sudden cough/sneeze and the drop in r's. Then it's fine, until it does it again. Anyone diagnosed this same thing and can tell me what I could/should check? Funny thing is that my brother has a Merc 90 hp OB, and I noticed his doing the same thing...so my guess is that it's sort of common with older motors. Yet, as I said, besides this, these motors are performing very well.

 

[emckelvy]

Re: Merc cough/sneeze while idling...what is it?

Sounds a little lean. Don't know if this motor has fixed idle jets or adjustment screws, if it has screws I'd back each one out 1/8 turn (CCW-rich) then run it and see if that helps. If not, try another 1/8 turn, see what happens.

One way to tell if it's a lean condition would be to hit the primer button for just an instant and see if that keeps the motor from 'sneezing'.

If you find this motor has fixed idle jets, you may need to remove and clean them; if they're readily accessible from the outside of the carb, you could try spraying some carb cleaner thru them in an attempt to clear them out.

HTH........ed

 

[JoeMan]

Re: Merc cough/sneeze while idling...what is it?

Thanks Ed!

 

[tashasdaddy]

Re: Merc cough/sneeze while idling...what is it?

use seafoam deep creep, rather then carb cleaner, deep creep has oil in it. carb cleaner does not.

 

[Texasmark]

Re: Merc cough/sneeze while idling...what is it?

A good decarb and keeping Sea Foam in with the gas helps to prevent that and improve idling. Seems that when you reduce the carbon buildup the engine runs better at idle. Don't know if it's because of improved combustion because the rings have freed up or what but mine does since doing it.

Mark

 

[JoeMan]

Re: Merc cough/sneeze while idling...what is it?

I'll tell you, I did do 2 major applications of Deep Creep late in the boating season this year, went right by the instructions, then I ran a lot of it in my gas as well. Still, thanks for the tip, because I keep that stuff around, and I believe it's a good product. I'm thinking that it might be time to pull all 3 carbs off and do a bench soaking and make sure there's not some very tough clogging going on in a jet or 2. Can't be too bad though, because this motor launches my boat very nicely! I'll also check for lean as well. I really appreciate the replies! This was my first 'full' boating season, I'm still learning, but I'm doing a lot better, and having a lot of fun!

 

[tmcalavy]

Re: Merc cough/sneeze while idling...what is it?

A sneeze through the carb means too lean.

 

[MrBigStuff]

Re: Merc cough/sneeze while idling...what is it?

My '91 135hp does that too. I rebuilt the carbs completely and it still does it. I adjusted mine to run a bit rich and you can tell from the exhaust smell. Now I wonder if one of the carbs is running lean. But it definitely "huffs" occasionally during extended idle conditions...

 

[andy6374]

Re: Merc cough/sneeze while idling...what is it?

Also, make sure the throttle shutters in the carb throats are completely closed at idle, if not it will sneeze all day long.

I just want to say that a lean condition isn't the end all, say all for a sneeze...it could be a weak or broken reed valve as well (not that I think it is in this particular case as it is rare). But if it is actually spitting gas back out the carb when it's sneezing then it's likely the reed valve.