2.5 rough idle

**redsnapper386** · 02-21-2018, 07:03 PM

guys i have an ongoing problem with a rough idle and rpm fluxuate between 900 rpms to 1100 rpms 1993 2.5 merc 200 hp off shore black max.i have installed new stator trigger carbs rebuilt ect any ideas of what to check next.this involves a twinn engine set up,the other motor is fine.starts fine with nice idle.thanks .

**Krazymaan** · 02-21-2018, 08:24 PM

Sometimes a rough idle with a carb motor can be a bad cylinder, broken ring, a broken or non shutting reed. But it can also just be blockage in a vacuum port in a carb, or a gasket problem on the front half. There are 200 small reasons to cause a rough idle. I had a rough idle in a 2.4 once and for some reason the metal ring in the head gasket rotted in just one spot. Thats all it took. Maybe you can elaborate on what you have tried, what you think it is, etc. Heck it could just be a bad coil acting up.

**redsnapper386** · 02-21-2018, 08:43 PM

thanks krazyman the heads were redone all electric parts tested coils volt regs ect.i think i will pull carbs and dbl check floats and idle jets.i agree its something of 200 items.thanks for input.

**Krazymaan** · 02-21-2018, 09:16 PM

A very crude test that I do is when its actually idling. I slap the palm of my hand over each carb one at a time and sometimes you can narrow it down to which cylinder/carb is acting up by how fast things change by choking out a cylinder one by one. I'm assuming you don't have a carbonator synchronizer in your tool box that shows how much each carb is pulling for vacuum.

**redsnapper386** · 02-21-2018, 11:11 PM

Originally Posted by Krazymaan

A very crude test that I do is when its actually idling. I slap the palm of my hand over each carb one at a time and sometimes you can narrow it down to which cylinder/carb is acting up by how fast things change by choking out a cylinder one by one. I'm assuming you don't have a carbonator synchronizer in your tool box that shows how much each carb is pulling for vacuum.

thanks again no on the syncro thats good info for the vacume test

**billybob** · 02-22-2018, 10:26 AM

You have a part, mt,d to stbd. head, called idle air valve. It lets no air into idle circuit when cold. Then when warm, it let,s air into idle circuit for better idle. When warm simply unplug it, & see if idle improves. It is probably just plugged up.