2.4L 200 H.P. Low RPM burp? For lack of better terms.

[Hydrodreams]

I have a 1989 2.4L 200 H.P. carb Merc. The engine is completely stock with only approx. 300 hrs. It's been running great at idle (for a carb Merc) through acceleration and at WOT. It's run smooth, sharp, and snappy all spring. Carbs were rebuilt, with stuffers and reeds installed all courtesy of Chris Carson this past winter. The other day I noticed it running a little rough at idle and when cruising under 3000 RPM. Over 3000 and she runs like a raped ape all the way to WOT! If acceleration up to 3000 is affected I cannot notice it. It just seems to burp when at a steady RPM under 3000. I put new plugs in the other day and it seemed to smooth out a tiny bit but quickly migrated back to where it was. The old plugs did not look bad at all. I have read about switch boxes and how they may affect the RPMs. I have also read about some little idle box or something you can unhook. Maybe I have a coil assembly going bad, I don't know. Forgive my ignorance when it comes to electronics. With anything I can see I can fix. Unfortunately I cannot see electricity. Any thoughts and help would be appreciated.

[Merc 2.5]

Grab a timin light shoot all 6 at idle. If u can index flywheel it can tell alot

[Hydrodreams]

Grab a timin light shoot all 6 at idle. If u can index flywheel it can tell alot

Something I have never done, nor have any concept of unfortunately. Give me the supplies, a ladder, a saw, my tool belt, and I can build a house. When it comes to timing and electronics I am an idiot. It's all what you are exposed to! lol! I am thinking of installing new switch boxes, then coils. After that I guess its a trip to the Marine Mechanic.

[SS Minnow]

Indexing: piston stop tool installed, and gently seat piston to stop. Mark that point in relation to timing pointer on flywheel. Spin motor backwards till it hits stop again (GENTLY) mark point on flywheel in relation to timing pointer. Dead center of those two points is TDC or top dead center for that piston. Repeat for all cylinders.

Then, with motor at operating temperature, verify with timing light that all cylinders at firing when they are supposed to. A few minutes on Utube ad you can be a timing checker/setter professional. A timing light is not that expensive and I get a bunch of use out of mine. Piston stop tool was like $10. If you have the money and a good mechanic near you, by all means. I do not have competent mechanics near me that are willing to work on the 2 smokes so I have no choice.

[Motv18]

Did you rejet after the reeds and stuffers? Sometimes you’ll need to check as you putting a good bit more air in. 3000 is the kinda sticky on jetting spot iirc depends on the carb and I don’t remember which ones get finicky.

Along with the electric stuff.

[Merc 2.5]

The way ss minnow explains is rite boit indexing , but u can use a dial indicator and pull up #2 to tdc and write 2 on flywheel where indicator pointing, do that with ea cyl , then put timin lite on #1 and shoot , go to #2 and shoot , do all 6 like that , u should only see the # the timin lite hooked to , say if its hooked to #3 and u see 3 and 4 in the flash then u got double /misfiring, hope that makes sense

[Hydrodreams]

Thanks guys, she ran perfect for over a month after the carbs were rebuilt. So I don't think it's the carbs. I will have to teach myself the indexing and timing deal. So tonight I went out and she ran perfect. I was thinking I ran a can of sea foam through this tank full of gas (Non-Ethanol) maybe some carbon build up was stuffing off and affecting the plug fire. I don't know we will see what we get next time out I guess.

[Motv18]

Conoco, Hess, petrolcom, Sierra still offer mtbe pump gas, and a few others run mmt (degrades into a fine rust colored sand). So it being fuel based is a high possibility. Branded stations typically have their sds online. Unbranded stations you have to ask the for brand or for a sds. To look for stuff that shouldn’t be in the gas.