Tower of power rpm problems

[Smacdatmerc]

Hello all I’m having trouble with my 1981 90hp mercury in-line 6. The problem is, I can only get 3500 rpm out of it and every now and again it’ll jump up to 5000rpm. I’ve rebuilt the carbs last season, found a gas leak in my return line and repairs that and put new plugs in it. After reading online, I’ve narrowed it to a couple things and would like to ask your opinions.

Spark plug cables? (Worth changing out?)
carbon build up in the leg, choking out my airflow?
gelled up fuel line?

and then the bigger purchases.

Switch boxes change out?
and if it’s that, how can I field test my switch boxes. And which box controls which cylinders?

any other info would be greatly appreciated. She fires up good. Idles good and trolls good. Cruises at 32-3500 rpm but sure would be nice if it would go at 5000

 

[roscoe]

Welcome to iboats.

You really do need a service manual to delve into these motors.
Could be one of several issues, or multiple issues.

Check for spark on each plug.

If no spark, then test coils by swapping one that is sparking, with the one that isn't.
If coils check out good, but still no spark, test switchbox - power in, and power out, this is where the service manual will be of great value.
If no power in, then you need to look at the stator.

One switchbox controls plugs 1-3- and 5.
The other controls plugs 2-4- and 6.
Follow the wires from the plugs to the coils to the switchbox.

The cylinders are numbered in order, starting with #1 at the top.

From my experience, it will run with one cylinder down, maybe 2, but will not stay running if 3 cylinders are dead.

 

[Chris1956]

Pump the fuel primer bulb when she runs slow and see if she will rev up. If so, look to the fuel pump.

Listen for spark plug wires arcing to ground. Replace all wires that arc.

I doubt it is carbon build up. Partially clogged fuel line or filters is a possibility.

If you have spark on all 6, the switchboxes are fine. Check spark plugs for very clean condition, as in steam-cleaned. That is a signal that you are getting water in your cylinders.

 

[jimmbo]

My in-line did that a few times. One time it was a bad switch box, another time it was a float that was stuck and the carb was flooding. Either is relatively easy to troubleshoot