Mercury Mark 55 idle

[outboardnut]

Hi Everyone.
I'm tinkering with a Mercury Mark 55 today.
Got it running pretty good.......but I think the thing could run just a little better
I can get the motor to run around 650 RPMS reliably in forward gear.

Here is the part that bothers me. I shift into neutral and the RPMS jump up to about 1050 which is a little to fast for my liking.
I'm feeling like it RPMS should move up about 200 RPMS.

Usually when motor does this it is because it is a little to lean(right?). I have the low speed jets backed out just about as far as I can get them and it still run halfway smooth.

Any ideas?

I kinda feel like I am getting to picky with a motor this old....

 

[Laddies]

Re: Mercury Mark 55 idle

Check the idle pickup tubes and some of the old 55s had way to much prop on them

 

[Chinewalker]

Re: Mercury Mark 55 idle

Hi OBNut,
What prop are you spinning? I've had similar problems with my old Mark series 4-bangers if I'm running a tall prop. In order for it to actually idle in gear I had to set the neutral idle fairly high so that it didn't stall when shifting.
I would adjust the low speed jets to where the motor runs best, and then adjust the idle stops and pick-ups on the mag tower to achieve your idle speed...
- Scott

 

[outboardnut]

Re: Mercury Mark 55 idle

Hi Laddies, Hi Chinewalker.
Glad to here from you two.

The prop being to tall didn't even dawn on me. I removed it and could not find a pitch on it. It is a two blade propeller. I snapped a picture of it next to a 10 pitch 3 blade I have...it looks a lot taller.

Well before I tore down the carbs and have them soaking at the moment...and am rechecking the point gap on the mag.

I wish I had the dwell plate for this......I don't feel good about using a feeler gauge but it is all I have. I don't know if any shops around here have the special tools for the keik mag either...I'm kinda out in nowhere.

It's going to take me a few days before I can follow up much on this...I'm ordering new points/condenser and a carb kits.

 

[outboardnut]

Re: Mercury Mark 55 idle

Well the RPM jump is confirmed due to it being a high pitched prop. I rebuilt both carbs and magneto with no change.
It is in fact caused by the load of the prop.