Logo
FULL SITE MENU

1991 Mercury 25 Plug gap

S

snookwhaler

Regular Contributor
"Anyone know the correct plug

"Anyone know the correct plug gap for a 1991 Mercury 25 HP? The engine is a short shaft, tiller, rope start.

We recently put a bunch of new parts on this engine after sitting for a few years. It runs good but it will not idle properly until warm.

We cleaned the carb. real good. It had a saturated float. Put in a new float and now it runs like a top. Just need to get the idle figured out. Seems the plugs are wet every time we pull them. What is the correct procedure for adjusting the idle mixture screw? I am familiar with OMC but not so much on Mercury's."
 
"AJ, factory plugs are NGK BP8

"AJ, factory plugs are NGK BP8HS-15 gapped at .060"

Your crappy cold idle is most likely due to the fact that Merc did not install a thermostat in the factory. So if you have never added one along the line, there will be nothing there.

Spend the 20 bucks at Merc and get the thermo and a new gasket - install it and your idle will improve considerably when cold....

The starting point for the idle mix is - lightly seated, then back it out 1 3/8 turns +/- 1/4 turn. Then 1/8 turns from there either clock/counter clockwise until it smooths out. Better a little too rich (counter-clockwise) than the other way. After any minute adjustment, give the motor a good 20-30 seconds to adapt to your change and then adjust again if necessary..."
 
"Hi AJ, my Clymer shop manual

"Hi AJ, my Clymer shop manual simply says "not adjustable" for the 25hp, since the primary recommended plug appears to be a NGK BUHW which is a surface gap design. A specified alternative is the Champion L76V, but no gap recommendation is listed.

The standard procedure for the idle mixture screw is as follows (from the manual):

1. Turn the screw inwards until seated, back out 1.25 to 1.5 turns

2. Start engine, run in fast idle (neutral) until warmed up. Throttle back to idle for approx. 1 min to stabilize revs.

3. Push primer/fast idle knob all the way in and turn all the way clockwise

4. Shift into FWD gear

5. Turn mix screw CCW until engine begins to run rough due to over-rich mix. Note position of screw.

6. Turn screw CW until idle smoothens out again, continue turning until engine starts to misfire due to lean mix. Note position of screw.

7. Set screw halfway between too rich or too lean. Don't set it leaner than necessary for a smooth idle; to be safe, it's better to err slightly on the side of too rich rather than too lean.

Hope this helps!"
 
Back
Top