1986 75HP 4 Cyl. Merc. Does not idel very well.

[ck85abc3]

I have a 1986 75 HP 4 Cylinder Merc. The motor does not idel well and dies when I try to drive forward. If I choke the motor quick every 5-8 seconds the motor runs pretty good. As soon as I get up to about half throttle, the engine will take off without me choking it.

I turned up the idel with the two idel screws on the intake faceplate. It is ideling way to fast (estimated 1500 rpms), but atleast the motor does not die all anymore. It does still die quite often while cold and every once in a while while warm.

Compression is 120 PSI on all 4 cylinders with a brand new gauge. All 4 cylinders are firing. I'll be replacing the spark plugs next weekend.

Any ideas on what is wrong and what I should do next?

I was going to order carb repare kits for the motor on here, but I can't find the right HP, year, and number of cylinders. The carb rebuild kits I find are for 3 cylinder motors.

Any thoughts on what I should do next? What about where to buy a carb rebuild kit?

Thanks

 

[emckelvy]

Re: 1986 75HP 4 Cyl. Merc. Does not idel very well.

The screws thru the faceplate that you adjusted are likely idle mixture screws. These were on the front of the carbs, ?yes?

Note that you adjust Mixture not idle speed with these screws. You probably have the idle mixture screws adjusted too lean, which of course increased your idle speed somewhat but doesn't help takeoff!

Turn the mixture screws all the way clockwise and gently seat them. Then, turn each screw CCW approx 2-1/4 turns. This should be close enough to keep it running in the water.

Note that the way to properly adjust idle speed is with the "Idle Stop" screw on the throttle arm.

To decrease idle, the screw is turned CCW, to increase idle, vice-versa. Once you have established the correct idle mixture and idle speed in the water, in gear, you'll lock down the idle stop and re-adjust the throttle cable.

The proper way to adjust the throttle cable is to have the control box in Neutral. Then adjust the brass or plastic Ferrule on the cable so that the throttle arm is positively pushed into the idle stop. Not too much, not too little. Just enough so that when you give the motor some throttle then go back to idle, the arm returns to the stop.

Now, back to idle mixture, warm the motor up in the water, idle in gear at the lowest speed it'll idle smoothly without dying, and adjust the mixture needles, one-at-a-time, for fastest idle. Then, back each one out approx 3/8-1/2 turn from that setting and it should be just about right.

Set the idle speed, adjust throttle cable (if required), and you're good to go.

Hopefully that does the trick, if not you'll want to service your carbs. Look up parts here, they have excellent diagrams:

http://www.mercruiserparts.com/selectSerailRange.asp?doc_nbr=75+(4+CYL.)

If you can't find the correct parts at iBoats, try Mercruiserparts or www.boatfix.com

Last thought, check the diagrams for throttle and spark advance linkage carefully; if you mess with the Spark Advance stop, you'll need to re-set maximum spark advance. If you find you have disturbed this adjustment, don't run the motor until it's reset or you could melt a piston. Bad way to end a day of boating!

HTH & G'luck..........ed

 

[ck85abc3]

Re: 1986 75HP 4 Cyl. Merc. Does not idel very well.

Thanks