Mercury only starts when I throttle up

I have a 76 Mercury 85hp. I can't start it by choking or with the throttle lever at idle speed. It starts easily when I raise the throttle level. It dies when I try to take it down to idle speed which I must do before moving into forward or reverse. It runs good at fast speeds but dies when I slow down to an idle speed. It may be not firing on all cylinders according to another boater. The last time I got it running by quickly dropping the throttle and placing it in forward gear. This is my first boat. I feel capable of making repairs as necessary but I don't know where to start. I feel like the idle speed needs to be higher. Please give me suggestions with details.

Re: Mercury only starts when I throttle up

Have the carbs and fuel pump been cleaned and/or rebuilt? My 79 40hp was doing similar things and I rebuilt both and it runs like a top.

Re: Mercury only starts when I throttle up

Mac is proably correct about the carbs being dirty. The carbs idle mixture needs to be as rich possible, consistant with a reasonable idle, in order not to die, when the throttle is pushed forward.

BTW - you will get real good at pushing the cold start lever down and droping the motor into gear and giving it a bit of throttle, in one motion....

Re: Mercury only starts when I throttle up

The carbs have been cleaned.

Re: Mercury only starts when I throttle up

How do I adjust the carb idle mixture?

Re: Mercury only starts when I throttle up

turn it clockwise until it is lightly seated. not completely tight. just snug. then back it out (counterclockwise) 1 and 1/2 turns. Some people say 1 and 1/4 or 1 and 1/8, but I like to go a little farther because it wont hurt the motor to run it a little rich. too lean can cause problems. You then need to put the boat in the water and put it in forward gear. while moving forward slowly, turn the screw clockwise in 1/8 turn increments. give the motor time to catch up and adjust. turn it in slowly until it starts to bog and misfire. then, turn it counter clockwise and allow it to catch up. it should begin to idle faster. somewhere between those positions, you will find the best idle speed.

Re: Mercury only starts when I throttle up

Adding to what Mac said, usually around 1-7/8 to 2 full turns open is the correct setting on the top carb, a bit less rich on the mid and lower carbs. Try for smooth acceleration and decent idle.

Re: Mercury only starts when I throttle up

Sorry I forgot it was only two carbs....

Re: Mercury only starts when I throttle up

I'm having a similar problem. I have a Mercury 30 hp 2 stroke. My motor will only start in fast idle or turn over once or twice in idle. I can get it in gear but even then I need to boost the throttle to keep the motor running. I took most of the carb apart already and cleaned out the varnish in the fuel well, and replaced the corresponding gaskets. I also replaced the plugs and fuel filter and played with the fuel ratio but nothing seems to keep her running. I don't know what else to do. Any suggestions on something I'm missing? Or is it time for a new carb?

Re: Mercury only starts when I throttle up

You shouldn't need a new carb. You should be fine with a carb rebuild kit. Get gaskets, new needle and seat, and a new jet. When I cleaned my carb, I took the entire thing apart, and soaked the carb and all the parts in a gallon of Chem-dip for several hours. Great stuff. You may also want to do a fuel pump rebuild if you havent already. I don't know what year model you have, but if it has the fuel pump on the left side of the powerhead (left side if you are looking at the carb) you likely need to rebuild the fuel pump also. It has 2 check valves inside that can dry out and lose their ability to close and open properly, which affects the ability of fuel to pump properly. Also the diaphragm on the fuel pump likely needs replacing. If you pull the pump off and the diaphragm is all wrinkled, it needs replacing, and that can make worlds of difference. Granted, this is all only pertinent to you if your motor has this fuel pump. But I would definitely rebuild the carb completely and replace everything. My motor had very similar symptoms and after a carb rebuild/cleaning, and a fuel pump rebuild/cleaning, it runs like a top.

Re: Mercury only starts when I throttle up

cool i'll give that a shot. thanks!

Re: Mercury only starts when I throttle up

Does your outboard have that fuel pump on the left side?

Re: Mercury only starts when I throttle up

i think so. its right behind the fuel filter on the left side like you suggested

Re: Mercury only starts when I throttle up

Okay I was just checking. Do those rebuilds and let me know how it turns out.

Re: Mercury only starts when I throttle up

It appears that a fuel filter is on top of the fuel pump. It has a slotted screw that is almost stripped. Do you have any ideas on how to remove it? It also appears that many of the fuel lines within the housing are cracking. It looks like much of the motor must be taken apart to access and replace these lines. Is this accurate? BTW, I did a compression check, all four cylinders were at 120-122. I replaced the spark plugs and I ran Seafoam though the carbs. It still dies a low speeds and will not start or run at idle. This makes adjusting the carb difficult since it is supposed to be adjusted at idle. I was starting to look at the fuel pump but I ran into the screw problem. Any help would be appreciated.

Re: Mercury only starts when I throttle up

Try giving it just a little more initial timing. If that solves it then you're probably sucking air through one of those cracked hoses. You probably want to think about replacing them. It's a common problem. I would let the carb rebuilds, etc slide until you try the timing advance trick. It's just something I was taught by an old pro.