Extreame Runaway Idle 78 Mercury Black Max 150hp

[jd0g255]

Couple things first, barn find. 78 Mercury Black Max 1500v 150hp. Amazing condition for the years it has been sitting. Mice ate through a couple wires in engine, already fixed those. Installed new ignition switch, yes the wires are in the correct spot. New battery, new plugs, wires are still great, carbs look good. Everything moves smoothly with no ruff or tightness in carbs, lines, or throttle cables. Pumped out all old gas, which was maybe 5 cups. Lines are clear of any debris. Full tank of 91 with 50/1 prem oil mix. New fuel bulb, new filter, engine can receive fuel fine.

Problem is the engine starts fine but then revs immediately to almost full throttle. Revs immediately up to about 3500 rpm and slowly climes higher. Obviously I didn't run it very long after this. My control box has a push to choke button which closes the front side of carbs. Just trying random things to see if anything was wrong or tight pushing this button restricts the air flow enough to bring the engine down to a normal level, but obviously choking it out soon after. As soon as i release the button it revs back up again.

I know my basics around mercs and helped plenty of people with their new merc engines but ive never owned or been around something quite like this engine. I'm astounded that it runs like it, if only i can figure out what is causing this problem i think she'll run great. Thanks.

 

[aussieflash]

Re: Extreame Runaway Idle 78 Mercury Black Max 150hp

Sounds like its running lean,possibly sucking air.Are you watching butterflies are closed under idle.....maybe stuck open.If sitting for a long time,rebuilding carbs would be a good move.Hope you drained old fuel out of carbs as well.

 

[CharlieB]

Re: Extreame Runaway Idle 78 Mercury Black Max 150hp

With the motor OFF, Disconnect the throttle cable on the motor, while looking down the carb throats at the butterflies move the throttle fully open and closed to ensure they move freely and together.

Have a helper move the Control lever to WOT and back to idle while you watch the cable end for smooth full travel.

While holding the linkage in the 'closed/idle' position reattach the throttle cable, the cable should have just a 'slight' amount of tension against the linkage to ensure it is held closed. Too much tension can create shifting problems in the Control box.

Was the cable adjusted correctly or did you have to make and adjustment?

Start the motor, will it idle now?

 

[jd0g255]

Re: Extreame Runaway Idle 78 Mercury Black Max 150hp

Alright, so took apart the throttle cable from the engine, moved the control arm if you want to call it that, to reduce the throttle further, and it idles fine. So the problem lies within the linkage. I adjusted the throttle cable to reduce the throttle all the way and its still not far enough/loose enough, for the engine to idle properly. Do a link/sink? Otherwise I'm just going to mess around with control arm and try to figure out why it won't release the throttle all the way back. Ideas?

 

[jd0g255]

Re: Extreame Runaway Idle 78 Mercury Black Max 150hp

The problem lies within the idiot who put the control box together before I bought the boat. :facepalm: Readjusted the box, and all the throttle stops in the engine and she runs on the water like a beauty. But about an hour in the driver side rear floor boards starting getting wet so I'm pretty sure we got a hole somewhere. And a little bit slow on the get up speed but off to a great start so far.