1978 Merc 800 won't idle below 1200 RPM

[1970Thunderbird]

Hey all,

My 1978 Mercury 800 won't idle below 1200 RPM. What I mean is, my idle adjustment screw is positioned to give it the lowest idle possible, and my motor still won't idle below 1000-1300 RPM.

Is there an adjustment somewhere else on the motor? The plastic piece on top of the top carb has a screw that appears to be for adjustment, and i've checked it. There's no room for adjustment there, either.

The RPM reading of 1200 is in neutral. I know it's supposed to idle around 750-800 in forward gear.

I appreciate the help.

Thanks!

 

[bgrave1]

Re: 1978 Merc 800 won't idle below 1200 RPM

Hello,

I am going through nearly the same thing with my 1978 800. The PO took it to a mechanic and he has it again to have a closer look.

He mentioned looking at the bottom of the powerhead at the seals to verify that it is not sucking air. His next place he is looking into is the reeds. These motors have a history of the reeds breaking off.

These are the 2 things that are on the table right now with mine.

Let me know if you find anything and I will do the same when I hear back from the mechanic.

Bob G.

Hey all,

My 1978 Mercury 800 won't idle below 1200 RPM. What I mean is, my idle adjustment screw is positioned to give it the lowest idle possible, and my motor still won't idle below 1000-1300 RPM.

Is there an adjustment somewhere else on the motor? The plastic piece on top of the top carb has a screw that appears to be for adjustment, and i've checked it. There's no room for adjustment there, either.

The RPM reading of 1200 is in neutral. I know it's supposed to idle around 750-800 in forward gear.

I appreciate the help.

Thanks!

 

[Chris1956]

Re: 1978 Merc 800 won't idle below 1200 RPM

Idle speed is set by idle pickup timing and throttle cable adjustment. You may loosen the idle speed screw, but if you do not adjust the cable to push the throttle closed....

Start by verifying idle pickup timing. When that is in spec, remove the throttle cable from the clamp and back off the idle stop screw, pushing the throttle closed, until you reach the desired idle RPM. Now tighten the idle stop screw and asjust the throttle cable to push the throttle closed.

 

[1970Thunderbird]

Re: 1978 Merc 800 won't idle below 1200 RPM

BGrave,

I'm pretty certain my seals and reeds are good, as I just rebuilt the motor. It has less than 15hrs on it.

However, I did discover that my number 4 cylinder is not firing, so I'm sure that is a major contributing factor. I'll update once I get it firing again and see if that alters anything.

 

[Chris1956]

Re: 1978 Merc 800 won't idle below 1200 RPM

The reeds on that motor hardly ever fail.... I think Bob G. may be getting some unnecessary work.

 

[1970Thunderbird]

Re: 1978 Merc 800 won't idle below 1200 RPM

Bob,

I replaced my switch box and ran the motor last night. All my problems were a direct result of a faulty box. Once I got all my cylinders firing properly, I got the motor to idle at 500-600 in neutral, and 700 in forward gear.

I got my box from Amazon, as it was the cheapest and fastest shipping method. Sucks that I'm now out $170, but my motor runs like new.

Hope that helps!