Mercruiser 140 high idle

[patbax2]

I just did a tune-up on my 85 crestliner with mercruiser 140 and alpha 1 drive. I replaced plugs, distributor cap, rotor, points, and condenser. When I fired it up the idle was around 1700 and would not idle down. I checked everything on the carb over and everything looked normal so I checked the idle speed screw and still couldn't get it to idle back down. Any ideas?

 

[grahamh]

Re: Mercruiser 140 high idle

Did you accidentally advance the timing in the process? I would check the timing first, since all the work you did was ignition system stuff. Advanced timing can cause a high idle.

 

[patbax2]

Re: Mercruiser 140 high idle

I wouldn't think so. Didn't have the distributor loose. But I'll check it out

 

[grahamh]

Re: Mercruiser 140 high idle

I think that replacing/adjusting the points can alter the timing too, but I'm not terrible familiar with points, I'm from the electronic ignition generation.

 

[myoldboat2]

Re: Mercruiser 140 high idle

hmmm, i don't think installing the dist cap turned wrong or hooking up the plug wires wrong would do that... but i would check those (they are easy to eyeball)...

 

[patbax2]

Re: Mercruiser 140 high idle

Ok I checked the timing. I had to disconnect the throttle linkage and hold the throttle closed to get the rpms low enough, but the timing is right on. The butterflies in the carb are fully closed, so its nothing there. The distributor only goes on one way, and the wires are definately in the right spot. I don't know of anything that I did that would cause the high idle. Any other suggestions?

 

[Don S]

Re: Mercruiser 140 high idle

Disconnect the throttle cable and make sure the carb linkage is against the idle stop screw. If it is, then you were WAY overdue for a tuneup, and now it's running right. Now you have to adjust dwell, timing, carb mixture and idle speed to finish the job.
Probably idle speed has been adjusted up to keep it idleing at the right rpm with bad plugs, cap, rotor, points and condenser for years. Now you made it all new again.

 

[patbax2]

Re: Mercruiser 140 high idle

Yeah i'm sure the boat was overdue for a tuneup. I just got it a few weeks ago and its been running pretty good. Now it runs great, except for the idle. I've adjusted dwell, timing is good, messed with the carb mixture, and tried the idle speed, but it won't go down. With the throttle cable disconnected, the idle speed screw all the way out and holding the linkage closed the idle will get a little bit lower, but there's no way to make it stay. What about the timing advance? Is there any way there could be a problem with it?

Disconnect the throttle cable and make sure the carb linkage is against the idle stop screw. If it is, then you were WAY overdue for a tuneup, and now it's running right. Now you have to adjust dwell, timing, carb mixture and idle speed to finish the job.
Probably idle speed has been adjusted up to keep it idleing at the right rpm with bad plugs, cap, rotor, points and condenser for years. Now you made it all new again.

 

[michaeli84]

Re: Mercruiser 140 high idle

did you try loosening the distributer bolt and turning the cap left or right to bring the idle down. If you do that while your turning it you should here the engine idle get faster or slower. If it does you need a timing light and you aim it at the timing tab by the crank. it should be 6 degrees btdc i think check the manual to be sure. Good luck i hope i helped!

 

[patbax2]

Re: Mercruiser 140 high idle

Yeah I ran a timing light on it when I held the throttle linkage down with my hand. The timing is right on where it should be at 6 btdc

 

[sockos]

Re: Mercruiser 140 high idle

Does your 140 have a vacuum advance? That makes a difference in how you time it.
Did you do anything to the dist. advance plate by wrestling the old rotor off?
Any vacuum leaks?

Is idle screw resting on high idle cam of choke?
Mixture screw too far out?
That's all I have for now good luck!

 

[cr2k]

Re: Mercruiser 140 high idle

Well as I see it the KEY to this problem was that he had to disconnect the throttle linkage to get the idle down to check the timing.

Adjust your throttle cable to put a very slight amount of push on the carb to keep it closed at idle.

 

[sockos]

Re: Mercruiser 140 high idle

I thought he tried the idle screw first off ?

 

[sockos]

Re: Mercruiser 140 high idle

The throttle is at idle, right?

 

[Don S]

Re: Mercruiser 140 high idle

Does your 140 have a vacuum advance? That makes a difference in how you time it.

Boats don't have vacuum advances.

Ok I checked the timing. I had to disconnect the throttle linkage and hold the throttle closed to get the rpms low enough,

Kind of explains why it won't idle doesn't it. Adjust the cable end to put some pressure/preload on the idle adjustment screw. Not a whole lot, just a little to keep it there. Adjust mixture, rpm, timing, as many times as needed to get it right.

 

[cr2k]

Re: Mercruiser 140 high idle

Then maybe someone messed with the butterflys when they had it apart and they are not seating. Or there is debris in the throat holding the butterflies open.

If so with the carb off loosen slightly the screws that hold the butterflies and close the throttle to seat them and re tighten.

If the butterflies are closed all the way the engine should die. If not there is a vacuum leak somewhere.

With engine running shoot a little carb spray around the carb base and intake to block areas and see if there is any change.

 

[patbax2]

Re: Mercruiser 140 high idle

What I was trying to say is, I took the throttle cable loose, I had the idle speed screw backed all the way off to where it wasn't touching anything, and I held the throttle closed with my hand. It took quite a bit if pressure to hold it to where it would idle down even close to where it needed to be. I don't think it should take that much pressure should it?

Kind of explains why it won't idle doesn't it. Adjust the cable end to put some pressure/preload on the idle adjustment screw. Not a whole lot, just a little to keep it there. Adjust mixture, rpm, timing, as many times as needed to get it right.

 

[patbax2]

Re: Mercruiser 140 high idle

Ok today I got it out in the water to check the idle in gear. The manual says it should be between 600 and 700 RPM I believe. Mine is around 950 RPM with the idle speed screw backed all the way out. I'm going to try what Don told me tomorrow and see if I can adjust the throttle cable to hold the throttle closed. Thanks for the input guys. I'll post back when I get it fixed

 

[patbax2]

Re: Mercruiser 140 high idle

Ok guys I figured I'd post back today and let you know what I found. The cable adjustment wouldn't work for me, so I pulled the carb off. When I got it off I found that one of the butterflies was bent open and on one side and wouldn't close all the way. The bend was in a place I couldn't see from the top of the carb. I guess before I did the tune up it wasn't running so well so it needed to be opened up more to acheive the right idle, like Don said. So I got the buttefly bent back down where it should be and i'm putting the carb back on today. Thanks again for the input.