Mercruiser 3.0 140 wild idle issue after carb rebuild

[NastyinSC]

Hello everyone just wanted to start with a quick thanks, this forum has been an excellent resource since I've owned by boat. I have a 1989 Citation bowrider with a 1988 3.0 Mercruiser 140hp 181cid linked to a mercruiser (pre alpha) stern drive.

First the back story:

I ran into some an issue at WOT where it would lean out and stutter. Using this form I tried to hunt down problems and as of now have:
-Replaced water fuel separator filter
-New distributor cap
-New plugs
-Cleaned fuel pump filter (fuel pump was new when I purchased it in '16)
-Drained gas tank and filled with fresh ethanol free gas from marina

And most recently re-built the 2bbl Mercarb with the Sierra rebuild kit that included new power jet, accelerator pump, idle mixture needle and gaskets.

I put the newly rebuilt carb on two days ago and thought I had messed something up with the rebuild, but realized I hadn't set the idle mixture needle anywhere close to spec which according to my rebuild sheet is 2 turns out from seated.

Got the boat to run with the needle set at 2 1/4 turns out, but this is where my issue starts.

The boat will crank fine, and idles a bit clunky dead on 700 rpm, but after about 5-10 seconds of that it revs up to about 2200 rpm, holds for about 3 seconds, then drops back to idle at 700 and sounding like it's going to stall. This issue was not present before the rebuild.

It's not limited to neutral idle, when the drive is engaged just out of neutral it will do "the reving thing" spooling up to around 1500 rpm then back down.


The boat runs better than ever at WOT at 3500-4000rpm(I don't really run it over 3800) the stalling issue is totally gone so that's good!

My gut is that this a throttle or shift linkage issue because now when I shift into gear the drive engages in what "feels" should be the neutral area and when it comes down off plane I have to pull it almost to reverse to get into neutral. I'm a bit lost, but feel like I'm close to having this licked. Any help would be awesome!

 

[Scott Danforth]

You have an air leak somewhere

 

[NastyinSC]

That was a thought of mine too, I know people have talked about the "thicker" bottom gasket where the carb sits on the block, I'll take some carb cleaner out there and see if I can hunt it down. Still wondering about the slack drive cable though...

 

[TurtleTamer]

Checking for the vacuum leak will probably expose the issue. Slack in the cable will give you that symptom but at a different time. It'll make it refuse to idle all the way back down once you reduce the throttle from higher RPM. It typically won't cause the idle speed to actually increase on its own. In fact, if it's a slack problem then I suspect that it would NOT do this because the vacuum in the manifold should help to suck the throttle plates closed.

I have found that the "thick" carb gasket is a bit misleading. I ordered the OEM under the correct part number and while the gasket is fairly hefty, it's not overly thick. Before this I actually made a custom gasket that was all in all about 3/8" thick and it made no difference in idle adjustability.

One other thing to check: I don't know if the threads in my carb body are worn, or what, but the new idle mixture adjustment screw in my kit didn't do as well as the old one. I'm pretty sure it's original to the carb because it has no extra slotted nipple coming off the back, just the hex head. I could grab the new one and literally move it up and down and side-to-side. So I installed the old one back in with some grease on the threads for better sealing.

 

[hoowahfun]

I believe the service manual calls for 1 1/4 turns out for the idle mixture needle as a starting point.

Did you verify that ALL adjustments were within spec per the service manual? Thinking float drop, float height, pump rod, choke, choke plate, etc.

Vacuum leak is also a possibility as previously suggested.

 

[NastyinSC]

Thanks for the reply’s guys. Put the old idle needle in and set the fast idle screw in a turn and that really helped. Was able to take it out and warm it up and adjust the mixture needle. Got it to idle without it reving up on its own. Idling right around 800-900rpm. Still having the “not wanting to go into neutral” issue when I bring the control arm back to center. It’s acting a lot better. RE-set the top half the carb and think that solved any vacuum leaks, should I try adjusting the drive cable barrel?

 

[Scott Danforth]

Got it to idle without it reving up on its own. Idling right around 800-900rpm. Still having the “not wanting to go into neutral” issue when I bring the control arm back to center.

idle is too high and your lower shift cable may need replacing

 

[TurtleTamer]

...should I try adjusting the drive cable barrel?

Only if bringing the throttle back to idle doesn't fully close the butterflies. With the throttle in neutral, try and push it closed. If the idle drops and dies then it's not closing all the way and you'll need to adjust the barrel. While the engine should run fine with some slack in this cable, it'll make idle rpm unpredictable.