Mercruiser 140 - new carb - why won't it hold an idle???

boozer1966 · Aug 29, 2013

Replaced Mercarb 2bbl on Mercruiser 140. Replaced with Rochester. Starts with electric choke it with very high idle. When choke is off, it will not hold an idle. Have tinkered with idle mix screws to no avail. No apparent vacuum leak. Any ideas?

achris · Aug 29, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

Start with the basics. Make sure the engine is mechanically sound. Compression test is the first check. Then check your distributor for slop and wear. Check the points dwell, and all the spark plugs are good, clean and gapped properly. Check the timing is correct and steady.

Chris......

thumpar · Aug 29, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

What what the reason for replacing the carb? Was there an issue before?

boozer1966 · Aug 29, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

Prior carb broke throttle linkage. Seemed easier just to purchase a remanufactured one.

NHGuy · Aug 29, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

I think there is a leak in the carb or base gasket. Squirt carb cleaner at the choke shaft ends and the throttle shaft ends, squirt it around the base, and at any spot where a leak could occur. When it revs with a closed choke it must be getting air somewhere.
Then when it warms it won't run right because it goes too lean when the choke allows air in. The carb cleaner will change how it runs and show you where the air is getting in. My bet is that it's the base gasket.
Once you fix the leak, go back through the mix and idle settings again.

thumpar · Aug 29, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

If it is the correct carb for the engine with the correct jets then like achris posted, back to basics.

boozer1966 · Aug 29, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

All very good stuff. Timing and related parts are fairly new and were working well before replacement. Didn't have a new base gasket. Will get one and reload. Thanks for all of the suggestions.

boozer1966 · Aug 31, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

I gave up. Took linkage from old and new carbs and made the old carb work. Very frustrating. Now engine idles roughly and has no power off idle. Almost like it has a cylinder down. Spark plugs look ok. Timing is good. Seems like one problem led to another.

thumpar · Aug 31, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

Did you do the compression check?

boozer1966 · Aug 31, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

I guess that's next. Will report back.

bonzoscott · Aug 31, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

It may be helpful to know what lead up to changing carb. Did the boat run well at some point this year? Is the boat new to you? etc...

boozer1966 · Sep 10, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

I ended up taking a step back, borrowed carb linkage from the re-manufactured unit and put the old carb back on. Was able to get idle back to baseline, but still was running rough. Compression readings were as follows:

105
0
0
105

Removed head to find gasket between cylinders 2 and 3 compromised. Will replace gasket and see if that gets me on the water again, although the compression readings on cylinders 1 and 4 seem to indicate a more involved rebuild is in order. Will report back.

chris650 · Sep 10, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

I would also do an old fashioned vacuum test also

chris650 · Sep 10, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

and on the comp test I think 105 is too low.

achris · Sep 10, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

chris650 said:
and on the comp test I think 105 is too low.

Personally, I think the two cylinders reading 0 are a lttle more concerning. Like a blown head gasket.... That might also account for the poor idle....

Chris....

thumpar · Sep 10, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

If you can get the other cylinders to 105 it should run fine but would be low on power. See if the head gasket will get you going. If it does run it. Not sure where you are at but most places are getting close to the end of the season. You could rebuild it over winter.

Volphin · Sep 11, 2013

Re: Mercruiser 140 - new carb - why won't it hold an idle???

Whats up with all these 3.0L head gaskets blowing? Seems fairly common... use a good quality gasket like Fel-Pro Marine.

Mercruiser 140 - new carb - why won't it hold an idle???

Petty Officer 2nd Class

More fish than mountain goat

Admiral

Petty Officer 2nd Class

Captain

Admiral

Petty Officer 2nd Class

Petty Officer 2nd Class

Admiral

Petty Officer 2nd Class

Senior Chief Petty Officer

Petty Officer 2nd Class

Petty Officer 2nd Class

Petty Officer 2nd Class

More fish than mountain goat

Admiral

Lieutenant