1981 120 Mercruiser idles rough when hot

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Dannysmyk

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Hello all!
I cant seem to figure this out for the life of me. I have a 1981 120 mercruiser that has been in the family for the last 20 years. I love this boat maybe a little too much lol in the last five years we have been through it. I redid the interior and basically every engine component besides the block and head( electronic ignition, new distributor, new cap rotor, new coil, new plugs, new wires, new starter, new fuel pump, rebuilt alternator ect...) shes a moody old girl but shes my moody old girl so thats why im here. The last two years ive noticed after it gets nice and hot it doesnt want to idle. Starts right up when its cold...idles out of the canal fine... ride around for 30 minuets (runs like a top) come back and she doesnt want to idle in gear through the canal. It will idle fine for about 5 minuets, then stuble, then stuble worse, then die (happens slowly). if i give it a few pumps of the throtle its idles fine for about 30 seconds then starts to sumble again. This winter i rebuilt the carb in hopes that an idler jet had some gunk in it. Started it in the driveway today and it ran great.... for about 20 minuets.... then again stumbleeee. Any help or ideas would be great. Thanks
 
flashback

flashback

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Sounds like a problem with the carb, that's where I would look, maybe a gunked up fuel tank.. try running it off a portable tank.
 
alldodge

alldodge

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It's most likely fuel related, either not enough or to much.

When it stumbles look down throat of carb to see if it appears to be flooding. If no flooding, need a fuel pressure gauge and Tee fitting. Measure pressure at the carb
 
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Dannysmyk

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alldodge said:
It's most likely fuel related, either not enough or to much.

When it stumbles look down throat of carb to see if it appears to be flooding. If no flooding, need a fuel pressure gauge and Tee fitting. Measure pressure at the carb
Click to expand...
what should the pressure be?
No flooding cant see and fuel squirting into carb at idle
 
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airshot

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Idle adjustment needs to be done while underway in the water....cannot be done sitting on a trailer in the driveway !! Do a compression check, these 4 cylinder engines are known for warped heads, if compression is low on 2&3 your head is probably warped and they don't idle well, especially when warm. Might very well be a carb issue, so use a complete kit from the factory, but, proper idle adjustment and a compression test is free and easy place to start.
 
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kenny nunez

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If the fuel system checks out you should look at the distributor. It only has 1 bushing and the other one is in the block above the oil pump. Also the posts that hold the advance springs and counter weights under the breaker plate tend to loosen up.
If the posts in the distributor are tight then the distributor can be repaired by adding another bushing in the bottom and replacing the top one.
The other thing to try is replacing the rotor. Get one from a Mercruiser dealer, the blade on those are slightly longer. What might be happening is the spark is going to ground when the engine gets hot. That is a problem that happened a lot in the past and brand new boats with the inline engines were acting like the symptom you are describing.

The rotor shaft is probably wobbling around and the dwell and timing are moving up and down.
 
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Dannysmyk

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kenny nunez said:
If the fuel system checks out you should look at the distributor. It only has 1 bushing and the other one is in the block above the oil pump. Also the posts that hold the advance springs and counter weights under the breaker plate tend to loosen up.
If the posts in the distributor are tight then the distributor can be repaired by adding another bushing in the bottom and replacing the top one.
The other thing to try is replacing the rotor. Get one from a Mercruiser dealer, the blade on those are slightly longer. What might be happening is the spark is going to ground when the engine gets hot. That is a problem that happened a lot in the past and brand new boats with the inline engines were acting like the symptom you are describing.

The rotor shaft is probably wobbling around and the dwell and timing are moving up and down.
Click to expand...
The timing jumps around at idle when i put a timing light on it. Suppose to be set at 8 and jumps irradically from 7-9 BTC. Do you think thats my issue and how do i fix it?
 
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dubs283

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There should be little to no play in the distributor shaft, fairly easy to check with the cap/rotor removed. Just try to wiggle it. As stated earlier the repair in this case is to install new bushings

Did this issue arise after the electronic ignition was installed?
 
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Dannysmyk

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dubs283 said:
There should be little to no play in the distributor shaft, fairly easy to check with the cap/rotor removed. Just try to wiggle it. As stated earlier the repair in this case is to install new bushings

Did this issue arise after the electronic ignition was installed?
Click to expand...
Okay ill check that. No it ran fine when i first out the electronic ignition in this is a new issue that seems to be getting worse.
 
Scott Danforth

Scott Danforth

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Define hot. The motor should be between 150 and 160 when warm
 
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dubs283

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Most aftermarket electronic ignition upgrade components work quite well out of the box however I've seen a few fail within a week or two of install. I've seen some behave exactly as you describe, work great first 20-30 mins but when fully warm/up to temp they start to fail. Would be high on my list of suspect components in your situation

When the engine starts to exhibit the issue check the idle timing, of its okay check the spark quality
 
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kenny nunez

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If you have a volt meter check the voltage at the + side of the coil, you should have 8-9 volts, anything less then the resistor wire is the cause. Try to measure the voltage especially when the engine starts to act up.
 
flashback

flashback

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kenny nunez said:
If you have a volt meter check the voltage at the + side of the coil, you should have 8-9 volts, anything less then the resistor wire is the cause. Try to measure the voltage especially when the engine starts to act up.
Click to expand...
Would he still have a resistance wire with the electronic ignition conversion
 
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