Mercruiser 3.0 sometimes dies and won’t start

[jtim108]

I have a 1984 mercruiser 140 with an alpha 1 gen 1 serial number: 6805225

everything on the boat is great however sometimes after driving around for a couple hours if I shut it off and go to start it it’ll start run fine for a minute and then the idle will start to jump around and get pretty low if I go to give it some throttle it’ll die out and then doesn’t want to start and idle again, also noticed sometimes under load it’ll want to bog under load for a second if I give it throttle to quick

we have changed the fuel filter and the fuel screen inline on the carb the carburetor is brand new and the fuel we have is clean 93 octane, we did spark plugs wires and a distributor cap but haven’t gotten around the do the rotor and points/condenser. The coil hasn’t been changed yet either.

it seems like a fuel issue to me by the way it’s bogging but it could also be ignition related but with it being a random issue it’s hard to diagnose any tips would be appreciated thank you

 

[jtim108]

also forgot to add if when it dies i pump the throttle 2 or 3 times to shoot some fuel in the carb itll try and start for a breif second otherwise it just cranks till no end

 

[merc120_81]

Have you checked the timing at all on the boat?

If it has points, it also has a mechanical advance, a least my 1981 120 has mechanical advance. I would start with doing the points and condenser, and checking the timing. when you do the points, go the extra step and pull the points plate out of the distributor, I believe it is two screws.check that the springs and weights are intact, clean and lubricated.

You can buy a lot of sets of points for what they want for the electronic conversion kits, but you still would have to check the timing too.

It never hurts to get that stuff spot on and correct.. After you have done the points, condensor, timing and dwell, chase the coil..

Everyone says its never the coil, and it usually isn't. I did have one go bad, and it was a bugger to chase down. Mine would run great for a while, and then start popping and complaining under load usually and when hot. the hot part was the key..

 

[jtim108]

Have you checked the timing at all on the boat?

If it has points, it also has a mechanical advance, a least my 1981 120 has mechanical advance. I would start with doing the points and condenser, and checking the timing. when you do the points, go the extra step and pull the points plate out of the distributor, I believe it is two screws.check that the springs and weights are intact, clean and lubricated.

You can buy a lot of sets of points for what they want for the electronic conversion kits, but you still would have to check the timing too.

It never hurts to get that stuff spot on and correct.. After you have done the points, condensor, timing and dwell, chase the coil..

Everyone says its never the coil, and it usually isn't. I did have one go bad, and it was a bugger to chase down. Mine would run great for a while, and then start popping and complaining under load usually and when hot. the hot part was the key..

So I went and checked dwell and it was at 52 I changed the points condenser rotor and checked plug gap my Dwell is at 34 now and timing is at 6 btdc we let the boat run for 45 minutes with some rpm raises and shut it off let it sit for around 30 minutes

Once it started back up it idled at about 5-600 and was a little rough until I blipped the throttle for a second then it dropped down to idle of about 825 and ran smooth

i did it one more time and had the same outcome. I rechecked timing and dwell and the numbers are the same, im pretty confused anything else to look towards?

 

[merc120_81]

timing advances pretty quick on my 120. If the boats in the water, can you throttle up with the timing light on, 2200 or so, you should see the timing advance, and then come back down when you bring it back down to idle.

You didn't mention if it started better and consistently??


Now that ignition is close /in spec. 34 dwell is on the high side of the spec. might want to look at the idle circuit. also, is your choke functioning properly? The choke on mine is thermal controlled, and opens up pretty quickly.

 

[jtim108]

timing advances pretty quick on my 120. If the boats in the water, can you throttle up with the timing light on, 2200 or so, you should see the timing advance, and then come back down when you bring it back down to idle.

You didn't mention if it started better and consistently??


Now that ignition is close /in spec. 34 dwell is on the high side of the spec. might want to look at the idle circuit. also, is your choke functioning properly? The choke on mine is thermal controlled, and opens up pretty quickly.

Do you want me to check timing at 2200 RPMs while moving or in neutral also the boat started quickly and then after it sat for a little bit it still started up right away haven’t checked the choke but I will see what it does later on on the water might take the dwell down A couple degrees

also what parts of the idle circuit should I be looking at?

 

[jtim108]

Also I’m gonna mess with my carb adjustments I’ve never check the acc pump adjustment which might explain the slight hesitation if you punch the throttle and the idle air/fuel mix and the choke adjustment

 

[merc120_81]

The timing should advance weather under load or not, just don't run it over 1200 or so on the muffs in the driveway. but the timing advances pretty quick on mine. check it in the water.. free rev in neutral is fine with a timing light you will see it pretty much go off the timing scale.

Could you post a video of the hesitation/so it could be heard by the community here?

Accelerator pump is not too hard on these carbs, I assume you have the same 2 barrel I have (You will either have a rochester of a mercruser carb usually) .

I also don't know if there is any adjustment for the linkage, its been awhile since I have messed with it.

If you haven't found the links to the manuals.
https://forums.iboats.com/forum/eng...miscellaneous-mercruiser-information?t=329915

( I am pretty sure you need manual 3) carb adjustments are laid out in the manual.

There are two idle circuit jets at the base of the carb make sure they aren't gummed up.. Idle should be set under load, so in the water and in gear.

I make it a point to put startron enzyme treatment into every tank of gas (ethanol stabilizer I think is better than stable), fights the gum and phase separation that ethanol gets.

For what its worth, mine has a slight hesitation, then takes off when I jam the throttle on.. (It seems like a 1 Mississippi maybe 1/ 2 a Mississippi being fully technical and accurate)

 

[jtim108]

The timing should advance weather under load or not, just don't run it over 1200 or so on the muffs in the driveway. but the timing advances pretty quick on mine. check it in the water.. free rev in neutral is fine with a timing light you will see it pretty much go off the timing scale.

Could you post a video of the hesitation/so it could be heard by the community here?

Accelerator pump is not too hard on these carbs, I assume you have the same 2 barrel I have (You will either have a rochester of a mercruser carb usually) .

I also don't know if there is any adjustment for the linkage, its been awhile since I have messed with it.

If you haven't found the links to the manuals.
https://forums.iboats.com/forum/eng...miscellaneous-mercruiser-information?t=329915

( I am pretty sure you need manual 3) carb adjustments are laid out in the manual.

There are two idle circuit jets at the base of the carb make sure they aren't gummed up.. Idle should be set under load, so in the water and in gear.

I make it a point to put startron enzyme treatment into every tank of gas (ethanol stabilizer I think is better than stable), fights the gum and phase separation that ethanol gets.

For what its worth, mine has a slight hesitation, then takes off when I jam the throttle on.. (It seems like a 1 Mississippi maybe 1/ 2 a Mississippi being fully technical and accurate)

I will get a video of what the boat is doing today or tomorrow took it yesterday and the dwell and timing adjustment definitely helped but I never got the chance to kill it and see how it ran after sitting a couple minutes, still gonna take apart the carb and check everything out