3.0 Lx Sterndrive

[jtomanelli]

I have a 1995 Spectrum 18 SD with mercruiser 3.0 sterndrive that has been intermittently very rough running, even after warm up. It's done this numerous times and often times it cleans itself up after I let it sit.

I've replaced the following:
- coil
- dist. Cap
- cables
- plugs
- both fuel filters (main and carb)

Also, I have set the timing using the proper procedure of bypassing the advance and the shift interuptor and setting to 1'BTDC and verified. I performed these procedures when running smoothly and at 650-700 RPM on muffs.
Tonight, when testing it out again on muffs, it started running rough again. Eventually after quite a few minutes it cleared up. Even after it cleared up it seemed to revert back and forth instantaneously, like something was being shorted or shut off and on... not like air/fuel mix being leaned out or getting rich which would sound more gradual.

I'm really out of ideas.
Any help would be appreciated.

 

[Scott Danforth]

check the contents of your fuel filter

when was the last time your carb was taken apart and cleaned?

 

[achris]

Yep, sounds like some dirty floating around in the carb. Pull it, strip and clean it, put it all back together...

Chris......

 

[jtomanelli]

Thanks for the quick replies!
I changed both filters and actually completely drained the gas tank and put in new gas (old gas from last year in the tank).
The main gas filter was actually a "stone". I replaced with the proper quicksilver filter, but the bowl had some white goop and a little rust. I cleaned it all and reassembled.

I have not disassembled the carb. Sounds like that's my next thing.
One other item I forgot to note, when running on muffs and going through the tuneup procedure I tried setting the fuel mix screw to the recommended 1 1/4 turns out but it ran terrible. The sweet spot for me was 2 3/4 turns. Is this related? Of course I set this when the engine was running seemingly right.

 

[Scott06]

That main turns out is not typical, probably indicative of dirt in the idle circuit. Cleaning and rebuild kit as recommended seems to make sense here. Because the float is plastic coated foam I'd eccomend getting one with your kit. I had to buy one because it cracked when I took mine out. Mercruiser float at a marina was more than the carb kit. Also check your choke operation, these carbs they can tend to hang up and stay closed. I adjusted mine so lean it never closed and ran fine for years with out it.

 

[Rick Stephens]

With all the fuel related issues you have worked on, the carburetor is virtually guaranteed to be a mess. The proper procedures for rebuilding and setting the carb are contained in the Mercruiser service manual for your engine. Main thing though, get it really clean.

 

[Scott Danforth]

you can keep throwing parts at it and keep changing filters, it isnt going to fix the fact that your carb has crud in it.

and no, there isnt a majic mechanic in a can you can pour in to make it work

 

[jtomanelli]

So I removed the carb and opened it up. Not very dirty inside. Took jets out and cleaned inspected all the little galleys etc.
Will have to get the rebuild kit tomorrow and see what the results are when it's back together as I need the gaskets at the very least to test. Regardless, it likely needed a rebuild.

scott - I had verified the choke and it was operating correctly at cold start and warmup.

Interestingly, most of the times that I had these issues, it happened 10-15 min into a boat ride. Never when it's cold.

 

[achris]

Remove all the jets, venturis, float and needle and seat. Soak the stripped carb in carb cleaner (without Chlorine in it!) overnight. Next morning/afternoon blow all the passages out with DRY compressed air, about 50 to 80 psi... You may not see any dirt, but it will be there. And remember to blow all passages, in both directions.

Chris........

 

[Scott Danforth]

I like to pass a piece of fishing line thru the passages. For the ones that are cross drilled, i like to spray carb cleaner or wd40 in them so i can see that every passage is clean and clear before i put the carb back together

 

[jtomanelli]

Soooo since it's Sunday and I live in a small town, what would be a good alternative to carb cleaner to soak it in? I want to take advantage of today so that tomorrow I can finish the job.
Any help would be appreciated. I was thinking paint thinner ?

 

[Scott Danforth]

if you have a walmart, they carry berrymans carb cleaner. that and 2-3 cans of the red crc brake cleaner and your set

 

[Rick Stephens]

I've seen Simple Green used. Almost anything that will dissolve the deposits.

 

[jtomanelli]

OK so no luck on the Sunday, so it'll soak in the carb cleaner I picked up today for 24 hours.
Hopefully I can reassemble tomorrow and test it tomorrow night.
Will post result here of course.

 

[jtomanelli]

OK so here's what I've done and the status:

- dismantled carb wiith everything that was removeable removed.
- soaked in TYME for 24 hours
- purchased the Merc rebuild kit which includes many new parts and of course the gaskets
- rebuilt it tonight per the instructions and set all per instructions... some tweaking was required for the float settings


results:

-after a few cranks (to fill the bowl), she fired up and smoked a bit to start
-I set the fuel mix screw to 2 turns out (which is where the merc carb kit sheet recommended)
-after a warm-up and the choke opening up, best sounding setting was around 2 1/4 turns out
-it sounds good but still not perfect, I will do final settings on the water, hopefully tomorrow if weather holds out


I'll be pulling the plugs when it cools off and either replacing or at least cleaning them as I suspect they are fouled.
I'm still a bit concerned as most items I've read say to set the fuel mix screw to 1 1/4 turns out. Only the rebuild kit states 2 turns out to start.

As of now, and I ran it for a while, it sounds decent on muffs and it seems the rebuild was successful. Best test will be on the water hopefully tomorrow. Any thoughts/comments on the above?

 

[Rick Stephens]

Those mix screw settings are strictly recommendations so you can be close enough to start the engine. Then you always need to set them for your particular carb and motor. It would be pure dumb luck if the recommended setting ended up being correct for your motor.

If you still have running issue, next step is to do a quick compression check. That motor is 20 years old. Could tell you something about the condition of rings and valves.

 

[jtomanelli]

Hmm. It never occurred to me to check compression because when it runs good, it runs very good. I will check it regardless I think, good baseline info.
Anyone know what compression I should be expecting?

 

[achris]

Around 130, depending on your gauge. The important thing is the variation. No more than 10% between any cylinders.

Chris......

 

[jtomanelli]

Compression check yielded 160-170 lbs on all 4.
Checked with 2 different gauges to be sure.
I think she's ready for a lake test, just have to wait on good weather which looks to be a few days away

 

[Rick Stephens]

Compression check yielded 160-170 lbs on all 4.
Checked with 2 different gauges to be sure.
I think she's ready for a lake test, just have to wait on good weather which looks to be a few days away

Nice! That's gotta feel good.