Merc 3.0 with 2BBL Mercarb starts and dies - details in post

[shrews]

Hi,

I've seen a handful of very helpful threads here over time and am extremely thankful for this resource.

Now I'm having a specific issue and have gone through all the testing I could following archived threads but I can't seem to get it right and am posting in hopes you can guide me specifically. Thank you kindly in advance.

I am a former foreign auto mechanic, so I have some competency, but this is fully escaping me.

This is a friend's boat that sat for 5 years (all because a faulty ign switch. I wasn't in town or it would've been running ages ago!). We rebuilt the carb once about 10 years ago and it ran like a champ until they parked it and let it sit.

Now the issue is:

Fixed ignition switch and fired it up. Ran pretty rough so decided to rebuild the carb. Now it requires starting fluid and only runs for a few seconds then dies.

Replaced the fuel filter and it ran for a full minute before dying, but now it's back to the same symptoms. Disconnected the choke and operated it manually and was able to keep it alive a little longer, albeit a bit rough. The issue with the choke is that it's electrical and starts to open as soon as the switch is on. Aside from checking the choke -- again -- do the coils go bad? Is a new choke something we should consider?

Merc 3.0L (serial not available at the moment)
2bbl Mercarb
Mechanical fuel pump

Checked float height 200 times (maybe only 5x) - always with in a mm of spec
Checked needle and seat a few times, both seem good as can be
Running gas straight from a clean bottle, not from fuel tank
Carb pumps 2 solid streams when primed
Replaced fuel filter

I may have forgotten something I've already checked, as the laptop and fingers covered in grease do not happen at the same time, but that's the gist.

Thanks again in advance!

 

[alldodge]

Howdy

First your working with the carb, so is it flooding?

If not flooding and getting gas, next I would check compression of the motor.

If compression is good, not flooding and you can keep it running by hand choking, it might be the fuel pump. Need a vacuum fuel pump tester and install and Tee between the pump and carb. The pressure should be 4 to 7 psi.

What do the plugs look like?

 

[shrews]

Thanks for the quick response! I'll dig out the vacuum gauge and check the plugs and compression ASAP (boat is not at my place).

It is getting gas; when I pump the accelerator I can see it jetting in. I don't see fuel leaking in between sarting attempts. How can I tell if it's flooding otherwise?

 

[Bondo]

How can I tell if it's flooding otherwise?

Ayuh,.... Welcome Aboard,..... Are the spark plugs Wet,..?? or dry,..??

 

[NHGuy]

It only runs on the starting fluid then dies. At that it runs rough. I'd say the roughness is "cobewebs" in the actual combustion chambers.

But the short externally fueled runs point to lack of fuel from the carburetor's normal fuel supply.

I did see where you are confident of the float level, the needle and the seat. And that is borne out by the functioning accelerator pump. It does seem that there is fuel in the bowl. If there was no gas there would be no squirt.

Focus on the circuit that feeds the idle and primary venturi system. It's gotta be clogged or closed off.

Check idle feeds, jets,and any variable metering system the carb may have.

After you rebuilt the carb did you set the idle mix screw(s) up off their seats so SOME fuel could get through til you fine adjust?

 

[NHGuy]

Oh by the way. Welcome!

Please honor us with your updates and solution. Others will find these posts and benefit.

 

[shrews]

Thanks everyone for the ideas, and for the kind welcome! I absolutely aim to continue posting until this problem is solved, including the solution. I'm currently scheduling a time to get over there and run down these tests. I'll return with the results. One thing I should clear up, however:

It only runs on the starting fluid then dies. At that it runs rough. I'd say the roughness is "cobewebs" in the actual combustion chambers.

But the short externally fueled runs point to lack of fuel from the carburetor's normal fuel supply.

I may have slightly mis-described the scenario. I'll have to check again, but in the past it would run after priming alone (no starting fluid needed), but dies shortly thereafter. I've been using the fluid because it sits for a few weeks between work sessions.

An earlier issue that seems to have turned into the current starting-then-dying problem was that it would idle okay indefinitely, but die on acceleration. I'm not sure what, if anything, changed other than a few times trying to start it about a week or two apart. It used to hold idle, but now won't even do that.

All that said, when we swapped the fuel filter last weekend, it idled for about a minute before dying. And when it's running, it sounds great! I checked and double-checked the jets a few times, and can't see any issues there. But I am open to trying again!

I hope that helps clear up the symptoms. In short: rebuilt carb, would idle, but then die on acceleration. Now doesn't hold idle, though did for about a minute after changing the fuel filter. Gas is coming clean from a temp source. Manually screwing with the choke kept it running (roughly) the last time I fired it up, even under mild acceleration. Pretty sure I pulled the idle mix back a little, but will re-check that, too.

Thanks again! I'll post back re: plug condition, vacuum readout for fuel pump, and mixture screw. I'll also re-check the carb but beyond checking every jet and passageway I can find, I'm not a carb pro and can't be certain I understand the intricacies as suggested below, though I seriously cannot find a clog. It wasn't in too bad shape to begin with.

Focus on the circuit that feeds the idle and primary venturi system. It's gotta be clogged or closed off.

Check idle feeds, jets,and any variable metering system the carb may have.

 

[NHGuy]

I'm glad you re commented on " Disconnected the choke and operated it manually and was able to keep it alive a little longer, albeit a bit rough. " I didn't take that into account with my earlier recommendation.
Seeing that makes me consider the possibility that there could be a leak between the carb and the engine. Or a leak in the carb itself.
Make sure all the intake manifold bolts are torqued and there are no open ports in it. Carburetor hold down bolts too. Then if it will run spray bursts of carb cleaner at the carburetor pivot points where the throttle shaft enters the carb and where the choke shaft does too. Or anywhere there could be undesired air entering. Also spray around where the manifold contacts the head and where the carb contacts the manifold. If any of those spots causes an increase in rpm you have found a vacuum leak.

Still it probably does have dirt in the carb. And likely in the fuel. Remove all the old fuel and get the tank perfect or you'll be chasing your tail.

 

[shrews]

Thanks NHGuy!

Just had a round of testing.

Plugs are a little black, but nothing major and a light touch with my finger took the black off. Also, after a few bursts of running, the plugs are dry.

Borrowed a pressure gauge but it didn't come with the right fitting for the carb, so I did the next best thing, ran the tube into a clear bottle. The pressure seemed to be fine and shot fuel into the bottle, but not too forcefully.

With a little goose-ing and pumping, I was able to get the engine to idle after starting (high, but smooth). This was long enough to shoot carb cleaner at the carb gasket, and indeed, the idle shot up. I killed the engine and tightened the carb, but haven't been able to get it fired again since then.

In my automotive days I used to smear grease on both sides of the gasket before putting it on the manifold, which I did not do this time. Still, with the carb pretty dang tight on the manifold, I can't imagine the leak being so drastic it won't idle. But do you think I should get a new gasket and add a little grease?

I hear you on dirt in the carb, but I've disassembled it 3x and can't find anything. Short of buying a new or reman, I'm not sure what else I can do.

Fuel is coming directly from a clean plastic bottle, bypassing the tank altogether. Idle is 2 turns in from seated, and mix is 1 1/4 out from seated.

Any ideas?
Thanks!

 

[shrews]

I bought a house that turned out to be MUCH more than a fixer-upper so it took me a while but the weather was perfect to spend some time with the boat.

I blew carb cleaner near the base gasket and I did rev up. So I pulled the carb, cleaned the heck out of the seat and base of the carb and installed a new one with a thin smear of grease on both sides.

Same problem: idled super rough, then died immediately when throttled.

Beyond pulling the carb off and apart for the 3rd time, any ideas? (In the meantime, I will get a new gasket kit for the carb and start over there next time). I can't even get it to idle well enough to check for more leaks.

Thanks so much!

 

[Bondo]

Beyond pulling the carb off and apart for the 3rd time, any ideas?

Ayuh,..... Seein's you've confirmed the leak, pull it again, 'n see if the throttle shafts are leakin',....

They can wear out like that,.....

 

[shrews]

Thanks Bondo!

How do I check once the carb is off? And are they repairable?