1984 mercruiser 140 missing/ rich idle HELP

[Cmhust89]

Boat is a 1984 17' Sunrunner with a 1984 mercruiser 140 inline 4cyl 3.0L engine.

Single adjusting screw mercarb 2bbl

I purchased The boat about 3 months ago, the first couple times I took it out it ran great, cold started good, didn't miss fire, idled smooth, restarted warm good.

I noticed one day gas was very minor seeping out of the float gasket between the 2 carburator half's. I purchased a rebuilt carburator, then swapped it out with another rebuilt carburator and can't get it to run right at idle or starting.

The engine seems to be running rich at idle, you can smell gas and iv tried different plugs and they are always all black. it will not start at all just caughts cold (when the choke is on). If you turn the key on to let the choke preheat and open it will start once choke is fully opened but still is missing at idle and smelling rich exhaust. Once off of idle it runs smoothly through out the throttle range. It will restart good warm if choke is fully off but still be missing and stumbling.

Idle mixture adjustment that's supposed to be at 1 1/4 out has almost zero effect on running condition except if you screw it past 2.5 turns out it will really flood and die. You can screw it all the way in lean and it has little to no effect. it will still be running and missing.

Cap and rotor look Good- somewhat new
Plug wires look recently replaced

Plugs are new oem

Distributor has petronix electronic ignition

Compression is 132-133-132-132

Spark timing has little effect on missing at idle/ rich idle. I've tried 6° through 14° advance. Spec is 8° seems to do best at 12° advance.

yesterday I installed the spring loaded needle and seat it didn't help

​​​​​​I've played with float adjustments and adjusted then with a micrometer and they were already right on spec

Can you help me find what I'm missing?

 

[achris]

Check the fuel pump pressure. Should be around 6psi. If it's too high, it may be forcing the needle off the seat and flooding...

 

[Cmhust89]

I disconnected the carb fuel Inlet line and deadhead it into a gauge. Key on it reads 5psi. It's an aftermarket electric pump. Oem mechanical pump has been blocked off.

 

[Cmhust89]

I'm going to try a valve adjustment and pull the distributor to see if I can see anything wrong with the timing advance mechanism

 

[fishrdan]

I'm going to try a valve adjustment and pull the distributor to see if I can see anything wrong with the timing advance mechanism

I wouldn't, since the engine was running fine before the carb swap, issue sounds like card/fuel. BTW, you don't have to pull the distributor to check the advance weights/springs, just remove the points plate with the distributor installed, then you can see the advance mech, clean and lube as necessary.

How did you adjust the carb and what is the idle RPM. 1 1/2 (1/4?) out on the mixture screw is it's starting point. Set the idle speed to 700-750 (should drop into spec when on the water), then adjust mixture to best idle, readjust speed, readjust mixture, etc, etc, until it's idling best. Final idle adjustment needs to be done on the water, with the engine completely warmed up, and the engine idling in gear, same as above mixture-speed-mixture-speed-etc-etc, until it's idling best.

Timing should be set to 8*, running the engine under load at 12* can damage the engine, detonation - melted pistons.

Does completely (lightly) closing the idle mixture screw kill the engine? It should, and your post didn't sound like the engine was dying with the mixture screw closed off. - Idle too high, fuel pressure, vacuum leak, internal carb problem.

 

[Cmhust89]

Idle speed is 650-700

With the idle mixture screwed all the way in, is when it idles best and has the least amount of random misfires. It does not die with the screw all the way in. If I start backing the idle mixture screw out, with the engine running by 1 turn out it will start sputtering alot more and will die by 1.5 turns out.

When mixture screw is all the way in if you place your hand over the top of carb when running ( acting like a choke it will sputtering heavily) it also feels like it has good strong vacuum at the top of carb and I've sprayed starting fluid at manifold and carb base make no change. So I think the gaskets are good.

If you mist for 1/2 second starting fluid over the carb when running it will kill it.

This is the second carb iv tried from some big carb online retailer in Florida.

Reason I was maybe thinking distribution was because with the timing light it runs better as it's advances it sounds alot better at 12 than 8. Also timing mark does not seem to stay put, with the soightesl movement of diatdistrib like 1/4 to 1/2 inch it looks like the timing advance goes up to and over 16° advance.

 

[achris]

...Reason I was maybe thinking distribution was because with the timing light it runs better as it's advances it sounds alot better at 12 than 8. ...

That's normal. As you advance the timing, the engine will feel smoother. Until you load it up at advanced revs, then it'll blow holes in pistons. Set it at 8 BTDC and move on...

Chris......

 

[devildogae]

I had a "pro" who owned a race shop build my carb. It was very gunky. He cleaned it and reassembled it. I installed it and it ran like poo. I decided to get another rebuild kit and do it myself. Bought a sonic cleaner, filled it with simple green and put the parts in. Used compressed air to blow out small holes, made sure the check balls were out. Sonic cleaner, then water, then compressed air. Kept doing that over and over. I got a ton of crap out of those little holes and small placed where the check balls reside. Put it together, making sure I followed the instructions on setting the float bowl settings and inspecting the operation of the parts, condition of needles. I put back on with new gaskets, dry, and torqued down to specs.

Its the difference between Day and Night. You will always do better doing it yourself, so you know it was done right.