5.7merc stalling at low RPM after warmed up.

[ScotWithOne_t]

Boat starts without too much drama. It's always needed a couple pumps of the throttle and a slight bit of cranking when starting cold. Starts easily once warm. It will sit and idle for 10-20 minutes in the driveway just fine, however after taking it 5 miles up the river at 3000ish RPM, it refuses to idle, or drive at slow, no-wake speeds. This becomes a huge pain coming into the dock. Oddly enough it didn't stall in reverse.

After doing some reading it seems like it might be dirty low speed idle jets in the carb. If that is so, wouldn't it have trouble idling in the driveway? Also, leaving the boat launch, I was able to go slow. Wouldn't this be inconsistent with dirty idle-jets?

Didn't have this problem 2 years ago (last time I used the boat).

I've never rebuilt or tuned a carb before, so this might be a "throw-money-at-it" type of situation. Unless I'm completly barking up the wrong tree and it's something else entirely. Ignition? Bad gas?

What are some easy things to check before I start researching how to rebuilt a carb, or start calling shops for quotes?

Thanks.

 

[alldodge]

Boat starts without too much drama. It's always needed a couple pumps of the throttle and a slight bit of cranking when starting cold. Starts easily once warm. It will sit and idle for 10-20 minutes in the driveway just fine, however after taking it 5 miles up the river at 3000ish RPM, it refuses to idle, or drive at slow, no-wake speeds. This becomes a huge pain coming into the dock. Oddly enough it didn't stall in reverse.

After doing some reading it seems like it might be dirty low speed idle jets in the carb. If that is so, wouldn't it have trouble idling in the driveway? Also, leaving the boat launch, I was able to go slow. Wouldn't this be inconsistent with dirty idle-jets?

Didn't have this problem 2 years ago (last time I used the boat).

I've never rebuilt or tuned a carb before, so this might be a "throw-money-at-it" type of situation. Unless I'm completly barking up the wrong tree and it's something else entirely. Ignition? Bad gas?

What are some easy things to check before I start researching how to rebuilt a carb, or start calling shops for quotes?

Thanks.

How does the boat shift?
Check the shift interrupter switch and determine if it needs adjusting.
Is the carb choke opening all the way?

 

[ScotWithOne_t]

How does the boat shift?
Check the shift interrupter switch and determine if it needs adjusting.
Is the carb choke opening all the way?

It shifts just fine. Interrupter switch seems to be working correctly. It doesn't really seem like that would be the problem though, since it likes to die even when throttle is pulled back to the first dedent (is there a technical term for this throttle position?)

What should the choke plate be doing? I can only really check it when the engine is cold, and it didn't have a problem at cold temps. Only after running a few miles under load. Does the choke act differently at different engine temps? Wouldn't the choke being open cause it to lean out and run faster, not bog and die?

 

[keith2k455]

The idle circuit is just that....for idling. Your carb jets are for throttle. Try warming it all the way up and adjust your idle circuit. It sounds like it may be leaned out. You will want a vacuum gage to set it correctly. You will aim for 1) a smooth idle at 2) the correct rpm, the only thing I don't know is 3) how many inches of vacuum for your motor.

 

[ScotWithOne_t]

Forgive my carburetor ignorance, but what is different when it is hot vs. when it is cold?

It didn't run like this 2 years ago, which is what led me to believe that the old gas has possibly clogged an idle port or something.

 

[keith2k455]

You may be right and have gummed something up from sitting, so maybe doing a whole bunch of adjustments isn't going to help anything. If you do adjust the carb, you want to do it warm because the choke will have fully pulled off, which is where the motor runs most of the time.

Speaking of choke, are you sure it opens up fully when the motor is warm? What about fuel, have you filled up with fresh fuel and still have this problem?

 

[NHGuy]

Go over the caburetor hold down nuts and bolts, also the ones on the intake manifold. Snug them, if you find any loose give the motor a run after tightening them. Try loosening the gas cap after that 3000 rpm run, if you hear a big vacuum sound there's a clog in your fuel vent. Also try shutting the boat off for a minute or 2 after the 3000 rpm run. Will it idle after a momentary shutoff? If it will, the fuel pickup could be temporarily holding some piece of debris on there that it is picking up during the high fuel demand.
One band aid that could work would be to get it running and kinda warm. Can be at the dock, or on muffs. Then shut it off, take the throttle linkage off. restart it and raise the rpms to 12 or 1400 with your hand on the lever. Now while it's revving shoot some bursts of carb cleaner in the throat of the carb, enough to stumble it a little, but not enough to kill the engine. You can also run some fuel system stuff through the tank, I like Seafoam. I think you want it in there with 10 gallons of gas. It will clean any varnish that's in there, including the idle jets and circuit if they are not completely clogged. If you are lucky it might fix you up, if not you might want a carb service lesson.

 

[ScotWithOne_t]

My coworker mentioned vapor lock. Any thoughts on that? Since I last filled it with regular pump gas and not the non-oxygenated stuff. He said regular gas will absorb more water or something.

 

[NHGuy]

Vapor lock is a very good possibility. Try opening the motor box to allow the whole engine area to cool after your 3000 rpm run. See if the motor can idle once it gets some extra cooling going.
Choke should be completely open when the engine is warm, that means the plate is not restricting air flow at all.