Mercruiser 3.0 won't throttle up

[Kailofsouls]

I have an older mercruiser 3.0 140hp sn 6720261 that has been giving me issues my whole camping trip.
So I've been trying to get it running correctly, but every time I throttle up in neutral it stalls. On the lake it won't go over 2000 rpm. If I go VERY slowly it will rev up and roar like a lion. But still not over 2000 rpm in gear.
Here's what I've tried. Checked timing with light it's right between 8-6° btdc, pulled carburetor full tear down and cleaned with canned carb cleaner. Reinstall carb at 1-1.5 turns out on idle mixture screw it will back fire though carburetor. After 2 turns and up it no longer back fires, but stalls on throttle up.
Any thoughts would be helpful thanks.

 

[Bt Doctur]

accelerator pump not working ? Too lean or choke not working correctly?

 

[Kailofsouls]

Accelerator pump works. I think my choke doesn't work, but it is full open. Running warmed up it Doesn't seem to matter how far I open the mixture screw it stalls on revving motor.

 

[Grub54891]

Start With the basics. Is there water in the fuel? Are the filters clean? Choke should be open when warmed up. Have good spark? Check the dizzy cap for corrosion. Was it ok the last time you used it and, how long did it sit till you used it now? Is it a points distributer or electronic?

 

[Kailofsouls]

Fuel is 3 weeks old rec with a can of b12 chemtool. Filters are clean. Choke is always open unless i close by hand. Distributor cap, rotor, points, and condenser are all brand new looking as well as wires and plugs. It seemed ok last year if a little boggy. Only thing I can find is a very slight ridge on the mixture screw. You can see it, but not really feel it.

 

[Kailofsouls]

Upon monkeing around I've found that after I rev slowly in neutral and back back off till just before popping it back to center the rpms stay at 1300-1800 and don't come back down till I put it back center. Is that normal operation? Center idle is 600-800 rpm. Motor does shake a fair amount at the lower rpm. I checked the distributor weights and springs seem to be fine.

 

[nola mike]

Check compression

 

[Kailofsouls]

I'll try a compression test. Will have to rent a gage tomorrow.

 

[Bondo]

I'll try a compression test. Will have to rent a gage tomorrow.

Ayuh,...... If you get good numbers, go back to the carb, 'n do a real rebuild on it,......

 

[Scott06]

Make sure there is not a vacuum leak at carb base gasket if non go back and rebuild carb again

 

[Grub54891]

Is the choke hooked to any springs, Or the heated element? Just a thought this morning. If it's floppy it could be slamming shut when throttling up and loading with fuel.

 

[cyclops222]

You have run the engine down to death.
Get a mechanic to fix all those problems.

 

[nola mike]

You have run the engine down to death.
Get a mechanic to fix all those problems.

Not sure if you actually realize the point of this forum, but it's DIYers looking for advice on how to fix their boats. "Get a mechanic" is NEVER the answer here. There is nothing on a carb'd 3.0 short of machine work that can't be tackled by a shade tree mechanic.

 

[nola mike]

Upon monkeing around I've found that after I rev slowly in neutral and back back off till just before popping it back to center the rpms stay at 1300-1800 and don't come back down till I put it back center. Is that normal operation? Center idle is 600-800 rpm. Motor does shake a fair amount at the lower rpm. I checked the distributor weights and springs seem to be fine.?

Are you doing all of this revving in neutral (not indicative of anything) or under load? The 3.0 is not smooth, but shaking makes me think that you're not running on all 4 cylinders. While fuel issue is most likely, a pooched valve/other mechanical issue could cause those symptoms as well.

 

[cyclops222]

Pass

 

[Scott06]

Not sure if you actually realize the point of this forum, but it's DIYers looking for advice on how to fix their boats. "Get a mechanic" is NEVER the answer here. There is nothing on a carb'd 3.0 short of machine work that can't be tackled by a shade tree mechanic.

Exactly, and the guy is on vacation using the semi working boat. Either leave a helpful answer or move on...

Only other thing I might add to OP would think it is carb related but also check that timing is advancing - Dont know the total advince of your exact engine but shooting from the hip if you have 8 BTDC at idle would look to be 25 BTDC at 2800 rpm. Dont think this is your issue but is a good rule it out check. Ideally done with an advance timing light but you could swag it by how much it moves on the balancer when rpms come up.

 

[ROY WILLIAMS]

I'll try a compression test. Will have to rent a gage tomorrow.

PISTON COMPRESSION 140PSI. TIMING 2DEGREE BTDC. 600 IDLE RPM.

 

[Kailofsouls]

Thank you for all the responses. I'll try to answer all of them in this post.

There shouldn't be a leak at the gasket it's new and has some gas rated rtv on it.

Unfortunately I can't get a compression tester, but it runs at 600rpm so I believe it should be at least passable, but maybe not.

The choke doesn't just flop around but I can move it by hand and it stays where I put it revving or not.

I'm hoping I don't have a pooched valve but it almost feels like it's not firing on all 4 till I get the rpms up over 1000. I am doing the revving in neutral because in gear it does the same thing, and the rpms fall off at 2000 in gear. I don't want to be stranded in the lake testing so I'm doing all this at the launch on my trailer.

I have a timing light with advanced from harbor freight, but I'm not sure how to use that feature so i keep it set all the way down. The stickers on my valve cover says 6° btdc if I go below that it won't start at all slightest twitch of the distributor and no start for me.

I hope I answered everything worth responding to clearly. Unfortunately now my battery is dead so I guess it's done and over till I get home and charge it. But I'm still looking forward to any more ideas from everyone. Thanks again

 

[nola mike]

Thank you for all the responses. I'll try to answer all of them in this post.

There shouldn't be a leak at the gasket it's new and has some gas rated rtv on it.

Unfortunately I can't get a compression tester, but it runs at 600rpm so I believe it should be at least passable, but maybe not.

The choke doesn't just flop around but I can move it by hand and it stays where I put it revving or not.

I'm hoping I don't have a pooched valve but it almost feels like it's not firing on all 4 till I get the rpms up over 1000. I am doing the revving in neutral because in gear it does the same thing, and the rpms fall off at 2000 in gear. I don't want to be stranded in the lake testing so I'm doing all this at the launch on my trailer.

I have a timing light with advanced from harbor freight, but I'm not sure how to use that feature so i keep it set all the way down. The stickers on my valve cover says 6° btdc if I go below that it won't start at all slightest twitch of the distributor and no start for me.

I hope I answered everything worth responding to clearly. Unfortunately now my battery is dead so I guess it's done and over till I get home and charge it. But I'm still looking forward to any more ideas from everyone. Thanks again

You can try putting the timing light on each plug wire while it's running to see if you're getting fire to the wire at least. Or use an in line tester if you have one.

 

[Kailofsouls]

If I can get my hands on a battery charger I'll try that.