50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

[jpate9]

This is my first post here and I don't know if this has been asked before. I have a 1971 Merc 500 thunderbolt, electric ignition. I just bought the boat a couple of months ago. I tried the engine out in neutral in a trash can of water. It seemd to run just fine. I could rev the engine at will. I tried burning off as much of the old gas as possible this way. I added 10 gallons of high octane gas with the 50:1 ratio of oil. Still ran great in neutral.

We took it out to the lake and it puttered around the no-wake zone. It seemd to sputter some but got us to the lake to open it up. When I finally gave it throttle in gear, it started to stall. I pulled back on the throttle and tried running in sputter-mode to burn of as much gas as I can. I tried the throttle again and it stalled completely. Then I had difficulty getting it to start again. It seemed like it was flooded.

After waiting a while. It started again and ran great in neutral. Each time I tried throttle in gear the cycle repeated. We took it to a certified mechanic and he changed the spark plugs which were fouled. We thought we had it fixed and went back to the lake. The cycle repeated once again. The mechanic suggested rebuilding the carbs and gave us a very scary quote.

I've looked at the oldmercs website and saw that the rebuild kits are pretty reasonable so I think I might try doing that myself. It would be a nice project.

Today a saw a thread that mentioned "timing is synched to carb/throttle advance" and was wondering if this may be important.

My first question: Is there something different with the throttle / timing / fuel when an outboard is in neatral as opposed to in gear? The control for this boat has a separate lever for trhottle when in neutral.

Next Question: Could this be something else? I am going to try taking out the spark plugs and see if there is any resistance in the lower unit when I am in gear.

Please Help and Thanks in Advance.

 

[flabum]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

Your mechanic is probably right in saying the carbs need to be rebuilt. If you want to do this yourself, get a service manual.

 

[JB]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

Howdy, jpate9.

Welcome to iboats.

Never believe an outboard running without a load on it. They can seem to be running fine on one cylinder.

You may be running on dead cylinders, you may not be opening the throttles on the carbs all the way, you may be out of synch between carb and ignition.

My crystal ball says you have one or more dead cylinders. With the engine in the tank and at idle, disable each spark plug in turn. If a cylinder is making power that will drop the idle or kill the engine. If it does not affect the idle speed it is a dead cylinder.

If you have a dead cylinder check it for a hot, blue spark across an gap of about 7/16". If it has good spark, squirt some fuel mix (NOT ether) into the carb for that cylinder. If the idle picks up that carb is not working correctly.

Let us know what you find.

 

[jpate9]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

Thanks for the input. This is great advice. I just purchased the Mercury Service manual last night. I can rebuild the carbs and fuels pumps without too much problems. I seem to remember that one of the spark plugs was less fouled than the other three. Maybe that cylinder is shot.

Here is the plan:

1. replace fuel: Lost out on 10 gallons Yikes!!!
2. check the fuel lines: The primer bulb seemd to stay pretty hard but I think I can go thru and make sure everything is all right.
3. Rebuild carbs
4. Rebuild pulse pump
5. check timing: I was exercising the throttle in gear, motor off, the other day and it seemed to me that the whole distributor was rotating or moving along with the throttle. What's all that about? Is that supposed to happen?

Am I missing anything?

As you can see I'm brand new to boating and outboards. I only paid $800 for the boat so I think this will teach me a lot.

Thanks again everyone.

 

[JB]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

I really think applying fixes before determining fault is a waste of time and money and often introduces additional problems, jpate9.

Please isolate what is wrong first. Then we can help you with a fix that fits.

 

[jpate9]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

I ran the motor again in the water tank to try and re-create the problem. First I took out all the spark plugs and put it in gear. There was hardly any resistance. At least it didn't seem to be unduly difficult. I bought a hand pump to replace the lubricant in the lower unit.

Then I saw that there were several greease fittings around the distributor. I added grease until I saw it coming out. It took some lubricant before it came out so that may have been dryer than it should have.

I started the engine. I put it in gear and it ran fine for a couple of minutes. I repeated the process and it stalled out on me. It put out a lot of bluish white smoke as I tried to modify the throttle and keep it going. It stalled out.

I waited a few minutes and tried it again and this time it chugged on through in gear. As I let it run more and more it seemed to heal itself. I let it run as much as I could but my water tank is such that If I keep it in gear I don't get any water in through the impeller.

I then removed the prop the next day. Repeated everything as before and it ran like a champ. I had a constant stream through the pee hole. I was in gear at varying throttle settings and I burned a quarter tank of gas.

Could the lubing of the distributor made that much of a difference? Or is it more likely that there was some water or krudd that I had to burn up before it started behaving?


Thanks

Jim

 

[JB]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

Reread my post, Jim.

With the engine at idle, disable each spark plug in turn. If the idle does not drop when you take that cylinder out, it is a dead cylinder.

Then test any dead cylinders for a hot, blue spark reaching about 7/16"

Then test any dead cylinders by spraying/squirting some fuel premix into that carb.

No-prop is the same as neutral. . no load, and means little.

 

[jpate9]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

Will do.. I'll post after I've tried it.

Thank You.

 

[tmcalavy]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

Aside from what has already been recommended to diagnose your problem, take the engine cowl off and check all the wires...they corrode/degrade with age but are easy to replace. If compression and the other basics check out, make sure the fuel lines/bulb are working correctly as well as the fuel pump. If you're going to pull/clean/rebuild the carbs, you will need to do a link-n-sync after you reinstall them...to synch the timing/throttle advance to the carbs. I had a 72 Merc 500 and it acted just like this after I rebuilt the carbs, not knowing about the link-n-synch. Paid our local marina/boat mech $50 for that and she's still running to this day...from what I hear from the guy who bought the boat/outboard.

 

[hkeiner]

Re: 50 Hp Merc Runs Fine in Neutral But Stall w/ Throttle

You have received some excellent advice that should help you solve your stalling problem. If it does not, I would suggest as an additional check that you test for fuel starvation during engine stall/bog. Mercury has issued a service advisory that describes how to do this. Knowing if there is fuel starvation during engine bog/stall will help you to either definitely exclude or include a group of possible fuel related culprits.


http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/2001/EN_17.PDF