Stalling Under Load

[denblaq]

I have a 1963 Merc 500 50hp that stalls as soon as you put it in gear while in the water. It will idle fine but not take a load. Sitting on the trailer with a water hook-up it will runs fine in gear and will throttle up to high rpm's (I do this quickly and do not keep it at high revs). I have a service manual and all the settings seem to be correct. any ideas ?
Regards, Denis

 

[Rayzor]

Re: Stalling Under Load

Sounds like you have some plugged jets in the carbs. Its getting enough fuel unloaded to rev, but starved for fuel to go up in rpm under load. Have you cleaned the carbs lately?

 

[denblaq]

Re: Stalling Under Load

Have not been into the carbs, do I need to remove them ?

 

[Rayzor]

Re: Stalling Under Load

Yes they need to come off.
Pay attention to how things come exactly to keep from messing up your settings. I would try pulling one carb off at a time. Remove the float bowls only to blow out the jets. safe glass are a big plus here as stuff can rocket out of little ports.

 

[quicktach]

Re: Stalling Under Load

Since it rev's up on the trailer but not in the water - loading could be an issue. Have your tried making the carbs less lean - 1/4 turn out at the same time - each ? Do the throttle plates move and position equally throughout the throttle range from idle to WOT? More details on the motor - did this just start happening ? Is the motor "new" to you ....if not, do you know it's history ... what's been done to the motor ? Done a compression test ?

 

[denblaq]

Re: Stalling Under Load

After mulling over the stalling issue I did a root-cause as to what could be the problem. The boat is new to me, I bought it up in Michigan from a former neighbor so I know the history. The day I picked it up we ran it around the lake awhile before loading it on the trailer. The only problem with the motor was that it would stall at idle either in neutral or forward gear. If you put the gas to it as soon as you put it in gear it would run like a scalded cat ! So I figured an afternoon with a service manual, which I have the proper one, I could check adjustments etc. and solve the problem. So I haul it to Florida and proceed to to install a new impellor, fresh gear lube and give the motor a good going over. As I thought, some of the carb adjustments were off, i.e. idle and throttle limit. The low speed needle screws were fine. After setting everything properly it idles fine on the trailer or in the water in neutral or forward gear but stalls when you put the gas to it. What gives ? If it ran good up north it should run good in Florida now that the carbs are adjusted properly, right ? So what changed, what was added or subtracted from the system after I got it to Florida ? Being a proactive type I added a spin on fuel/water separator in the gas line. The motor never had any external fuel filter in the line and relied on the strainers in the carbs to do the job. I'm thinking the filter is acting as a restriction under load or the fuel pump is not up to the task. I will isolate the filter from the system and see what happens. Any thoughts ? Regards, Denis

 

[CharlieB]

Re: Stalling Under Load

Idle mixture is critical to get a decent idle quality.

Idle mixture MUST BE adjusted with the boat IN the water and IN Forward gear, this is MANDATORY to have the correct exhaust back-pressure and the correct load on the motor.

One at a time, adjust each idle mixture screw very S L O W L Y so to give the motor plenty of time to burn off any excess fuel, adjust too 'Best Idle Speed' IN Forward gear.

If the motor idles too fast then use the idle speed (timing) to reset idle speed.

Once at this 'Best Idle Speed' IN Forward gear, the motor needs even more idle fuel mixture to provide the additional fuel necessary for crisp acceleration.

Open each idle mixture screw an additional 2 widths of the screwdriver blade slot in the screw.

Now you are ready to test acceleration, IN Forward gear at an idle, rapidly open the control and accelerate the boat.

ANY bog, lean cough, miss, hesitation, shows the motor needs even more idle fuel, again, open each idle mixture screw just one blade width and repeat acceleration test.

Repeat to perfection.

Done.

If the carbs are not spotlessly clean, take them apart, clean and reassemble using NEW gaskets. Make double sure the float heights are all the same and within the range for your serial #.

 

[quicktach]

Re: Stalling Under Load

The fuel filter shouldn't cause any significant restriction. I think you're still starved for fuel on acceleration. Try increasing the idle speed too. Sounds like nothing changed between Michigan and Florida since it was stalling up there. One thing you may eventually want to try before you go taking off carbs is a spray tin of decarbonizer or "mechanic in a can"....Go at the idle adjustment again, under load. If none of this and the advice others have given, you will probably be into a good carb cleanup and maybe even timing adjustment.

 

[denblaq]

Re: Stalling Under Load

I plan to take the Rear Admiral's advice regarding the adjustment procedure. I will also reattach the fuel/water separator at the same time. If all goes well my problem will be solved, if not a spray tin of decarbonizer will be on my shopping list. A last resort will be removal of the carbs. I am well versed cleaning old carbs as I have been a motorcyclist for 40+ years mostly riding old Harleys. It looks as if the hardest part will be getting the carbs off the motor.