'60 Merc 400 runs great for an hour then starves for fuel

[jiveturkey1]

Hi. loving my new old boat.
Having an issue though. She runs great for about an hour then at full throttle bogs down and dies. It'll start right back up and idle fine. Pumping the bulb will get it back up to speed for a while but not very long.

Here's some things I've done to it before or after it started to do this:
Installed a water separator (before)
Changed the bulb from horizontal to vertical (still using old bulb) (after)
New fuel lines to motor but not in the motor (before)
Did a Seafoam shock treatment (before)

My ideas or friends suggestions:

Rebuild fuel pump
New fuel lines in motor (maybe collapsing after getting warm?)
Gasket from carb to motor (letting air bypass after getting warm?)
Rebuild Carbs
All of the above

The mystery to me is why it runs good for an hour then gives up??

Thanks!

 

[Chinewalker]

Check the vent in the tank. Make sure it's open and actually vented. I had a similar issue recently where the pinhole in the cap was clogged. Cleaned it out and all was good...

 

[DeepCMark58A]

Is the gas tank full every time you go boating? If it runs fine for an hour then has issues the first place I would look is at the fuel tank. Pull the float/intake stem and inspect.

 

[jiveturkey1]

Thanks guys!
The vents are clear and it did the exact same thing with both tanks. I'll pull the stems though and inspect both.
I went ahead and ordered a fuel pump rebuild kit and two carb kits. I ordered them from a certain online store because that's the only place I could find the pump kit. Hopefully that wasn't a mistake after reading negative reviews and I couldn't get ahold of them. Anyway, I'm going to change the lines in the motor, check the tanks and the carb gaskets and report back. Thanks for the help so far!

 

[jimmbo]

You say it runs for an hour than dies? Was it at full throttle for the entire hour, or at part throttle for most of the time, then when you goose it dies. How low are the tanks when it dies?

 

[jiveturkey1]

The first time it was full throttle for about 20 minutes, idle for 30 full throttle 5 then died. Second time it was full throttle for 30 idle for 10 full throttle for 10 then died (approximately).
The first time I had both tanks hooked up to the water separator and they were about 2/3 full when it died. I read where using two tanks might be a bad idea so I re-plumbed it for one tank. The second time it was tank #2 and it was about 1/4 full when it died. The common denominator is using tank #2. The first time I had it out I used tank #1 with no problem (no separator either). Only had it out 3 times

 

[jimmbo]

Remove the fuel pickup assembly from the tank#2 and examine it very carefully for a pinhole leak. Try running it on tank #1 and see of it dies?

 

[jiveturkey1]

Will definitely do that. Thanks. Where do you get the clear pickup hose to replace?

 

[DeepCMark58A]

Couple of things I have learned with my 400 concerning the fuel pump kit. 1 only buy genuine mercury parts 2 make sure the fuel pump rebuild kit includes check valves.

 

[jiveturkey1]

Thanks. Not sure if it is or does. I'll find out if I ever get it.

Update:
I changed the fuel lines in the motor, bulb, cleaned screens in the carbs out.
Tried to start it up and nothing. Now not getting a spark.
I didn't mention that the third I tried to take it out it wouldn't start. Fired a couple times. I figured it was super flooded from changing the tank lines and pumping the bulb a bunch. It started right up the next day. Anyway, this old Merc is getting frustrating.

 

[DeepCMark58A]

Oh I hear you on the frustration I spent about a year trying to figure out exactly what was wrong with my 400.

 

[jiveturkey1]

I'm going to try cleaning the points. .020" gap?

 

[jimmbo]

my service manual says Point Gap 0.008" - 0.010"

 

[jiveturkey1]

Thanks jimmbo!

 

[jiveturkey1]

Do I need to take the whole magneto column off or just drop the cap from the bottom? It doesn't seem like there's much room to work under there.
Thanks for the help! This forum is great

 

[jimmbo]

To do any real work on the merc distributor it pretty much has to be removed. Do you have a service manual for your engine? If not get one

 

[jiveturkey1]

Yes. I downloaded one. I'll try referring to it more. There seems to be gaps in it because I'm not a mechanic.

 

[Canufixit2]

OK, a quick 2 questions - when it does die do you still have spark and is the flywheel too hot to touch? Flywheel may have become loose and rubbing causing heat that causes the magnets to fail until they cool off .... Happen to me ..

 

[Chinewalker]

OK, a quick 2 questions - when it does die do you still have spark and is the flywheel too hot to touch? Flywheel may have become loose and rubbing causing heat that causes the magnets to fail until they cool off .... Happen to me ..

Flywheel magnets are not involved in ignition system on these motors. Motor's have a separate self-contained magneto.

 

[Canufixit2]

"Flywheel magnets are not involved in ignition system on these motors. Motor's have a separate self-contained magneto."

Not true. Magnets provide the trigger pulse to the trigger pickup for the switch box.

Magnetic field passes through the Magneto coils and is picked up on the inside of the magneto coils by the trigger coil plate.