TOPIC: 1958 Mercury Mark 78 won't rev past half throttle

We are working on a 1958 Mercury Mark 78 dockbuster and when under load, it won't rev past half throttle. The throttle blades and choke plates are all the way open but there is no response past half throttle. It's running 3/8" fuel line and the primer bulb is firm when running. We did a compression check that came in about 80# cold on all the cylinders. We also went through and disassembled and cleaned the carburetors with the exception of the welch plugs. The prop is the original QuickSilver A-48-28269A1. When it's running with no prop and using a hose for the cooling water, it appears it revs to full throttle. We also adjusted the trim on the motor to both extremes and in the center with no change. We're using it on a 15' 1958 Redfish.

Does anyone have a suggestion as to what might be the cause?

It 'sounds' to me like she is running on just (3) cylinders.
Every other cylinder is fired on one half of the dual coil - dual point ignition.
Start pulling spark plug wires while she is idling & see which (3) are not firing.
doc

Thanks for the quick reply.

We did check each spark plug for spark while cranking (not running) the motor and they all had spark.

Pull spark plug wire under load to make sure each cyl is firing and carrying a load.

Too little spark advance can cause issues, you either need the Merc special tool or setup a dial indicator to check/adjust max spark advance. Distributor must rotate and come up fully against its spark advance stop.

www.maxrules.com/timingindex.html


A prop with too much pitch can cause slugging operation. A light flat-bottomed 15' boat should plane out fast with 70hp, check to see if the prop is larger than 15" pitch, if so it's probably too much. I was running a 12" prop on a Merc 700E with 13.5' Sea-King runabout. It was great for skiing and went 35 mph.

Check for carbon buildup in the exhaust, you could pull the L/U to peer up the exhaust tower to see if there's anything clogging the exhaust plenum area.

Actively troubleshoot on the water by pulling the cowling then placing fingers over one-carb-at-a-time to block airflow. Speed-up indicates lean condition; slow-down indicates pulling its weight; no change means cylinders may not be getting any fuel and that's bad news!

BTW does this motor have fixed main jets, or adjustable mixture screws at the front bottom of each carb? If adjustable, may not be set correctly. Must be set on the water with throttle.

HTH & let us know what you find.........ed

Thank You for reply.

A few answers and clarifications before I go tearing back into engine.
Has adjustable jets, Initial setting 1 1/2 turns out. Adjusted each leaner and richer under load, no noticeable difference, same with idle mixture screws
Did not choke carbs individually. Pulled choke with choke rod [all 3 choke plates on] Under load. The engine stalled and died immediately
I had the Lower Unit off for water pump impeller replacement. Although I was not specifically looking for exhaust problems it not appear to have excessive carbon or exhaust build up.
Double checked prop specs ok. 13" dia 15" pitch. probably original prop
Under no load had good spark on each plug using its respective plug wire
Distributor move all the way one way and then the other with cable
My gut feeling is a fuel supply problem at or in carbs. I say that because who ever set the ignition has it correct as the forward/reverse works and it fires right up. There is evidence of this from sharpie marks on distributor. But then again I have read if its running on 3 you wont notice. he also could have messed it up as well.
Also what forces fuel into cylinders crank case pressure, vacuum reed valves ?
Thanks Again

3 Reed blocks are concentric with the crankshaft, each reed block has reeds above and below to feed 2 cylinders. You'll definitely know the difference underway if only 3 cylinders are firing, but running the motor out of the water with the prop removed, maybe not. There's very little load in that situation and to the untrained ear, the engine sounds pretty healthy beating away on only 3 cylinders.

You can clip an inductive timing light (or use a "firing indicator) around each spark plug wire to see if that cylinder is sparking while the engine is running. That's why I recommended a "drop test" where you pull one spark plug boot at-a-time off the spark plug while it's running; if that cylinder is contributing, the motor rpm's gonna drop very noticeably. You'll note no difference if that cyl isn't firing. S

Since the old Six has essentially 2 independent ignition systems, it's very possible (as the Doc said) for one side or the other to be out.

You can also check for spark off each coil wire, if both coils have fire going into the distributor but either 1,3,5 or 2,4,6 are dead, you probably have a bad rotor, which is shorting one side to ground.

Although my money is still on the ignition, check the fuel pump diaphragm & gaskets for flaws.
I stopped using Sierra Marine fuel pump diaphragm kits for that style pump, because thier cork gaskets quickly distort and have resulted in two cylinders flooding.
doc

ed-mc wrote: 3 Reed blocks are concentric with the crankshaft, each reed block has reeds above and below to feed 2 cylinders. You'll definitely know the difference underway if only 3 cylinders are firing, but running the motor out of the water with the prop removed, maybe not. There's very little load in that situation and to the untrained ear, the engine sounds pretty healthy beating away on only 3 cylinders.

You can clip an inductive timing light (or use a "firing indicator) around each spark plug wire to see if that cylinder is sparking while the engine is running. That's why I recommended a "drop test" where you pull one spark plug boot at-a-time off the spark plug while it's running; if that cylinder is contributing, the motor rpm's gonna drop very noticeably. You'll note no difference if that cyl isn't firing. S

Since the old Six has essentially 2 independent ignition systems, it's very possible (as the Doc said) for one side or the other to be out.

You can also check for spark off each coil wire, if both coils have fire going into the distributor but either 1,3,5 or 2,4,6 are dead, you probably have a bad rotor, which is shorting one side to ground.

Yep on inductive timing light! Just found real fast a bad coil on a motor. Also infrared temp gun will show a cold cylinder..shoot it at the plug.

We took the boat to the lake today to do some testing

Before we left, we cleaned the cap and rotors and checked continuity of the wires.

While on the water under load, we started by covering each carb individually and noticed a drop in the engine speed each time.

We then pulled each spark plug wire individually and noticed a drop in the engine speed with each one and just to be sure we pulled each coil wire at the coil and there was a very noticeable drop in the engine since it killed 3 cylinders.

At one point, it very briefly ran slightly better (2mph faster) then it gradually dropped back to the poor performance not developing full rpm.

What should we check next?
Could it be the fuel pumps not providing enough fuel? We are running 24:1 fuel, 1qt to 6 gallons. When we pulled the spark plugs none of them looked fouled.

Also, 3 of the spark plugs have a sleeve on them to adapt them to a larger hole. My guess would be sometime in its past stripped the threads. We did a compression test on the 3 cylinders that aren't drilled out (don't have an adapter for the larger 3) and they all came out about 80#. Not sure if that helps diagnose our situation at all.

Does anyone know what the fuel pressure should be coming out of the fuel pumps?

I don't know that,...Maybe a couple psi??
If you suspect low fuel pressure,...Continually and slowly squeeze the prime bulb, and see if the beast picks up RPMs.
I tested a prime bulb and found I could develop 21 psi, way more than even a monster carburetor v6 requires.
doc

Decided to recheck former ignition settings. I am pretty certain
Crank to Distributor timing is off, I think the belt has slipped. Need 2nd opinion. The first pic is out of manual showing correct timing. 2nd pic is showing how it is currently set with timing marks aligned. 3rd pic shows what I think it should be if timing marks were in alignment and in alignment with center-line of crank and distributor as per factory pic. Pic 4 and 5 show how loose belt is. Pic 6 is belt part #. Motor sat for 10 years before I started it up. Maybe belt lost its elasticity or was installed wrong to begin with.

pic 1

having pic issues will have to resize and repost

to install the belt just bring top cyl to top dead center. then aim the arrow on the did pulley at the crank don't worry about the marks. the arrow is hard to see but is on one of the webs.

These are the pictures

It still looks to be one tooth off in the last picture, but it might be because the belt is so loose.
The belt is a lot looser than any I just checked.
Is there any sideways play to the flywheel or the top of the distributor?
doc

Thanks for the reply. We ordered a new belt and will set the timing correctly and see what happens.

The flywheel is on their TIGHT! we've tried a steering wheel puller. is there a trick? or just keep working at it?

Well,...Remember,...You asked
I'm a wee bit off the charts on some of my "techniques", so you may want to wait for further insight.
BUT,...I really load up the puller to the point that it surely will self-destruct.
I use grade 8 cap-screws, and I place a brass bunion between the end of the crankshaft & the puller.
When you would swear that something is going to bust,...The flywheel will surrender - sounding like a 30.06 going off.
I have been known to smack the puller's center bolt with a large hammer too, but that practice is commonly frowned upon.
It works though, but the concern is the risk of fracturing the crankshaft. I haven't yet, but that is the concern.
I've considered applying MAPP heat to the flywheel while the puller is loaded up, in an effort to expand it away from the tapered crankshaft, but haven't done that,...very often.
doc

Thanks again for the continued help.

I finally got the flywheel to surrender this morning. I loaded the puller like you said and just kept at it till it popped. Now to wait for the new belt and hopefully we'll be on our way!

We got the new timing belt on this morning and took the boat to the lake. It ran much better!

Thank you everyone for your help/advice

Good news! Damn, gotta think that motor was built when I was 8 years old, and been though a few "belts" myself!

It was built the year I was born. I don't even want to go there. Most of them, if they still run at all, still have some good years left in them. Don't tell my wife that I talked about old girls like that.
Bill