1977 470 Mercruiser running problem

I'm having some issues with my 1977 Stury with a 470 Mercruiser.
It will run fine most of the day pulling skiers and tubing etc.
I will then start hard and then on one of the pulls with a skier it will pick up a miss and won't get over 3000 RPM's.
It will straighten out sometimes as well.
The problem is the same when it had point ignition and when I changed to the Hot Spark
I've checked the fuel tank and it is clean.
Changed fuel line
Replaced fuel pump.
Replaced carb
Changed fuel filters
Changed over to HotSpark ignition and their coil.
(Checked voltage under way. It is 10.29 volt when it is screwing up and 10.33 when it is running good.
Changed plugs and wires.
Replaced the solinoid
New cap and rotor.

Had it to a dealer. First time I've ever had to take a boat to a dealer as I'm a pretty good mechanic.
They replaced a couple of wires that they thought were causing the problem but it didn't change.
Changing the carburator made it run better when it is running right.
I ran it all weekend with the hatch open figuring it may be a heat related issue and it still picked up the miss.
Problem is that it is intermittent and with no reason.

Any ideas would be appreciated.

on one of the pulls with a skier it will pick up a miss and won't get over 3000 RPM's.

Click to expand...

When you converted the ignition did you inspect the mechanical advance in the distributor? It may be hanging up. Does the distributor shaft have a wobble...bushing wears and the shaft will move side-to-side.

(Checked voltage under way. It is 10.29 volt when it is screwing up and 10.33 when it is running good.

Click to expand...

Those numbers aren't right. Idle should be 12 to 12.5 vdc and WOT should be 14.5 to 14.8 vdc when it's charging.

Sorry, I didn't give you all the infomation.
The voltage check was at the + terminal of the coil.

I disassembled the advance, cleaned it, gave it a light film of white lithium grease to protect from corrosion.
The advance weights worked free.
BTW- there are only 600 hours on the motor.

I'm wondering if it is not related to either a tach issue. Although the tach works fine or something with the resistance wire.

Check the spark at the plug wire end...need a loud snapping blue spark on each one. If the carb is a 4 BBL are the secondaries opening? Engine has 600 hrs...won't hurt to check compression and you can check the color of the plugs...medium brown is what they should look like. Stick a timing light on it to see if there is timing advance.

One issue i had once with this motor was the radiator cap leaked, not much but enough that the coolant went straight down into the spark plugs. It shorted out the plugs when the motor was normal operating temp.

Took me a few days to figure this one out....replaced all ignition components one at a time and nothing corrected the issue. Once I figured out the issue I then replaced the cap and all was good.

If this is the issue you will see light traces of dried coolant leading down to the spark plugs, Mainly the two center cylinders.

Also if you suspect the tach pull the grey wire off the coil - and retest,

What wires did they replace? Was it the resistor wire? There is or maybe was a ~ 3 foot long wire in the harness attaching to the coil+ (purple). it is a different core than all other wires, more like a solid core/bright metal vs copper. The wire is typically attched within the harness at a junction point with other purple wires and it is long enough that it is folded over on itself (this is to achive the ~ 2 OHMS of resitance) needed for the point ignition.

If your replacement ignition can do without this resistance wire maybe find it and remove it and replace it with a new NON resistance wire. This all depends on the coil you are using. external resistance needs the resistance wire, Internal resistance then no need for the resistance wire.

You were in the dist. and didn't replace the whole dist. ?

Did you wiggle the shaft ?

Weights move free you say but the shaft wiggles ?

The new upgrade didn't fix the problem... cause it wasn't the problem.

But if you threw in a new dist it may have.

Merc used a resister wire ?

Funny i only saw a resister wire on omc's.

http://www.marineengine.com/parts/m...-thru-5520114/voltage-regulator-and-rectifier

You have the upgrade alt ?

Or is it still original ?

Ok i'm not going to beat you up, my own inexpert opinion about your engine setup won't help you.. cause i feel your pain you just want the engine to run right, and go boating.

Willing to listen and try to learn something ?

With a switch between the coil pos. and the battery pos. side run a wire.

Again a switched hot wire, in essence your gonna hot wire this old byatch.

Place the switch at the helm.

In the water you flip the switch start the boat.. more on that latter.

You run the boat if the problem goes away good post back we will go from there.

To turn off the boat you turn off the key and it will still be running so you flip the switch to kill the engine.

Here's how to run a carbed engine:

http://www.marineengine.com/boat-forum/showthread.php?t=380996

Master mechanic said:

You were in the dist. and didn't replace the whole dist. ?

Merc used a resister wire ?

Funny i only saw a resister wire on omc's.

Ok i'm not going to beat you up, my own inexpert opinion about your engine setup won't help you.. cause i feel your pain you just want the engine to run right, and go boating.

Willing to listen and try to learn something ?

http://www.marineengine.com/boat-forum/showthread.php?t=380996

Click to expand...

Not sure what your drinking in the morning...........

Your link is to a rectifier/regulator.............nothing to do with the ignition circut..........

Yes all mercruiser point ignitions had a resistor wire...........


I thought OMC had a ballast resistor?????

Lets keep to the facts!

Here is the schematic............