1981 Mercruiser 120 cutting out

[81 Dixie]

Boat cuts out at higher rpms. Tested it on the water with engine cover removed the past 2 times to keep the heat down incase it is vapor lock. I have had the boat 10 years and never experienced it.

Ran fine for a total of around 50 miles one weekend (BP 87 octane),then started the issue the next week. (Had refueled with Shell 87 octane)

New fuel filters at the pump and the carburetor this year. Running engine temp around 150. 13 volts on the ammeter. I have replaced the coil since it went bad trying to chase this problem. Cleaned and reset the points.

Have taken it out 4 times with this issue. Takes anywhere from 5 minutes to 30 minutes for the issue to show up. Once it starts cutting out (feels like its starving for fuel) it becomes almost constant. I can ease into the throttle and get some speed but it will start cutting out again, sometimes killing the engine.

Thanks in advance for your input.

 

[alldodge]

13 volts on the ammeter

Probably a voltmeter with 13V, but if its close to accurate then look at the alternator. The ALT should be putting out 14.5V running or close to it.

Have you changed the condenser?

 

[81 Dixie]

It is a volt meter, not sure why I put the other. Haven't changed it, considering changing it to electronic Ignition.

 

[alldodge]

Electronic ignition while good can have their own problems. A good points distributor does quite well on its own. You could change, but right now I'm not so sure that is your issue

Also your issue is not vapor lock

and have you replaced the condenser when replacing the points??

 

[81 Dixie]

Didn't replace either, didn't want to spend the extra money if I did decide to change to electronic.

 

[alldodge]

OK, I would replace points first, or electronic
Look at connections and get a good meter to check ALT output

 

[81 Dixie]

Will check output at alt. Also going to check fuel pressure. Will be changing the Ignition one direction or the other.

 

[81 Dixie]

At half throttle voltage at the alternator is 13.8 - 14.4.
I decided to check at the hot wire leading to the coil, it was 16.6 - 17.5. (Resistor wire by passed using flamethrower 1.5 internal ohm coil) (resistor wire is bad)

Fuel pressure is 5psi at idle and 4.5 at half throttle.

 

[alldodge]

voltage at the alternator is 13.8 - 14.4.
hot wire leading to the coil, it was 16.6 - 17.5.

Something is wrong with your meter of the ALT, you should never see more voltage at the coil then the ALT/Battery. If it was 16+V at the coil, then the regulator has gone out in the ALT and that much voltage will burn things up

 

[81 Dixie]

Checked each 3 times with same results. Puzzling how the voltage is higher further away.

Do you think that when increasing throttle, hence voltage, its getting the coil to hot causing it to not fire correctly?

I will change the regulator hopefully today and check again.

 

[alldodge]

If coil is actually getting that much voltage it would get hot. The difference in voltage may also be how well its grounding at that point. Try placing the ground lead on the battery negative and then measure the points

 

[81 Dixie]

So you're saying run a lead from coil negative directly to batt negative with a jumper wire. Then measure voltage at the coil positive, correct?

 

[alldodge]

No
Attach the negative lead of you voltmeter to the negative post of the battery. Then measure the voltage of the ALT and at to coil.

 

[81 Dixie]

OK, Measured voltage at apx.1/2 throttle grounded to the battery, at the alt it showed 14.77.
At coil it showed 15.1 - 16.

Still have fuel pressure gauge hooked up and now that outside temperature is 92 and boat is sitting in the sun the fuel pressure is down to about 2.25 - 2.5.

 

[alldodge]

I think its the points and condenser causing your meter to read the pulses, which in turn makes a meter reading DCV show more, so you should be good.

The fuel pressure is not good, should be at least 3 psi and in most cases its above that at WOT

 

[81 Dixie]

Ordered a new pump. Will update, supposed to be here Sunday.

 

[81 Dixie]

Ordered a new fuel pump. Will update.

 

[81 Dixie]

Okay, put on new fuel pump, 3 ohm flamethrower coil, 1.6 ohm ballast resistor, all new fuel lines and filters.
Measured 14.78 volts across the battery, 14.5 before the resistor, 14.1 after, and 13.1 on the negative side of the coil.
​Tested it and much better but not totally right.​​​​​​

Had my alternator rebuilt (had bad diodes) and just got my electronic Ignition.

Hope to test this weekend.

 

[alldodge]

With electronic ignition you no longer need a ballast resistor

Don't pay any attention with voltage on the neg side of coil, if your getting spark its working. Now if a tachometer is attached do disconnect it until everything is working correctly

 

[81 Dixie]

Wondering if the number on the right is the gear ratio?