Mercruiser 470 runs rough after 15 Minutes

[Alterry]

I have a 1985 Searay with an I/O 470, mercruiser 2bbl (170). I installed a SEI 106 out drive about 20 hours ago, last November. The boat start and runs well cold or warm. It accelerates smoothly and idles just fine, shifting seems to working well. After about 15 to 20 minutes at cruising speed (3200-3500 rpm) the engine misses and the rpm deteriorates to idle. The next day, same drill, it runs fine for 15 to twenty minutes then begins to miss.

No evidence of overheating, water temp is just above a quarter travel which is normal for this boat over the course of many years. I also ran the boat completely uncowled trying to track this down and the problem developed in the same manner is when the engine covers were on. There is no evidence of hissing or burning either when the cover is removed. Oil consumption is normal, oil appears normal on the dipstick and coolant is not leaking.

Changed the following over the last month/ten hours

Anti siphon valve
Fuel pump and filter
Carburetor inlet filter
Plugs
Wires
Condenser (2times)
Points
Cap
Rotor
Coil (multiple times)
Cleaned, lubricated mechanical advance
Checked timing, 8 degrees
Checked wires for loose connections, rerouted wires away from exhaust.
Disconnected shift interrupt switch
Also ran boat with tachometer/ shift interrupt wire disconnected from distributor, no change in the problem.

Carburetor was rebuilt within the last two years, has about 75 hours since last rebuild.

Any thoughts on what to do next would be helpful.

Thanks!

 

[kenny nunez]

With a volt meter connected to the + or bat side of the coil run the boat and watch the voltage compared to when you first started out. There should be 8-9 volts when the engine is running. If the voltage starts dropping below the starting point when the engine starts slowing down then the resistor wire is the problem. I have encountered what you are describing before. Another thing to try, when the problem starts, stop the boat and replace the rotor that is also a problem with the Delco distributors when the rotor gets hot.
You could be having a fuel starvation problem. Get a 6 gallon outboard tank and set it up so that you can switch it to the fuel filter when the problem begins. Older boats suction tube in the tank do get plugged up as there is a very fine mesh filter in the tube that needs to be removed and pitched.

 

[cyclops222]

When it starts next slow down ? Remove the gas tank fill cap. To release any vacuum in the gas tank.

 

[nola mike]

Throw a timing light on it when it starts missing to see if you're dealing with electric or fuel issue

 

[Alterry]

Great ideas, thanks! I just bypassed the resistive wire with a resistor and there was no change. I’ll double check with a voltmeter as suggested and I think I’ll try to start it with the starting wire to the coil removed, then run it and see if that changes anything

I’ll try the timing light as well. It sure would be nice to narrow down the problem to fuel or electric. I have cleaned out the fuel vent, but I’ll open the gas cap next time as suggested. Is the fuel suction tube filter something that can be changed without removing the tank? I was not aware of this and it’s probably worth replacing anyway?

Thanks again all!

 

[Alterry]

Great ideas, thanks! I just bypassed the resistive wire with a resistor and there was no change. I’ll double check with a voltmeter as suggested and I think I’ll try to start it with the starting wire to the coil removed, then run it and see if that changes anything

I’ll try the timing light as well. It sure would be nice to narrow down the problem to fuel or electric. I have cleaned out the fuel vent, but I’ll open the gas cap next time as suggested. Is the fuel suction tube filter something that can be changed without removing the tank? I was not aware of this and it’s probably worth replacing anyway?

Thanks again all!

Just an update, there was no filter on the fuel suction tube, but the tube appeared clean with no debris. I tried the timing light, but couldn’t find anything conclusive when the problem would occur. With the engine running, I checked my voltages again in the low tension coil circuit, I was only getting a two volt drop at the coil positive. I had one volt dropping across the coil and the rest between coil negative and ground. When I checked the resistance of the breaker point wire I had about 5 ohms and it would fluctuate higher at times.

I replaced the breaker point wire and I found the same voltages as before, so I thought I’d hit another dead end, but when I ran it, the problem did not reoccur. I probably won’t no for certain until I put more time on it, but this is as good as it’s run all summer.

The voltages that I have are:

14.5 system voltage on the alternator, engine running starter solenoid terminal to ground
12.5 positive coil terminal to ground
1 volt drop across the coil
11.5 volts coil negative to ground.

It seems like the voltage at the coil is still high compared to other posts, I’m just not sure why

 

[nola mike]

If you have spark you can stop chasing your ignition system. Is your choke opening?

 

[Alterry]

If you have spark you can stop chasing your ignition system. Is your choke opening?

Thanks, I’ve checked it as engine is warming up and when I’ve had the problem I pulled the flame arrestor and it was open.

 

[ROY WILLIAMS]

Great ideas, thanks! I just bypassed the resistive wire with a resistor and there was no change. I’ll double check with a voltmeter as suggested and I think I’ll try to start it with the starting wire to the coil removed, then run it and see if that changes anything

I’ll try the timing light as well. It sure would be nice to narrow down the problem to fuel or electric. I have cleaned out the fuel vent, but I’ll open the gas cap next time as suggested. Is the fuel suction tube filter something that can be changed without removing the tank? I was not aware of this and it’s probably worth replacing anyway?

Thanks again all!

My Wellcraft 20 ft the engine MCM470 year 1978 ... TODAYS !!! 40 YEARS !!!
SO the engine is gasoline vent clean it . the fuel pump and the carb 2 filters.
the 650-800 rpm ignition 4-8 degrees . the gasoline 91-93 octane .
the engine oil 30WT ... the engine oil zinc plus 6 OZ ...
THE coolant antifreeze 50% and the distilled water 50%...radiator ..
the ignition coil positive resistor wire 1.8 ohm .......
the no resistor of the starter key .. now the engine runs 1.8 resistor ..