Losing Power after 20 to 30 minutes of perfect operation

[smdellin]

We have a 1985 Invader with the Volvo Penta AQ125/270 I/O.

The issue is that the engine runs flawless for about 20 minutes and then surges power numerous times and then stalls. This has been so consistant that you can put a stop watch to it. The motor will start up immediately with no issues but you can only idle as it will die.

If I wait 20 minutes, the engine runs perfect again, but for a slightly shorter period of time.

I have ruled out fuel as the siphon valve has been temp removed and tank and lines are clear and filters and fuel pump are new.

I suspect the coil (points and condensor replaced), but never had one react in this manner. The last outing I actually poured water on the coil to cool it (I know sounds silly) and it worked, engine ran fine again for 20 minutes. I pulled old coil and it passed inital ohm readings. It is ineresting though that marked on the coil it stated external resistor required and to my knowledge the boat does not have a ballast resistor in line. Anyway replaced the coil with brand new and same failure, crap!!!!

So am I overheating the coil and need a ballast installed (means prior owner had wrong coil???) or does the ignition system need a internal resistor coil? Or am I chasing wrong issue?

At a loss and summer is passing fast and no boating fun, just frustration.

 

[Don S]

Re: Loosing Power after 20 to 30 minutes of perfect operation

Re: Loosing Power after 20 to 30 minutes of perfect operation

Sure it could be the coil. Get the proper coil for it and give it a try.

 

[smdellin]

Re: Loosing Power after 20 to 30 minutes of perfect operation

Re: Loosing Power after 20 to 30 minutes of perfect operation

OK, tried new coil and have exact same issue. I expected to see a ballast resistor as shown in my manual but could not locate one. Can some one help me to determine if I have a ballast or resistive wire? I have only one wire to positive side of coil. I expected to see two, one for 12 volt start and one for resisted voltage during run (at least what I know from autos). Voltage to plus side of coil key on is 12 volts, expected 9 volts. Coil states use with external resistor.

Is all this stacking up to a possible removed ballast. Wiring looks stock and unmodified but who knows after 28 years.

 

[aerobat]

Re: Loosing Power after 20 to 30 minutes of perfect operation

Re: Loosing Power after 20 to 30 minutes of perfect operation

I have ruled out fuel as the siphon valve has been temp removed and tank and lines are clear and filters and fuel pump are new.

maybe you ruled you fuel supply ou too quickly. when an engine looses spark it normally stalls immediately and not surges before stalling .

you may deal with somekind of vapor lock due to hot engine compartment. when it stalls next time pull a plug before restarting to look if its wet or dry ( no spark or no fuel ) and try to run with engine room hatch open to see if it makes a difference.

 

[bruceb58]

Re: Loosing Power after 20 to 30 minutes of perfect operation

Re: Loosing Power after 20 to 30 minutes of perfect operation

Voltage to plus side of coil key on is 12 volts, expected 9 volts.

You have to measure this voltage with the points closed. If they aren't closed, no current is flowing and no voltage drop across a ballast resistor if there is one.

 

[QC]

Re: Losing Power after 20 to 30 minutes of perfect operation

maybe you ruled you fuel supply ou too quickly

Almost sounds like a clogged tank vent. Acts a little like a 5 gallon tank with vent closed. Have you tried loosening the cap?

I can see vapor lock as well, high ambient?

 

[Maclin]

Re: Losing Power after 20 to 30 minutes of perfect operation

I was typing in about the fuel tank vent and had to exit then came back and see QC's post about it, that is sure worth a look. Remove the gas tank cap and listen for air rushing in next time right when this happens.

 

[smdellin]

Re: Losing Power after 20 to 30 minutes of perfect operation

When this all started I thought fuel also, all indications point to it. Oddly enough the boat ran perfect for several weeks prior to all this.

I did look for vapor lock, no rushing air and also ran with cap off, no change. I have removed siphon valve. I also installed a fuel pressure gauge and it maintains fuel pressure throughout (electric pump). I will try this weekend with cover open to see if it is heat related.

The odd part is how it runs perfect on initial use and after cooling down, driving me nuts. I restore old muscle cars on the side and in all my years have never been stumped like this before.

Anyway, rebuilding carb today (unrelated, just looks like it is needed) and will try and determine if ballast resistor is instlled somewhere or not and also run with engine cover open. If I don't get this soon my family has threatened to get themselves another boat mechanic.