Runs crappy after 15 minutes

[75 tique]

1976 Chevy 305 OMC, EI conversion by previous owner.

Boat runs great for 15 minutes. Then starts to miss, run rough, backfire.

After a few minutes of that, will idle but not run in gear.

After a few minutes fo that, will not start.

Next time out, or even an hour or so later, everything is good again, for 15 minutes.

This problem appears to be time related, so what can change after 15 minutes? Not choke, choke is open as it should be. Not engine temp as engine is up to temp in a few minutes but failure is not until 10-15 minutes.

Had carb rebuilt, no difference.

Tried new coil, no difference.

Found out recently (I had never been into the cap in 4 years of ownership) that boat has EI conversion.

I am thinking of getting to cap and rotor, maybe plugs today, aven though I can not think of a time related problem with those. I polished up rotor contact last time it quit and it went for another 10 minutes, and died again.

 

[erikgreen]

Re: Runs crappy after 15 minutes

Have you checked your fuel system?

It almost sounds like you could be building up a vacuum inside your tank, and after a certain period of running your fuel pump no longer has enough power to suck fuel to the engine. Or it could be a clog in the fuel pickup that's building up junk while the engine sucks fuel, then slowly restricts it until the engine dies.

 

[Don S]

Re: Runs crappy after 15 minutes

Try is test.

http://forums.iboats.com/showthread.php?t=167561

 

[Maclin]

Re: Runs crappy after 15 minutes

Has it run that way since you were the owner?

I am not discounting any fuel delivery problems, but your mention of EI conversion made me think of the types of problems encountered during and after a conversion like that.

I would want to verify that there is or is not ballast resistance in the coil primary circuit. I have heard of and seen problems with coil saturation when there is no ballast resistance. If the coil is designed for ballast resistance then it has to be there no matter what type of trigger there is for the coil (points or electronic).

Also the electronic unit needs a full 12 volts which it will not get if it is connected after the ballast restistor or resistor wire.

Find out as much as you can about what the EI conversion entailed, what brand, type etc, then try to verify it is wired correctly.

 

[75 tique]

Re: Runs crappy after 15 minutes

Maclin.

It ran great for four years up until last fall. No changes to engine ahead of change in performance. The ballast resistor is still in the loop. Someone has just suggested bypassing the ballast resistor, thinking that it may have gone bad and when gets hot, cuts voltage. That is my next test. I guess the next thing after that would be replace EI module. Right now I dont know anything about the EI system in it. I have a mallory flat cap dist, but dont know the model. I will have to do some research to identify/replace the module if I end up going that route. Any other thoughts?

Dont think it is a fuel delivery problem, given the absolute predictability in the timing of the failure each time I am out. I would think a fuel problem wouldnt kick in after time, but would be more consistent.

 

[Maclin]

Re: Runs crappy after 15 minutes

I would verify that the ignition module has a reliable full 12 volt supply that is on with the ignition switch. If it is wired in after the ballast resistance then that could be a problem, evern moreso if the resistance component has a heat-related problem. If it was not wired correctly then I suppose the module could act up after this length of time. Bypassing the ballast resistance could be a good thing to try, but I would definitely find out what everything is wired to.

 

[Eye Spy]

Re: Runs crappy after 15 minutes

I am by no means an expert on this, but i had a similar problem last year. I called the repair shop and they ended up replacing the coil and some sort of a sensor. Took 30 minutes of work and haven't had any problems since.

Ryan

 

[shdw2001h]

Re: Runs crappy after 15 minutes

I had a problem close to this, this year, What happen was the altenator went bad, with that it took battery. the battery would read 12 volts but would not hold under a load. Found this out after I bought the alt. I also cleaned the ground connections on the motor. since I did replace these 2 items and cleaned the connections I have not had a problem with the boat since. I hope this is some help.

 

[buggiedad_67]

Re: Runs crappy after 15 minutes

still sounds like your fuel tank maybe vacum locking, a easy test is just remove your fuel cap and start it on muffs and let it run, and see if it stays running, i had a dirt dobber build a next in my vent tube and did the same thing, the screens had a hole in them and let the little boggers in.

 

[bomar76]

Re: Runs crappy after 15 minutes

Or, he could do the tests that Don suggested in post #3 and he would know exactly what the problem is and plan a repair.
Or, keep on playing this endless guessing game and keep throwing parts at it and maybe some month stumble on a solution by blind luck.