Here are the symptoms. Where would you start?

[BradleyPLKD]

Hi Mariners,
I have an intermittent engine problem and would like to get your opinions on possible causes. Have a 1996 5.7 Volvo Penta gas motor, carburated, with electronic BID ignition. The motor runs perfectly 99% of the time, but every once in a while when trolling at 2000rpm/9knots it stumbles, then powers up again and keeps going as if nothing happened. I have not been able to recreate the problem while on earmuffs no matter how fast or slow I rev the motor. I'm thinking the spark is jumping somewhere periodically (coil tower/spark plug wires etc) but wanted to know if you guys think there are other things that could be causing it. Thoughts? These types of engine problems drive me crazy because they're so hard to pinpoint.
aloha,
Brad

 

[Bondo]

Re: Here are the symptoms. Where would you start?

Hi Mariners,
I have an intermittent engine problem and would like to get your opinions on possible causes. Have a 1996 5.7 Volvo Penta gas motor, carburated, with electronic BID ignition. The motor runs perfectly 99% of the time, but every once in a while when trolling at 2000rpm/9knots it stumbles, then powers up again and keeps going as if nothing happened. I have not been able to recreate the problem while on earmuffs no matter how fast or slow I rev the motor. I'm thinking the spark is jumping somewhere periodically (coil tower/spark plug wires etc) but wanted to know if you guys think there are other things that could be causing it. Thoughts? These types of engine problems drive me crazy because they're so hard to pinpoint.
aloha,
Brad

Ayuh,.... Welcome Aboard,.... When it does it, does the Tach stumble,..??

Try disconnectin' the tach,...

 

[mikethefixer]

Re: Here are the symptoms. Where would you start?

Hi Mariners,
I have an intermittent engine problem and would like to get your opinions on possible causes. Have a 1996 5.7 Volvo Penta gas motor, carburated, with electronic BID ignition. The motor runs perfectly 99% of the time, but every once in a while when trolling at 2000rpm/9knots it stumbles, then powers up again and keeps going as if nothing happened. I have not been able to recreate the problem while on earmuffs no matter how fast or slow I rev the motor. I'm thinking the spark is jumping somewhere periodically (coil tower/spark plug wires etc) but wanted to know if you guys think there are other things that could be causing it. Thoughts? These types of engine problems drive me crazy because they're so hard to pinpoint.
aloha,
Brad

Hello Brad...I see by the date that you have more than likely figured your problem out...At any rate I think you might find that your carb float is set a bit too lean...At idle the pump doesn't supply enough fuel for jets that are working probably too well.Bring the engine rpm up and more fuel is supplied.If this is the problem you should be able to go to a high rpm state for a bit and then go to idle and time how long till it acts up...Do it 2-3 times and you will see a time factor brought in...Bad wires and connections will not be consistent.
Wish ya luck and if you did fuigure the problem out,drop me a line to know either way...Thanks mikethefixer...

 

[BradleyPLKD]

Re: Here are the symptoms. Where would you start?

Hi Bondo,
I haven't noticed the tach jumping around when the symptoms occur...though my first instinct is to pull back on the throttle when I feel the stumble so my eyes aren't looking at the tach at that moment. I'll see if I can get the problem to occur by playing with the connection on the wire.

Hi Mikethefixer, I just posted this yesterday so still gathering comments. The motor runs perfectly on earmuffs at all rpms so I assume if it was a float problem, it would show up then. You mention bad wires and connections, too. Perhaps when on the water bouncing around trolling, the motion causes a momentary bad connection somewhere?

 

[Bayou Dave]

Re: Here are the symptoms. Where would you start?

I think Mikethe fixer looked at your Join Date of Jan 2012, not your thread post date.

 

[hall832]

Re: Here are the symptoms. Where would you start?

Can you re-create the problem on the muffs.

 

[hall832]

Re: Here are the symptoms. Where would you start?

Hi Bondo,
The motor runs perfectly on earmuffs at all rpms so I assume if it was a float problem, it would show up then. [/QUOTE

No, there is a difference of pressure from running on the ear muffs to having the boat in the water.

 

[Walt T]

Re: Here are the symptoms. Where would you start?

Sometimes if you are trolling at a low rpm for an extended period of time the spark plugs can start to foul a little and you'll get the stumble, then they'll clear up a little. With a carburetor that type of operation can put the carb into idle/ power valve/ accelerator pump mode and it's possibly over fueling it a little. See it all the time with fishermen. Outboards and inboards, engines don't like trolling and really aren't designed for it.

 

[BradleyPLKD]

Re: Here are the symptoms. Where would you start?

Hall832 and Wait T, no haven't been able to make it stumble on muffs when on my trailer. When trolling at 8.5 knots/2000rpm, we can go for hours with it running perfectly. Then stumble for a few seconds. Then back to perfect again. This is usually in normal 3-5 foot seas. We don't get many flat days so haven't noticed if the problem goes away when water is calmer.

 

[Pesimist]

Re: Here are the symptoms. Where would you start?

If you have any spark jumping between plug wires or coil tower and a ground you should be able to hear the static on an AM radio (AM channels hate interference). Also, next time you crank the engine while on muffs - do it at night, look for arching.

When was the last time you changed your plugs?

 

[BradleyPLKD]

Re: Here are the symptoms. Where would you start?

Hi Pesimist, Interesting idea with the Am radio. I will fire up the motor tonight and see if moving the plug wires around (as if at sea) can cause the stumble. Plugs should still have plenty of life left...

 

[bruceb58]

Re: Here are the symptoms. Where would you start?

This sounds pretty normal for an engine that idles for a long time. A boat we had a long time ago always did this. We just chalked it up to be a slightly rich idle mixture.