4.3 Mercruiser runs great cold, no power and sputters warm

[V-Tech]

1995 4.3 Mercruiser, 2bbl carb. Runs great when engine is first started cold and after 10 minutes begins to lose power and will get worse as engine warms up. Here are the parts we know are good, or are at least new: Ignition coil, Ignition module, spark plugs, plug wires, distributor pick-up coil. Ignition timing is at 8 degrees BTDC, Dist. cap and rotor fairly new with no visible cracks, choke is working properly, fuel pressure 6 -8 psi., fuel volume - 1 pint in 15 seconds. Fuel filter clean and water seperator new. Fuel tank emptied for cleaning within last year. Oil pressure correct and oil level correct. Oil pressure switch for electric fuel pump by-passed for testing purposes (so fuel pump does not shut-off due to any potential oil pressure concerns). The carburetor float bowl is clean and float level appears correct.

Again, this boat runs absolutely great when first started and driven.
Please give any thoughts on the cause, I am stumped for now.

 

[Bondo]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

Ayuh,... My guess is, yer Carb is AFU....

When cold, the choke is richening the mix...
When it pulls off, it goes Lean...

 

[V-Tech]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

I did have another person drive while I manually applied the choke and it made it worse. Thank you for your thoughts on this. Any other ideas?

 

[Scott Farley]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

Have you checked the cylinder compression? Worn rings/pistons will do what you are describing. Hope you get it fixed!

 

[V-Tech]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

No I have not checked compression. Good suggestion. Not sure I want to know the results though.

 

[jones56]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

Bad plug or coil wire could be arcing.. Mine did this saturday. Thank goodness easy fix. We even heard the spark arcing..

 

[V-Tech]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

I finally found the ignition coil was only receiving 11.2 volts to the positive side (purple wire) when the engine was running. The battery voltage was 14.3 at the same time. I jumped a wire from the positive terminal of the battery to the positive side of the ignition coil (purple wire) and it runs great again. Take the jumper off it runs bad. Reconnect the wire and it runs great. Now I need to locate the source of my voltage drop and correct it.

Anyone have a good suggestion as to where the culprit might be located that is giving me high resistance in the purple wire?

 

[V-Tech]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

In case anyone is interested, this problem was completely fixed by replacing the purple ignition wire from the back of the ignition switch all the way back to the positive terminal of the ignition coil. The original wire also powers the gauges, so I spliced in a new wire only for the ignition coil and ran it back to the engine. The original wire ran up high behind the dash board and is quite difficult to access so I went the route of an all new wire. In conclusion, I went from a voltage drop of over 2 volts through the circuit to a drop of just .3 volts. Anything less than .5 is considered acceptable. The original wire had numerous splices and connections that must have built up high resistance over the 16 years of the boat's life. Added up, it was too much resistance and gave a symtom of a boat that gradually lost power over the years, particularly under load and high speed. Thanks for the input.

 

[Bondo]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

The original wire had numerous splices and connections that must have built up high resistance over the 16 years of the boat's life.

Ayuh,.... Glad ya found the Gremlin,...

Those are 'bout Impossible to see on the internet,....

 

[paultjohnson]

Re: 4.3 Mercruiser runs great cold, no power and sputters warm

In case anyone is interested, this problem was completely fixed by replacing the purple ignition wire from the back of the ignition switch all the way back to the positive terminal of the ignition coil. The original wire also powers the gauges, so I spliced in a new wire only for the ignition coil and ran it back to the engine. The original wire ran up high behind the dash board and is quite difficult to access so I went the route of an all new wire. In conclusion, I went from a voltage drop of over 2 volts through the circuit to a drop of just .3 volts. Anything less than .5 is considered acceptable. The original wire had numerous splices and connections that must have built up high resistance over the 16 years of the boat's life. Added up, it was too much resistance and gave a symtom of a boat that gradually lost power over the years, particularly under load and high speed. Thanks for the input.

Its so nice when people post the fix, as you just did, as opposed to those that receive a bunch of time and info from other forum members and then just disappear...... Kudos for the follow up ..... Nice work... Thx