4.3 backfires when cold.

[Gosub]

Hi there,
I'm looking to buy a Four Winns 180 Horizon with a 4.3 Volvo Penta V6 fitted. The boat is year 2000 but the motor was replaced in 2009 with a new factory unit. Approximately 50 hours used since the new engine.

The motor starts on the button from cold and is fine at marina speeds, but when the throttle is opened it starts to backfire. As the engine warms it does this less and is fine when it is fully warmed up.

Any idea why this is happening?

Thanks in advance.
Steve.

 

[Bondo]

Re: 4.3 backfires when cold.

Ayuh,.... Welcome Aboard,....

Backfires through the carb are generally from a Lean condition....

If yer lookin' at this boat to buy,...
Have the owner fix it, Before ya buy it...

 

[Gosub]

Re: 4.3 backfires when cold.

Thanks Bond-o. If the mix was lean wouldn't it still backfire when it warms up? When warm, it runs flawlessly. My initial thought was ignition system related, maybe distributor cap problems. And when it heats up the damp(?) clears... but I'm far from expert.

Slainte,
Steve.

 

[Maclin]

Re: 4.3 backfires when cold.

If it is a carb fed engine then the backfiring is due to a lean condition as mentioned. It is probably from a combination of choke setting, accelerator pump shot, and idle mix settings.

You can check the accellerator pump by looking down the throat of the carb then open the throttle, should see 2 very solid streams of fuel squirting in immediately when the throttle is moved and continuing until past half throttle.

As for the choke, if you have to hold the choke plate open (on cold engine) to look down the throat of the carb then it is probably set ok.

Idle mix settings, usually 1.5-2.5 turns out from all the way in.

All of this assumes a carb that is in good condition and not missing any internal parts, like check balls, etc.

As an engine reaches full operating temp the pump shot is slightly less important because the fuel is flowing and atomizing better in general.

 

[Maclin]

Re: 4.3 backfires when cold.

Important safety note, only check the accellerator pump like that with the engne off, just sayin'...