Backfire through carb

[proline2550]

I have had a problem with my 94 Mercruiser 4.3 liter 205 HP since last year. The problem is an intermittent pop through the carb, then right back to running smooth and strong. I've changed the entire ignition system from the plugs to the distributer and switched out modules with a friend with the same engine a HP.(Thunderbolt ignition) A friend told me it may be alcohol in the fuel when I filled up at the street pump a few times, or water in the fuel. (I have a water separating fuel filter and change it twice a season. Sometimes it'll do it quite often, Other times I won't hear it for two trips out. Seems to be more prevelant when it's warm outside as it didn't act up at all this past spring until the days became hotter. I have even left the fuel cap off with a standing hose coming out to rule out vacuum lock. I now want to sell the boat and get another one, but don't want to sell my problem to some poor slob who thinks it's not a big deal and he can fix it.

 

[Haut Medoc]

Re: Backfire through carb

I would think an intermittant pop is a fuel condition....
Either a little water in the fuel, or something else creating a temporary lean condition....
Does it do it right after a filter change.....
Have you checked the filter screen @ the carb?
Doubt it is an ignition problem, unless you have cross fire from old wires....
Have you checked their condition & routing?
The only other thing would be a sticky intake valve....
But I don't think that would be all that intermittant........JK

 

[Bondo]

Re: Backfire through carb

Probably Crap in the High Speed circuit of the Carb......

Usually,....... Backfires are caused by a Lean condition.....

 

[Limited-Time]

Re: Backfire through carb

Could be a fuel delivery issue, or carb problem Is there a specific rpm or condition (like under hard accellration) the backfires occurs? Could also be a valve train problem, sticking intake valve or such.

EDIT: Dang you guys, Haut Medoc and bondo, sneaking yer post ahead of mine. Makes me sound like an also ran.

 

[proline2550]

Re: Backfire through carb

Routing is fine, the engine runs top until the misfire/backfire. No specific time when it happens and curiously enough, it happened just as bad with another enging that was in the boat. The carb was changed about 3o hours ago and has no secondary filter inside the housing nor is there a provision for one. It happens as often at normal cruise (2700 RPM) as at higher speeds, but never at idle, such as a no wake zone. Only under the load of being on plane and runnin'. The electric fuel pump has also been changed due to failure and the problem remains. The carb is a Weber for Mercruiser four barrel. My main thoughts are fuel related. Would it harm this Vortec V6 if I put a fuel additive designed to treat any water that might be there?

 

[wrench001]

Re: Backfire through carb

I am having the exact same problem, for two years now since an engine swap. I have done the same repairs as you as well as cam, lifters and timing chain. mine is a 1979 merc 260 (350 cheb engine) i was just talking to a guy at the marina who said that its possibly a riser issue. water getting into the valves. what do you think?

 

[DaveM]

Re: Backfire through carb

Popping through the carb usually points to a fuel starvation issue. The fact that this happens in warmer conditions would tend to indicate corrosion on the fuel pump electrical connector terminals since impedance (resistance) rises with temperature.

Just a thought.

 

[proline2550]

Re: Backfire through carb

Wrench,

I replaced my risers already so that takes care of that one, for me anyway. As far as fuel pump wires, that is a thought, I will check but I'm pretty fussy when I wire things. I subscribe to do it right the first time rule. Heat shink butt connectors so it's straight from the source and not through any kind of bullet connector.

 

[gwhjr98]

Re: Backfire through carb

I had the same problem... it turned out to be a short in the wiring to the fule pump. It would run fine and then start to knock at the high end of the RPMs. Voltage was dropping causing the pump to not push the right fuel. Ran a new wire to the fuel pump to solve.