Mercruiser backfiring - Stumped

[marsharc]

I have a 1992 Pursuit 255 with a Mercruiser 454, Bravo 2 outdrive, Thunderbolt 4 ingition, 1405 hours.

At the end of last season it was hard to start, fouling plugs and would backfire when trying to get up on plane. After several bangs it would smooth out and run.

This year I had the following done. New plugs, plug wires, distributor cap, rotor, carb worked on (the mechanic said the choke was unhooked and it was set too rich), timing set. Parts all Mercury.

Launched it last weekend and it started great, idled fine but when I went to put it up on plane it backfired so bad it scared me. Would not smooth out and run like last year.

Oil pressure=40-50, Temp 155-160, Amps 12-14, Fuel had 1/4 tank from last year with Stable in it, filled the rest of the way with premium on way to launch.

If anyone has an idea where to look next I would greatly appreciate it.

 

[zbnutcase]

Re: Mercruiser backfiring - Stumped

You dont have points and condenser so how were they changed? If your mechanic said he changed them and charged you for it you need a new mechanic badly. And if this clown touched the carb....

 

[PitchFork]

Re: Mercruiser backfiring - Stumped

Maybe check the secondary butterflies spring maybe it is too loose. The same thing happened to me when I way adjusting the secondary spring and it broke. I was unable to accelerate hard. I had to slowly work the throttle up to 3K+ RPMs.

 

[marsharc]

Re: Mercruiser backfiring - Stumped

My mistake, I assumed it had points and condenser. First time i had used this guy but he came with good references and since the backfire never happens until it is under a load and he never had it in the water i figured he didn't get to see if the parts and work had fixed it.

 

[imported_TheMan]

Re: Mercruiser backfiring - Stumped

I'm with ZB.

If it used to smooth out and run then it is probably a carb issue and not a timing issue.

The carb should be rebuilt if it wasn't.. How do your plugs look this year? Are they fouled or is the engine running too lean?

 

[marsharc]

Re: Mercruiser backfiring - Stumped

I'm with ZB.

If it used to smooth out and run then it is probably a carb issue and not a timing issue.

The carb should be rebuilt if it wasn't.. How do your plugs look this year? Are they fouled or is the engine running too lean?

I didn't pull a plug yet. I can do that this weekend. What should i be looking for?
Thanks

 

[6meter]

Re: Mercruiser backfiring - Stumped

Go for a coil. Match for the boat, ie internal or external resistor. $20

 

[tschmidty]

Re: Mercruiser backfiring - Stumped

Typically backfires are because it is running lean. Your statement about the mechanic saying it was running rich makes me think he leaned it out too much. Curious too how he determined that since really it all comes down to ballpark/judgement unless you hook an o2 sensor up to it and run it at different loads. Also how did he adjust the 'rich' setting? Guessing he just cranked in the idle mixture screws so it may just be a matter of turning those back to a recommended setting.

 

[Hansolo99]

Re: Mercruiser backfiring - Stumped

Typically backfires are because it is running lean. Your statement about the mechanic saying it was running rich makes me think he leaned it out too much. Curious too how he determined that since really it all comes down to ballpark/judgement unless you hook an o2 sensor up to it and run it at different loads. Also how did he adjust the 'rich' setting? Guessing he just cranked in the idle mixture screws so it may just be a matter of turning those back to a recommended setting.

I agree it sounds like it is too lean. I hate when people adust a carb and just give it to you. I always look for someone that can run the boat under load (like on a lake or in a tank) and really throttle up. They guess give it to you and this is what happens. This is something you want to address right away. Too lean is not good.

 

[Bondo]

Re: Mercruiser backfiring - Stumped

At the end of last season it was hard to start, fouling plugs and would backfire when trying to get up on plane. After several bangs it would smooth out and run.

This year I had the following done. New plugs, plug wires, distributor cap, rotor, carb worked on (the mechanic said the choke was unhooked and it was set too rich), timing set. Parts all Mercury.

Ayuh,... I agree, it sounds like it's going Lean at higher rpms...
The Carb needs Rebuilding, or Replacing...

 

[wca_tim]

Re: Mercruiser backfiring - Stumped

Ayuh,... I agree, it sounds like it's going Lean at higher rpms...
The Carb needs Rebuilding, or Replacing...

if car is seriously full of crap it could be dumping gas at idle and lean once you give it a little throttle. I would bet on carb and either a careful rebuild or just replace it with an aftermarket if it's tired and in bad shape.

I've seen engines pop and backfire under throttle with coil / ignition system problems as well, but in completely different applications.