350 Magnum Back Firing Under Load

[kvkon]

Been helping a friend get his engine issue worked out. Mid 90s 350 Magnum Alpha 1 Gen 2. Back fires and misses from about 2500 rpm and higher only when in the water and under load. Runs perfect on muffs on the trailer. Changed plugs, dizzy cap and rotor and ignition sensor. Put a timing light on it and timing looks good, advance is working. Plug wires are nearly new. Any suggestions? Coil? Carb?

 

[QBhoy]

The usual reason would be water in the fuel. Even if you’ve topped up with fresh fuel. Any amount previously in there with a little contamination will affect the rest of it.

 

[Bt Doctur]

Or a broken valve spring, burnt valve, low fuel delivery,

 

[kvkon]

Or a broken valve spring, burnt valve, low fuel delivery,

Did a compression test. All from 180-190. You're thinking fuel? Carb?

 

[Bondo]

Did a compression test. All from 180-190. You're thinking fuel? Carb?

Ayuh,..... I'd start with checkin' the Contents of the fuel filter, 'n changin' it,.....

 

[Scott Danforth]

Did a compression test. All from 180-190. You're thinking fuel? Carb?

first, check your gauge. anything over 150 psi on a new stock motor is a bad gauge or water or oil in the combustion chamber. to get 180-190 psi on a used motor would make it about a 14:1 compression drag motor and not a 9.4:1 boat motor or you have a junk gauge

then as Bondo stated, check the health of your fuel

backfiring under load is generally called a "lean sneeze" and can come from any of the following (no particular order):

water in fuel
contaminates in fuel
dirt in fuel delivery system (carb or EFI issues)
stuck AF valve
improper timing
broken valve spring
burnt valve

 

[alldodge]

Mid 90s 350

Depending on ignition, it may need to be placed in base timing mode to set
Fuel pressure low

 

[Bt Doctur]

how come nobody will believe broken springs. GM motors are usually 2 springs an inner and outer, They both need to be intact to develop the pressure needed to seal .When all else has been checked, remove the valve covers and check, with a broken spring you will see it or you will be able to push the retainer down by hand

 

[kvkon]

"to get 180-190 psi on a used motor would make it about a 14:1 compression drag motor and not a 9.4:1 boat motor or you have a junk gauge" Off to the shop to check that gauge. When I ran the compression test I expected to see about 135 but was more concerned about all being about equal. Gonna pull the carb and put a kit in it today. Boat sat for a year and a half and carb is probably full of gunk. He didn't tell me this bit on info until today.

 

[alldodge]

how come nobody will believe broken springs. GM motors are usually 2 springs an inner and outer,

Don't see anyone disagreeing that a broken spring is not a possibility, just listing other things that it might be

 

[kvkon]

how come nobody will believe broken springs. GM motors are usually 2 springs an inner and outer, They both need to be intact to develop the pressure needed to seal .When all else has been checked, remove the valve covers and check, with a broken spring you will see it or you will be able to push the retainer down by hand

Agreed! Will do, thanks!

 

[QBhoy]

Drain the fuel tank completely and fresh fuel in it. New filter and be done with it. The odds would say it’s a water in the fuel thing. As said earlier. Even if you have only a gallon of contaminated fuel there, you add another 20 gallon of fresh on top...that gallon can still ruin the rest of your fresh fuel. Even if you’ve built the carb recently...a little dud fuel through it can mean you need to do it again.
check that fuel filter and empty it into a glass. Leave it over night and look for water.

 

[kvkon]

Drain the fuel tank completely and fresh fuel in it. New filter and be done with it. The odds would say it’s a water in the fuel thing. As said earlier. Even if you have only a gallon of contaminated fuel there, you add another 20 gallon of fresh on top...that gallon can still ruin the rest of your fresh fuel. Even if you’ve built the carb recently...a little dud fuel through it can mean you need to do it again.
check that fuel filter and empty it into a glass. Leave it over night and look for water.

Fuel tank was out of the boat and drained dry with fresh tank pumped from local gas stn last week. Fuel filter is new.

 

[kvkon]

I'm looking at the carb now. It looks like an Edelbrock (1410 maybe?) or a Carter AFB. No tag on it. 350 Magnum motor. Anyone know what Mercruiser was using about 1995? It's definitely the original carb from new.

 

[alldodge]

I'm looking at the carb now. It looks like an Edelbrock (1410 maybe?) or a Carter AFB. No tag on it. 350 Magnum motor. Anyone know what Mercruiser was using about 1995? It's definitely the original carb from new.

It was either a Carter AFB or Rochester Q-jet

Here is your motor, maybe
https://www.mercruiserparts.com/350-mag-alpha-gm-350-v-8-1987-1995-0b

 

[Bondo]

Anyone know what Mercruiser was using about 1995?

Ayuh,...... The Merc/ Weber/ AFB, 96XX,...... An Exact copy of the Edlebrock 1409,.....

Which is Great, 'cause Edlebrock parts are plentiful,.....

 

[kvkon]

Ayuh,...... The Merc/ Weber/ AFB, 96XX,...... An Exact copy of the Edlebrock 1409,.....

Which is Great, 'cause Edlebrock parts are plentiful,.....

Yep and the carb kit for it cheap also. Thanks!

 

[Bt Doctur]

Have you run on an outboard tank directly to the fuel pump to eliminate the fuel system. Only because I had a customers boat that would literally stop running at high speed. Eventually found a screen INSIDE the fuel pickup tube.

 

[Bt Doctur]

click to enlarge

 

[kvkon]

click to enlarge

Thanks, I'll check on that. Wondering if some crud flaked off the inside of the tank when it was sitting dry for about a year.