OMC 460 backfire under load

[Trevthefox]

Sounds to me like the jets are too small. 650 could be fine for a 460 with the right jets. Fords are thirsty motors, maybe better off with a 750 in the end

I would do a plug burn to determine proper jet size

I thought about that but everything seemed to work fine the past couple of years. Maybe a backfire here and there but wasn't a huge issue. The jets are the size specified in the manual.

I've only made a couple changes from last year to now.
1) Lowered fuel level in both fuel bowls because I was having a flooding issue after coming down from higher throttles. But that change has now been reversed

2) added a check ball that was missing to the vacuum secondary diaphragm. ( i read that this could have been causing my flooding issue)

3) added a 1 inch phenolic spacer between carb and intake manifold

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[Scott Danforth]

Ignore the manual when it comes to jetting. That is the starting point for reading the plugs

 

[Trevthefox]

It was definitely the carb. After messing with it so many times and wasting time going down to the boat in this LA traffic we figured we just bite the bullet and buy a brand new carb. The carb was undersized at 650 cfm anyways. Ended up getting a pretty good deal on a 770 cfm marine holley. Slapped her on and ran like a dream last night.

Maybe I'll take apart the old carb again and try another rebuild and then sell it. But at least the boat is up an running with no backfire.