454 Backfiring issue

« **on:** March 24, 2015, 07:29:05 PM »

Working on a friends boat.....ran great last year. He put a new cap/rotor/wires on it....and it is now backfiring out of the exhaust on both sides at idle. Put the old cap/wires back on and it's still happening. Triple checked firing order....swapped the coil......we are stumped. Only thing hes changed since last year is the cap, rotor and wires.

« **Reply #1 on:** March 24, 2015, 07:32:46 PM »

Maybe carb gummed up? From sitting in the off season?

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« **Reply #2 on:** March 24, 2015, 07:33:31 PM »

Just had the carbs rebuilt.

« **Reply #3 on:** March 24, 2015, 07:33:39 PM »

Forgot to mention that.

« **Reply #4 on:** March 24, 2015, 07:35:20 PM »

The backfire is rhythmic going from one side to the other...which point to ignition in my book.

« **Reply #5 on:** March 24, 2015, 07:39:44 PM »

Check plugs

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« **Reply #6 on:** March 24, 2015, 07:42:48 PM »

When ever I get back firing like that . I pull the # 1 plug and and get it to blow.
It sounds like 180 out.

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« **Reply #7 on:** March 24, 2015, 07:47:24 PM »

Could also be a stuck valve. If all else fails pull the valves covers and make sure all the valves go up and down.

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« **Reply #8 on:** March 24, 2015, 08:04:57 PM »

Pulled the distributor... Checked the teeth on both sides. Set it at TDC....same thing.

I would say stuck valve as well but its back firing on both sides.

« **Reply #9 on:** March 24, 2015, 08:25:28 PM »

Well you did say it has been sitting for a year??? Stranger things have happened!

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« **Reply #10 on:** March 24, 2015, 08:26:08 PM »

Could be 2 valves one on each head

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« **Reply #11 on:** March 24, 2015, 08:26:13 PM »

Re check plug wires / firing order, maybe some crossed plug wires

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« **Reply #12 on:** March 24, 2015, 08:38:21 PM »

I would run the valves real quick like. If you have to, back them all off a turn and see if it still pops.

Try the damp towel trick too, take a couple of damp shop towels and start to block off the carbs creating a rich condition.

Do you have the right carburetor base gaskets? Are the floats set right ?

« **Reply #13 on:** March 24, 2015, 09:28:36 PM »

Is it possible that you guys didn't put the wires on right???

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« **Reply #14 on:** March 24, 2015, 09:35:09 PM »

Checked the firing order a million times...put in a brand new battery....been on the phone with Glenn (gt jets) and hes even stumped.

« **Reply #15 on:** March 24, 2015, 09:42:10 PM »

Almost has to be a broken ground wire or something just wrong enough to allow it to run.

Have you tried removing the alternator belt?

I am grasping at everything I can on the way down.

GT

« **Reply #16 on:** March 24, 2015, 10:10:15 PM »

What happens when you rev it up???

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« **Reply #17 on:** March 24, 2015, 10:13:07 PM »

Carbon build up on the heads??? Try a little water down the carb when reving it up. Like a tea spoon!

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« **Reply #18 on:** March 24, 2015, 10:13:09 PM »

Compression test and or leak down could verify or weed out a sticky valve.

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« **Reply #19 on:** March 24, 2015, 10:17:51 PM »

Compression test is 120 across the board

« **Reply #20 on:** March 24, 2015, 10:19:45 PM »

Quote from: mobboss on March 24, 2015, 10:13:07 PM

Carbon build up on the heads??? Try a little water down the carb when reving it up. Like a tea spoon!

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LOL we used to pour like a liter down em....LOL

Ensure the pick up wires are not backwards......

Green from the 6A to the black/purple and purple to the black/orange... Also too grounding the box is muy importante...

Just out loud for the listeners...

GT

« **Reply #21 on:** March 24, 2015, 10:22:51 PM »

Maybe pulled the plug wires away from the boot?! Bad connection? Cap on right? Lol I'm just fishing here

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« **Reply #22 on:** March 24, 2015, 10:23:11 PM »

Are you guys running a 6al box?

If so try to disconnect it. I have herd of them going batty and doing shit ilike that.

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« **Reply #23 on:** March 24, 2015, 10:23:58 PM »

Check exhaust temps to see if you have a cooler cylinder. Will at least help you identify what cylinders are the problems.

« **Reply #24 on:** March 24, 2015, 10:25:46 PM »

Quote from: mobboss on March 24, 2015, 10:23:11 PM

Are you guys running a 6al box? If so try to disconnect it. I have herd of them going batty and doing shit ilike that.

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Maybe change the rpm chip as a test.

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