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454 Backfiring issue

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Brad @ SCJB

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« 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.
  • Boat #1: 1978 Liberty
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Riverlife_

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« Reply #1 on: March 24, 2015, 07:32:46 PM »
Maybe carb gummed up? From sitting in the off season?


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Brad @ SCJB

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« Reply #2 on: March 24, 2015, 07:33:31 PM »
Just had the carbs rebuilt.
  • Boat #1: 1978 Liberty
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Brad @ SCJB

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« Reply #3 on: March 24, 2015, 07:33:39 PM »
Forgot to mention that.
  • Boat #1: 1978 Liberty
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Brad @ SCJB

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« 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.
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« 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.
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mobboss

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« 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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mobboss

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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 ?
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
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« 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.
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« 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
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
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« 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
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« Reply #20 on: March 24, 2015, 10:19:45 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  :thumbup:


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
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
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« 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.
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« Reply #24 on: March 24, 2015, 10:25:46 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.
  • Boat #1: '77 Hondo Pantera GT Jet
"Racing is life!  Anything that happens before or after, is just waiting"    Steve McQueen

The problem with America is stupidity. I'm not saying there should be a capital punishment for stupidity, but why don't we just take the safety labels off of everything and let the problem solve itself?


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