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Back fires and Stumbles

Old 05-30-2003, 02:52 PM
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Default Back fires and Stumbles

454,s/470 HP
Symptoms, Start good, idles great.
In neutral , real easy acceleration, slight stumble on both motors and then smooths out at 2600 rpm on up.
In gear, smoth acceleration on port motor but, stb motor pops through carb one time now and then , but run fine at 4000 - 5000.
Does not always do it , but sometimes it will pop every time you give it a hard acceration.

What do you think?? Ignition or fuel
8 deg initial timing/mark iv box 26 degree curve
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Old 05-30-2003, 02:57 PM
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Had a similar problem. Was an easy fix for me. Can of carb cleaner and a K&N air filter. Carb was all carboned up.
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Old 05-30-2003, 03:26 PM
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I just went through this same thing and all I had to do was to use fuel treatment. It cleaned right up.
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Old 05-31-2003, 01:30 AM
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Popping through carb is usually one of several things-

1) Engine needs more fuel (carb adjustment). Swap carbs and see if the problem changes sides. If it does, check carb- jets, accelerator pump diaphragms, pump nozzles etc. for proper operation. If it does not change sides, check fuel pressure and filters.

I don't know what carbs you have, but some have in-line filters in the carb. The Rochester Quadra-jet is one.

2) Bad exhaust lobe on cam (don't panic!)- probably not the case if it clears up at higher RPM.

3) check timing at idle and at full-in RPM- eg. 32 degrees at 2800 RPM

4) change plugs

Good Luck!
Bulldog aka Ronnie
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Old 06-01-2003, 04:45 PM
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Check the obvious
Sounds like spark plug wires could be breaking down. or perhaps something in the cap?
Weird it is on both engines, That may be fuel???
 
Old 06-02-2003, 12:48 PM
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Well, long story short...
Went over to check engines, but guy that owns boat was not down for the weekend. He had to work. So, I uncover the boat and start to work. Seems he decided to change plugs and crossed up a couple of wires. Fixed that, and found a bad crimp on the + side of the coil. Fixed that. Found HE wire on coil to fit sorta loose, Fixed that. Refired engine idles better. Adjusted mixture so that it sounds like it is not singing soprano any more. Noticed that if I move the accel pump down to the next hole, less fuel, (Edelbrock Marine Carbs) that it tranistions from idle to mid range much smoother. Did not go for a test run yet. Will try next Saturday and let you know. Thanks for the input...We're getting closer!
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Old 06-10-2003, 10:37 PM
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So Helmwurst

Did you test ride?
 
Old 06-11-2003, 09:59 AM
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Reset accel pump on both motors to middle hole. Transitions off idle much smoother. Re-calibrated both carbs idle mixture. Boat sounds strong and pulls hard through entire power band. Installed an old coil from my street rod on the backfiring motor. Did not detect any backfires or stumbles. It was so windy on Sunday that you could not hear anything! Suggested that we install new coil and wires on both engines to eliminate any possible problems from arching or breakdown.
Definitely runs better, but has room for improvement.
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Old 06-13-2003, 03:51 AM
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Helm....Im having the same problem with my new Holley I just installed and that was one of the reasons I changed from my Edelbrock....So your saying that by leaning out the shot you got rid of the stumble and backfire? Seems like everything Ive read sent me going richer on the shot I will try to lean out my shot and see what happens.....

BH
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Old 06-13-2003, 05:52 AM
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I had same probs here but there were several things wrong - each contributing to similar symptoms. Once I fixed all the stuff the shop had done, I had solved most of it. Here's what I found and fixed:

Fuel pumps were hooked to coil, cutting voltage to entire ignition system, not enough spark, eventually burned up coils too. Installed a fuel pump relay from PainlessWiring.com

Carb gasket was skewed off center, reseated the carb.

Bolts on carb were too long and not creating a seal at the intake, got shorter bolts to ensure good tight fit.

Checked timing, no problem.

Spark plug wire on #2 was not connected.

Reset idle mix (Holley 800's, Dart Intake on 502 stock block)

Just a little load up while running 6MPH zones, will sputter on hole shot unless I blip the throttles a bit to clear it out. Other than that, no more backfire and misses from throttle up through 3800, still breaking them in.
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