Back fires and Stumbles
#1
Registered
Thread Starter
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
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
#4
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Join Date: Feb 2001
Location: Abita Springs, La.
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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
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
#6
Registered
Thread Starter
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!
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!
#8
Registered
Thread Starter
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.
Definitely runs better, but has room for improvement.
#9
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
BH
#10
CBPBA's Walmart Greeter
VIP Member
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.
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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K2
-----------------------------------------
CHESAPEAKE BAY POWER BOAT ASSOCIATION
'http://www.cbpba.com'
"Experience is something you don't have until just after you need it."