Carb backfiring and hesitation. Please help!

[jmznshana]

I apologize for the length but want to be detailed. The boat 95 monterey montura bowrider , here's what it's doing and what I've done so far. The motor- Merc 5.7 w/ thunderbolt ign. Motor is new (crate motor) to boat approx 5 yrs ago w/ maybe 100 hrs on it. Last season it developed a carb backfire that occurred when I dumped throttle for hole shot and occasionally while cruising at around 75% throttle and any time I added throttle from that it would pop. Didn't get a chance to run boat this year but about 3 times. The first time out back in March it ran fine for a short 20 cruise, it sat for a few hours and on the way back it was backfiring pretty bad at lower rpms I couldn't get it over 2000 w/o it backfiring repeatedly... After that trip i did a lot of reading here and believed the problem to be a lean fuel issue related to the accelerator pump on the quadrajet. I replaced the carb with a new completely rebuilt carb, new fuel/water separator, checked the plugs, cap and rotor. All looked good w/ no issues. Got it out after that and it seemed to run much better. It did backfire a few times still, although nothing like it was and only when I rapidly dumped the throttle from standstill.... WOT had no issues. Took it back out this past weekend and had issues again but an added one this time. While no wake cruising at maybe 1500 rpm it was backfiring and also could a sluggish hesitation. Could not hear the motor missing or changing its tone but could feel it in the motion of the boat if that makes sense.... I started the day putting in 2 new batteries as they were dead. Noticed for first time that day also the volt meter on dash had a lot of movement... When sitting idle it showed 12-13 volts. As soon as I put in gear and had a load on the motor the needle dropped to 9 +\- volts and this is when the backfiring started.. Also at about 75% throttle it felt like it was not getting full power, like I had to use more throttle than usual to keep the speed I was running 30-35. Water was flat so no wave or wake issues. I took the alternator in and it tested good today.... I'm out of ideas at this point.... Please help Thank you

 

[jmznshana]

Forgot to add it does fire right over even when cold, by florida standards. Idles perfect

 

[NHGuy]

double post

 

[NHGuy]

Your charging system isn't keeping up. It should show somewhere between 13.8 and 14.3 volts once the motor is above idle. At 9 volts the ignition can barely fire the plugs.
Make sure the batteries are hooked up right and any battery isolators or switches are oriented correctly.
Check the batteries states of charge and recharge them.
Check for corrosion in the alternator, starter and battery wiring. Clean the ends with medium or soft metal brushes or with emery cloth.
Then run the engine again. Now if your voltmeter reads low take the alternator out for a test at the auto parts place. But don't buy one there, if needed you HAVE to get a marine alternator. Auto alternators are not spark proof and can cause marine fuel vapors to ignite.
BTW does your boat have a big honkin' sound system or lots of electronic instruments? If you have big amps and a normal alternator that can cause this kind of stuff too.

 

[jmznshana]

Your charging system isn't keeping up. It should show somewhere between 13.8 and 14.3 volts once the motor is above idle. At 9 volts the ignition can barely fire the plugs. Make sure the batteries are hooked up right and any battery isolators or switches are oriented correctly. Check the batteries states of charge and recharge them. Check for corrosion in the alternator, starter and battery wiring. Clean the ends with medium or soft metal brushes or with emery cloth. Then run the engine again. Now if your voltmeter reads low take the alternator out for a test at the auto parts place. But don't buy one there, if needed you HAVE to get a marine alternator. Auto alternators are not spark proof and can cause marine fuel vapors to ignite. BTW does your boat have a big honkin' sound system or lots of electronic instruments? If you have big amps and a normal alternator that can cause this kind of stuff too.

It has brand new batteries, cleaned all connection contacts. Alternator is only a few years old and it was bench tested today at a place that rebuilds starters and alternators. Starter is also only a few years old. Everything stereo/accessories are shut off when the issues happen. Could a failing coil cause these symptoms?

 

[jmznshana]

Anyone else with ideas?

 

[alldodge]

Agree with NHGuy, you have an electrical issue with the 9V. If there is something else going on the voltage reading needs to be fixed first. Your first post

Motor is new (crate motor) to boat approx 5 yrs ago w/ maybe 100 hrs on it.

Start by checking your grounds, remove the main cable attached to the block and make sure it is clean and tight. Check to see if any of the grounds (small ones) had been inadvertently left off. Missing a ground wire behind the engine can happen very easily. Next pull the cannon plug and check/clean the contacts.

What is your engine serial number or at least the year

EDIT: Find out what your fuel pump pressure is by putting a tee fitting in line between the fuel pump and carb

 

[bruceb58]

Put a multimeter on the output of your alternator and find out what your true charging voltage is. Don't trust the dash gauge.