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Merc 454 black plugs

Old 05-23-2021, 05:54 PM
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Angry Merc 454 black plugs

I have a 96; 454 Mag with a 440 HP build. Edelbrock intake and new Holley quickfuel 750cfm carb. Distributer cap and rotor on thunderbold IV ignition.

Back story - on land ran great, no smoke, good idle, no issues, on the lake under load it backfires and stalls out of you push it. So I did the following: replace edelbrock 650 card with the holley, new sea water fuel mechanism and pump, coil, dist cap and rotor, plugs, wires, gas is non ethol and new, new fuel water separator. Did a compression test and all cylinders between 140 and 147. Timing was checked and set at 8 degrees off.

When the new carb was first installed the boat idled at 650 and ran solid while in the boat slip. no smoke or anything. Next day, will not idle, lots of smoke white/ grey/ black. Pulled plugs and 1 and 2 were just wet with gas. 2,3,4,5,6 were black, 3 and 6 especially oily. 7 and 8 just a black coating. I am at a total loss, any help will be appreciated.
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Old 05-23-2021, 08:58 PM
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Back to the basics..

You mention something about needing to adjust the timing 8 degrees. Or something like that.

And help me understand what you mean about it "running fine" out of the water. Did you run it under load in a car or a dyno?

Then I read that you swapped the carb. Was this before it started running badly or after it ran badly? A detailed timeline will be much more useful than comments out of sequence.

Your compression test - was that done cold or warm? Were all the plugs out and the throttle held wide open? Or was the throttle shut? Or some tested with a few plugs still in??

Very likely, you are experiencing flooding due to trash in the carb inlet needles, improper float level, intake obstruction, dirty flame arrestor, excessive fuel pressure, a recent adjustment to the carb, etc.

But if you give more details and a timeline, I'm sure you'll get some better guidance.

MC
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Old 05-24-2021, 07:31 AM
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Originally Posted by mcollinstn View Post
Back to the basics..

You mention something about needing to adjust the timing 8 degrees. Or something like that.

And help me understand what you mean about it "running fine" out of the water. Did you run it under load in a car or a dyno?

Then I read that you swapped the carb. Was this before it started running badly or after it ran badly? A detailed timeline will be much more useful than comments out of sequence.

Your compression test - was that done cold or warm? Were all the plugs out and the throttle held wide open? Or was the throttle shut? Or some tested with a few plugs still in??

Very likely, you are experiencing flooding due to trash in the carb inlet needles, improper float level, intake obstruction, dirty flame arrestor, excessive fuel pressure, a recent adjustment to the carb, etc.

But if you give more details and a timeline, I'm sure you'll get some better guidance.

MC
Hello,
Let me timeline this:
Day 1: First start of the season on trailer in my driveway. Idled fine, no smoke, ran cool, good throttle, ran it for 30 minutes. Before starting it I replaced the mechanical fuel/water mechanism, plugs, cap/rotor, oil, thermostat.

Day 2: Idled boat in driveway. All seemed very good. Starts right up, no smoke, ran fine. Took to gas station and put 30 gallons of non ethanol 90 premium in it. Took to marina and put in the water. Went on the lake. As soon as I tried to get on plane it started sputtering and backfired out of carb. Pulled boat back to my boat well.

Day 3: A mechanic met me at the boat. I pulled the plugs and all black and wet. I replaced the plugs, wires, coil, fuel/water separator, the mechanic checked the timing and told me it was at 8 degrees. My engine build is a 440HP not stock 330 or 360. Not sure why 8 degrees but he has same builds on his scarab. Didn't question it. Boat started right up, idled fine, good throttle. Took on the lake and exact same issue when under load. Went back to boat well.

Day 4: Pulled plugs again, all black and wet. Since I had the brand new quick fuel, holley 750, and have had issues with the edelbrock 650, we decided to replace it. Put new carb on boat. Boat ran a little rough at first and minor white smoke. Once it warmed up and a few adjustments idled and rev'd perfect. Didn't take on lake do to time constraints.

Day 5: Went to boat. tried to start it. very rough and lots of smoke. Wouldn't idle and didn't want to push it for fear of doing damage. Did put at 1/4 throttle and it wanted to run but again smokey. Pulled the plugs: 1 and 2 just wet with gas but clean, 6 and 4 were very black with soot and oily. Rest just black that wiped right off. Did a compression test on all cylinders. WOT and pulled one plug at a time. I cranked for 6 cycles on each cylinder. Waited a minute for loss of compression. All cylinders between 141-150. Didn't see any loss of compression. Checked oil, very clean no signs of water. there is a milky substance in my engine bilge.

Talked to mechanic and he said the holley needs jetted down so ordered a jet kit.

At a loss other wise. Motor was pulled last year and a new coupler installed. While out gaskets replaced and pistons, valves, etc. inspected. All looked fine. 900 hours on boat.
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Old 05-24-2021, 05:09 PM
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So...the engine/boat ran perfectly fine last year, correct? This "440 hp" build? You've had it for...how long? And it's run fine ever since built?

Then the engine was pulled to put in a new coupler, and replaced "gaskets"? Which gaskets and why? What was this "inspection" of the pistons, valves, etc? You pulled the heads off at some point?

Just trying to understand why you'd start replacing all the things you mentioned on an engine that had been running great...?
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Old 05-27-2021, 06:12 AM
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Originally Posted by [email protected] View Post
I have a 96; 454 Mag with a 440 HP build. Edelbrock intake and new Holley quickfuel 750cfm carb. Distributer cap and rotor on thunderbold IV ignition.

Back story - on land ran great, no smoke, good idle, no issues, on the lake under load it backfires and stalls out of you push it. So I did the following: replace edelbrock 650 card with the holley, new sea water fuel mechanism and pump, coil, dist cap and rotor, plugs, wires, gas is non ethol and new, new fuel water separator. Did a compression test and all cylinders between 140 and 147. Timing was checked and set at 8 degrees off.

When the new carb was first installed the boat idled at 650 and ran solid while in the boat slip. no smoke or anything. Next day, will not idle, lots of smoke white/ grey/ black. Pulled plugs and 1 and 2 were just wet with gas. 2,3,4,5,6 were black, 3 and 6 especially oily. 7 and 8 just a black coating. I am at a total loss, any help will be appreciated.
hello brian,,,did you figure out anything here???
sounds like carb issues....
good luck..
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Old 05-27-2021, 07:14 AM
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All good replies above.

Awaiting info to those questions asked, ill add:

Ignition MSD by chance ? Or still Merc Tbokt IV ?

Id be checking fuel psi. If good, check the fuel for water.

Few carb notes: surely that Edelbrock 650 was really a 750 ? That Quickfuel 750 should not be needing smaller jets on a 440hp 454 big block. Most 750 are set up out if bix for lower hp big blocks and decent hp 350/383 small blocks. So be careful with that advice.

Last edited by SB; 05-27-2021 at 07:19 AM.
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Old 05-29-2021, 09:08 AM
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I have a marine Quickfuel 750 too. On a 10.3:1 383. Mine is sensitive to fuel pressure. If it's over 7 lbs it wets spark plugs. Thankfully I put a gauge on there. Once around 6 psi it's happier.
Not saying that's all you are up against, but it matters. Also, you have float windows on that carb, so be sure the level is right in the glass viewer. Easy to change too, as you probably already found.
What cam is in your build? If it's even remotely performance you might need added timing. Most TB IV's add 24 degrees all in. So if you were 11 degrees idle you'd be 35 at 3000 rpm.
I don't speak big block, but I'd think 35 degrees is ok?
People who know more will correct me if I'm wrong.
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Old 05-29-2021, 01:26 PM
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Originally Posted by NHGuy View Post
I have a marine Quickfuel 750 too. On a 10.3:1 383. Mine is sensitive to fuel pressure. If it's over 7 lbs it wets spark plugs. Thankfully I put a gauge on there. Once around 6 psi it's happier.
Not saying that's all you are up against, but it matters. Also, you have float windows on that carb, so be sure the level is right in the glass viewer. Easy to change too, as you probably already found.
What cam is in your build? If it's even remotely performance you might need added timing. Most TB IV's add 24 degrees all in. So if you were 11 degrees idle you'd be 35 at 3000 rpm.
I don't speak big block, but I'd think 35 degrees is ok?
People who know more will correct me if I'm wrong.
hello,,,a comment on the timing...
i was running total 38 degrees,,,but then read some comments and suggestions in here and on line,,,some were saying 32-34 total,,,so i have adjust my total down to 32 --33.
the 38-40 came from my super gas car racing days,,,,hope this helps
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Old 05-29-2021, 03:49 PM
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Looks like the OP ditched ship from here. Writes the OP and only posts 5/23.
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Old 05-30-2021, 09:55 AM
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Originally Posted by SB View Post
Looks like the OP ditched ship from here. Writes the OP and only posts 5/23.
no,,he posted on 5/24. you need a thicka pair of glasses too.
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