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Discussion Starter #1
I have a 1996 7.4.
I noticed the motor was missing/popping last year so I changed the plugs and wires, cap and rotor. That didn't seem to solve the problem.
We're out on our first trip and the boat ran great the first day out but started doing the same thing on the second day. We could hear the motor
popping on the left side so I changed the 1,3,5,7 plugs. They were all slightly rich and the 7 was very rich.
With the new plugs, it ran better for a half day and then got worse. Today my top speed went from 45 down to 30-35 with a lot of popping/missing while the motor is under a load.

Thoughts on what could be the cause?
 

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Discussion Starter #3
whoops.
Motor is carbed. Holley DP. It has Imco Powerflows and a K&N filter. Other than that, it's stock.

Motor was opened up a few years ago due to an impeller problem. Heads were sent out to be checked and I believe they did a valve job on them. The Imco's were added at that time.
Motor has been hard starting since then but did not have any problems with missing/popping until last year.
It usually turns about 4900 rpm at wot and trimmed up. Less now that it's acting up.
I've read other post that suggest a leak down test. Not sure if that is something to start with. I've never done one before but after doing research, it doesn't seem to difficult.

Any help/ideas are appreciated.
 

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Discussion Starter #5
do you know how to check float bowl levels? sounds like a carb thing/ fuel delivery. but i'm just guessing -Jas
We checked the float level last season when I first started having problems. It was ok. Also through in a new power valve.

Would that make 1 cylinder continue to foul/gunk up a plug?
 

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Where?

We checked the float level last season when I first started having problems. It was ok. Also through in a new power valve.

Would that make 1 cylinder continue to foul/gunk up a plug?
Where were you boating last, and where are you now? If an engine is getting more fuel than the air available it will pop unburnt fuel in the exhaust. IF you're super rich the excess fuel CAN puddle up in the floor of the plenium and drain into the back runners, or runner, if it is a dual plane intake. Is #7 the lowest runner?............Just an idea...MP
 

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Discussion Starter #10
Where were you boating last, and where are you now? If an engine is getting more fuel than the air available it will pop unburnt fuel in the exhaust. IF you're super rich the excess fuel CAN puddle up in the floor of the plenium and drain into the back runners, or runner, if it is a dual plane intake. Is #7 the lowest runner?............Just an idea...MP
I was having the problem last year at Mojave, Havasu, and Lake San Antonio.

The intake is stock Mercruiser. Not sure if it's single or dual plane.
Carb is not stock. Maybe it's to big for the motor.
 

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Discussion Starter #11
Twist

Ok, here's a twist.
Saturday we went from Havasu to Park Moabi. Boat was spitting a lost and not running at full power. We cruised up there and did not push it to hard. Sat at Park Moabi most of the day with a quick cruise to the store to get the kids Ice cream.

We left Park Moabi after being there 5-6 hours and on the way back, the boat ran great.

One thing I have been noticing is that my amp guage is sometimes at 9-10. On the way back down river when it was running good, the amp guage was showing 13-14 amps. I'm wondering if I have a bad connection or an alternator that is on it's way out and maybe not allowing my ignition system to get enough juice????

Thoughts?

I think I may do a leak down test this week just to see if anything shows up. Also will check the alternator.

Thanks again for the suggestions.
 

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I had a case with Mercruiser ignition where the boat started fine and ran OK but it popped under hard load only. It turned out to be the coil. As soon as I pulled the coil wire out of the coil I could see it was burned.
 

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I'm taking it 'amps' was really meant as 'volts.'

All ignitions are sensitive to alternator/batteries since ignitions are electric devices that run off those.

Get your alternator/battery/and or electrical woes work'd out before you start throwing parts at the wrong area.
 

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Most ignitions need at least 10 volts and up to run properly. I would check the electrical system, I think you answered your own question.
 

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Discussion Starter #17
So I installed the new alternator before our trip last week.
1st day off the trailer the guage was up to 12-13v. Good.
I didn't notice it missing at all. We cruised and I finally opened her up, trimmed up and my RPMs were up to about 5500 at top speed. I've never seen the RPMs up that high with this boat. I was happy as the boat was full of gas (100 gallons) and was almost hitting the top speed I get when it's empty. So it looks like the alternator helped. I was also surprised to hit that speed as I just installed some silencer tips that are pretty restrictive compared to what I had before.

Here's the twist. Sometimes the RPM's would max out at 4800 and other times 5400-5600. I'm thinking I still have a part that is on it's way out or a bad connection somewhere.
Think I'm going to replace the coil. The boat is due for it and we'll see what that does. Also going to check the connection on my tach.

I was really happy with the silencer tips though. I can hear the stereo a lot better when cruising at 40ish. :D
 

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Due for it?

So I installed the new alternator before our trip last week.
1st day off the trailer the guage was up to 12-13v. Good.
I didn't notice it missing at all. We cruised and I finally opened her up, trimmed up and my RPMs were up to about 5500 at top speed. I've never seen the RPMs up that high with this boat. I was happy as the boat was full of gas (100 gallons) and was almost hitting the top speed I get when it's empty. So it looks like the alternator helped. I was also surprised to hit that speed as I just installed some silencer tips that are pretty restrictive compared to what I had before.

Here's the twist. Sometimes the RPM's would max out at 4800 and other times 5400-5600. I'm thinking I still have a part that is on it's way out or a bad connection somewhere.
Think I'm going to replace the coil. The boat is due for it and we'll see what that does. Also going to check the connection on my tach.

I was really happy with the silencer tips though. I can hear the stereo a lot better when cruising at 40ish. :D
Is there a "life time" in the manual for the COIL?......Coils will usually last 50,000 miles or more in an automotive application. Your variation of voltage/charging system is probably where you trouble is. Check ALL the connections, especially ground connections, AND your charging system goes THRU the gauge to the battery. A loose or corroded connection can cause you months of grief, not to mention a small fortune replacing the wrong parts.......Good Luck.........Oh, how many batteries on board?.......Maybe a bad one?.......MP

PS: Maybe your RPMs are not changing, maybe its the tach, which could explain a lot...........There is a hidden message here, ignition impulses is what makes the plugs fire, AND the tach work...Hmmmmmmm......Look under the dist cap.........
 

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Only with an amp gauge does the charge feed go THRU the gauge. Volt gauges get hooked up ACROSS the gauges.
 
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