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MERC 7.4L Problem

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Old 07-22-2010, 01:07 PM
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Default MERC 7.4L Problem

I am having some trouble with a mercruiser 7.4L motor. the motor starts and idles fine, it even runs fine to a point.

the motor will idle all day, but once you are moving, anywhere above roughly 3000 rpm, every so often, the motor basicaly cuts out for a split second and then just goes back to running, the motor is running very well, then that split second it essentially stops, but picks right back up and keeps running good. that is my problem.

it was doing this before I changed some parts and did a few things, figured that would fix this problem along with the other problems I was having, but I was wrong, so recently I have replaced the cap, rotor, ignition sensor, plugs, and plug wires, the carb was also rebuilt- that was due to ethanol/water problems..

I checked a few things, main ground is tight and secure, battery leads are secure and positive blocks are secure.

what could this be? what can i check? thanks, does not mater whether water is rough or glass it happens both ways.

thanks for responses in advance
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Old 07-22-2010, 08:12 PM
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Well you checked all the stuff i was thinking what about fuel filter and pump is the pump electric?
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Old 07-22-2010, 08:49 PM
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no mechanical fuel pump, which incidentally was replaced at the end of last year.. i basically had a ton of small problems that all masked each other, now just left with this one.

I thought about it some more, and I guess it could be fuel, but it is too perfect in my eyes to be fuel. it does the same exact thing whether its at 3000 or 4000 or 4500, and it does it in smooth water, rough water, anything. cuts for that split second and is running perfect again, the prop stops turning too because the boat feels like you are hitting the brakes...

i was thinking, could it be something with my kill switch or shift interrupter, not really even sure about the 2nd option, it is a bravo 1 drive, not sure if they have that like the alpha's. mechanics around here are really backed up, and just trying to figure something out on my own.
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Old 07-22-2010, 10:25 PM
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Do you have a rev limiter could be acting funny weird that it does it at 3000rpms and then acts fine what kind of ignition
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Old 07-22-2010, 10:29 PM
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Change your coil. I had something similar on my 454 Mag.
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Old 07-22-2010, 10:43 PM
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it is thunderbolt V, also, changed the coil last year, at the end of the season as part of my whole trouble shooting thing, figured it was old and cost was very cheap so i just did it. lol I am sorry I forgot to mention that, as you can see, as far as ignition goes, I have changed everything except the module for the ignition, what about the rev limiter, how would i know whether my motor has it or not?
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Old 07-23-2010, 12:10 AM
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Quote:
Originally Posted by firehawkcat View Post
Do you have a rev limiter could be acting funny weird that it does it at 3000rpms and then acts fine what kind of ignition
+1. Just had a rev limiter go bad awhile back. It would only do it intermittently at first, then it got to the point where the motor wouldn't fire back up until it cooled down.

Same symptoms, we would be cruising along, then the motor would cough and pick right back up.
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Old 07-23-2010, 12:13 AM
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Take a couple of pics of your ignitiion. Most TBV's have rev limiters.



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it is thunderbolt V, also, changed the coil last year, at the end of the season as part of my whole trouble shooting thing, figured it was old and cost was very cheap so i just did it. lol I am sorry I forgot to mention that, as you can see, as far as ignition goes, I have changed everything except the module for the ignition, what about the rev limiter, how would i know whether my motor has it or not?
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Old 07-24-2010, 06:21 PM
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Last edited by Timeless61; 07-24-2010 at 06:28 PM.
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Old 07-24-2010, 06:22 PM
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images are not showing up for some reason, i will put up a link. but, I ran the boat on the hose today and fooled with a lot of things while it was running, and it will not do what it does in the water on the hose... i was wiggling every wire for the ignition at the motor including the cannon plug, all connections are good, only does it while actually using the boat. if that tells anyone anything.
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