7.4L Carb stutter issue, I'm at a loss.

O

osubob

Member
Aug 8, 2012
31
West Michigan
Boat Info
1997 330 Sundancer
Engines
7.4L VDR
Hello,

I've been chasing a problem with my port side mercruiser in my 1997 Sea Ray Sundancer. Its a 7.4L carburated inboard connected to a V-Drive. When I'm up on plane (3200-3400RPM), I periodically get a stutter in the engine. The RPM's drop 1000-1500RPM for a split second and then the engine returns back to the cruising RPM. I have performed the following with no impact:

-Replaced all spark plugs, gapped to the recommended 0.035"
-Replaced spark plug wires
-Replaced Distributor Cap
-Replaced Distributor Rotor
-Replaced Distributor Pick-up
-Inspected the main engin harness. Unplugged, and re-plugged it in a few times.

I did have a mechanic take a look at the issue and they confirmed that it is loosing spark when the engine stutters, so I am fairly confident that it is on the electrical side.

The only items I can think to replace next are the coil or the ignition system. I'm hoping it is the coil (Cheaper of the two components).

I do see the stutter at no-wake speeds, however, it does not drop RPM nearly as much at this speed (Maybe a few hundred RPM)

Am I missing anything else? Any idea what could be causing this other than what I already replaced?

Thanks for your help!
Bob
 
Just a few thoughts:

-Remove engine ground, clean, replace.

-Remove main engine electric plug at harness......inspect, re-seat.
 
Having very similiar issue. Please post if you find solution and I will do likewise.

My problem started like your and got worse.................... mechanic looking at it soon.
 
Same issue here....yesterday my port 454 7.4 mercruiser sputtered and stalled. I could not get her back without giving significant revs on the throttle. Thought it was a fuel issue so changed: fuel filter, water separator and played with the choke on the carb which has given me issues in the past. Still had issues staying on at idle. So then I went to electrical. Checked sparks, changed wires, roter, cap, and coil. Still had an issue.....but not stalling out any more just sputtering. Then adjusted the idle a bit under a load at the dock (while heavily tied down) and she began to run great again at both high and low speeds. The took her out for a rip, put her on plane for 30 seconds then bogged down again and stalled. I couldn't get her to fire. limped back to the marina on one engine. very weird.....could be as simple has something small is inside the system and simply needs to be flushed out? Thanks guys.
 
I'm going through the same issues with the stb engine. On the dock and in neutral, it idles perfectly, I then run the 7.4 up to 2,500 RPM without a sputter, but when I take her out for a run and put her under load, I cannot get it over 1,100 RPM before it sputters and loses power.
 
Changed the coil. There was an improvement, but still get a hick up every once in a while. Now thinking ignition system. :(
 
All,

I have an update to my issue. I learned that the thunderbolt ignition system had been changed on the port engine. I was not able to swap between port and starboard because the updated one had different connections. The mechanic and I were able to narrow down the studdering issue down to the ignition system. I have a new one on order and will be installing it soon I hope.

My understanding is that the new systems have a knock sensor and the ignition system retards the engine. The current theory is that the ignition system is thinking there is a knock and is stoping a spark periodically. It is not the sensor, because we swapped knock sensors and the issue remained. I'm happy and sad that we found the problem. Its another $500 I need to spend to get it fixed! Oh well, its boating.

Bob
 
I just changed my coil too. Same problem ... won't rev over 1,100 RPM. Now I am getting a periodic low noise. Where are my exhaust shutters / flappers located? I removed the port exhaust elbow on my stb engine, but it is not in there.
 
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