1994 Mercruiser stumbling above 2700

[tonyiiiafl]

1994 Carver 390 CPMY with mercruiser 7.4L carb engine. The carb was rebuilt by the marina this past winter. When I brought it home, it would get up to around 3800 and that was it. I also noticed a slight vibration/shudder akin to a misfire. I did a shake down run today. Seems OK up to 3200 then the engine sync gauge was a little sporadic. With that shudder, that was on the port side. I dropped anchor and changed the gas filter. No improvement. Now it is smooth as heck up to 2000. However, once above that it has no power sync guage jumps and shakes. Engine got new plugs, cap, rotor and module last fall. Seemed to run great. However, not so this spring. I checked cap and rotor and looks great, no pitting or flashing. I am lost. The only change was the carb rebuild by marina. Engine has 1100 hours and has even and strong compression per the marina last fall.

Any ideas? I am lost.

[waybomb]

I'm guessing they set the float too low.
Did you change the gas filter in the carb?

[tonyiiiafl]

I don't know what and how they did the rebuild. I am bringing it back tomorrow for their look see. I will report back.

[bud37]

I agree with Fred......just one engine or both.....did the marina remove and reinstall the carb ?....if so they have probably done something odd...lets see what they come up with...

[tonyiiiafl]

UPDATE: I unplugged and re plugged in each and every spark plug wire and distributor wire. Then I saw on the port side of the port engine the spark plug wires were out of the separator loom, and had some oil on them. I wiped them off and re established the wires within their respective places in the separator. Then I had a beer and knocked off.

Sunday AM I had my crew, and we set off for the trip to the marina that did the carb work over the winter. I came out into the Cape Cod Canal and gave both engines the throttles. Both engines, including the suspect one came right up and smooth as I remembered. I got to the end of the no wake zone, and did what i NEVER do, and NAILED both throttles forward. The old girl got right up and galloped. Smooth, and up to just over 4000 rpm and 29 KTS. I brought the throttles back down, and then gave it the throttle more sanely as I usually do......again, both engines, came right up to WOT with no issues, and super smooth. I ran at 2800, then 3200 and then 3400, before bringing up to WOT again. NO issues. I got a call from the marina today and they said they went out on a long sea trial, burning 55 gallons of gas. NEVER missed a beat. A gauge was placed on the fuel pump and it tested great. The scan tool was hooked to the harness Nd no issues were found. The diagnosis: Either I had a loose wire (none seemed loose) or a piece of crap was in the gas line and worked its way out. I am bringing her home Wednesday afternoon, so we shall see!

[tomschauer]

Tony, you may need to replace your wires. If they were out of the separators and you had a performance problem they may be cross firing due to cracked insulation. Consider news ones for next season.

[RGrew176]

Sounds good. Hope the rest of your season is uneventful as to mechanical issues.

[tonyiiiafl]

I picked up HOLODECK from the mechanic today and they found absolutely nothing. The fuel pressure tested as it should and the module was also good. The explanation was either a loose wire(Allit takes is ONE!) or the spark plug wires were arcing from not being separated. Anyway, she ran like a champ, no vibrations, and walked right up to 4100 on both engines, and slurped gasoline at 65/gph. I had it up to 31KTS! We cruised back to Taylors Point at 3200 rpms and 21 kts. Lets see how it goes the rest of the summer!

[mjk1040]

Sounds like wires to me. Good Luck!

[bud37]

Good catch...... thanks for posting,wow another wiring issue....I know with ours I found the plug wire looms/holders were very tight and on both engines they were jammed against the ground studs.....