1997 Mercruiser 4.3 LX not running steady steady at idle or higher RPM's

[tedwards516]

Hi, I have a 1997 Mercruiser 4.3 LX Gen+ starts fine, but when at idle or at higher RPMs the engine does not run consistently smooth and the RPMs will waiver. Seems like I tried everything but still having the same problem.

• Checked gas for water
• Replaced fuel filter
• Disconnected from main tank and ran off a portable tank
• Compression tested the cylinders
• New Coil, rotor and distributor cap
• New fuel pump
• Cleaned and rebuilt the carb

Probably starts better than ever, but haven't fixed my issues. Any other suggestions? I would also add that the tachometer seems to read higher that what I get on a test tachometer.

 

[Scott06]

Hi, I have a 1997 Mercruiser 4.3 LX Gen+ starts fine, but when at idle or at higher RPMs the engine does not run consistently smooth and the RPMs will waiver. Seems like I tried everything but still having the same problem.

• Checked gas for water
• Replaced fuel filter
• Disconnected from main tank and ran off a portable tank
• Compression tested the cylinders
• New Coil, rotor and distributor cap
• New fuel pump
• Cleaned and rebuilt the carb

Probably starts better than ever, but haven't fixed my issues. Any other suggestions? I would also add that the tachometer seems to read higher that what I get on a test tachometer.

have you set the timing on it and is the timing jumping around. Also disconnect the tach to see if it changes things. Bad tachs can mess with the spark

 

[nola mike]

also doesn't sound like you actually checked the spark

 

[tedwards516]

Thanks for the input. Tried setting the timing also, but it is jumping around. Also, I only have a single timing notch to match time timing mark to. Wasn't sure if that would be the 8 btdc setting, or if I need to spend more on a different timing light. I'll try disconnecting the tack and checking for spark.20

 

[alldodge]

Are you grounding the Pur/Wht wire before setting timing?
Being a 1997 you should have TB5 ignition, and timing should be at 10* BTDC

The inconsistent rpm is usually caused by the carb. You cleaned it but it may need more cleaning or adjustments not done correctly

 

[tedwards516]

Are you grounding the Pur/Wht wire before setting timing?
Being a 1997 you should have TB5 ignition, and timing should be at 10* BTDC

The inconsistent rpm is usually caused by the carb. You cleaned it but it may need more cleaning or adjustments not done correctly

Thanks. Its a TB4. My boat is 97, but the engine itself may technically be a 96, so I didn't ground the pur/wht wire. I was thinking about recleaning the carb. I tried to get to everthing as best I could. I didn't change out the accelerator pump ball valve since they peened the body to hold in the retainer t-piece. I would have to grind that out a little I guess. I'll recheck all my settings again.

 

[Scott06]

Thanks. Its a TB4. My boat is 97, but the engine itself may technically be a 96, so I didn't ground the pur/wht wire. I was thinking about recleaning the carb. I tried to get to everthing as best I could. I didn't change out the accelerator pump ball valve since they peened the body to hold in the retainer t-piece. I would have to grind that out a little I guess. I'll recheck all my settings again.

yes TB4 will not move at idle like a TB5 will. The accelerator pump ball wont affect idle quality, if you have a good pump shot and it doesnt bog when you stab the throttle leave it alone.

If you adjust the idle mix screws does change the idle speed or quality? if not likely the passages didn't get clean. I started using an ultrasonic cleaner for my rebuilds and really improved the success rate. Just a $100 HF one. Bigger carbs like a webber/afb/edelbrock you have to move around a couple times.

might also check for vacuum leaks too spray starting fluid at carb base see if idle changes.