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Merc 4.3 TKS Low RPM Power Loss

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Brandon465

New member
I have a 2012 20” Stingray with a Mercruiser 4.3 TKS (carbureted) stern drive. The boat is new to me but seems extremely well cared for.

I am noticing some power loss when slowly advancing the throttle from stop to around 2000 rpms. As I slowly inch the throttle forward, the boat accelerates normally but as I continue to advance the throttle I find places where I’ll lose rpms and power. A little more throttle, more power, but a little more throttle and a power loss again. The power loss is not complete but is definitely noticeable. At certain places along the throttle I’ll get some rpm wobble without even moving the throttle. It has stalled once during one of these power losses but in general does not cause the engine to stall. It will do this up until about 2000 rpms and then everything is fine and there are no more power losses.

I only experience this when accelerating slowly up to about 2000 rpms. I can push the throttle immediately from stop to 3000 rpms and don’t notice any power loss and accelerate very quickly, no problems. Full throttle, the boat goes upwards of 50mph without issue. There are no issues at idle. My current fix is just avoiding those rpm ranges but obviously that means I can’t travel in the 10 to 20 mph range without experiencing weird power losses.

What could be going on here? Feel free to ask questions and I will do my best to clarify.

Thanks,
B. Smith
 
Brandon465 said:
Jeff, would a faulty accelerator pump cause these issues even under slow acceleration?
Click to expand...
It looks like the idle transition circuit is plugged. As the throttle opens - Carbs run off the idle ports at idle, then there are slots or a series of holes that make up the idle transition circuit before there is enough airflow through the venturi to suck gas out the main jets. If you go past this quickly and its ok i suspect you need to strip clean and rebuild the carb paying attention to getting the idle and transition circuts clean.

if you remove the flame arrestor look down the carb while increasing throttle, you will most likely see at the rpm when fuel dribbles out of the main venturi your issue goes away.
 
"Jeff, would a faulty accelerator pump cause these issues even under slow acceleration?"

It definitely be more noticeable under slow acceleration. If you have access to an ultrasonic cleaning machine clean the carb in it. It will vibrate all the stuck dirt until it washes out. It does a great job vs. a dip tank.
 
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