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Thunderbolt 500 lacks power

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Pdanno

New member
I picked up a 1972 Mercury Thunderbolt 500 outboard in pretty fair shape. The engine runs smoothly, however the throttle does not increase the engine speed until the lever is at about 12 o'clock, at "full" throttle it runs probably at 1/3 of expected speed. Adjusting the gross throttle adjustment under the cowal a bit allowed higher speeds, but the idle increased also. It appears possibly that the throttle cable is not allowing the engine to adjust to higher speeds. If this sounds plausible, can and how is the cable adjusted to allow full range of operation.

Had it on the water yesterday and could only go a couple mph, got back to dock and started looking around under cover and discovered the above issues. It seems to be related to throttling and not getting enough advance at full throttle. It was recently tuned up and carbs cleaned.
 
Sounds to me like someone didn't get the throttle linkage on correctly. Had the same problem when I bought my boat. Last time I ever take someones word for anything I'm buying.
 
I ran into the same problem on my 78 Thunderbolt 500. if you have the dual lever control (small lever on the back and forward/reverse lever on the font), the back lever is just a warm up lever. When you want to throttle up, Put this lever all the way down (toward ignition switch), and the forward/reverse lever will move farther forward (almost to 9 Oclock position), and will open throttle all the way open. If you have the cover off and watch the carbs from the drivers seat (with engine off) you can see the butterflys open to the wide open position.
 
I finally got the motor to run well after rebuilding the carbs and replacing the wiring harness. The linkage problem was just the tip of the iceberg. The fuel distribution yolk needed new fuel line and the hose and bulb needed to be replaced. Took time but it is ready to go fishing.-Fuddyduddy
 
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