75 sprint Johnson no high end

I'm looking for info. on a carb. rebuild to get the high speed performance back. I've tried all basics ie spark plugs,cleaned carbs,changed fuel line and pump, new and different fuel,spark to three cyl. good--a little light on #3,compression even at 110,the engine runs fine idle and slow but boggs at full throttle any ideas any body thanks. RD I think the year is late 60's or early 70's

If the engine hits full throttle for some length of time then bogs out, suspect a fuel restriction.

If it has a tendency to die out immediately when throttle is applied, I'd suspect that you've overlooked something within the carburetors (clogged high speed jets) OR the timer base is sticking under the flywheel.

Thanks for the reply- I'm not familar with a timer base (mechanical/electrical?)could you elaberate on how to check and fix? also do you know of any service bulletin that addresses this no top end problem with a carb. rework or special rebuild? I agree that it seems to be something amiss in the carbs. With further digging I beleive the unit is of the late 70's early 80's if that matters. Is a manual available and would it be helpful for my problem? My tests are when the boat is trailered in a ramp so it has max. load. In the past in the water I have had it suddenly go full ahead like its suppose to but not lately. Its really is messing with me.

Does the engine start to die out immediately when throttle is applied?
OR.......
Does the engine hit full throttle for a time, then start to die out?

Having the rig secured on a trailer in the water at a boat ramp, the engine would encounter a scenario that we call "lugging". A situation whereas the throttle butterflies might be wide open, and with the spark advance set to full BUT it would be impossible for the engine to obtain its full rated rpm due to the fact that it isn't going anywhere....... unless you have a factory test wheel installed. However, the test you speak of is understandable.

The timer base I mentioned previously is the item that moves under the flywheel as you slowly move the throttle lever forward. In forward gear (engine not running for safety reasons), as the throttle is moved to full throttle, the timer base should move smoothly all the way up against a black rubber bumper (full spark advance stop).

The high speed jets would be located in the bottom center portion of the carburetor float chambers..... in back of the screw/bolt that you'll see at the bottom front portion of the float chambers. Either remove and clean those jets OR clean them where they're located with a piece of single strand steel wire.

Thank you for your help. I'll be back on line when I have some results to give. Really appriate your time. RDD

Received your PM..... reply sent.

I went to the river with carb. cleaner, tools, and fuel in a bottle to see what the engine would do. With just me and a freind the engine came up to speed and ran very well (kind of smooth and slow). My conclusion was that one carb.was off line and previously too much weight in the boat. Maybe this engine is a little under powered or old for this amount of boat( I don't run as fast as I used to either!). Does an engine loose some zip with age? I used to pull my young kids skiing that I don't think it would do now. I really appreciate your in put and advise. Hope you have time to enjoy the water! RD