Mercruiser 4.3 idles at 3k rpm

[stevenusmith]

I bought a 1992 19' Thompson Carerra with the 4.3 and alpha one. this is my first inboard boat however im fairly fluent in engine mechanics.. that being said when i purchased the boat it did not run. i discovered the gas was old and full of rust. i grabbed a clean little 5gal tank and added fresh gas. now when you pump the accelerator, it will fire, but proceeds to run at 2800-3k rpm in neutral. the previous owner stated it had sat for 4 years prior to selling it to me and that he had just "rebuilt the carb", well from what i can see the choke is missing pieces and he also added an aftermarket in line fuel pump. i wonder if he did a crappy job of reinstalling the carb, or if there may be like a crack in the intake and or some kind of leak? does this sound possible? i have already verified that the idle speed screw it butted all the way against the fast idle cam. the throttle butterflies seem to be closed all the way, yet it idles at 3k...the choke butterfly is basically loose as the electric choke was disassembled. Everything else seems to look pretty good. Could it maybe have to do with ignition timing? the distributor doesn't seem to look like the guy had been messing with it anytime recently, although i know that doesn't matter.. Or maybe i'm looking in all the wrong places, any suggestions comments concerns would be welcomed, thanks.

p.s. Is it wierd that it only fires if you pump the accelerator, it will NOT start otherwise. Im assuming this could be contributed to the non functional choke?

 

[Bondo]

Ayuh,..... Welcome Aboard,...... A carb that sat for 4 years with gas in it, is no longer a rebuilt carb,.....

I'd pull it, 'n do a full-on rebuild on it, fixin' the choke while yer at it,.....

 

[stevenusmith]

thanks for the quick response! I guess i will be pulling to carb today to see just what i'm dealing with, thanks. Its just interesting to me because other then the high idle it runs smooth. Also I've never heard of a dirty or clogged carb, causing a ridiculously high idle (3k rpm). To me in the automotive world, (which is relatively irrelevant), an idle like that would be attributed to a big vacuum leak etc.

Nonetheless, I will be pulling the carb today and will update the post. before removal, would it be a bad idea to try spraying carb cleaner around, to source any vacuum leaks, or is that practice frowned upon for use in engine cabins?

 

[Rick Stephens]

Almost a guarantee that sitting for years the idle ports are clogged solid. The only way it might even run is if the butterfly is open enough to reach midrange ports in the venturi giving it the rough approximation to a useable mixture.

If missing parts, it may even be best off finding a rebuild replacement.

 

[stevenusmith]

]

 

[stevenusmith]

 

[stevenusmith]

after disassembly i am willing to bet that the previous owner did indeed attempt a rebuild of the carb before sale to me. the accelerator pump parts look to be new as well as all the gaskets. Every piece of brass in there is chewed to hell. and i even found a little C clip in the bottom of the bowl, however it is not the clip that holds the accelerator plunger on, as that clip was still present. i can not figure out where this loose clip is from. i removed the clip, cleaned everything really well and reinstalled the carb. the engine still runs the same, about 3k rpm. as mentioned before the choke butterfly is completely loose as the choke mechanism has been gutted/disconnected. i noticed if i were to close the choke butterfly the rpm at idle drops. is the choke being fully open at idle my issue? Everything else aside i will be buying a new rebuilt replacement. In the meantime, ive noticed that i can not turn the steering wheel, not even with the engine running. The steering parts look to be in relatively good shape, and suggestions on what to look for?

 

[stevenusmith]

Update : I have put in an order for a rebuilt replacement carb. I also came across good information on diagnosing and fixing any steering issues and will try and tackle that tomorrow. one other issues i need to address are that i can only seem to engage drive at the engine, and the original fuel tank under the floor needs to be flushed and cleaned somehow..