running horrible

I have a 1988 GM 350 inboard marinized by Mercruiser. I changed the spark plugs and mechanical fuel pump and now it won't idle. When I run at 1500-1800 rpms it smokes and there is fuel and carbon in the exhaust water. I adjusted the idle screws on the carb to a half turn out and it idled better. Factory spec. for the idle screws is two turns out. I am confident the spark plug change is not the problem and the mechanic who recently rebuilt the carb checked it out and found no problem. The dealer stated the fuel pump he sold me is the same one used regularly (QuickSilver). Any ideas what went wrong? Tomorrow I am putting the carb back on to try again. Should I put the old fuel pump back on? This all started when the boat would not come up to full power and I did a tune up, carb rebuild, fuel pump replacement. Now I've got this new problem.

Re: running horrible

I'm pretty sure you already did this, but make sure the spark plugs are properly gapped.


(Yes, I've forgotten, that's how I know)

Re: running horrible

firing order also

Re: running horrible

...and correct order and that distributor is in it's proper position.

What carb?

..and the carb setting for the low speed screws are just a starting point to get the engine to start. Correct setting is highest adjustment/reading (between17-20 "/hg) with vacuum gauge.

Re: running horrible

Maybe a couple of things to check. Check fuel pump pressure, this will rule out fuel pump. Double check spark plugs, did you gap the new plugs, I have seen new plugs with no gap. Check your spark plug wires, make sure that you have the proper wire on the proper plug. If all of this checks out ok swap the old carb back on and see what happens. Do not change more than one thing at a time or you woun't know what the problem was.

Re: running horrible

Type of carb is important, if it's a Holley you most likely have a blown power valve. If a quadrajunk your floats may be set too high.

Re: running horrible

i had a bad idle, not exact same symptoms as you, but worth checking.

first time... behind my carb on top of the manifold i installed one of those, what do you call it, it's a nut with a copper tube sticking up out of it, to let you check the vacuum when adjusting the carb. it's a great thing to have. i seal it with a rubber vacuum cap. years later i forgot to check the condition of the rubber cap, and it had developed a crack, so i had a vacuum leak. idled really poorly.

second time... the idle rpm wasn't acting right, would vary too much. this was about two years after having the carb rebuilt. i eventually noticed that the body of the choke was loose on the carb. when i took the choke apart, the two phillips screws that hold it to the carb were about ready to fall off.