Newly rebuilt carbs and last years fuel...

Finally made time this spring to pull the carbs off my Merc 454s on my 1989 340 Sundancer. Took them to a trusted auto mechanic for a rebuild. He said the fuel inside was pretty nasty and he suggested I drain my tanks. I trust him to work on the carburetors but he is not a boat guy.

The rebuild was prompted by an issue at high RPMs that I was plagued with all last season. Otherwise boat ran fine at idle and up to about 2800 RPMs. I put it away last Fall with about a quarter tank in each side with appropriate amount of Sta-bil. Started and ran the same this season before pulling carbs.

So, should I attempt to pump out and dispose of maybe 40 gals of fuel or should I just add fresh on top of it and move forward with the fresh carbs? Is there an issue where the old fuel will harm the rebuilt carburetors?

For what it’s worth, all fuel filters were changed late last season to try and alleviate the issue.

And if you are wondering why I have an auto mechanic working on my boat then you have never tried to have your boat worked on in Louisville. If it’s not small enough to be pulled on a trailer to a shop then it’s difficult to get work done and the pool of qualified mechanics is pretty slim.

Thanks in advance for any help.

I had a similar issue 9 years ago when I bought my 390. Before I ran the boat I installed Racor filters so I could easily drain some fuel off to check for contaminants. I'd add some fresh fuel (maybe 89 or 91 octane) along with some Stabil or Startron and run them. Keep a close eye on the fuel for a while. BTW have your Seatow/BoatUS paid up.

If you had good fuel in the tanks at the end of the season I would just change the filters (dump them into a clear container to see if there's any water) and fire it up and burn the remaining fuel before filling up w/new.

I would not add fuel as it complicates getting rid of bad gas if you end up going that route. I would start with new filters and use the 40 gallons that you have. Once you are through that change the filters and fill up. Do another change of filters when that fuel is gone. Fill up and change again when empty. After that you should be good to go. Getting rid of 40 gallons of gas is not something you really want to get involved with if it means draining tanks. I ran into this situation years ago when work caused our boat to sit for almost an entire summer without use. The gas had chunks of brown crude in it. Changing out the filters as I described worked fine and I never had any issues when I started using the boat again on a regular basis.

Thanks for all of the advice.

Does it make sense to set both of the tank selector switches to one tank and run it dry? Then switch to other tank. I worry about trusting the gauges at such a low level but would like to be able to use as much as possible.

I’ll keep you posted on the results.