Carburetor Rebuild

Rebuilding carburetors has been a thing since right after carburetors were invented. If it isn’t grime and sediment in the fuel clogging things up, then it is varnish left over from evaporating fuel. Most will have gaskets dry out and leak. In any case, at some point, you will be rebuilding the carb.

In my case, there are three of them. One for each pair of cylinders.

I’ve watched my dad rebuild more than one, and I rebuilt one for a lawnmower about 20 years ago. Most recently I took one apart for my nearly new string trimmer. That one was an easy fix since I found a little speck of something blocking the fueljet. Suffice it to say, while I’ve never done a rebuild on carburetors this large, I felt confident I could do the work.

So, what’s involved with a rebuild? Normally you disassemble the entire unit. You take every single part off of the main body as you can, then you soak everything in a carb cleaning solvent for at least a day. Once done with a soak, you give the parts an extra scrubbing, including internal parts. The goal being to get rid of any debris or varnish that is left. You want them as near spotless as possible.

Me, being me, didn’t want to do all that. These carbs looked pretty good. They had no varnish or dirty spots on them. In fact, they looked like they had been recently rebuilt. I decided that I’d do a partial rebuild. I’d focus on cleaning what I could without fully taking the carbs apart and soaking them. There are several pressed-in caps and other things that I didn’t want to mess up. For instance, I don’t have a replacement jet, should I break one. They don’t make a lot of new parts for these. If I mess up a major component, I might was well toss the whole thing in the trash and buy another hopefully good used one.

Assembling these would replace the gaskets, the idle mixture screws, the bowl floats, and the little valves that controls the flow of fuel into the bowl. These are common wear parts.

I neglected to make a video of the process. My work bench isn’t well lit (yet), and I don’t have a good camera and light setup (again, yet). Perhaps a future project will allow for some close up video. Plus, you didn’t miss anything, except for some light swearing by me.

I did use several YouTube videos to help me learn what to do. If you want to see the process, click below.

There was something interesting I got from this video. He removes the hose that goes over the top of the carb that connects to the fuel bowl and to a vacuum passage. This thing is called the economizer, with the goal that it would lean out the fuel mixture when the motor gets to a higher RPM. Supposedly, it will use less fuel this way and save you some money.

To me, this is a foolish idea. Lean fuel means less 2-cycle fuel. Less 2-cycle fuel means less dissolved 2-cycle oil. This will lead to additional wear of the motor because high RPMs are whe it needs the most lubrication. It also helps to cool the motor as the oil passes through.

Anyway, he removes the economizer line and puts a little heat-shrink over the connectors to seal them off. I decided I’d do the same, but I’d also add something to help seal it off. My choice was a steel BB, which the heat-shrink pulls inward to seal the hole, but can’t fall into the hole. Will it work? I guess we will find out.

Here are a few photos I took during the rebuilds. Yes, I wear gloves. They are cheap from Harbor Freight and they keep the nasty stuff off my hands. Carburetor cleaner is a bit tough on the skin.

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Which brings us to this little guy. This is the float and I think this is the reason I was having trouble at the end of the day. This thing was filling with fuel and allowing fuel to flood the bottom “C” carb. With that one flooded, it didn’t produce any power, making my 6 cylinder into a 4 cylinder, with the extra 2 cylinders stealing horsepower.

Like I mentioned, many of the parts looked almost new, so it was rebuilt at some point recently. My original concern with replacing the little float bowl valves was that the original design of these had a little bit of rubber that helped to seal it. These would deteriorate quickly with modern fuel being at least 10% ethanol. Mercury stopped selling the older version in favor of what was installed on this motor, and what I replaced it with.

I kept the old, but still mostly good parts. One never knows when one of these will need some work.

It was an afternoon well spent. I had been waiting to do the rebuild for quite a while, but it was hard to find the time with so much stuff to do with the kids during summer, as well as all the various household projects that always seem to crop up.

Now I just need a few hours to get them back on the boat. I hope I remember how I took them off. It has been a few months! See you soon with an update.