Thursday, 28 May 2015

DIY: Rebuilding an outboard carburetor

Today I'm going to show you how to rebuild a carburetor using a carb kit. The patient today is my 1961 Johnson JW-17 3 hp. The engine wants to go, it starts right up and fires well but then it revs really high and dies. Those symptoms indicate mainly an air leak somewhere, but it may also indicate a clogged up carb. So, the next step is to rebuild and re-gasket the carburetor.

Here is the engine as it sits. It is missing the lower part of the engine cover, so if anyone has got one please contact me.

Start by removing the face plate. Notice that someone at some point has attempted to repair the carb using some ordinary automotive body filler. Luckily the damage behind the filler is no big deal, it's just the air box fastener plate that has broken off. That issue will be addressed later.

Remove fuel line

Remove the air box bolt

Remove the two nuts holding the carb to the engine. Do remember to remove the throttle returning spring.

Realize that this is so much easier to do without the tank in place. Remove tank for better access.

Remove carburetor.

Get yourself one of these. Make sure the kit fits your engine in question. This kit fits almost all 1950's and 1960's Johnson and Evinrude 3 hp engine, plus a few more, your local dealer knows whether it fits your engine or not and can supply you the correct one.

Remove the fuel bowl. Inspect for damage, dirt, varnish, anything that looks suspicious. This bowl happened to be surprisingly clean.

Inspect the carb kit, make sure it contains all the parts it should.

Install clean high speed nozzle and brand new nozzle packing. Install new float pin and seat.

Install float. I used the original one after making sure it floats. I will replace it with a brand new one some day. Should have done that while rebuilding it, but you know. Oh well.
The carb kit comes with a new float pin, but strangely enough it is about 0.5-1.0 mm too long, which doesn't seem like a problem when installing it, but the fuel bowl won't fit when using that pin. So I had to re-use the old one after cleaning it thoroughly.

The camera flash made the carb look whiteish under the float. That's just an optical phenomenon, the carb is perfectly clean. Normally, white powderlike substance in the carb indicates water in the fuel system.

Set float level.

Install new fuel bowl gasket and reunstall the fuel bowl.

Install new needle packings, the red ones in this picture. Be sure to remove the old packings first!

Install needles and needle packing nuts.

Admire handywork!

Reattach carb to engine and hope for the best. 

This is a simplified version of rebuilding the carb. The process is really quite straight forward and you shouldn't encounter any real difficultues. The only hard part might be removing the float valve guide, which sometimes is really stuck in there! If you destroy the guide, it's no problem per se since the carb kit comes with a new one but be careful not to destroy the carb.

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