Aftermarket Honda CT90 Carburetor Drain Modification

Relocating the Drain on a Aftermarket Honda CT90 Carburetor 

I recently picked up an aftermarket carb for one of my CT90's off of eBay that is very similar in design to a Klein 556A carburetor except for one detail and that was the orientation of the drain fitting.  The drain fitting pointed straight down verses straight back and is a common issue with aftermarket CT90 carbs.

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Adjusting and Jetting a Honda CT90 Carburetor 

Here is a picture with the old carb on the left and the new carb on the right.

I purchased the carb off of eBay instead of fixing the old carb because it was only $28.00 and I always wanted to try one of these carbs out.  I thought this one which I purchased from eBay seller Carbstar, was a deal as most of the aftermarket carbs with the correct choke lever configuration were always in the $50.00 range.

The only real issue with the new carb was that the drain was pointed down instead of to the rear like a stock CT90 carb, so I decided to modify the carb to relocate the drain to the correct location.

I knew that the fuel line and drain fittings were pressed and or shrink fit into the carb body, so I figured I should be able to remove the drain fitting by pulling it out.

Pulling out the drain fitting wasn't going to be easy, but the way I decided to approach removing it was using a pair of pliers and a small square of heavy gauge vinyl to try and protect the brass drain fitting.

To really be able to pull on the fitting I removed the lower bowl of the new carb and secured it in the wood working vise on my bench as shown below.

My technique was to pull had and apply a very slight wiggling motion to the drain fitting.  The drain fitting is brass so I minimized the wiggling to avoid breaking it and with a little bit of work it finally came out. 

The next step was to drill the new hole at the rear location. I measured the old hole and figured that a 5/32" bit would give me a hole that was a few thousandths smaller than the diameter of the drain fitting, so I would end up with an interference fit. The first step in drilling the new hole was to remove the actual drain nob from the bowl and then use a center punch at the location I wanted to drill my hole.

I next drilled and then chamfered the hole.

I next blew out all the passages with air to make sure all the chips were gone and then I was ready to install the drain fitting in the hole I just drilled.

Before I installed the drain fitting I put it in my freezer for about a half an hour so that it would cool down and shrink ever so slightly to make the installation easier.  To install the drain fitting in the new hole I just use a plastic mallet to tap it into place so there was about a 1/4" of engagement which is about the same as when the fitting was installed in the previous location.

With the drain fitting installed in the new location I now needed to deal with the old hole.  To plug the old hole I made up a small aluminum plug like shown in the picture below that was a few thousandths

larger in diameter than the hole and then I tapped it into place.

With the plug installed I reassembled the bowl back onto the carb and now the new carb had the same drain configuration as the stock carb.

With the carb modified, I bolted it up on the CT90 the old carb came off of and it worked like a champ and I am very happy I picked up this carb as $28.00 is only a little bit more than a rebuild kit and the modification to relocate the drain is not really all that difficult.

I hope you found this information useful!

Helpful Links (Shop Manuals, Wire Diagram, Model Information, etc.)

Link to page with listing of CT90 and CT200 parts available on Amazon