Honda CT90 Engine Reassembly

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I have one 1971 CT90 where I was going to rebuild the clutch because the kickstarter just wasn't turning over the engine like it used too,  and I also had a Big Bore kit from DrATV that I wanted to install. My intent was to just do the top end and rebuild the clutch with new disks, but when I opened up the cases to rebuild the clutch pack I found a slick black slime in the bottom of the case.  I didn't like the idea that there could be black slime in the rest of the engine, so I decided to disassemble the entire engine to make sure I cleaned everything out.  Since I was tearing everything apart I thought I would document step by step how everything goes back together in this post in case others were interested in knowing what the inner workings of a CT90 engine look like. Links to Related Posts: My Honda CT90 Clutch and Headset Nut Tool Cut Away of a CT90 Engine - CT90 Engine Exposed! Repairing Damaged CT90 Spark Plug Threads Using a Time-Sert Thread Insert CT90 Clutch P

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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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.

CT90 Carb Drain

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.

CT90 Carb Drain

CT90 Carb Drain

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.

CT90 Carb Drain

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. 

CT90 Carb Drain

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.

CT90 Carb Drain

I next drilled and then chamfered the hole.

CT90 Carb Drain

CT90 Carb Drain

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.

CT90 Carb Drain

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.

CT90 Carb Drain

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.

CT90 Carb Drain

CT90 Carb Drain


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.

CT90 Carb Drain

CT90 Carb Drain

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.

CT90 Carb Drain

I hope you found this information useful!

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Comments

  1. I still have my 1966 CT200 that I bought when I was 14. It hasn't run in years and needs a full restoration. About 10 years ago, I bought a late '66/early '67 CT90 with the overhead cam engine. It was running at the time. Fast forward to today, I'm trying to get the CT90 running as a dry run to restoring my CT200. The carb leaks all over. Despite a carb kit I'm looking all over for the o-rings etc. for the Keihin carb and nobody seems to have them. So I bought a "K0" carb off of ebay and learned the hard way that the intake manifold and carb mounting holes didn't match. My manifold is exactly 42MM. I would just buy another aftermarket carb but I don't trust ads anymore that claim compatibility when they aren't. Advise? Thanks, Frank Baylor

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  2. Great write up and clear photos! Thanks again Michael!

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  3. I love this. As much as I love my original carb, I've never been able to tune it just right. I'm going to try 1 more time before I give up. I've used the aftermarket ones that are slightly different because of the drain. Do you have a recommendation on making the plug? I don't have a lathe or anything that could help me make a perfect plug like you did.

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  4. So happy I found your blog. Great work!

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