Honda CT90 Air Filter Element Differences

I have a 1971 CT90 that I plan to install a Big Bore cylinder and piston that I picked up from DrATV and thought that since I am adding a bigger piston I should look at doing what I can to open up the air flow going to the carb.  So, I started digging through my parts bin for air cleaner parts and noticed that the elements that support the foam air filter are not all the same and actually have have evolved quite a bit with the most recent configurations designed to allow greater air flow.

From the parts I have in my bin, I think I have figured out what air filter element parts I will use with my Big Bore project so that I have the least restriction and therefore the greatest air flow, but since I was spending the time looking at the differences between these parts I figured I would to do a post here to share what I have observed, focusing on the air filter supporting elements used on the K2 and later versions of the CT90 and CT110.

The specific parts that I want to focus on are the metallic parts within the air cleaner assembly that support the foam air filter and are the last parts in the the path the air takes getting to the carb.

The basic parts used in the K2 and later air cleaner assemblies are shown & labeled in the following photo:

For reference, here is a general schematic of a K2 CT90 air filter assembly that is similar to the K2 and later years of the CT90 and CT110.

The parts in the picture below (excluding duplicates) reflect the variation I found for the parts I have available to use with my Big Bore project.

Given there seemed to be significant variation possible with the inner element pipe and the air cleaner element holder I wanted to understand all the possible variations so I spent a little time on the web and ended up building the table below.  I also included info on the K1 air filter assembly and on the rubber boots that connect to each end of the air filter housing along with all of the common parts used  across the K2 and later years. I can't say the info in my table is 100% accurate, but its the best I could do based on what I found using the various parts finders at several Honda sites.

After building the above table it indicated that there were was a inner element pipe and air cleaner element holder that I didn't have examples of from my parts bin, but I did find pictures of the parts and they are shown below.

Pipe, Cleaner Element (Inner) 17222-102-790

Holder, Air Cleaner Element 17223-102-720

Since none of the parts I own have any part markings I did the best I could at figuring out what parts mate up to what part numbers, but here is a photo of the parts from my parts bin with what part numbers I think are correct focusing on the inner pipe element and air filter holder element.  If you see an error please let me know.  I show two parts with the same part number, but that was what the data was telling me that I found on the web, so again if someone knows different please let me know.

With a basic understanding of the differences between the various parts and which part number is which (I think...), it is time to get down to the real task of figuring out which collection of parts will give the greatest air flow and be used with my Big Bore kit.

Focusing on the inner pipe air cleaner element first the decision was pretty simple  and I plan on using the 17222-102-000 that has the two simple legs connecting the front and the rear parts of the assembly together.  This part has the greatest open space of all the configurations I have, so by observation it should flow the most air.  The inner pipe used on the CT110's, 17222-102-790, is slightly better due to the holes in the legs, but don't own one and don't plan on buying one. 

I took the picture below that gives a "carb eye view" looking down each of the inner pipe air cleaner elements and provides an additional perspective of how the 17222-102-700 from the early K2 and later CT90's is probably the most restrictive and that the two leg version of the 17222-102-000 is the least restrictive.

With the outer part of the assembly, the air cleaner element holder, the choice is not as obvious until you look into the details.

For the two configurations I have, 17223-102-000 and the 17223-102-790, both of the parts look almost identical as shown in the picture below (17223-102-000 is on top and 17223-102-790 is on the bottom).

I slipped a piece of white paper towel down the center of each of these parts in the picture below to show how the pattern of holes is actually different between the two parts.

And here is a close up of the hole patterns between the two parts.

Besides the hole patterns being slightly different, the holes in the later built part, 17223-102-790, are actually slightly bigger.  The holes in the older 17223-102-000 are .186 inches in diameter and the holes in the later 17223-102-790 part are .194 inches in diameter. While the .008 inch differences in the hole diameters may not seem all that great it does result in the holes in the later 17223-102-790 part having almost 9% greater area! So given that both the parts seem to have the same number of holes (I didn't count them) I plan on going with the later 17223-102-790 part which seems to have 9%  more are for air to flow for my Big Bore build.

I would like to get my hands on a 17223-102-720 air cleaner element holder which looks to use a screen mesh instead of the perforated sheet metal to see if it looks like it might breathe all that much easier then the earlier configurations.

I hope that this information is useful to others working on their CT90 air cleaner assemblies.

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

Link to page with listing of CT90 parts available on Amazon