Getting a CT110 Carb on an ST90

A while back I bought a CT110 Carb from DRATV to put on my ST90 after I installed the big bore kit and performance camshaft also from DRATV. I quickly found out that the boot that goes from the airbox to the carb would not fit on the new larger carb. I emailed DRATV about this and here is our exchange.

-Me: Hello. I am going to install your big bore kit and high performance camshaft on my 1974 ST90. would the OEM CT110 carburetor you sell be a good choice to run with those upgrades? If not, what carburetor would you recommend using on an ST90 with the big bore kit and performance cam? Thanks. -Will

-DRATV: that would be a great choice
i don't know if the jetting is correct, but it should be very close if not
thanks
jack

-Me: Hello. I went ahead and bought the OEM CT110 carburetor. What intake manifold and carburetor insulator do I need to make the OEM CT110 carb fit on my 1974 ST90?
Thanks. -Will

-DRATV: what are you going to do about the air filter ? do you want to keep using the stock one ?

-Me: Yes Id like to keep using the stock air filter if possible? -Will

-DRATV: then you must use the stock manifold and try to rig up something to adapt the larger rear of the new carb to the smaller hose coming from the stock air box

-Me: Any suggestions on how to do that? -Will

-DRATV: not really , sorry

-Me: Would one of your carb to air box boots do the trick? like 17E or 42C? -Will

-DRATV: Sorry i don't have anything on hand that will reduce it

Does anybody know how I can "rig up something" or is there someplace that makes custom airbox to carb boots?

Thanks,

-Will
 

kirrbby

Well-Known Member
I'm thinking you can rig up something easy enough with a piece of rubber hose. The trick will be in making it look good. You might find a cheapo, generic, air filter with a good lookin boot that will fit your carb...then mate that boot to your original boot, or right to your air box..?

Pics and dimensions will help a lot, so folks can see, and know, what you're working with.
 

b52bombardier1

Well-Known Member
I can send you a 3D printed spacer in PLA material between the exit of the manifold and the entry throat of the 110 carb. If you can find the same spacer for a 72 Honda CL100, our 3D printed spacer is modeled very closely after that spacer. My oldest son is big into 3D printing and this was the first ever part that he modeled and printed for me. Since then, he has done a throttle cable clip on my LS-engine equipped 70 Chevy El Camino. Cost for the carb spacer to you . . . . zero.

Not sure but the spacers below might work. You'd have to ask the vendor about the width of the mounting post holes.

https://www.ebay.com/itm/1965-Honda...d4e6310:g:7QEAAOSwOLVbR60n:rk:17:pf:0&vxp=mtr


https://www.ebay.com/itm/HONDA-1972...03ce7fa:g:nd4AAOSwytJZ7MkE:rk:29:pf:0&vxp=mtr

This same spacer can be copied in genuine NHL hockey puck rubber. The rubber has good squish properties for no vacuum leaks, resists heat and gasoline fumes.
Rick
 
Well I can't say no to that offer. I'm not familiar with exactly how a spacer would work and I can't quite make it out from the pics in those links but I'm sure once I have one in my hands it will become apparent. Where would I take it to be copied in hockey puck rubber?
 
Here's some pics of the situation. I have a CT110 carb boot as well. It and my original ST90 boot are pictured here. I was thinking of cutting the two boots and melding them together to create a Franken-boot that would fit correctly on the air box and the carb. Would 3M weather stripping adhesive work to hold my Franken-boot together?
 

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69ST

Administrator
Staff member
Administrator
Just a reminder, make sure that no location between the carb & airbox has an ID smaller than the carb...or you'll create a flow restriction and counter the gains you're seeking.
 
Just a reminder, make sure that no location between the carb & airbox has an ID smaller than the carb...or you'll create a flow restriction and counter the gains you're seeking.
Seeing as how the entrance to the air box where the boot attaches is a smaller diameter than the carb I don't understand how that would even be possible?
 

69ST

Administrator
Staff member
Administrator
The carb venturi should be the narrowest place of the entire induction tract. Otherwise, it's like running with the choke partially applied...or with a smaller carb. FYI, this is not my opinion, it's basic physics.

Since you're looking for increased power/speed it's a key consideration.
 

69ST

Administrator
Staff member
Administrator
Maybe it's just me, but I would be looking at some auto radiator hose if that will be out of sight.
Polymer hose, including rubber, can get kinda testy about tight-radius bends. IOW it tends to collapse easily, beyond a certain point. Two simple workarounds come to mind. One would be inserting a spring inside the hose, to support the walls. The other would be using a rigid bend (45, 60 or 90 degrees, as needed) made of PVC or copper and readily available at any hardware store, with straight sections of rubber hose used as couplers.
 
Polymer hose, including rubber, can get kinda testy about tight-radius bends. IOW it tends to collapse easily, beyond a certain point. Two simple workarounds come to mind. One would be inserting a spring inside the hose, to support the walls. The other would be using a rigid bend (45, 60 or 90 degrees, as needed) made of PVC or copper and readily available at any hardware store, with straight sections of rubber hose used as couplers.
Now we're talking! What about the air box? Do I need to cut where the boot attches so it's the same diameter as the carb?
 

69ST

Administrator
Staff member
Administrator
Now we're talking! What about the air box? Do I need to cut where the boot attches so it's the same diameter as the carb?
Without seeing the actual parts involved, I cannot give you specific cutting advice...and have it mean anything. Parts, such as that airbox, are a lot easier to cut than to replace, if cut incorrectly.

That said, some general guidelines that are useful. If there's going to be a size mismatch, then the IDs should get larger, downstream from the carburetor (i.e. intake same size, or larger than the carb, intake port same size, or larger, than the intake), to realize maximum airflow and without unwanted turbulence. Upstream of the carburetor, ideally, I like to see at least the same ID as the airhorn of the carburetor; that not only insures unrestricted airflow but also keeps the airbleeds (located inside the airhorn/inlet side throat) fully exposed to uniform airflow...which insures proper fuel deliver delivery, at speed. Now, Honda sorta broke that rule, with the CT70; the inlet side airbox boot had a restrictor cone with an ID similar to the carb venturi itself. Keep in mind that the outlet side airbox boot, the one that attaches to the carburetor was larger than the airhorn/throat ID.

What size is this carb, what was the original? Those dimensions are critical. If you want to remain on the safe side, then I'd suggest keeping the ductwork, between the airbox & carburetor at least as as larger as the carb airhorn/throat inside diameter. Looking at your photos, it appears that the opening on the airbox...the one into which the rubber hose fits, is at least 22mm; the question is what's the ID of the hose that fits inside of it?
 
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