Does anyone have suggestions for a larger than 24mm carb for a CT1 cylinder?  I plan to add a Webco head and an aftermarket expansion chamber.  The CT1 cylinder has a flanged intake with a 28mm hole and about 50mm spread between the carb mounting bolts.  I like Mikuni carbs.  Any ideas?

If you intend to keep your cylinder stock, the 24mm carb will be fine. The transfer ducting on these engines is quite restrictive, and unless that can be improved, a bigger carb isn't likely to make a great deal of difference.

If you were able to convert from a flange to rubber spigot mount carb, that would be a good idea, as heat/vibration dont make for good carburation.

A Yamaha 248M (vs stock 248A) carb is 26mm, bolts right on, and usually runs pretty good as jetted.  (It was stock on the piston port AT1 MX)

I might have one around here someplace.  Let me look. 

A 248M2 carb with a webco head and chamber works well. I usually go with a 200 main jet with that combo.

Yam-fan said "If you were able to convert from a flange to rubber spigot mount carb, that would be a good idea, as heat/vibration dont make for good carburation."

Great option, but I have not found a spigot flange that fits the CT1 cylinder without serious modifications.

MarkT said "I might have one around here someplace. Let me look."

Please please please!

Capt.Dave said "A 248M1 carb with a webco head and chamber works well. I usually go with a 200 main jet with that combo."

Thanks Captain! Good to know!

Asco said "I have a 248M2 on eBay. It’s the last one for sale I own."

Is that the $180 one or one of the $300 ones?

Thank you all!

www.aliexpress.com/item/32968009848.html...eLogUid=vvFbZxvINccn

Hundreds of intake adapters available for low cost. If you upgrade to rubber mounted Nibbi 26mm carb, you will have better performance, improved starting, lower fuel consumption.

There is very good reason Yamaha used rubber mounted carbs, on all the later bikes.

I still intend to find a 248M carb somewhere, but I did find a possible alternative.

Found an Amal 627 in my basement that would bolt right up to a CT1 cylinder.  The only problem is that it's too tall and the bottom of the carb will hit the top of the engine cases before everything lines up.

This led me to look for spigots that mate to an Amal flange.  I found this part on the Mikuni Oz website: AFA49-59/35/28 .  They also sell the same part on the Mikuni Oz ebay store.  This part will mate with Mikuni VM24, VM26 and VM28 carbs with a 35mm spigot.  I still question if the bottom of this setup will clear the engine cases.  These carbs are not as tall as the Amal 627, but I don't see a measurement from the spigot centerline to the bottom of the carb.  If the modern VMs spigot centerline to the bottom of the carb is the same as vintage carbs, then this would work.  The bottoms do look different, and there is not much clearance on a CT1/AT1 from the bottom of the carb to the top of the case.  If I resolve this, I will follow up.

Just off the top of my head...


175cc = 0.00618007 cu. ft.
0.00618007 X 8,000 rpm (redline) = 49.44 CFM
0.00618007 X 10,000 rpm = 61.8 CFM
In theory, you want a carb that will flow, say 50 to 60 cfm?

There's a lot more science in using larger carbs that affects performance, but this is a baseline.
I didn't consider air velocity, altitude, mixture, yadda-yadda...
I was thinking of my drag racing days and using larger carbs. Just cause it's bigger doesn't mean it's better. Some larger carbs caused performance to drop.
 

On TY175, the bottom of the float bowl is very close to the top of the crankcases, when a more effective aftermarket carb is fitted.

There are hundreds of flange to spigot type adapters listed on sites such as Aliexpress. If you can find one with the correct bolt centres, look at the NIbbi US website, which shows several suitable carbs for your application.

This post is NOT a troll, simply reflective of my own first-hand experience!

Anyone used one of the AT1/CT1 Webco carb adapters for the larger bodied carbs? There are some Victor adapters on ebay if you look up the below.

Yamaha Victor Products AT-1-CT-1 Racing Carb. Adaptor
eBay item number:293654062397