Stock ct70 k0 sprocket sizing
- Thread starter Bad Dax
- Start date
cjpayne
Well-Known Member
The stock carb on the K0(AT7B) had a 65main jet.
This is a great example of why details are critically important when it comes to carburetion. First off, jet sizing by itself cannot be compared between different carburetors...unless you know that they are exactly the same, in every detail. Jet sizes can vary almost beyond belief...until you understand what other factors are in play. Early CT70 & ST70 carburetors, which can be described as the "rectangular float bowl era" came fitted with mains as small as #58 and as large as #65 and that goes back to the very beginning...1969-70. This is listed in the first edition Honda shop manual, which lists specs for both CT & ST 70 models.
Every top end part...piston, cam, head, etc is the same. Each of these engines needs the same amount of fuel, under the same operating conditions. And Keihin machining tolerances have first-rate precision. Thus, the key question is "why are different jet sizes required?" The answer is that a main jet can flow varying amounts of fuel, depending upon the rest of the carburetor assembly. The emulsion tube, needle jet & jet needle taper profiles, airbleeds, and slide cutaway can, and do, have substantial effect(s). For the sake of brevity, as well as the sanity of anyone reading this, I'll limit my ramblings to the main change...the emulsion tube. The earliest design had fewer wall orifices and they were all in the same plane. The later versions of this carb got more pairs of these orifices and they were in two planes, spaced 90-degrees apart.
So...while Keihin jets can be relied upon to be accurately sized (by number), that means almost nothing once you're dealing with a different model number OEM Keihin carb of the same size & series. Once you go for a different manufacturer's metered air & fuel leak (a.k.a. carburetor
) going by OEM Honda specs, regarding jet sizing, becomes useless. If your aftermarket carb can use Keihin jets, that's cool, you know where to source good quality jets. At the same time, consider this unknown carb to be a world apart, meaning it will have to be tuned from scratch. Until it's been installed & tested, on your motor, you don't know what settings & jet sizes will be optimal. It is possible that it could deliver more fuel through that #58 main than one of the OEM Keihins could with a #65 installed, for example. I wouldn't expect that. If you sourced a well-known aftermarket carb, it should be reasonably close as-delivered. And, I'd experiment with different jet-needle heights. If you can get it to run overly-rich, just by dropping the C-clip to the lowest groove, then installing a larger jet would not make sense. OTOH, if it's lean and that cannot be corrected with a jet needle height adjustment...then a larger jet is the logical next step.
Thanks everyone for the help....managed to get hold of another speedo in mph...took the bike for a spin and hit 40-45 mph...so thats good news...not sure what to do with my new km/hr speedo...maybe it will come good after some use...any ideas...also i have put indicators on my bike...its a k0..and will be mainly riden on the street, i have wired them in they are single wire 6 volt indicators i am using a k3 indicator switch but the k0 battery doesnt seem powerfull enough...if i use the circuit that the headlight uses will they dim at idle and go bright when the engine is revd? How can i safely wire them in without blowing the globes...any help would be much appreciated.
You'll be feeding the turn signal circuit from that source, regardless. Fact is, it's merely a question of where to make the physical connection.
Make a "Y" connection, using bullet connectors and you can make the new turn signal circuit literally plug & play...using either the hot lead from the horn, or brake light leads, as best fits your bike.
As for battery power, the answer is "it depends". While the engine runs, DC is fed to the battery. If...and that is the operative word here...DC input exceeds system draw (output to the bulbs & accessories) then the battery charges. If not, it discharges. That balance is dynamic. At idle, brake light powered, the battery is discharging, alternator output can't keep up. Once the revs build, the battery begins charging...or at least discharging much more slowly. Once the brake light deactivates, the battery charges as you ride.
Turn signals draw a lot of power, in short pulses. There's no way a stock alternator can keep up; it's okay. There's maybe 10w going to the battery. The biggest problem with OE turn signals is getting a consistent flash rate, from an old type relay. That requires constant draw, voltage and temperature...imho it's a dodgy system, always was. Hence the suggestion to go LED + electronic relay. That'll also allow you to use an original battery setup.
Getting more battery capacity is easy enough...source an SLA battery, which is not only smaller but has double the charge capacity of the old leaker. If that's sufficient, problem solved. If not, you'll have to either use a larger battery or fit two batteries, wired in parallel. That's a bit of a challenge.
Make a "Y" connection, using bullet connectors and you can make the new turn signal circuit literally plug & play...using either the hot lead from the horn, or brake light leads, as best fits your bike.
As for battery power, the answer is "it depends". While the engine runs, DC is fed to the battery. If...and that is the operative word here...DC input exceeds system draw (output to the bulbs & accessories) then the battery charges. If not, it discharges. That balance is dynamic. At idle, brake light powered, the battery is discharging, alternator output can't keep up. Once the revs build, the battery begins charging...or at least discharging much more slowly. Once the brake light deactivates, the battery charges as you ride.
Turn signals draw a lot of power, in short pulses. There's no way a stock alternator can keep up; it's okay. There's maybe 10w going to the battery. The biggest problem with OE turn signals is getting a consistent flash rate, from an old type relay. That requires constant draw, voltage and temperature...imho it's a dodgy system, always was. Hence the suggestion to go LED + electronic relay. That'll also allow you to use an original battery setup.
Getting more battery capacity is easy enough...source an SLA battery, which is not only smaller but has double the charge capacity of the old leaker. If that's sufficient, problem solved. If not, you'll have to either use a larger battery or fit two batteries, wired in parallel. That's a bit of a challenge.
Texan
Active Member
How would a K0 perform with a 15/35 combination? Just curious, because I screwed up and ordered a 35 sprocket. In the process of refurbishing the wheel assemblies, so I haven't attempted to run the bike with the new sprocket. NBD if I need to purchase the 38 sprocket.
It'll be overgeared and likely struggle at both ends...from a dead dig and at top end. By the numbers...stock gearing works out to 4.78mph/1000rpm, in top gear, across all models, K0, K1-later & 4-speeds. The different sprocket combos are determined, mathematically, by crank-to-countershaft gearing, to give the same final drive (crank-to-wheel) ratio. You've got roughly a 3% "window". Beyond that, you're either undergeared or overgeared. Undergearing can be okay for specific, low-speed, riding conditions...like a swap meet, parade, really tough trails. But, overgearing only works if a.) you weigh a LOT less than than the average rider, b.) you ride with tailwinds and c.) downhill. Honda really did their homework when it came to gearing, including the transmission ratio spread. Overgearing will leave you feathering the clutch on takeoff and with weak performance in top gear, with the slightest incline or headwind.
15/35 on a K0 means ~9% taller gearing. That's more than a 52mm bore-up kit (88cc) can pull efficiently.
BTW...I'd recommend against sourcing a 14t sprocket, as chain life would be shortened.
15/35 on a K0 means ~9% taller gearing. That's more than a 52mm bore-up kit (88cc) can pull efficiently.
BTW...I'd recommend against sourcing a 14t sprocket, as chain life would be shortened.
. I'm gonna set aside the 35 sprocket and buy a 38.