CT70 K0 - sapphire blue - sleeper build
- Thread starter CT70 K0
- Start date
darrel gunderson
Active Member
Looks great, keep the pic coming!
fatcaaat
Well-Known Member
Did you decide on your carb? As someone that's built and run lots of these motors and experimented with different carb sizes (including the tb roller rocker head) I would recommend the Mikuni VM20 carb. If you have money to burn, the MJN TM22 carb. I once ran a vm26 on one of these and it lost so much bottom end/off throttle. The absolute largest I would recommend would be the 22, but really, a 20 seems to provide the best all around performance . I am running a VM20 on a 106 TB setup, one on a 105cc trx90 conversion and they are perfect.
CT70 K0
Member
I got the Mikuni VM26 that came with the whole engine. Interesting the feedback you have for the VM22 or VM20.
I will be able to check how it behaves after I receive the exhaust and install it.
fatcaaat
Well-Known Member
Well, with a VM26, the off throttle low-end was less responsive. ON the top end, you might get a little more from it...really that equates to 1-2mph max with the engine screaming...and where you will spend less than 1% of your riding.
The VM20 has to be the best in terms all all round. Good off throttle response, good midrange, and fine on the top...and very easy to tune.
The VM22...it's been a very long time on using a VM22 on this size engine. However I have a TM22 MJN that will be mounted on top of a 110 engine in the very near future.
All around, for this size motor, the VM20 has to be my pick. Again, I've run prob every motor combo out there, with every carb and the VM20 and 106-117cc is a sure bet. Alternatively people really like the PE20 carb on them too, but I'm a mikuni guy.
The TM22 MJN is probably the best carb in this size range. That said, the deal-breaker for me is not the price tag but the uncertain parts availability. I wouldn't mind an expensive carb, if it delivers improved performance, i.e. seamless throttle response to go with top speed, as long as it can be serviced when/if needed. It's a JDM part, no authorized US distributor...whom, ostensibly, would stock parts.
If the rumors of the VM20s imminent demise are true, service parts (seals, gaskets, inlet needle/seat, jets) are likely to be available for years to come and cheap enough to stash a few for the future; it's marketed through Mikuni USA and Sudco.
Only downside to the PE20 is its cost...double that of the VM20 and it's a spigot-mount. A rubber-coated mounting flange has to be sourced separately and they're not always easy to find.
BTW, I agree with Jarred on the top speed issue. On a 60mph bike even a 10% flow restriction (which is huge), caused by an undersized carburetor, would only cost ~2mph at the extreme top end; a stiff headwind would have far more impact. On an engine making less than ~12hp+/- (enough to easily reach 65mph), there's no way a 20mm carb would be flow restriction, especially if port size is 23mm, or less. On the other hand, just upsizing from 20 to 22mm does increase venturi cross-section by 21% with a corresponding 21% decrease in airflow velocity through the carb. That's what impacts throttle response, especially at light-throttle/low rpm.
If the rumors of the VM20s imminent demise are true, service parts (seals, gaskets, inlet needle/seat, jets) are likely to be available for years to come and cheap enough to stash a few for the future; it's marketed through Mikuni USA and Sudco.
Only downside to the PE20 is its cost...double that of the VM20 and it's a spigot-mount. A rubber-coated mounting flange has to be sourced separately and they're not always easy to find.
BTW, I agree with Jarred on the top speed issue. On a 60mph bike even a 10% flow restriction (which is huge), caused by an undersized carburetor, would only cost ~2mph at the extreme top end; a stiff headwind would have far more impact. On an engine making less than ~12hp+/- (enough to easily reach 65mph), there's no way a 20mm carb would be flow restriction, especially if port size is 23mm, or less. On the other hand, just upsizing from 20 to 22mm does increase venturi cross-section by 21% with a corresponding 21% decrease in airflow velocity through the carb. That's what impacts throttle response, especially at light-throttle/low rpm.
CT70 K0
Member
great knowledge Guys sharing the details between the various carb size options and pro/cons.
The way I am expecting to drive that bike is not to run 60mph all day long but more in low end/middle rpm driving on small roads. I am not expecting to drive on highway what that little bike.
So looking at above feedback I might potentially in the future switch to a VM20 or 22, however I check 1st how the bike behaves with the current setup.
The way I am expecting to drive that bike is not to run 60mph all day long but more in low end/middle rpm driving on small roads. I am not expecting to drive on highway what that little bike.
So looking at above feedback I might potentially in the future switch to a VM20 or 22, however I check 1st how the bike behaves with the current setup.
OEM carbs were typically 4-6mm smaller than the intake port. IMO, 2mm smaller is typically the best balance. Once the valve & guide are factored-in, there's roughly that much unobstructed port diameter remaining. The one basic rule that will never change is the upper limit of carb size...it should never exceed port diameter. Oversizing the carb will only increase the difficulty of getting decent throttle response and operating efficiency; you'll have to go richer than ideal to cover the transient lean spots.
All things the same, horsepower is horsepower...gotta move the same amount of air & fuel...regardless of displacement. That said, given two engines of the same peak hp, but different displacements, the larger of the two will move more air at low rpm...and tend to deliver crisper throttle response down low.
There's another problem faced by tuners in 2016, virtually no manufacturer takes any carb under 28mm seriously anymore. Even the VM26 tuning kits have gone extinct. They used to be available with different throttle slides, needle jets, jet needles and airbleed orifices. If you knew what you were doing, you could really tailor a carb to great effect. Just a smaller slide cutaway would often richen the light-throttle mixture enough to eliminate the low-rpm stumbles. Alas, we're now down to pilot jets and mains as the only tuning parts available. Fortunately, size-matching the carb to the motor will still deliver acceptable results.
If you do decide to go with a smaller carb, the VM20 is the better choice imho. The VM22 really needs 125cc minimum, to get seamless throttle response with the limited tuning parts available. For anything but a race machine, I'd not fit a VM26 to anything with less than 140cc displacement.
All things the same, horsepower is horsepower...gotta move the same amount of air & fuel...regardless of displacement. That said, given two engines of the same peak hp, but different displacements, the larger of the two will move more air at low rpm...and tend to deliver crisper throttle response down low.
There's another problem faced by tuners in 2016, virtually no manufacturer takes any carb under 28mm seriously anymore. Even the VM26 tuning kits have gone extinct. They used to be available with different throttle slides, needle jets, jet needles and airbleed orifices. If you knew what you were doing, you could really tailor a carb to great effect. Just a smaller slide cutaway would often richen the light-throttle mixture enough to eliminate the low-rpm stumbles. Alas, we're now down to pilot jets and mains as the only tuning parts available. Fortunately, size-matching the carb to the motor will still deliver acceptable results.
If you do decide to go with a smaller carb, the VM20 is the better choice imho. The VM22 really needs 125cc minimum, to get seamless throttle response with the limited tuning parts available. For anything but a race machine, I'd not fit a VM26 to anything with less than 140cc displacement.
CT70 K0
Member
I am planning to buy a velocity stack filter with a pre-filter and screen from PJM.
they have a model that should fit VM26 carb with a diamater of 45mm.
the kit with following parts if at 86.95 USD:
- Velocity stack
- screen filter
- prefilter
they also sell foam filter to put in between the screen and pre-filter for 8.5 more
here they webside:
http://www.pjmotorsports.com/velocity-stack-filters.html
and the filter kit
anyone have experience using this product ?
thanks.
they have a model that should fit VM26 carb with a diamater of 45mm.
the kit with following parts if at 86.95 USD:
- Velocity stack
- screen filter
- prefilter
they also sell foam filter to put in between the screen and pre-filter for 8.5 more
here they webside:
http://www.pjmotorsports.com/velocity-stack-filters.html
and the filter kit
anyone have experience using this product ?
thanks.
Last edited:
CT70 K0
Member
The Guy I bought the engine from had it running and tuned with the VM26, so I am wondering that it will work for me as well. So in the assumption that it behaves correctly I will keep that carb. Also this is the carb that comes with the tb 108cc kit.
darrel gunderson
Active Member
I would run the K&N air filter that Dr ATV sells. Its for that carb. Its what I run on my VM26.
darrel gunderson
Active Member
K&N filters are quality goods and they'll last for years.
Forget the V-stack, at this power level it's purely window dressing; the only noticeable difference it will make on this motor will be to shorten its service life. That's with the optional filter installed; without it, you'll shred the cylinder/piston/rings in no time. If you're itching to see for yourself, find a super-clean stretch of road then give the V-stack a test, back-to-back with a real air filter installed. $90 is far cheaper than an engine rebuild.
Forget the V-stack, at this power level it's purely window dressing; the only noticeable difference it will make on this motor will be to shorten its service life. That's with the optional filter installed; without it, you'll shred the cylinder/piston/rings in no time. If you're itching to see for yourself, find a super-clean stretch of road then give the V-stack a test, back-to-back with a real air filter installed. $90 is far cheaper than an engine rebuild.