Thread: 19mm carb with 88cc Racehead?

I have decided to pursue a more stock appearance for my KO which already has a TB 88cc bore w/ race head kit. This means that I'd like to go back to stock bars and throttle but I now have to make a choice and have 2 questions:

1. Will a stock throttle assy properly control a 20mm carb?
Some say it will not provide enough lift for my carb and others disagree. I've seen a lot of conficting info on this subject.

2. Would the $30 19mm carb provide adequate performance for an 88/RH motor with stock exhaust? If the 19 is good enough, I'd love to go back to internal petcock and stock look.

I should mention that this bike is used for running errands around the neighborhood and must maintain 40 mph.

If you've already been there, please share your knowledge. Any help would be greatly appreciated.

I measured the cable actuation of both the stock throttle and the Kitaco throttle last night. The stock throttle provides about 18mm of cable travel from full open to full close and the Kitaco throttle provides about 60mm. This large difference was quite surprising and makes me think that a stock throttle assembly could not possibly provide the necessary lift to fully open an aftermarket carb. I still can't help but wonder if raising the piston 18mm from it's resting position might allow it to clear a 20mm throat at WOT, but that is a big assumption and I really don't have a good way to measure the actual piston travel of the 20mm carb.

If any one has tried to use a stock KO throttle with Kiehin 20mm carb, please help me out here.

thanks

19mm should be large enough to feed 88cc. Most of the low-dollar PRC carbs I've seen are smaller, in reality, than their nominal (stated) sizes. That usually works out well, because most guys tend to order oversized carbs for their modest performance tunes. When in doubt, use intake port diameter as the limit for carb size. 2mm smaller is usually the best balance between flow capacity sufficient to feed those extra CCs and ease or tuning.

FWIW, I'm running a 22mm Mikuni carb on my daily rider and the OEM internal twist throttle has just enough travel.

For someone with the right combination of skills, drive and outright stubborness, I believe that it should be possible to modify the assembly to provide at least 26mm of travel. Most guys who've fitted engines that truly require carbs this large would probably opt for a better quality aftermarket twist throttle (and think that I'm nuts for even considering using the admittedly cheezy OEM throttle on an 80mph bike)

Thanks racerx, Good to know what my best options are. I appreciate the sound advice and am pretty committed to using the cheezy throttle on this bike.

and think that I'm nuts for even considering using the admittedly cheezy OEM throttle on an 80mph bike)

80 MPH??? Are we still talking about a ct70?? And if so, who's nutz enough to go that fast on these little bikes?

Originally Posted by racerx

19mm should be large enough to feed 88cc. Most of the low-dollar PRC carbs I've seen are smaller, in reality, than their nominal (stated) sizes. That usually works out well, because most guys tend to order oversized carbs for their modest performance tunes. When in doubt, use intake port diameter as the limit for carb size. 2mm smaller is usually the best balance between flow capacity sufficient to feed those extra CCs and ease or tuning.

FWIW, I'm running a 22mm Mikuni carb on my daily rider and the OEM internal twist throttle has just enough travel.

For someone with the right combination of skills, drive and outright stubborness, I believe that it should be possible to modify the assembly to provide at least 26mm of travel. Most guys who've fitted engines that truly require carbs this large would probably opt for a better quality aftermarket twist throttle (and think that I'm nuts for even considering using the admittedly cheezy OEM throttle on an 80mph bike)

Wow what kind of motor is on your daily driver?

Not trying to hijack the thread but .....IYO, what can be done for a stock 107cc Lifan w/a 19mm carb?

The last thing I want to do is overcarb the motor.....it reminds me of those idiots that put a 750CFM 4 barrel on their stock V8 and it ends up running like crap.

Originally Posted by Adam-NLV

Wow what kind of motor is on your daily driver?

Not trying to hijack the thread but .....IYO, what can be done for a stock 107cc Lifan w/a 19mm carb?

The last thing I want to do is overcarb the motor.....it reminds me of those idiots that put a 750CFM 4 barrel on their stock V8 and it ends up running like crap.

Exactly the mental picture I get when someone talks about running a VM26 on a stock motor with an 88cc kit. Takes me back about 35 years when I used to see twin 650 Holleys atop a tunnel ram intake...on a 289 Mustang

You should be able to run a true 22mm carb on an L110. I'd probably be happy with a Keihin or Mikuni 20mm. It'll likely be larger than your current 19mm unit (unless yours is different from what I've seen of PRC carbs), enough to give you a bit more power. IMO, the stock motor won't really gain anything from the 22mm unit, and will be a bear to tune below about 40% power, where you spend a lot of time on the street. A true 22mm carb is adequate to support 70mph power easily. Mikuni corp shows their VM26 good for ~25hp, enough for 90mph.

Originally Posted by Haas

80 MPH??? Are we still talking about a ct70?? And if so, who's nutz enough to go that fast on these little bikes?

I've ridden a couple of 160cc Nice-powered customs into the mid 80s, pushed my own purple Dax custom to 77mph. All of these bikes were heavily modified, top-to-bottom, including suspension & brakes. Brad has taken his 137 Nice powered Monkey to 81mph. I reckon it could be argued as to whether, or not, you consider these bikes CT70s, or not. Fatcaaat has taken his 177YX-powered CT70 up to 87mph and it's close to being a "sleeper" in terms of outward appearance. There are a few of us nutbars out there

Just my own personal opinion but, mid-50mph cruising is a lot more enjoyable. That said, pretty tough having too much passing power and, not everyone shares my geezer-ish views.

Wow, that's incredible. I would love to feel what that feels like, but I'd be too scared to do it.

Carb size is not so much constraint on cc's than on air flow rate, IMHO. A small cc engine running at high revs will sometimes need even more breathing capacity than a larger one reving lower. The drawback is that a large carburator will be much more difficult to adjust at low rpms and idle if its not designed to work properly at slow air speeds. Some brands do better than others, however.

Changing the carb of my green Z50 (128cc) from a (reasonably tuned) Mikuni 26 to a (perfectly adjusted) Keihin 28 gained a nearly 1hp dyno boost with much better throttle response and perfect idle and mid range behaviour (yes, I admit: I'm one of those 'the larger, the better' ).

This engine has its peak performance at 12700 rpm and pumps nearly the same amount of air at this engine speed than its sibling in my orange Z50 with 146cc which has its peak power at 11500rpm.

So my answer would be: it depends.

If you want an easily tunable, well behaving motor and are willing to sacrifice a little top end power to that, go 19 or 20mm. If you want to go crazy and squeeze the ultimate (which will probably need a little more than just a large carb) out of your engine, get yourself a Mikuni 24mm flatslide.

Regards,
Markus

I have a 20mm carb with 70cc cylinder using the stock throttle cable on my z50ak2. I had to use the slack adjustment nut and double it up on the cable guide to take the excess slack out of the cable, and still have room for adjustment on the 20mm carb's adjustment nut.