special thanks to racerx and Pony Express

[milspec ct]

Hi guys,
Just wanted to thank all those who helped with questions Ive had regarding my hopped up ct70 (120cc) bike.Special thanks to racerx and PonyExpress.I hit my top speed goal today on the highway,early this morning,no car drafting,no wind, No BS speedo reading,all GPS . Flat 2 mile between entrance and exit section of the highway.tucked with throttle pinned at 70.5 MPH on a build up of a stock 70cc motor! this with my 6'1" 203lb frame on top.Bike ran like a champ.For those interested it has a 54mm bore,52mm stroke,ported ultrahead,Mikuni vm26 ,and a bearing cam with 7mm lift both sides........

 

[bracmon]

so you put on a bigger top end on a stock motor(Honda)?

mike

 

[milspec ct]

Pretty much,along with a stroker crank,and high output oil pump.....

 

[MSZ]

How'd the bike feel at 70mph? It's fun flying on a CT as long as you have the brakes to stop the bike if you get into a pickle..

 

[steampick]

To brake at that speed just sit up straight.

I'd hate to be going 55 mph in my car and be resoundingly passed by a CT70. That said, I applaud your new land speed record for, hmmmm, what class would this be? I guess, Open Modified CT division.

 

[milspec ct]

How'd the bike feel at 70mph? It's fun flying on a CT as long as you have the brakes to stop the bike if you get into a pickle..

The bike felt good,engine humming along at 11k plus,brakes,I need them,hooking up with Bob for a set of nissin 220mm.I scope out this section of freeway prior to taking out the ct,had a fan club of landscapers in a pick-up cheering me on LOL! You were correct about my set-up being undercarbed with the vm22,the vm 26 made a HUGE difference( about 7MPH top speed).Thanks! Bob gave me advise earlier when the bike was at 110cc with a milder cam,said to try a vm22,worked perfect with that set-up.Now at 120cc,way larger cam,and port work,it was holding me back.

 

[milspec ct]

To brake at that speed just sit up straight.

I'd hate to be going 55 mph in my car and be resoundingly passed by a CT70. That said, I applaud your new land speed record for, hmmmm, what class would this be? I guess, Open Modified CT division.

Yes,sitting up helps alot LOL,but might not be quick enough in a panic,need hydraulic front discs.Yea,I passed a few cars,was in the fast lane for maybe a mile or so,if I stayed in the slow lane I would reel cars in everytime,get caught in there draft,and just get too close for comfort.I dont think I will do it again until I have real brakes mounted....

 

[daxman]

If I remember right, you wrote that you had troubles getting the vm 26 dialed with 110 cc engine. How does it work with the 120 cc engine, was it easy to get running right?

 

[milspec ct]

If I remember right, you wrote that you had troubles getting the vm 26 dialed with 110 cc engine. How does it work with the 120 cc engine, was it easy to get running right?

You are correct,I had problems for sure.Difference with this build besides the extra 10cc is the cam,and some custom head port work I did (got lucky).The new cam has way more lift,both exhaust and intake,7mm,and more duration,those two changes being the key.The vm26 was purchased from Hondatrailbikes with a tapered intake which I opened up to match the head port.Ive heard you can get a vm26 with all sorts of internal jetting differences,so I dont really know what I have but it ran right from the get go.Clip in the middle position,never touched it,pilot at 22.5,and main jet so big its crazy.Now Im going to get it on a dyno,check torque,HP and air/fuel before the weather gets too hot.....

 

[PJ_ST70]

I also have such a type of VM26. Main Jet 190 and pilot jet 22.5. Runs perfect with a 165 main jet in it on my cd113 superhead engine

 

[milspec ct]

I also have such a type of VM26. Main Jet 190 and pilot jet 22.5. Runs perfect with a 165 main jet in it on my cd113 superhead engine

I purchased a wideband 02 meter,and spent this week playing with it.Tells everything in realtime about your air/fuel ratio,allowing you to see main and pilot jet change influence on the AFR.Dialed my bike right in.My combo ended up with a tiny #15 pilot,and a #210 main jet,running between 12.5 and 13 to 1 AFR,couldnt be happier....

 

[69ST]

I also have such a type of VM26. Main Jet 190 and pilot jet 22.5. Runs perfect with a 165 main jet in it on my cd113 superhead engine

When it comes to setting up Mikuni carbs, there are two "wild cards" in the mix: engine displacement and needle jet profile. Horsepower is horsepower, regardless of displacement, however both pilot & main jet sizing is still affected. A larger displacement engine moves more air at idle/off idle and for that reason, pilot jet size is closely related to displacement. That jet/displacement ratio is less clearcut at WOT peak power, but still exists. All else being equal, displacement equals air flow at any given rpm. With less air flow per firing, the smaller engine needs to fire more times (higher rpm) for a given power level. Less air moved per firing requires less fuel (smaller main jet), per firing, to maintain the same A:F ratio.

The biggest challenge to pre-jetting is the variation in needle jet profile and different air jet sizes right from Mikuni. The needle jet affects A:F at least as much as jet size. It is possible to run richer with a 120 main jet than a 220, with the same carb, if the needle jet has a larger orifice. The seem to be 2 or 3 different VM26 configurations available from Mikuni. Most seem to require main jets in the #200 or #120 range. This also seems to apply to the VM22 series. Air jets affect midrange metering circuit transitions and are used to tune-out stumbles. None of these parts are readily available, unfortunately, so you're stuck tuning around lean spots with pilot circuit, main jets and needle adjustments only. The point here being that one will have one or the other and the main jet requirements are vastly different.

As Jeff has pointed out, an wideband O2 -equipped EGA allows one to quickly figure out what's going on, saving hours of guesswork.