[SOLVED] Feedback on purchase/jetting/install for going to 70cc BBK

[245luigi]

Hello all,

Warning, many questions ahead! Now that the riding season is calming down in my area (Baltimore, MD), I have decided to install a BBK on my Like 50 during the winter. I have done alot of research on the forum here as well as other places. Now im looking for some feedback on my shopping list as well as how the heck Im going to tune this thing once all the parts come in.

First off, current setup: 2012 Like 50, Stock Kiehin PB 16 with 86 MJ and 35 PJ, Tecnigas Next R Exhaust, Stock Airbox

For this upgrade I plan on getting:

Malossi Cylinder Kit
http://www.racingplanetusa.com/cylinder-malossi-sport-with-head-70cc-kymco-horizontal-p-228-1.html?cPath=2_26378_26380&sessID=fdb133b4fb42f31519cb44d0a0acc53e

Malossi Carb Kit (Come with 19mm Dell'Orto PHBG)
http://www.racingplanetusa.com/carburetor-malossi-phbg-kymco-sf10-p-314-1.html?cPath=2_26378_26380&sessID=fdb133b4fb42f31519cb44d0a0acc53e

20mm Intake manifold
http://www.racingplanetusa.com/intake-manifold-20mm-racing-kymco-p-250-1.html?cPath=2_26378_26380&sessID=fdb133b4fb42f31519cb44d0a0acc53e

20mm Intake/Carb Spacer
http://www.racingplanetusa.com/spacer-20mm-intake-manifold-insulator-spacer-20mm-with-oring-p-2284-1.html?sessID=fdb133b4fb42f31519cb44d0a0acc53e

I also would like to go to Open Airbox during this conversion but I have no idea what kind of connection there would be between the Dell'Orto and the Open Air Filter. I dont know the size, as I havent been able to find any specs on the opening. Any input?

Now to jetting. I have no idea what jets the Dell'Orto comes with installed. I also dont even know where to begin with jetting trials. Im looking for some input as to a range of jets to pickup. I do know this, on the stock PB 16, with open airbox, I was using a 105 - 108 MJ, getting good chops and 38 PJ for a nice idle. Moral of the story here is I have never started from 0 with a carb. I've just had the experience of having a tuned carb and changing jets when I've messed with the airflow some how (exhaust duh!). Also how does one do plug chops during break in? I know WOT is bad but how else am I supposed to dial in my main jet?

Sorry for the stream of consciousness here but any and all input is extremely appreciated!

[245luigi]

I also would like to make sure the squish band is at the right spacing but Im having trouble finding which gaskets to use. I dont know if the stock kymco gaskets would fit or if I have to change over to Malossi ones. Also I havent found a resource for gaskets that I know would fit the 70cc kit and have options for different thicknesses.

[245luigi]

Actually I believe the cylinder head has an o ring type gasket so forget the squish band question

[Haarek]

What is the purpose of the tuning? Is it it get more power, increase max speed or everything at once? Personally I would do one upgrade at the time and not everything at once. That way you have more control on what is happening. A 70cc will run just fine on your stock carb as long as you re-jet it and move the needle. My advice is to install the bigger cylinder first, run it in and then consider the bigger carb. The mallossi is a cast iron cylinder so it will need to be broken in for some time before you can start to fine tune. As to the main jet size I won't recommend a specific size as I have worked on different scooters than yours and results may wary. Sound and feel are also good indicators on jetting. A few years since my last two stroke, but when I was at my game I only needed plug-chops as a confirmation and tuned by sound and feel exclusively. There are several guides on the net, so I won't reiterate it here. Either way I would jet on the rich side until your cylinder is properly broken in (and that can take a while) Do you have a unrestricted CDI? No point in huge air supply if the rpms are hard capped.

[Vivo]

Haarek is right.. a step at a time so you can really feel the difference of each upgrade and know what to do next...

[245luigi]

The purpose I'm tuning is because I have fun messing with my scooter! I'm looking to increase max speed. I find it thrilling trying to squeeze every last drop out of this thing. I absolutely agree with the 1 step at a time idea; always works whether its diagnosing issues, CVT tuning, or jetting after playing with your exhaust or airbox.

Haarek I wish I could tune by sound/feel. Thats very interesting.

I will install each part starting with cylinder and stock carb (I have MJ 78 - 120 and PJ 35 - 40). The reason I included the carb in my list is because all of the shops selling the cylinder kit + Like50 owners in the KF archive have said you need 17.5mm at least when going to 70cc. My guess is ill have to richen the stock  carb settings when I put on the 70cc cylinder. When I first bolt on a brand new carb how do I figure my starting point with jets? Just ballpark and go?

I dont know if I have an unrestricted CDI. I probably do. Thing is its really hard to find Like50 info out there; this forum has definitely been my best source. I still have yet to find a Like owner, here or some other forum that has messed with the stock CDI. When I did so open airbox jetting trials on the stock cylinder and tecnigas exhaust I was able to get ~5mph more out of my stock setup.

Thanks for the feedback

[Haarek]

Here are some tips on how to tune by "feel". I copied  it a few years ago from a site I don't remember the name of. Credit goes to "that" site 

The following advice is targeted at Keihin carbs, but the principles are the same for every carburettor. Notice the tuning is done without plug-chops until the very end 

Rich Jetting Symptoms and Solutions

- Motor won't idle well and idle set screw won't work effectively. Try leaning out the air screw by turning it counter clockwise a quarter turn at a time. If the air screw has no effect, install a leaner pilot jet and return the air screw to 1 turn out. Proper air screw settings are usually between 1 and 2 1/2 turns out. The correct pilot for your bike is one that allows instant off-idle throttle response, and allows the air screw to be effective between 1 and 2.5 turns out. If the problem still exists, check and clean the choke circuit and carb vent lines.
- Motor stumbles off idle to 1/2 throttle, then cleans up. If everything in part A above is correct, check the needle. Stock position is usually in the middle, but check your owner's manual to be sure. To lean out the needle, you need to raise the clip. If the needle is dropped all the way lean and problem still exists, try a leaner needle.
- Motor will run through the mid range then becomes gurgley (technical term) at full throttle and power is sluggish. This is a text book example of a rich main jet. This problem often occurs when an air filter becomes clogged and gets worse as the motor heats up. Choose a main jet with a smaller number and lean out the air fuel mixture one step at a time. If the problem still persists even though it's improved a little, reinstall your original main jet and lean out the needle one position. Now fine tune the main jet with plug checks.

Lean Jetting Symptoms and Solutions

- Motor hesitates off idle with a "Boooooooooowang" sound. This is a lean symptom and often occurs when a motor is cold. Try turning the air-fuel screw clockwise a quarter turn richer and check throttle response again until motor revs without hesitation. If the motor "hangs up" or doesn't come right back down to idle, install a richer pilot, and reset air screw.
- Motor knock knock knocks at idle when hot. Try solution A. If problem persists, perform a "leakdown pressure test" to check for ignition crank seal leakage.
- Motor revs clean and crisp but runs really hot and lacks power. This symptom indicates a lean main jet and/or needle. Use a richer main jet and/or needle setting. This is the most common misconception about two stroke jetting: When a motor runs excessively hot and lacks power, it is on the verge of seizing. If you are on the road and don't have the right main jet, try raising the needle by lowering the clip. It won't be exact but at least you won't seize.

Note: To be certain that your problem is lean, pull on the choke and see if the problem gets better or worse. If better, your problem is a lean condition.


The following has worked for me: (Usual disclaimer about not taking responsibility for other peoples tuning (You silly Americans...))

Premise: A  2 stroke scooter with restricted CDI, a big bore kit (70cc) and a unrestricted exhaust and variator.

Start by lifting the carburator needle from the middle position to the top. Install a new main jet that is around 10-15% larger. It might need to be bigger depending on air box modifications Remember that with the stock carb it wont be able to suck that much more air and it is the ratio of air/fuel that matters. Start it up and listen to the sound and feel of the scooter. Re-jet as necessary.

With a original carburettor and a 70cc you should experience a good deal more torque, but should not expect higher top speed. That comes down to the ability to rev high - depending on the rev-limiter in the CDI and the air supply.

Personally I have never bothered with top speed. When my scooter does around 70km/h I'm happy with that and tunes for torque. In these cases it is not certain that a bigger carb will give value for money. It will certainly bring up the fuel consumption.

[Haarek]

Here follows a more detailed instruction on how to tune a carb by feel (following is not my work, but I have used this method for several years with great success):

You can only begin re-jetting your carburetor if the following conditions are met:

    Top end is in good condition.
    Bottom end is in good condition. Crank seals.
    Spark plugs, air filters, reeds and so on.

If your bike's motor is not mechanically sound, then all the jetting in the world will not help. With all of the above conditions met, you should be able to jet your carburetor following these steps:

Step 1: DETERMINE THE CORRECT NEEDLE AND OR NEEDLE JET.

Whether or not your carburetor is a MIKUNI or a KEIHIN, it does not matter. This is the most important step in jetting your carburetor--period!

    Remove the main jet.
    Place needle clip in mid-position.
    Start motor and run it on the stand.

Condition: Motor running and main jet out. Needle or needle jet is correct: Carburetor should run clean to approximately 3/4 throttle. From 3/4 throttle to full throttle, the motor should start to break up as a result of too rich condition.
Correction: None needed.

Condition: Needle or needle jet is too rich. Carburetor runs clean to approximately 1/2 throttle but breaks up before 3/4 throttle as a result of too rich condition.
Correction: Mikuni replace needle jet with next leaner and test again. Keihin replace needle with next leaner diameter and test again.

Condition: Needle or needle jet is too lean: Carburetor runs clean beyond 3/4 throttle and has an erratic throttle response.
Correction: Mikuni replace needle jet with next richer and test again. Keihin replace needle with next richer diameter and test again.

The emphasis here is to find the correct needle or needle jet diameter, which will allow more fuel to pass than is needed but not so much that the needle itself has no control below 3/4 throttle.

Step 2: DETERMINE THE CORRECT PILOT JET.

    Make sure the bike is warmed up if at all possible.
    Main jet out.
    Needle clip in mid position.
    Turn air screw all the way in then 1/4 turn out.
    Start motor and run it on the stand.
    Adjust idle so the bike will just barely idle.

Condition: Motor running and main jet out.

PILOT JET CORRECT: 
With one hand on the throttle maintaining RPM at approximately 1/8 throttle, turn air screw 1/4 turn at a time clock wise until you bottom it out. Motor should become slightly erratic and you should have to play with throttle to maintain RPM. Start turning air screw counter clock wise, 1/4 turn at a time until you have reached 2 3/4 turns out. Between 1 1/4 and 2 1/4 turns, your motor should have reached its highest RPM maintaining a steady throttle. Adjust air screw again between 1 1/4 and 2 1/4 until you have determined highest RPM. Quick throttle response should be clean without bog.

PILOT JET TO RICH:
RPM does not reach a peak between 1 1/4 and 2 1/4 turns, stays the same or keeps rising out to 2 3/4 turns.
Correction: replace pilot jet with next leaner and test again.

PILOT JET TO LEAN:
RPM does not become erratic and motor maintains throttle when air screw is turned all the way clockwise.
Correction: Mikuni replace pilot jet with next richer and test again. Keihin replace pilot jet with next richer and test again. Remember, with a steady throttle approximately 1/8, there should be a distinct difference in RPM from 1 1/4 turns to 2 1/4 turns if the pilot jet is correct. The emphasis here is to find a pilot jet that will run crisp without bog and without the main jet.

Step 3: DETERMINE THE CORRECT MAIN JET.

The main jet selection process is easy once you have the correct needle diameter or needle jet. You now only have to correct a rich condition from 3/4 throttle on up and you know what a rich condition sounds like. Your pilot circuit is correct and without bog.

    Replace main jet with one that is at least two sizes smaller.
    Needle clip in mid position.
    Start motor and run it on the stand.

By replacing the main jet with one that is too small, you are looking for a condition that is too lean. You adjust your main jet from a too small to lean condition.

Condition: Motor running and main jet in.

MAIN JET CORRECT:
Carburetor should run clean and crisp to full throttle.
Correction: None needed.

MAIN JET TOO RICH:
RPM reaches a peak slowly with a deep sound. Excess fuel and oil mixture at end of silencer. Spark plug fowls easily and is dark in color.
Correction: Mikuni replace main jet with next leaner and test again. Keihin replace main jet with next leaner and test again.

MAIN JET TOO LEAN:
RPM reaches a peak quickly but erratic. A quick full snap open of throttle causes the motor to hesitate. A BEWAH sound or a complete bog. Motor sounds like it has a ring to it. End of silencer white. Spark plug is white in color.
Correction: Replace main jet with next richer until the BEWAH bog just barely goes away, then replace the main jet with the next richer and run it. The emphasis here is find a main jet that is just rich enough to allow you snap the throttle wide open without the motor bogging as a result of the main being too lean. Should be a quick crisp throttle with no hesitation.




Jetting Tip: pilot jet and air screw

To check to see if your pilot jet and air screw are adjusted right, warm up the motor and let it idle as low as possible while still running smooth. From idle, whack the throttle wide open then let it close completely. Listen to the motor. If it bogs right when you whack the throttle open, then revs up, turn the air screw in 1/2 turn then try again. Do this until it revs up crisply. After the motor revs up, listen to it revving back down. If the revs drop quickly, and the motor starts to bog, and/or die, then you're too rich on the air screw, back it out 1/4 turn at a time. If after you let the throttle off the motor tends to run on and on while revving down very slowly, you're too lean and need to turn the air screw in 1/4 turn at a time. You want the revs to come up from idle quickly and smoothly, then drop back down to idle the same way. If you turn the air screw all the way in and it still needs to go further, then you will need a larger pilot jet. The opposite is also true: if you are backing the screw out so far that it darn near falls out, you will need a smaller pilot jet.

[245luigi]

This makes me even more excited to get his project started! Thanks!

[JJJoseph]

I have decided to install a BBK on my Like 50

It's difficult to get extra speed by installing a BBK & rejetting alone.  The safest mod for extra speed is a BBK + gears.  Gear sets that result in about 9:1 final drive ratio will give a nice speed increase with a 70cc BBK.  The BBK gives the extra power needed to twist the lower ratio. Gears are cheap and easy to install.  You only need to change 2 out of the 4 gears in the case. In some cases, cheap Honda Dio/Elite gear sets work. Even genuine aftermarket Kymco gears shouldn't cost more than $40-$50.

[245luigi]

Interesting...Im still in R&D regarding gears. Mostly because I do all my work on the sidewalk (City living!) and obviously dont have a shop press 

[JJJoseph]

how the heck Im going to tune this thing once all the parts come in.

When upgrading a 2-stroke, just do one thing at a time as others here have suggested.  The Kymco 2-strokes are similar to the Honda Dio, so I can suggest that you switch the jug but leave the stock carburetor except for changing the main jet from a #75 to a #90.  You can add a tuned pipe at this stage, and 9:1 gears perhaps, but that's about it.  Then ride it carefully for 500 miles or so to break it in, sticking to the original top speed.  You'll have enough fun with the increased acceleration.  The main problem with running 2-strokes at WOT is that (unlike 4-strokes) they can quickly overheat and melt the piston.  Then you have to start all over again.  This is NORMAL when tuning/racing 2-stroke scooters & dirt bikes.  You're trying to go as fast as you can WITHOUT melting a hole in the piston. But sometimes you miscalculate.  A Cylinder Head Temperature gauge is useful, more useful than plug chops. 

[245luigi]

Joseph

I think you have a great point and I think that is what is best for me to do. When I have the engine out, do you think it would be OK upgrading the intake manifold but sticking the stock carb on it? Only reason I ask is because the engine is already off the bike and it would be so much easier to put it in at that time.

And if it is OK, will it let more air in despite being on the stock carb, perhaps upjet a little bit more to stay rich?

Muchos Gracias!

[JJJoseph]

do you think it would be OK upgrading the intake manifold but sticking the stock carb on it?

A bigger bore manifold won't make much difference.  Similarly, a bigger carb won't make much difference if it's mated to a stock manifold.  An upgraded manifold only makes sense if you upgrade the carb at the same time.

[245luigi]

Coolio!

Finally recieved my 70cc kit and Malossi Dell'orto phgb 19mm carb. Will install in approx 2 weeks. Current gameplan is to change the cylinder and intake manifold as I suspect it will be easier to change the manifold with the whole bike apart. I will break in the engine on the stock carb as I know how to tune it. Post break in the dell'orto will go on.

 Luigi