70cc BB kit main jet size #85 normal or not By Tawas (Read 5,028 times)

[Tawas]

I have a 50cc VIP 2010 and upgraded to a 70cc BB kit this thing normally runs outstanding never had any issues at all I let my son take it to MSU as freshman cant have cars first year of collage so in September of 2015 he used it all the time running great but soon as it got colder out problems with it starting and stalling what I believe is the air is dense in the colder weather and these issues started ..I have been working on these GY6 scooters all the time in the summer as no place likes to mess with them and I am a certified mechanic when I was 23 so I do know these scooters good enough to fix many in past years this is what I believe my issue is the air being denser causing issues because I might be borderline on my main jet smaller/bigger instead of alot of things I will be checking I just want a opinion on my main jet size as I believe Iam borderline of what it really should be it has a #85 .033 main jet runs great in the summer all I want to know is that a good jet for this 70cc kit or do most run a bigger jet the carb is the original is this to small for this or is that size what I should be running with I will be checking all the necessary things to check just looking for the answer to is this to small for this size engine or should I run a #90.035 or even a #95.037 main jet size ..and yes I will checking everything else also to get to the problem fixed just looking to verify running a #85 is right or not for this size 70CC BB KIT ...all emissions removed the right way ....thanks in advance is #85 main jet right/wrong or should it be correct for this size engine.oh I live in MICH if that helps I forgot to say blue CDI performance filter and exhaust

[larryhobman]

I run a #88 or #90 I keep switching trying to see which is better. I have tried a #92 which was not too good. I run a 72cc (47mm piston kit) with a A9 cam
I would check if you have the 44mm or the 47mm if it is a 44mm stay with the #85. Also I run a #34 idle jet it is better in the cold than the #31 I tried. Adjust your A/F screw too.

[Alleyoop]

Since your a certified mechanic you know in colder weather motors need more fuel and in hot weather less fuel. But your complaining about starting and stalling. The starting problem you need to richen up the fuel ratio mixture on the carb, turn it counter clockwise to richen it.

The stalling IF this happens coming to stops after riding around a little that is a classic case of the valves being tight and the motor is loosing compression and will not hold low idle.

If the stalling is when it finally starts then richen it up will fix that porblem as well.

As far as the MAIN JET it is not set in stone which size will work, all motors are different some have more compression than others and produce more power and some have less and produce less power. But for a 70-72cc BBK most will take a #85-#90, your 80cc+ bbks will take #92-#98.

A tach on it will help determine if it needs more or less fuel by what RPMS it runs at WIDE OPEN THROTLLE.
Alleyoop

[Tawas]

ok sound good I believe my valves are to tight I remember I didn't adj while hot and on my Yamaha G16 golf cart that made a huge difference adj hot ..I will stay with the 85 main jet and trying to adj the mixture screw thanks for the heads up I believe thats my problem ..I will do this on Monday and give a update to help others if they run into this issue thanks again

[Alleyoop]

Yea most folks do not even think about the valves being tight and they go right for the carb and start to adjust it to compensate for the tight valves. But then it valves get worse and no adjustment of the carb can help at that point. So then they adjust the valves and THEN what happens is they forget to re-adjust the carb now the valves are adjusted. So adjust the valves then test out the carbs fuel ratio mixture and adjust as needed.
Alleyoop

[Tawas]

looks like I read to review and says to adj cold so I will do this cold correct

[Alleyoop]

Yes adjust the valves when cold. Also a little tip, before you put the valve cover back on and button it up ROTATE the motor over by hand several more times and RE-CHECK you gaps a .005 feeler SHOULD not slid in and the .004 should. Many adjust them but when tigthen the lock nut the stud moves a little and your gap is off.
Alleyoop

[Tawas]

ok sounds good I will do exactly that thank you for the help

[Tawas]

Going today to fix the issue above and had few more questions I now have the Blue CDI pictured in other posts but did some reading about them not being so good should I keep that box if working or switch to stock box I do have a orange castol CDI and I have that after market boost bottle from cab mount boot should I remove that I dont know if that is a helping or just a item that does nothing..just to verify on my 70 BB KIT listed above my valve gap should be at .004 not .002 cold set like stock 50cc says .002 ..just wanted a tip on if I should change any of that or just use it ..thanks again for helping me your great people on this site it really helps me ALOT ..

[Tawas]

Working on the scoot and want to ask if the stock 50cc idle jet the one I going to try is the #32 and I compared it to the stock ones I have the bottom holes are smaller dia than the #32 which is marked 32 and the stock one has no marks and holes look bigger clearly is that correct stock bigger looking holes is what I have been running

[Alleyoop]

Well if the problem is just harder to start in the cold you do not need to upjet just richen the mixture up by turning the fuel ratio mixture counter clockwise. Normally the stock pilots are 31-32 on the 50s so use which ever one looks like it is a little bigger, Then adjust, if to much fuel you can always lean the mixture out by turning the screw clockwise. Just as long as your not to far off on the size in either direction you can get it dialed in with the mixture screw.
Alleyoop

[Tawas]

ok thank I will use the 32 because I know what it is by marked and the other is not marked and cant tell what it even is thanks again