Slam 125 Pit Bike 4th gear running problem

[r3ed0]

Hi All,
Please please please can anyone help me with my sons pit bike.
I purchased this bike for my son about 9 weeks ago and everything was great while running in the bike, then when he finally opened the bike up it ran like a dream in 1st,2nd,3rd gear but when you get to 4th gear and hold it at 1/4 throttle the bike boggs a little and splutters and farts and sound really rough, twist the throttle out of 1/4 and the bike runs great again . I have tried lots of thing to rectify this but to no avail here is a list of things I have tried :- new air pipe on fuel cap, looked at filter in fuel tank, cleaned the air filter, used unleaded and super unleaded fuels , new plug, new HT lead, new CDI box, checked all gaskets and seals, removed the inline filter, timing is right, tappets are set correct, needle clip is on middle notch and fuel screw is on 1 1/2 turns from lightly seated and tickover is i think set right. This problem is not helping me get my son into motor sports as everytime it does it he thinks it might cut out and has left him with no confidence on his first bike. i have taken the bike back 3 times to two different dealers of Slam and each time they tell me they have fixed the problem. i then take it out with my son and the problem is still there.
Many thanks
Steve Reed

Ps thanks for any replys in advance.

[bigdom]

If it was me mate I'd start from the beginning, Get a brand new NGK plug then go through the timing making sure its not retarded, How did you check yours?
Do the tappets again on the firing stroke on top dead centre TDC 4 thou inlet
6 thou exhaust.
then check the air mixture screw same as the idle 1.5 turns.
then look at the stator gap you want to be looking at the same as a debit card thickness.

[r3ed0]

Originally Posted by bigdom

If it was me mate I'd start from the beginning, Get a brand new NGK plug then go through the timing making sure its not retarded, How did you check yours?
Do the tappets again on the firing stroke on top dead centre TDC 4 thou inlet
6 thou exhaust.
then check the air mixture screw same as the idle 1.5 turns.
then look at the stator gap you want to be looking at the same as a debit card thickness.

Hi Bigdom,
Fitted a new plug still the same, timing/ i just alige everything on the fly wheel, sorry i dont know how to check stator gap and i will recheck tappets 2moro night.
As a teenager i always had 2stroke mx bikes for racing so 4stroke is a new thing to me so sorry if i am a bit slow on the uptake.

[bigdom]

Did you check the cam markings with the fly wheel?.
If you go on youtube search (OOracing valve clearances) that will help, if your bike timing is 90% out the bike will still run so it has to be set on the firing stroke.
As far as the plug you can buy duds and they will still spark but not enough.
What about the air mixture screw not the idle screw.

[r3ed0]

[QUOTE=bigdom;949184]Did you check the cam markings with the fly wheel?.
If you go on youtube search (OOracing valve clearances) that will help, if your bike timing is 90% out the bike will still run so it has to be set on the firing stroke.
As far as the plug you can buy duds and they will still spark but not enough.
What about the air mixture screw not the idle screw.[/QUO

The fuel screw under the makuni carb is 1.5 turns out from seated, idle screw is on the side is that right?

[bigdom]

Sounds right, have you had a look inside to check for blockages in the jets?

[r3ed0]

Originally Posted by bigdom

If it was me mate I'd start from the beginning, Get a brand new NGK plug then go through the timing making sure its not retarded, How did you check yours?
Do the tappets again on the firing stroke on top dead centre TDC 4 thou inlet
6 thou exhaust.
then check the air mixture screw same as the idle 1.5 turns.
then look at the stator gap you want to be looking at the same as a debit card thickness.

Hi all,
just went over some of the things bigdom said to recheck so I seated the air mixture and turned it back out 1.5 turns (happy days) then I did the same with the idle screw, seated then 1.5 turns out but when I started the bike the revs were really high, so I had to turn it out to about 5 turns for it to tick over nice. is this right? if not how do I correct?
thanks Steve

[bigdom]

So does the bike rev ok now?

[r3ed0]

Originally Posted by bigdom

So does the bike rev ok now?

Yes on 4 turns out

[r3ed0]

bigdom,
do you think I might need a smaller pilot.