I recently put a 72cc kit in from dratv. Put on a k&n airfilter and a #62 main jet. Still have stock exhaust and everthing else. On the top end everything seems fine, pulls hard all the way to redline/limiter. The bottom end is where i'm having lots of problems. I've read all the tuning guides and tried every combination of airscrew and needle settings and it only seems to make slight differences. But I CONSTINTLEY have bog off of idle. If i go from idle to wide open and hold it it will bog and die. Sometimes when coming from a hard pull to a stop it will also die. I'm currently at one notch below stock on the needle clip, the airscrew is 2 turns out. Yet I still get bog. Any ideas???

 

big bore, k&n, bigger mainjet, but with a stock pipe? maybe the bike just can't breath. you're moving more air and fuel into the motor, but restricting the flow out of the motor.
i'm not saying for sure that's what it is, but sounds like a good bet to me.

 

stock pipe is what you want for that set up. id put the stock filter back on and keep playing. you will get it dialed in.

 

You think that 62 is an ok jet? I was thinking maybe it was too big and thats why it was boggin off teh idle, maybe a 60 would be better? I have the stock pipe also because i ride street 90% of the time, and loud pipes might save lives but they also get calls to da popo...

 

yeah a 60 wouldnt be a bad idea. ive had good luck with 62s tho

 

main jet has nothing to do with boggin off of idle
you have increased the air flow through the carb with the air filter that you put on and now the pilot jet is to small for that much air.two remedies
put the stock airbox back on as fab says, which will work and is the easiest to do,or you can get a xr 70 carb and intake,the difference is the 70 carb you can change the pilot jet on the stock carb you cannot

 

More proof that i know nothing lol. Alright i'll throw the stock filter back on and see how that works. Thanks for the input guys.

 

just out of curiosity, wouldn't a bigger (free flowing) exhaust better match the flow out of the motor to the improved intake flow and do pretty much the same thing, with a slight power increase?

am i off somewhere here? i understand why the k&n makes in bog with a stock exhaust, and why the more restrictive stock airbox would make it run smoother, but if the exhaust was more free flowing, wouldn't this have a similar effect?

(and yeah, i am slowly realizing that these small 50 motors are alot more sensitive to tuning than the motors that i'm used to dealing with, so don't write me off as being incompetant, i'm just curious)

 

the stock head can only flow so much. it is limited by valve size and spring tension. you can put a 3" primary on it if you want. but you will still be limited by the head. what im saying is, the stock pipe is more than enough for what a stock fiddy head will ever be able to flow

 

thanks, thats what i wasn't aware of. most larger motors that i deal with aren't as limited by the head. i'm not used to taking that into account. i know the motor wasn't designed to be high output, but i didn't know how limited the flow of the stock head really was.
most of what i play with day to day are much more performance orienated motors, so i still get stuck in that mindset sometimes.
learn something new every day.

looks like i need to find a nice stock head to play with on the bench.

 

i wouldnt waste yer time really. race heads and Xr70 heads are so cheap its not worth messing with puttin in larger valves and porting

 

Stock airbox fixed the problem. Thanks

 

in all actuality, it'd be for my own amusement, just to see what kind of flow i could get out of it. i have too much spare time apparently.

 

you want the correct intake velocity that is the key to making them run
you have to have the correct velocity for the carb, that is why the stock airbox and pipe works the best, it is matched to the way the cam and ports and intake manifold are set up on that head, the problem with putting an aftermarket pipe on there is now you have lost the airwave holding the intake charge from escaping during overlap of the valves,which means you get less of a charge in the combustion chamber which means less fuel to burn
the same goes with the an aftermarket air filter now you are allowing more air into the venturi of the carb and it isnt flowing enough fuel to mix properly and causes a lean condition which essentially does the same as the pipe does not alowing the proper mix into the combustion chamber and not having enough fuel to burn resulting in it makes no power.you just moving air with no fuel in it.
heres the trick if youre going to run a pipe carb and airfilter you want to gain velocity so that when your intake valve opens your stuffing more than it can handle into the combustion chamber therefore getting a full charge, the fiddy with all stock stuff is getting about 80% of what it can actually handle, on mine i run an 00" xr70 intake and carb with a k&n filter with a velocity stack built in and a D&D exhaust before we swapped everything out it ran 27 mph with me on it radared, now im getting 34 mph. its all about volumetric effeciency, the more effiecient the more power it makes.

 

yup. i'll second that. i thought about playing with velocity stacks, i used them on the bandit1200 motor in my gsxr, worked awesome, really smoothed things out.