Question for knowlegable mechanics?

[richirich1113]

i recently made a discovery, riding my motorized bike one day the end cap on my muffler fell off. the motor became very loud( it sounded like a full on dirt bike) but boy i was impressed with the power! being to loud to ride without getting a ticket i took a cap from another exhaust and punched it out a bit. alittle less power than uncorked but less noisy. now my question is, after i punched out the exhaust, when i would be reved out and let off the throttle upon deceleration i would get a pop, pop, pop kinda like a back fire. today i read a thread about tuning the nt carb. so playing around with it i richened the mixture now i dont get the backfire poping. did i do something right?? for a change...

 

[machiasmort]

You don't want to run your bike w/o proper restriction (exhaust). You'll burn the motor! The air jacket inside your pipe gives the piston something to push against. By taking the cap off, you let the gas by-pass the baffle, making the bike louder.

You are probably not gaining anything with an open exhaust except attention and a headache!

 

[richirich1113]

i think you might have misunderstood, i didnt purposly take the cap off it fell off when i was riding. i replaced the cap with another that i just drilled a couple of holes into. its not much more louder but the bottem end power definetly increased. im just wondering why the popping stoped while decelerating when i richened the fuel mixture.

 

[motorpsycho]

the pop-pop-pop is because with an open exhaust, you create a leaner air-fuel ratio.
your motor was actually breathing better, leaning out the air-fuel ratio and giving you more power.
if you run an open exhaust, expansion chamber or anything that flows better than the stock muffler, you will have to richen the air-fuel ratio (by jetting, not by moving the clip on the jet needle.)
any time you create a lean mixture, you will get popping out of the exhaust. richening the air-fuel ratio will make the popping stop.

 

[richirich1113]

Thanks so much for the info! this forum site is the best!! i have learned so much and believed to had a few headaches stopped. everyone keep up the great work!!!

Now does anyone know maybe where to get jets for a nt carb?

 

[machiasmort]

There's some good thread for drilling your own with refference for sizes to use. Run a search!!! You are going w/ a bigger hole, not smaller! Pop Pop = hot hot, not good!

 

[richirich1113]

Thanks for reminding me, i do believe i read that thread. its just im the the type of mechanic that can replace just about anything with ease but if i try to open something up ....I sware there are gremilns afoot. lol but maybe ill give it try.

 

[motorpsycho]

the first thing to do is determine what size jet is in the carb now. the majority of the jets in the stock n.t. carbs are not marked with a size number, but i think someone estimated them at being a .70 jet. if you have the cns carb, i have no idea what jet is in it or what jets will fit it.

the thing about drilling out a jet is that it's real easy to go too big with the hole size.
it's all trial and error and if you do go too big, you can always solder the hole closed and re-drill it smaller. but a smaller hole will make it leaner and you don't want that. a slight change in the hole size can make a huge difference, especially when you are heading toward the lean side.

I bought delorto jets from sick bike parts instead of drilling out my stock jet. HOWEVER, the jets do not screw into the stock n.t. carbs easy. the pitch of the threads is slightly different and the jets screw in tight. They fit, but they are tight and you take a chance of messing up the threads in the carb where the jet screws in.
for example, on one of my engines, my stock main jet was (i'm guessing) a .70 and it was running too lean with my modified exhaust. so to determine that i had around a .70 jet in it, i threw in a .68 jet to give me a baseline.
well, the .68 made it leaner,(popping, bogging at w.o.t. and my plug was a silvery color) so i knew that the stock jet was around a .70.
from there i went up in jet sizes starting with a .72, which made a slight difference.
then i went to a .74 which made a huge difference and it ran great.
then, i went up to a .76 just to see if i could fine tune it a little more, and the .76 was too rich (it started 4 stroking at w.o.t., and my plug was black). i dropped back down to the .74 and that's where i left it. my plug is always tan, no 4 stroking and no popping. i think i have my jet needle clip set right around in the middle, but it's been awhile and i kind of forgot where it's set.
I also have a high flow air filter on it which helps the engine breath a lot better than the stock filter.
now, this jet change didn't make me gain 10 mph or anything crazy like that, but it did make a difference in how the engine starts, runs and accelerates.
i may have gained 5 mph from the jet change, modified exhaust and high flow filter, but it may not even be 5 mph...i'm not really sure.

 

[richirich1113]

Awsome Information!!

Thank You for the awsome information!!!!!!!