80cc big bore popping noise???

[myyz]

Hi there just installed 80cc big bore kit and a 21mm cvk carby with 98# main jet and a foam uni filter great power upgrade only thing I noticed was right up top when top speed is achieved at about 70kmh it starts making a popping noise almost like back fire and speed ceases to gain any one know why it would be making that noise?

[myyz]

Any ideas any one??

[whiteknight]

i think it might poss be that the engine has reached its limit but the wierd thing is your top speed. 70 kph is 43.95 mph. my agility goes faster and its still a 50cc i average about 45-50 mph most days so i dont quite understand how yours is going that speed.  Did you replaced the exhaust when you did the filter? because that could cause problems with the flow, plenty going in but not enough getting out.
dunno if i've been of any help but if you give a lil more info i might be able to puzzle it out.
hope things are sorted soon.

[whiteknight]

i've just read that you havnt done anything to the drivetrain. that might be the cause of your problems. make sure you have a smooth boss fitted. i have a Race Cdi and Race Variator with 1000rpm torque spring. im going to rebuild the engine soon to race spec but still keep it 50cc (going to be a challenge)

[myyz]

Ok thanks for the info I think you might be right cause I haven't done a smooth boss yet I also wanna do 5g rollers and clutch springs for more of the line as well. Do you know where I could get a smooth boss from in Australia? Thnks

[juice]

A machine shop with a could cut it down for you . Good luck .

[adamsd11]

mine did this, i installed a "race variator" cut the cdi and brought new rollers now mine does 57mph down hill 52on flat. it doesnt make the noise anymore but i thik the noise will come back when i have run the variator in.

The noise is when youve hit top speed. I dread hearing it because then i realise i cannot go no faster

[baddi]

My agility, although a 50 ccm makes a popping noise when the exhaust or the SLS is unattached you'll have to detach both when changing cylinder, check if both is properly attached again

[spidrgr]

Funny thought here: Ever noticed if this is an interference engine? What I mean is, will the piston be able to hit the valves if they are not closed when the piston reaches its top position. Important fact to know if your engine easily reaches its rpm maximum, the speed at which the valves open and close might overwhelm the springs, therefore valves might be floating and the noise you might be hearing could be the piston hitting the valves. If so, quickly get a fix or you will soon be faced with a seriously damaged engine. On the other side, what is perplexing in your case, is the somewhat conservative top speed for an 80 cc engine. You dont mention if the 70 kph is speedo indicated or GPS. If speedo, keep in mind that they are usually very optomistic, about 10%. So, if you read 70, you are actually going about 63. My people 50 cc 4 stroke with a dealer unrestricted cdi and nothing else changed, easily and regularly reaches an indicated 68 kph with no funny popping noises, even wide open throttle, going down a steep hill at about 72-73 kph. What you describe as the popping noise could also be the sound of the engine simply ruinning with the valves floating without the piston hitting the valves, in which case, no real harm should occur if not happening for prolonged periods.

Like someone mentioned previously, have your variator checked to see if it is limited there. An 80 cc engine should easily get you to about 80-85 kph with heavier rollers in it. Hope this helps!

[Peters]

The (stock) restricted boss won't allow the transmission to reach top gear. Kinda like only having 1st and 2nd gear in a car. You're over revving the engine. That boss makes a huge difference even if the rest is stock.

[zombie]

It sounds like you are too lean. A 4t will "pop" thru the exhaust. The sound is really detonation. You will also hear it thru the intake. Try raising your needle to the richest clip to see if it helps. If not go to a richer main, and lower the needle to a leaner setting as you appear to have decent mid range. One more thing that will help with avoiding detonation is to polish the head of the piston. Use 100 grit paper on a true flat surface to get the casting marks out, and finish it w/ 280/320/600gr. It takes a few minutes, and will add many, many miles to your kit.

[streido]

Why would you go to the bother and expense of fitting the 80cc kit before even derestricting the scoot properly? I dont get that

Like Whiteknight, i can also hit 45mph most days and get it just over 50mph on the gauge downhill, and im also only running it as a 50cc, no kit. If you have the 80cc kit then you are well slow - hardly surprising tho if you still have a restricted boss fitted.

Better swap out to a smooth boss asap or dont ride it WOT. You may need to check your belt too in case its worn. Scooter Assassins sell bosses on their own or ebay used to do them too, most scoot shops will do them or ask your Kymco dealer to get you one.

I honestly am shaking my head here trying to work out what possessed you to think fitting the 80cc kit to a restricted scoot was a good idea? Do the boss and you will fly. IMO clutch springs do little to improve take off speed at all. I run 1500rpm ones but tbh there is not much difference in performance really. Do recommend doing the torque spring tho to maybe 1000 or 1500rpm one.

GET A SMOOTH BOSS IN BEFORE YOU DO ANYTHING ELSE 

[blue]

x 2 or you well be singing that song( another bits the dust)

[wordslinger]

GET A SMOOTH BOSS IN BEFORE YOU DO ANYTHING ELSE

..lmao..true that!