24” mountain bike project

[Greg58]

Jet numbers confuse me that’s why I say what drill I’m using,,I think the stock 66 nt has a jet at #71, I’m going to a 73 and open it up from there. I remember 2door saying that in Denver he had to run a #73 drilled jet because of the altitude. It has alway seemed odd that two engines the same size and the same location could need different jet sizes.

 

[allen standley]

Am I correct in understanding the Higher the # the bigger the hole?

 

[Greg58]

No, they are backwards. When talking jet numbers the lower the number the smaller the hole is, with drill bits the higher the number the smaller the hole. I think when all of this was conceived the guy was probably laughing.

 

[Greg58]

Here's a drill size chart.

https://bestdrillbit.com/drilling-reference-guide/main-front-page-bit-and-sizes-chart/

 

[allen standley]

Thank you Greg.

 

[Greg58]

The jet that was in the carb was a #70 drill size or .28, I couldn't convince myself to go smaller than a #72 on the first try. The #72 was much better but still flattens out at WOT, I soldered up a spare jet and drilled it to a #73 drill or .24, it runs the best it has so far. I'll do a plug check to see where I'm at color wise but by feeling no difference in engine heat I don't think its too lean. On the plus side performance was improved, it gained 3 mph up the hill of the road we live on. So sometimes less is more.

 

[allen standley]

Excellent. Thanks for making sense of it for me. I ordered a pin set and will be drilling my own soon.

 

[Greg58]

Allen I mic the drill bits in the range I need to use to see what the size really is, mine are fairly close but some are .05 off. The only drill bits I've used fromm the set are 69-73.

 

[Jimmy Bloodmaker]

I've always just bought a spread of jets. the drilling was to much for my fat shaky hands. if i told y'all how many bits i have broken before just dropping the $20 bucks on a 60 -80 5mm jet spread, you'd think i was insane. plus it makes it way easier for my old man hands to just grab a flat head and pop in a new jet. i've become so good at rejetting, i don't even spill a drop from the float bowl anymore. speaking of jetting, i have to rejet my love ( fav bike). i just put a custom pipe on her. WOW!! got my power band to kick in at about 3k rpms and shuts down around 5500 rpms. ik, i'm spinning her so slow. just love this bike, and want her to last. what is your header length greg? i'm trying to find out as much as i can about others 24" builds. what size pipe? the pipe i welded up today has a 23" header, goes from stock 3/4 and transitions at about 7" to a 7/8 pipe for the rest of the run. then to a double 40 degree cone ( front and back are same on the chamber) for the expansion chamber, then it's plumbed with a 12" stinger at 1/2, with a muffler can on the back. i'm lifting the front off the ground if i pedal and hit the gas on take off. i have to pedal then slowly give her some throttle or she wants to jump. curious about if you are gonna be tuning your exhaust anymore or just leave it as it is.

 

[Jimmy Bloodmaker]

Yeah, Greg's thread plus Jimmy's input equals Allen building with what he has. Doesn't get much better than taking stuff off your shelf and putting them on the road! Good going guys.

Now what else can we put together? I've got a list somewhere...….

Rick C.

i'm sure if we all knocked our heads together, we could rule the motor bike world with our collective parts lists and imaginations. lol

 

[Greg58]

Jimmy I did a lot of pipe building and testing about 5 years ago, the pipe on my silver bike is the best I could come up with, it has a 16" head pipe with a deep bent to not restrict flow, a 12" chamber and a 2", 1/2" I.d. stinger. That's on a 66cc modified engine, totally different than what the 48 cc needs. I'm applying what I learned as a kid in the early 70's working on Suzuki and Yamaha motocross bikes, no two are or respond the same. Trying several pipe types till I find what this bike likes will be what I do, the pipe on it now is a stock 66cc kit pipe, it has a unusual large stinger. This pipe is better than the first 48cc kit pipe I tried, I've cut the 48cc pipe up to start building a custom pipe. I'll post results as I go.

 

[indian22]

Jimmy I agree with Greg. Though same dimensions will get you close try, ride and change is the only way I've found that works with selecting expansion chambers. I have two 66cc engines set up identically in every way and one performs much better with the same expansion chamber than the other. Change the stinger length & diameter a bit and it's vice versa on performance.

Rick C.

 

[Greg58]

I found a pipe on sale on Amazon, $53 shipped. With prime it will be here Monday.

 

[Greg58]

I'll test it on this bike, anyone else tried one?

 

[Jimmy Bloodmaker]

i've only used my own creations or stock/modified stock mufflers. i made this modified poopoo motcross pipe, worked really well on a 49cc, but only like 1500 to 2000 rpm band on a 66cc, an only higher up the rpm's( hit's like 4500 and runs to 5500-6000) are you really on the pipe. on the 66cc, when it hit's, it hits. take ya up a hill 28 mph, for blocks on end. top speed of the bike is 32mph. the bike runs best when loaded, like into the wind, and holds it's 32mph if ya let it. i got some steep hills around here. it was an almost 40mph bike till i wanted more torque for the hills.

 

[Greg58]

I was going to test the new chamber on this bike but it hits the fuel petcock, so I tried 4 different pipes on my black bike. The expansion chamber is the best performing but will wake the dead, I'll have to add a silencer. My modified stock pipe that came in a grubee 2010 kit was 2nd, a stock pipe I ordered off eBay that has a longer head pipe and a short fat body was 3rd and the stock 2014 no name kit pipe for a 66cc was slowest.

 

[indian22]

I was going to test the new chamber on this bike but it hits the fuel petcock, so I tried 4 different pipes on my black bike. The expansion chamber is the best performing but will wake the dead, I'll have to add a silencer. My modified stock pipe that came in a grubee 2010 kit was 2nd, a stock pipe I ordered off eBay that has a longer head pipe and a short fat body was 3rd and the stock 2014 no name kit pipe for a 66cc was slowest.

I also had to put a can on the "Miami" long ...style expansion chambers I run on two of my bikes...I'm too old to enjoy the noise or making enemies of my neighbors and cops, but there was a day...

Putting the same muffler on both bikes made each run differently which is 'kinda strange because both are identically modified reed motors but noticeably hit these pipes at different points in the power band, with or without the mufflers. Getting the right ex-chamber for any 2 cycle engine is the key to big performance and really make the rest of your power mods come to life, but it's science mixed with voodoo to get one that really performs; in my case I just experiment & call it luck when one really works!

Rick C.

 

[Greg58]

Rick that's what I've learned as well, I did a lot of this in the 70's on motocross bikes, you try several pipe/jet combinations till you get the power band where you need it. To my surprise the chamber helped more climbing the hill on the road we live on, it gained 3 mph on the hill.

 

[indian22]

Same here Greg Husky's, Bultaco's in '60's and early '70's & Yamaha, Kawa's later.

I use hills as a guideline as well 'cause they eat up the majority of powerband and hold speed down while pulling through the rpm's. plus I live in a valley so lots of contour to climb out of it. I have a friend with a small engine dyno but don't try to abuse it's use for much of my tuning typically just to verify what my butt already knew!

Fun stuff.

Rick C.

 

[Jimmy Bloodmaker]

y'all can make your own pipes with a pressure washer and a welder. you can just use the blown shapes and cut and adjust them with an angle grinder if you need to. they don't have to be perfect, i mean they are just little engines and all. any expansion chamber helps these tiny gals out.

what i've found is, big belly pipes make more all around, cones only make more on a certain " band". sharp high cones for a short band and a hard hit, wide sloping for more low end and not as hard a kick.