New Member, first build. 80cc china girl on a fixie

jerrywelz

New Member
Local time
11:14 AM
Joined
Jul 23, 2014
Messages
15
Location
Cali
Hello everyone!

Here is a quick introduction, then onto the pictures =). I'm jeremiah, lived in southern California for most of my life. I spent four years traveling the world in the USAF doing fuel systems repair. I'm out now and currently working for the grocery store. this is my work commuter bicycle. 8 miles one way every day.

I bought a kent fixed gear from walmart for $120 and ordered the 80cc 44t china girl kit from ebay.

Originally i tried to run the 18t fixed gear and it barely ran. I had to pedal up the most minor of hill to keep the engine from stalling out. she was also four stroking badly all through the RPM's with the C clip on the leanest setting on the NT carb needle.

Couldn't get to work like this so i swapped the 44tooth and it was ride-able.

After i adjusting the float to the proper 21mm, opened up the obstructed steel intake and exhaust manifolds, she ran even better but was still four stroking.

I decided more oil would help with the overly rich engine. My understanding is that the main jet is too large and causes the engine to bog at almost all throttle settings. adding oil makes the fuel more dense therefore less can flow through the main jet leaning out the engine.

Anyways i mixed some 12:1 tcw3 and did some minor port work. i thinned out the transfer ports and took a few mm off the top of the exhaust. Also cleaned and slightly enlarged all the ports. in hindsight i probablly lost torque. I also polished the exhaust port and abraded the intake.

The bike ran so damn good like this until i crashed it. hit a curb head on and bent the frame and rear wheel. I decided to just get a new identical bike from Walmart because this is how i get to work.

Engine ran so good i decided to try the 18tooth again. the bike is slow off the start but can still climb hills. I hit 39 tonight on a small downhill road.

I need a permanent air filter solution and would like to jet smaller so i don't have to run sooo much oil.

I'd imagine the manic mechanic sprocket adapter with a 30tooth sprocket would be scary fast. thanks for reading guys. all input is appreciated. looking for tuning advice or tips. however i am constantly searching the forum and reading about areas i want to improve on and i', headed in the right direction =) thanks guys!
 

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here is the best picture i could get of the plug. my camera died
 

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I'm glad someone tried the flip flop hub idea, that's something I've been considering for a long while but didn't want to invest that much in a potential flop. I might try the same thing on my next build.

what's with all that red junk packed between the fins?
 
^^^ the red goop apparently reduces noise but doesnt really ;) you see it on old school air cooled twostrokes, proper rubber things to stop those super thin fins from singing.

do NOT try a 30T, especially if they be 700c rims, which they appear to be? nice choice in tyres but! nice and solid looking :) dont use them skinny racer ones! (edit...they are, i see the sticker on the frame now)

i know from experience, you can *just* get away with a 36T. welded it directly to the inside of the fixed sprocket.

at the moment, i smell like wd40 and am cursing bikes again, as im just lacing a 3speed hub in, and have just realised i have 36 (also a 28 in the pile?!) hole hub with 32 hole rims... i guess i have to pull my only 36 hole 700c rim apart.... a real nuisance, but rather normal when dealing with these silly things! hoping i dont need to replace my spokes when i do... im thinking i might invest in a thread roller, as i get annoyed every time i lace up a wheel. $160 or so on fleabay. OUCH! but worth it as spokes are around 60 or so a set here...

ill be using a 44T this time. i was thinking of just replacing the current 36T with the 44T, but it appears the hub is starting to strip out, and its only been a few weeks. considering the sprocket runs on a right hand thread which will loosen off under power, and is locked by a smaller od left hand thread, i cant rely on it. im going back to plain old rag mounts.

unless i make a hub adaptor thingy, which i really couldnt be bothered doing. dont worry, i shall regret being lazy later on as normal, but hey... the spoke issue already has me annoyed enough as it is for one evening!
 

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i thought my frame was cheap! it is! but it has v-brakes! anyways...note the size of that sprocket...it was ok for 26" rims, but is slightly lacking on hills with 27"... :( still good for 70km/h but!


im surprised you can even get it started with that sprocket you have!



if you wanna go for "rag mount" sprockets, just note the number of spokes you have....need 36, or need to get a sprocket with 8 holes.


oh yeah. check all the frame welds THOROUGHLY. possibly even think of stripping it down and gussetting them, like i will be doing soon. hopefully BEFORE it collapses on me! :giggle: i noticed a few, erm....blow holes in the headtube/downtube welds....can you say CHEAP and NASTY?


i hope that spring on your tensioner is just an assistant and you tighten the bolt up good! good idea but ;) ive had a tensioner tear out half my spokes at speed before, and its not the most pleasant of experiences. so make sure its tight!

replace the chain, which youll have to do anyway now, but use a heavy duty bmx chain, as space is limited if you try using 410 or the chain they supply... in fact, im going to be finding out if theres actually enough clearance for the 44T soon! more than likely, the answer will be no, but hey...only one way to find out!!!!!


my choice of chain on these things, note it only goes on one way(really? im just saying it does), and requires a chain tool. in reality its the only HD chain my local shop stocks and i couldnt be bothered looking for alternatives :p also available in neat-o colours :) the half link means you can sometimes avoid the tensioner altogether, but its good for taking up slack. really, i prefer standard type links...

ch-YBNhdBLK.jpg
 
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Headsmess, thanks for your insight! i was planning on replacing the chain and will go with the heavy duty BMX. I've also read horror stories where the tensioner kissed the spokes.I made sure to install it more than snug, it's super solid where it's at. the spring does tension the chain, but the bolt and lockwasher are what's holding it there.

i just ordered the 36t sprocket and rag joint, but after reading your post i will try to run it without the rag joint if possible. I deliberated between the 41, 36, 32, and 30 tooth. If i can get around and even make it up hills with this 18t i'm sure any choice will be an upgrade, but the 36 seemed ideal.

I cut ramps into my piston where the transfer ports are. I was hoping to improve mid range and torque. the ramps help but need to be perfected. the angles are not quiet exact because i don't have the tools or knowledge to measure and cut each ramp exactly the same. one of the ports is opening a hair earlier than the other causing vibration.

I ordered a new piston kit and will try to perfect the one i have. pictures on all these internal mods will be posted next time i take the jug off again.

how does that pipe run headsmess? did you fab it yourself? it looks like it performs well. ever consider wrapping it for a cleaner look?
 
wellll, i maded a video the other day, that didnt really turn out as well as planned,(i got potholed at the least....i posted it on here none-the-less) but...

the pipe rips :) when i get this 3 speed on ill make another one. video, that is. i need a decent camera mount.

used 2 stroke wizard pro for the dimensions, v5, with his "duel power" extension. basically inside that reversion cone is the cone that was initially calculated, but made out of perforated sheet. in theory the internal cone sets the main rpm power band, and the extended outer cone gives a lower rpm band to boot. i found stuff that was around 50% open area, so i get about equal bands. ie, if it was 30% open id get more of the high band, if it was 70% open, more of the low band. i havent really done much fiddling on that side of things, takes long enough to make one as it is!


but i did make it without the extension as well, and there is a definitely noticeable effect down low! i got two power bands :)

i think, from memory, i tuned it for 8Krpm. puter blew up and i lost all my notes :cry: it feels happy there, anyway. like it hits a wall going any faster. i blame that on porting.

erm... i sorta painted it black the other day? :)


oh. i prefer letting the thing rev out, i like the sound of a 40T, but its either 44 or 36. im going for 44. what i have on hand, of course :)

just waiting for the postman, then i can sink my dentist drill into some ports :)
 
My choice of chain on these things, note it only goes on one way(really? im just saying it does), and requires a chain tool...

i prefer standard type links...

View attachment 53617
1/2 link chain, I sure didn't like the stuff.
1/2 links are a pain even on regular chain, a whole chain made of them was no fun, and yes they are directional, the skinny part of the links faces the direction of travel for much the same reason you always put the closed end of master link clip facing the direction of travel, if it hits something it won't catch.
 
the spark plug definetly shows signs of not enough oil in the fuel mixture. check carb throttle pin adjustment if u know how to set this. also add more oil to your fuel mixture. i put 3-4 caps of oil per liter of fuel. also i like the spring idea on the idler but im not sure it will last especially on bumpy roads like here in africa, where i live, where the roads have many pot holes :p
 
some pics of where we are at currently.

got the banana pipe and sha carb

I put the NT carb jet into the SHA as the jet was too large. still four strokes at WOT, but the NT carb used to do it as well. maybe i need a smaller jet? I'm running a 12:1 mix and the plug looks about right. Is there any reason why i need this much oil for the engine to run right?

I put this tubing on the banana pipe and it seems to cut low end power, but increase top speed. any thoughts?

thanks!IMG_0085[1].jpgIMG_0088[1].jpgIMG_0089[1].jpgIMG_0079[1].jpg
 
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