Schwinn High Timber hi speed build

I had about 3/4 a gallon of fuel and it was mixed about 36:1 and i added 1 1/2 oz of oil to it for break in for my motor. Im not sure if that isnt too oily but it seems to like it. Its oiling a lot though. my pants, my pipe, everythign.

Also i was using 34t sprocket wiht my 50cc but the chain was too loose with the shortened mount. Instead of taking out a link, i put back on a 26" wheel with a 36t sprocket.

Also, im still running NT carb with 66 jet, it feels pretty strong but it runs very rough at low rpms. after about 3000 rpms it surges with a lot of power. it can pull me from 3000 to 6000 rpms relatively easy. it seems to want to cruise around 25-29. I can pull 30+ but it gets kind of twitchy. I really would like to get back to 24" tires but its working right now and im goign to leave it alone.

changed my needle clip posish, to 2nd from the bottom richer. it runs great. My arrow carb just got shipped but for now im happy wiht this. It pulls well on all throttle positions and there's no longer a rough spot.

Here is my own answer to my own post



I have a schwinn High timber 26" bike with:
solid bike choice good sturdy steel frame
26 x 1.95 semi slicks
415 chain with 41t sprocket and rag joint
415 kit chain is junk, replace it asap

thatsdax premium f80 engine 66cc(60 something cc)
buy the f50 if you want a smooth engine, buy an arrow if you want a fast engine dont waste too much time tuning, it will end up costing you more time and a lot more money and you'll be chasing a dream. Don't put reeds on a f80 just buy the upgraded engine and leave it be.
upgraded chain tensioner with a 10t sprocket off ebay
skipdu spring tensioner is good if you want to drop the dinero, find a #41 10t sprocket and you'll be smooth sailing dont spend too much time fiddling with the kit tensioner
nt carb, stock jet size #70, clip is in center position
nt carb is okay, buy a 66 68 72 jet and that is all you will probably ever need to tune this carb, get a better air filter
low profile SBP air filter
this one works

the intake i modified with a 90 deg copper pipe jb welded to make the motor fit in the frame
buy the z intake or change the mount angle.
ngk iridium bpr7hix plug SBP
just buy the b6hs the irridium isnt any better
BGF expansion chamber
torquer 2 pipe is the best, banana pipe is too loud, stock is too restrictive
I have the tank double bungeed to a rear rack due to issues with cable routing.
the tank will break if it isnt securly mounted, also the weight in the back will make the bike less stable in corners, just make it work on the frame you wont be sorry

Ive seen reports of speeds ~ 35-40 mph but i can only squeeze on 28 on the flats and the engine will get me to 33mph top downhill screaming bloody murder.

1: What can I do to hit my targeted high 30 speed goal?
buy an arrow
2:will a #41 chain increase my top speed?
yes and no, it wont directly increase speed but it will make it much smoother thus enabling you to be braver and hold down the throttle longer
3: will a 24 x 1.75 tire and 24" rim increase my top speed?
no but it will lower your center of balance/gravity make you feel more stable turning
4: will changing the carb jetting increase my top speed?
yes get your jet running right for top speed up hills and then reset the clip position so its smoother backing off throttle
5: how about taking the dremel to the intake pipe?
clean up the seams but leave the port work to the pros


As is, it runs rather smoothly, it just lacks that top end I feel would make me comfortable in town. Not trying to break any land speed records, just want 35 mph WOT on flats.
arrow motor 35+ WOT on flats more top end power with 36t sprocket you'll easily get 35

oh yeah, and im running 91 octane pump gas 20:1 mix.
too lean, and too much oil
I weigh 190lbs.
lose weight

trying to tune this carb that came in. 70 jet was too rich, 66 didnt run right either until it was warm and then it was slower. 64 jet was very weak, now trying 68 jet.

So, im at race week for formula sae this week. I rode my new build to the van to go to michigan.

But im making a virtual wishlist for when i get back. This may will be free tuning month.

Here is my wishlist although slightly off topic it will still apply to this build.

I am going to put the nt carb back on, it had no air leaks and i was able to achieve 35mph.
I would like to try using the rt carb air filter on the nt carb it looks a bit nicer than the HD sick bike parts air filter.
I want to put back on my 24" wheel to lower my center of gravity.
I also want to chop my torquer 2 muffler and point it downward so i can lower my seat into a position where i can touch the ground with my feet easier.
I bought my 1800 lumen headlight that works awesome! I will start using that when riding late so i can see holes.
Hopefully when I get back i can get back up to 35mph, the rt carb gives me air leaks at full throttle i can only go 25mph

ive been riding my second bike, the diamondback sorrento with the bgf 50cc and ive realized that the cheap suspension fork i have does very little in the way of comfort. Also ive realized that the torquer 2 pipe is brilliant.

Today i rode for a few hours with 93 octane gas. Hit 40mph on a long slow decent and then backed off. This bike will easily cruise up slight hills at 30mph and seems to cruise in the lower 30s. Might have to watch it as my rear kenda tire is showing signs of wear.

webbee said:

I read this thread tonight, off and on over a period of hours, so perhaps I missed/forgot about if you have experimented with rolling your own CDI. It seems to me from the discussion on this thread, that if you improved the ignition curve on the top end you might get more top end power/RPM. This is speculation, not real world experience, on my part. It just seems you've about tried everything else.

It's interesting to note that so far the lowly 50cc seems to be the most reliable, although slower, than the other engines. Not probably what one would think would be the outcome of the tests so far. That Schwinn seems to be a great choice as a base for the experiment. When was that bike made, approximately?

Great thread by the way. Your exacting experimentation is well appreciated as is your concise write-ups! Two thumbs up and a tankard of virtual beer to you.

Click to expand...

Sorry for the late reply. Maybe thats why i dont have so many fans of my thread. I got the bike for free as my bike got stolen and my friend felt a sense of responsibility for not preventing the theft. I have no idea what year its covered in oil for me to look up a serial. Yes, it has turned out to be a solid frame, nothing has broken yet. I also have started a build on a diamondback frame, it seems pretty solid as well, the geometry is there and its cromo steel.

I havent given much thought to rolling cdis, because ive found the stock one is sufficient. I tried the jaguar cdi and could never get it to work.

I finally settled on getting the arrow motor and it makes the power i was looking for with the nt carb. mid 30s easy and pulled all the way up to 40 down one long winding hill. the nt carb is a bit rough on low rpms and the rt carb seemed to work a little better, but i was having a rough time getting it to run wide open throttle. Ive got a few more tricks im going to try wiht this build next week, we'll see if i can get more performance out of the arrow motor.

Got a few ideas for next experiments:
free wheel disk brake kit from jnm
velocity stack from jnm with big cone filter from my rt carb
nt speed carb from bgf
put 34t sprocket back on bump up my top speed to 43
per suggestion of webbee i might look into my box of electrical goodies and try to roll my own cdi. I havent had much luck with the jag cdi so i might save that for last.

We set up a cone course for the formula race cars to practice driver training and i bring my bike out and do the slalom runs and teh tight corners. I havent really learned how to do high speed cornering yet i find myself overshooting and i dont have much awareness, hopefully that will get better. I find myself being too safe in teh car, which isnt a bad thing on the bike, just slow.

I might ask for some heavier duty gear for my birthday to protect me from injury from crashes due to mechanical failure which i keep envisioning when i start reaching speeds above 35.

Probably time to service my rims too, ive noticed a wobble in both wheels.

Yesterday i placed my second order for nt carb velocity stack. I ordered the wrong one last time.
Also, the speed carb i bought had air leaks, so ill have to take that and probably open it up a little. its a little too tight to fit on the intake and the hole is a little off center. My experience has been that when the carb doesnt slip on you can't ever get it snug. I tried to jam one on and it broke.
I took apart the chain and regreased it, after probably 50-100 miles of 30+ there was no grease and the rollers were chattering. I also cleaned metal shavings from the case and inspected and regreased the clutch and drive sprocket.

When i started it up this morning it was much smoother and i rode around until it warmed up and took it up to about 33mph going up a slight hill.

It was suggested to me to use a hotter plug, i was using the irridium 7 plug from SBP, I'll try that out probably on wednesday. Its just got a funny way of not running completely smooth at high rpms. it'll get up and go 27 -30 then it will start coughing and wheezing. I want it to be smooth all the way out to 40, 40 is as fast as i want to go on these tires with this motor.

Im also looking down the road at what to do next, if i want to buy a ktm clone and if that will give me similar performance with more longevity. I dont really expect this motor will last forever.

Still thinking about buying a sprocket adapter and mounting my 34t sprocket on a 24" wheel. I dont like sitting so high and the 24 with a bigger tire seems like it would be more fun to ride. if someone sold a 7speed wheel with the hub adapter already mounted i would buy it so quick.

i ran 87 octane ethanol free gas and it runs real bad. I also tried running the ngk b6hs plug and it ran even worse. I will now run 93 or higher exclusively and run b7 irridium but i might try the b8hs

got my bp8hs in the mail from sbp, pulled my plug this morning and it is dry and a light grey/brown color. After talking to dave rust, I think ill stick to 32:1 oil mix with the bp8hs plug, provided it runs smoothly when i crank it up tommorrow. I really liked the bp7hix but im not sure its running smoothly and want to be sure. Either way ive noticed my top speed hasnt gotten back up to 40 in a while and want to see if these changes will correct it or worsen it.

ive got an airleak, most likely from the spark plug hole, but all in all the bp8hs is a winning plug. able to go from 32-40 on my speed hill without any stumbling or roughness.

was riding today like normal 32 mph then "pop" and rattlely sound coming from the engine. I shut it off, and pedalled home and looked at the engine. everything looked okay but there were two light scratches on the head gasket and a lot of gunk in the spark plug hole and seemed like there was also some gunk in the clutch pin. i dont know if i have a crankcase leak that is leaking into the clutch. Honestly i wasnt really sure what to do. I bought some permatex copper gasket spray and hope that its just a head gasket.