Higher top speed


New Member
Oct 22, 2013
Denver, Colorado
So it's been a while since I've posted anything, but now I'm trying to get to 40mph. I've had some success with my first MB, and even managed to hit 40mph on a massive downhill but I want to be able to get to 40 on the flats. I live in Lakewood, CO, so big hills are more common than potheads. Lol. With that said, TORQUE! is a necessity!
So I have a genesis 29er mtn bike with a ?66cc chinagirl?...ebay... maxxis hookworm tires, upgraded wheels/spokes, cut piston skirt only enough to clear intake port. modified '03? Yz80 exhaust, ht speed carb, 48t sprocket from http://www.bicycledesigner.com
With the adapter to the disc brake hub. I have a parts bike that a buddy gave me so I will soon have rear disc brakes as well as fronts. Woot!
So here's the problem... I thought my header was too long, (4-5") felt like the powerband came on at 17ish mph then topped out at 33mph. Which was great! Very good low/mid, but then it just stopped pulling? So I rejet the carb... again and again thinking this will help. No luck.
Shortened the header to 2" and lose 2mph? Rejet carb again and again... lose another 2mph? Wtf?

Anyway, I would like to get a little faster

So please tell me what I'm doing wrong.
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Well-Known Member
Jun 3, 2012
Vancouver, British Columbia
If you're getting good torque then why not go with a smaller rear sprocket? Like 40 or 36 tooth. Somebody on this forum will know a formula to calculate/estimate what you can get from a 36 tooth sprocket if you're topping out at 33mph with a 48 tooth. Not me, I'm just a chimp.


Active Member
Oct 21, 2012
Hey EarBiter,

Off-topic: I have the same rear-wheel. I ended up machining my sprocket to fit over the six brake-disk nubs. I only use 4 bolts to keep the sprocket seated; the torque is transferred by the sprocket's mating cut-outs to the nubs. What adapter do you have there?


Well-Known Member
Nov 28, 2012
sf bay area
Nice bike.

1. Change to a 40t rear same as a guy with 26" tires running a 36t
2. Cut head gasket out of a coke can (but not a bigger or different can!! Coke can is .0043" aluminum use only regular 12oz can)
3. Increase carb size to 19 or 20mm and run a pulse line to the left lower transfer port.
4. A longer header before the expansion chamber will help lower end torque. 2-strokes don't have much top end, so... I dunno. You decide.
5. Get rid of the yz80 exhaust and just weld a washer on the end of exp chamber with hole around 5/16" to 3/8" or increase ending pipe length
6. run at least 45psi in those tires the higher the better, whatever your a$$ can handle
7. Have the exhaust exit at least 6" away from the wheel off to the side so you don't get that grime and oil all over your tire
8. Wish really hard for it to go faster


Well-Known Member
Mar 15, 2012
Santa Barbara, CA
You should try a high compression head with a squish gap of about .325". That should give you enough to hit 40mph. Also, can you tell me what pipe that is?
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Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
Los Angeles, CA.
My first impression (looking at the pictures) is that the pipe header is way too long.
It should be exactly as long as it was stock from the factory. (remember, they spent millions of $$$ perfecting it).

Even just a inch or two too long will put the pipe in a very low RPM range & will start to lose performance as the revs go out of it's range.

Next, i'd use a reed valve & a bigger carb. (& all the internal piston & port machining)

Then add billet head.

Next would be a aftermarket CDI. (I've found that the stock CDI works best on 99% of bikes, but when you add all those parts (& especially that pipe) the timing needs to be changed a little).

Last would be to re-jet the carb to match all the work you just did. (^)
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Well-Known Member
Sep 28, 2010
40 mph is pretty easy to achieve. Getting your bike to last very long when you do it is a different story.

All my 66cc china girls would do about 40. All you need is a good expansion chamber, like a SBP, and a 36T sprocket. Don't expect much hill climbing power with this set up though.


Nov 3, 2015
Nova Scotia
You should try a high compression head with a squish gap of about .325". That should give you enough to hit 40mph. Also, can you tell me what pipe that is?
0.325"? About 1/3 of an inch? Over 8mm? Something wrong there.
Squish should ideally be about 0.030" or roughly between 0.015" to 0.040".

Adjust your base gasket thickness with different types of paper or eliminate or double up the head gasket. Measure the squish with 0.060" (1.5mm) electronics solder thru the sparkplug hole.

If you eliminate the headgasket, try Teflon tape. Twist it into a thread and use a bit of grease to hold it in place as you bolt down the heads. I thank Headsmess for this advice and it works well, even with high compression.

My recipe for 40mph with an otherwise stock Grubee GT5a and 44t sprocket is:
1) file head surface flat and cut a squish band in the stock head. Set squish to 0.020" to 0.035"
2) widen the intake and exhaust ports about 2mm each into the cylinder only.
3) port match the exhaust pipe and gasket to the exhaust pipe (but not the intake).
4) lean out the carb (by main jet or float level) checked by plug chops.
5) run 32:1 synthetic oil and regular fuel.

This will just barely give 40mph but the cost was less than $10.

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New Member
Oct 22, 2013
Denver, Colorado
Wow, haven't been on in a while, lots of new info, ty.

I just finished adding a bracket to hold caliper on rear, just need a longer cable and I'll be good to go... or stop hehe. Needed to brag about it.

I have decked the head and cyl. A little, just enough to make it flat. I didn't want it to over heat. Helped a lot, but still no 40mph.

I'll be getting a better plug, the high comp head from dax, and smaller sprocket this winter, so I guess I'll just ride it as is for the rest of the season.

I think my pipe is pretty close to the original length now, maybe an inch longer....? What did you mean by get rid of the yz80 pipe? I will shorten it again.

I also bored the speed carb, i forget how big it is now, a little more than15mm, I think.

Will a pulse fuel pump really help?


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