Tip & How-To about Motorcycles

How to tune / de-restrict a 2 stroke

2 strokes in lower ccs are usualy slow, so heres a list of things you can do to de-restrict and tune your 2 stroke.

1) Get a racing exhaust, Key part too most 2 strokes is a good exhaust, this will remove some restrictions. Gianelli, Big One, fmf and many others are available on ebay or your local bike shop.

2) Big Bore kit, adding a big bore kit can give up to 30% more performance, and will usualy de-restrict the bike. search ebay for (Bike Model) (year) Big Bore kit. it will come usualy with everything you need for the upgrade.

3) Carbon Reed Valves, are a key part too your acceleration, also removing the little cages that stop them opening all the way and replacing with big washers is another good step, make sure theres nothing for your reeds to nock on when removing the cage.

4) Boost Bottle, a boost bottle will help 2 strokes with no reed valves more then reeded bikes but will give alot more low and high end performace. search ebay for (bikes cc) boost bottle.

5) Jets, are a key thing when getting a bore kit or race exhaust. you MUST change the jets if you change exhaust, get bigger ones if you get an exhaust go a few sizes over and check the spark plug if its a brown tan/ grey collor you got it all right, if its black it means you have to much fuel so go smaller, and if its white it usualy means not enough fuel. you can buy jets from any motorcycle shop usualy around $1 a peice. you onyl need to change the Main jet.

6) Air Filter, you dont need it on most bikes but it could add extra acceleration, measure your carbeurator intake and search it on ebay for a K&N style filter, Power filter.

7) Restrictors in the intake, some 2 strokes have restrictors between the carb and the cylinder, it is key to remove these or drill them out to the size of the carb/ cylinder hole size.

8) Sprockets, if your lucky enough to have a chain and sprockets, its a nice trick to change sprockets on your bike.
For Top Speed - Bigger front sprocket & smaller rear sprocket
For Acceleration - Smaller front sprocket & bigger rear sprocket
by bigger or smaller i mean more or less teeth on the sprocket. if you go for speed your acceleration will lack and vice versa if you go for acceleration you will loose top speed.

9) Electical Restriction, Some bikes are restricted in the CDI, if yours does not want to reach a good RPM search up CDI, de-restriction. usualy it involves cutting a wire between the CDI and engine. and will unlock that slow engine into a screaming 2 stroke.

If you follow all these steps your 2 stroke will no longer be restricted and slow and will be the fastest arround.

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1 Answer

Is my 2005 qiangiang gmi 108 a 2 stroke or a 4 stroke

Hi, David engine design is very easy to identify:
1. Two strokes red line around 12,000-16,000 RPM.
2. A reed valve is in between the carburetor and cylinder head.
3. The exhaust has an expansion chamber.
4. The muffler has a small diameter tail pipe.
5. The exhaust has a high pitched Bap Bap sound.
6. Small scooters and small dirt bikes are generally 2 Stroke.
7. Older models had adjustable throttle actuated oil injection.
8. Older models had a premix ratio of 2 stroke oil and gas.
1. Four strokes red line around 6,000-8,000 RPM.
2. Four strokes have camshafts and valves.
3. Mufflers have large diameter outlets.
4. The exhaust has low pitch deep rumble sound.
5. 350cc and larger engines are usually 4 strokes.
6. Four strokes have separate oil tanks that feed an oil pump.
For more information about your issue and valuable "FREE" downloads that you will need for viewing and printing please click on the blue links below. Good luck and have a wonderful day.
2005 GMI Motor Sports GMI 108 Scooters
Qianjiang Scooter Parts Racing Planet USA
Full size Scooter Parts

Aug 02, 2017 | Motorcycles

1 Answer

yamaha wr125r 2012

Yes. Spend 1500 GBP on the parts and then fit them. You will need to replace the exhaust and the ECU and as you will have spent a lot of money on it already you will have about another 4 bhp. To get anymore you will need to get a tweaked head and cam plus a bore and piston kit to take the engine out to about 185cc. You would be better selling your bike and spending the extra money on a bigger machine or an older one. The Yamaha RD125DX has 16bhp and the early RD125LC made 21bhp. If you really want to tune an engine you should look for a 2 stroke. Tuning is easier and cheaper than 4 strokes. With 4 stokes the tuning involves cam timing and head and piston crown shape.

Jun 08, 2013 | Motorcycles

1 Answer

increasing top end 125 4 stroke

de-restrict the exhaust , leave the engine alone, may cause more problems.

Mar 20, 2013 | Motorcycles

1 Answer

125 2 stroke bogging down between idle and 1/4 throttle

A 2-stroke engine with an expansion box tends to have a narrow power-band, a much narrower band than with a stock exhaust and outside of that narrow band often an engine hardly develops enough power to keep itself running. Earlier 2-strokes are particularly "peaky" with a tuned exhaust and must be ridden accordingly.

The design and dimensions of a tuned exhaust can be tweaked to produce different power-bands at different speeds but it is all almost in the realms of the black arts. A half-inch here or there can move the band by 500 rpm.

Rejetting might improve matters or might not but riding above 2000rpm shouldn't take much effort and in the world of 2-strokes is fairly normal.

I suggest you do some reading on the subject and the Haynes manual is a good place to begin. The link below leads to the second edition which I haven't read but I think the first edition would probably be better if you can find one.


Sep 28, 2017 | Motorcycles

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