Chinese moped build

[classictwiggy]

Hi all,

Basically I've been driving for years but wanted to play with a moped so let me introduce my first ever nifty 50!

It's a battered 50cc Baotian/BTM Chinese scooter but all seems solid, engine has had a bit of work and runs nice, MOTd til September.




First on the list is a quick spray in matt black, a bit of tuning (seller gave me a spare carb and head so will see what we can do there).

[classictwiggy]

Last night's work involved a drill and some cable ties...



Final outcome of a broken panel... fixed



Panels painted and refitted with some much better panel gaps to before.



Next up to see what we can do with these bad boys...

[mylittletony]

Nice work with the blue shoe laces!!

Feb 20, 2014 11:00:04 GMT classictwiggy said:

Next up to see what we can do with these bad boys...

I don't know enough about those bikes, but that looks like an expansion chamber exhaust and therefore a 2 stroke. But the head has valve holes and a cam chain hole, i.e. 4 stroke. If that's the case then you might as well start making the head into a nice desk ornament, cos it won't fit

[classictwiggy]

Feb 20, 2014 12:27:12 GMT mylittletony said:

Nice work with the blue shoe laces!!

Feb 20, 2014 11:00:04 GMT classictwiggy said:

Next up to see what we can do with these bad boys...

I don't know enough about those bikes, but that looks like an expansion chamber exhaust and therefore a 2 stroke. But the head has valve holes and a cam chain hole, i.e. 4 stroke. If that's the case then you might as well start making the head into a nice desk ornament, cos it won't fit

Would be a kool desk ornament wouldn't it!

Bike has a 4 stroke engine so head should be fine, did you mean the exhaust already on it?

[mylittletony]

yeah, that exhaust is a 2 stroke expansion chamber (or looks it at least)

If the engine's 4-stroke, get a proper exhaust, whip out the restriction (google should help find out where) and enjoy!

[forde]

if that bikes a 4t and u have an expansion chamber on it ur gonna blow it up

[dodgerover]

Feb 21, 2014 2:08:38 GMT forde said:

if that bikes a 4t and u have an expansion chamber on it ur gonna blow it up

Its not going to blow up! It might run/ go curse word, but its not going to explode, 4 stroke doesn't need much if any back pressure from the exhaust to run well a 2 stroke needs a specially designed exhaust to perform well - hence the expansion chamber, back pressure is very important so the old tuning trick of removing the baffles on a stock 50cc moped back in the day usually used to loose you a few miles an hour rather than gaining them.

[forde]

i am aware that its not going to literally explode like a bomb but he could easily damage it so it wont run i.e. blow the engine. its a figure of speech

[Fungus]

It'll run a lot better with a proper exhaust. If that one's any good, it will be worth something so sell it and get something else. Looks like a Yasuni rep or something.

I had a chinese 4 stroke ped when i was 16, those 4.5 gram rollers you have are great for performance. I also put on a 72cc kit and upjetted it, it went well for what it was (did about 55mph). But then, of course, i sold it and bought a mid-race spec 2 stroke aerox

[classictwiggy]

Moped knowledgeable guys.... help!

Would the 2 stroke exhaust pictured work without the expansion chamber on the end? Surely this would act fine as a normal exhaust then?

Options:

- Fit standard exhaust and put rollers back to standard
- Find a correct 4 stroke performance exhaust
- As mentioned, chop end off 2 stroke exhaust lol

Which is best?

Correct me if I'm wrong but... is this not sold as a 4 stroke exhaust with an expansion chamber?
www.ebay.co.uk/itm/Racing-exhaust-chrome-50-cc-GY6-4-stroke-50cm-XINGLING-YAMASAKI-YIYING-ZHONGSHEN-/321214912071

And I've found the exhaust currently on the bike:
www.ebay.co.uk/itm/Baotian-BTM-BT49QT-11-50cc-Retro-Crossover-Race-Exhaust-Clear-/301070886382?pt=UK_Motorcycle_Parts&hash=item46193925ee

Seems to be correct - sold as a 4 stroke exhaust...

[mylittletony]

the expansion chamber part is the bulgy bit, the end can is a silencer as per 4 stroke exhausts

This should give a decent explanation of the shape: wikipedia

a 4 stroke exhaust is basically a bit of pipe with a silencer (google pit bike exhaust). The length of a 4t exhaust can be tuned so the pressure pulses draw the burnt gases from the cylinder, although this is more common with multi cylinder engines (e.g. 4-2-1 manifolds)

One of those listings says "it looks like a 2 stroke exhaust", it could be a decorative skin over a piece of plain pipe???

a proper expansion chamber starts to increase diameter closer to the exhaust port than those 2 listings you've shared (see theseebay pipes)

if your current exhaust is the same as one of those, and it rides ok, I'd say keep it

[forde]

the expansion chamber isnt on the end the actual big part where it starts narrow gets wide in the middle then narrows again is the chamber. the little cylinder is just a silencer

that naruku one you have posted is a fake expansion chamber. they arent sold as having a chamber, just looking like they have one. it looks like one but has a straight pipe inside. so it ok for your bike but the whole concept of a fake spanny is the dumbest thing ive ever heard so I wud change it anyway

[classictwiggy]

Re: Exhaust. A fake expansion chamber does seem rather pointless!

Could I just run a straight bit of pipe then if a chamber ain't needed?

Thing's burnt all its oil off now so need to check that out hehe.

[dodgerover]

Mar 25, 2014 16:28:48 GMT classictwiggy said:

Re: Exhaust.
Could I just run a straight bit of pipe then if a chamber ain't needed?

Yes but put some sort of silencer on the end of it somewhere to save your hearing,,,,

Mar 25, 2014 16:28:48 GMT classictwiggy said:

Thing's burnt all its oil off now so need to check that out hehe.

Ouch did it nip up (sieze) or get noisy? If not you might have got away with it, once those type of engines get really hot they can go through oil pretty quickly, especially if its cheap oil or never been changed - just topped up. Either way you know now so you can keep an eye on the level.
Do you know the correct procedure for checking it - I know it sounds stupid but on some of them you have to screw the dip stick in to check the level, others you just unscrew it - wipe it and then drop it into the hole to check how much is in, doing it the wrong way round is usually bad. If it has a clear glass plate to check the level in you need to have it bang upright on flat ground otherwise it can be very misleading.

[Fungus]

I bought one really cheap that burnt all its oil in 2 miles of riding. A full top end kit (Cylinder, piston with rings, gaskets, gudgeon pin) is under £20

[classictwiggy]

Mar 26, 2014 19:34:41 GMT dodgerover said:

Mar 25, 2014 16:28:48 GMT classictwiggy said:

Re: Exhaust.
Could I just run a straight bit of pipe then if a chamber ain't needed?

Yes but put some sort of silencer on the end of it somewhere to save your hearing,,,,

Mar 25, 2014 16:28:48 GMT classictwiggy said:

Thing's burnt all its oil off now so need to check that out hehe.

Ouch did it nip up (sieze) or get noisy? If not you might have got away with it, once those type of engines get really hot they can go through oil pretty quickly, especially if its cheap oil or never been changed - just topped up. Either way you know now so you can keep an eye on the level.
Do you know the correct procedure for checking it - I know it sounds stupid but on some of them you have to screw the dip stick in to check the level, others you just unscrew it - wipe it and then drop it into the hole to check how much is in, doing it the wrong way round is usually bad. If it has a clear glass plate to check the level in you need to have it bang upright on flat ground otherwise it can be very misleading.

Thankfully it didn't seize just started getting noisy and made me think. Did an oil change, cleaned up carb and plug and we're all sorted

[classictwiggy]

My new stainless exhaust, sorted

[dubmeister]

Throw that whole exhaust away its not suitable ,then go on ebay and search for bits for a QMB139 motor
I got an 80cc kit ,high lift cam,big valve head and racing variator with rollers (much heavier than yours )for less than £75
The same bike with the 80cc barrel took off and a standard piston and barrel on was ridden from john o'groats to lands end with no trouble at all despite being thrashed without mercy for 10 hrs a day

[forde]

the part of the exhaust that u have replaced is just the silencer. all you've done is make it stupidly loud.

its the other part that is the expansion chamber.

[dubmeister]

When i did john o groats to lands end i prepped my spare bike with a new head ,cam,125cc carb and one of the best things i did was to take off the standard exhaust and cut off both ends and beat the honeycomb catalytic converter bit in the silencer out ,made the bike no louder either really