139QMB Low top speed By chemicstry (Read 4,854 times)

[chemicstry]

Hi everyone,

I have baotian scooter with 139QMB engine. Originally it was 60cc and it ran ~55mph. Then I purchased 72cc kit, did 400mile break-in.

Acceleration is now much better but top speed doesn't reach 45mph down hill and it feels that engine is braking whole scooter. I tried changing variator rollers 4-6g and 6g were the best when driving with 1 person, with 2 persons acceleration was very slow.

I have changed my crankshaft bearings before installing 72cc kit because they broke.

I didn't do any modifications to the carb except top needle settings but they didn't change anything at all.

I hope someone could help me with this problem.
Thanks.

[chemicstry]

I just tested variator and belt climbs to about 3mm from edge.

[sprocket]

Could be the exhaust is too restricted for the additional ccs. Perhaps your CDI or ignition coil are old and getting weak?

You need to do a plug read and see if you are running lean...you may need to up jet. Which way did set the diaphragm needle?

Try removing the air filter and see if it makes a difference.. maybe the stock air box is not giving you enough air flow...

[chemicstry]

Diaphragm needle is now on middle setting, tried setting it in both ways but didn't notice any difference.

CDI and ignition coil are stock, i'll check plug and spark tomorrow.

[sprocket]

The needle should make a very large difference... I suspect if you saw no difference your main jet is partially plugged.. need to give the carb a really good clean and run wire through both jets...

[Bong]

I didn't do any modifications to the carb except top needle settings but they didn't change anything at all

Just my opinion but, I agree with everything Sprocket said. If you added a BBK without rejetting That could be the problem. You may be running to lean. The stock jet is only like a 75 or 80.

Riding 2 up on Qmb is asking a lot from the engine even at 70cc. Unless its 2 100lb teenagers. ;D

You may want to lose the airbox for a freeflow filter and upjet. And like Sprocket said check for restrictions in the exhaust. A new CDI and Coil package may help too for reliability and the advanced timing curve.

You should see a little more power after the BBK is fully run in though. I noticed a little more speed between 600 and 1000 km on my scoot when it was new.

[chemicstry]

Apr 18, 2011 12:52:40 GMT -6 Bong said:

Riding 2 up on Qmb is asking a lot from the engine even at 70cc. Unless its 2 100lb teenagers. ;D.

You say 2 is too much?
8 persons is my record...
This is how i killed my 60cc ;D



End of offtopic. I will check everything tomorrow.

Thanks for help.

[Bong]

Ok you win!! Eight is Enough!!!! No wonder ya needed the BBK!!!!!! Thats normal driving in Taiwan, without the trailer ;D Getting to 25mph may take a few blocks. (I know someone has a pic to insert here)

Let us know how it goes!!!!!

[Cruiser]

Welcome to Scooter Doc, chemicstry. I believe your scoot does not have a tach. A tachometer would be a very good instrument to have on your scoot not only for tuning, but to ensure you don't over rev the engine which would shorten it's life. A common problem with over reving is the destruction of the crankshaft bearings. If you did achieve a true 55 mph with your scoot, you were waaaay over reving the engine. An occasional run to an actual 45 to 48 mph is OK for short spurts. You should try to keep it below 8500 to 9000 rpm max. I have a Peace VIP50 which is most likely a 60cc and 45 mph (gps) is about tops on a flat road with little wind. 72cc should give about 2 to 3 mph more. There is a good possibility that the main jet may be to small as a larger jet is usually used with the 72cc kits. Let us know how you do with your scoot.

[chemicstry]

I cleaned carb, adjusted A/F correctly but still the same. This might help: when I set A/F too low engine was bogging and when I set it too high acceleration was a bit slow.

Spark isn't big but blue. Here is how my plug looks:

I think it's ok, yes?

I don't have tach to check rpms but I will buy one if I find any cheap one or I will make it by myself with avr.

I also noticed that it drops idle rpms after full throttle.

And one more thing: could it be because of too high compression? Starter hardly turns engine and kick starter sometimes hits back.

[sprocket]

The plug looks OK...a bit lean...

Try raising the diaphragm needle in the top of the carb...You may have to use a little washer if it is not adjustable...

[zugzug]

Sounds like your cam chain is off a tooth.

[chemicstry]

Cam shaft chain is shifted to a one by a half tooth (due to crankshaft repair and big bbk gasket (~2mm)) but i don't think it could make that big difference.

Today I had a very strange problem.
You should listen:
I checked all carb, cleaned air filter but it was still the same and after few moments it magically fixed O.o

Tomorrow I might buy a tachometer and test rpm.

EDIT: I guess I will pick pre-jetted carb with high flow air filter and new gear set from scrappy dogs. My current carb isn't in very good standing...

[sprocket]

Boy it is not sounding healthy... I'm hearing cam chain rubbing in the guides... are you certain you have tension on the chain?

I agree with Zug.. sounds like the chain is off a link or isn't tensioned right

[chemicstry]

Apr 20, 2011 11:27:25 GMT -6 sprocket said:

Boy it is not sounding healthy... I'm hearing cam chain rubbing in the guides... are you certain you have tension on the chain?

I agree with Zug.. sounds like the chain is off a link or isn't tensioned right

You were right. My cam chain tension screw was broken.

Now it runs as it was before. I inspect this as carb problem because it throws a lot of smoke and it normalizes when engine warms up.

Can't find any suitable tachometer, all shops are full of infrared ones.