Chinese scoot won't idle when warmed up. Some help needed.

[tomhyyt]

Been tinkering with wifes scooter for a while now and cant' get it to idle when warm. It drives okay with this setup but just won't idle when warmed up. This thing is Baotian BT49QT-20B3 that had a 4 valve 139QMA engine from factory. Head changed to 2 valve one

before it came to us so basicly 139QMB now.



let's start with the things done list:



- 72 cc bbk with new head and valve lifters

- Smoothed out new intake manifold and gaskets

- Smoothed out head ports (nothing big just smoothed out the casting marks)

- Original cam. Timing set and checked couple times.

- New carburetor 19 mm since it was cheaper to replace than by new parts.

- Tried main jets 85,88, 90, 92 and 95 sizes have , pilot jets 30, 32 and 34. Red ones in at the moment

- New accelerator cable adjusted to have small free play in it.

- Tested the choke. It seems to work after sitting in freezer and putting 12 V it moves around 10 mm and gets warm.

- Has good compression.

- Valve clearances adjusted to: intake tight 0,1 mm and exhaust 0,1 mm. Tried intake 0,1 mm and exhaust 0,15 mm no difference but more noise from valves.

- Oil changed to engine (10W40) and transmission (Motul scoot gear 80W90).

- New sparkplug (NGK CR7HSA) installed prior to bbk installation.

- New coil installed prior to bbk installation.

- New clutch and variator with slider weights.

- Original exhaust that seem to flow ok.



Ok now to the symptoms:



This thing just don't want to idle when warmed up. Engine runs when idle is set high as 2500++ rpm but still dies after small throttle movements and when coming to a complete stop. It is otherwise driveable but dies at idle when coming to a stop or letting it idle and warm up from cold start. Oh and it won't start up without WOT when warm.



- Starts ok when cold. Not straight up but does still start within couple seconds.

- Keeps the revs high for around 10 minutes on idle (2500 to 2800rpm)

- No noticeable choke turning off effect

- If you touch the accelerator it tends to bog a little on 1/4 and 2/3 - 3/4 throttle.

- If you just throttle it slightly it will die pretty quickly if throttle not kept open

- Goes over 50 km/h pretty easily and revs on the stand WOT easily

- Spark plug turns black pretty quickly when not driving WOT

- Mixture screw with 34 pilot had to keep it almost closed. 32 3 turns out 30 almost all out.

- Tried to lower and raise the float. No difference here

- Checked the fuel tap and it is working fine.

- Adjusted needle up and down. No in the lowest position and seems to work best other than idle.

- Smokes with main jet 92 and up and makes plug very very black quickly.



So if anyone has a good help to offer I will gladly try something out. I'll take photos if needed too. I'm not that experienced with carburetors so I might have missed something very basic thing. What comes to the build otherwise mechanically everything should be fine. I've set the timing when piston is at TDC and cam markings parallel to head and bigger hole pointing up. Tried to set valve gaps a bit bigger and smaller but no difference there.



Probably missed something I've done to it already but you get the point hopefully. Thanks for help in advance !

[90GTVert]

Sorry I'm late. Been busy.

It sounds basic compared to all of things you've listed, but have you tried adjusting the mixture screw once it's warm to see if you can improve it? How about something as simple as partially blocking the airbox inlet? That will richen it up. Would need to be a small step, like a piece of electrical tape just barely covering part of the opening. That could tell you if it wants more or less fuel based on how it reacts. Maybe not a bad idea to make sure there are no vacuum leaks as well.

[GrumpyUnk]

I'd start by lowering the idle rpm, a bit, and adjusting the idle mixture screw. I think going leaner(CW turn if idle jet adjust). If the idle rpms increase as you lean it out, lower the rpms a bit more, adjust, etc.
I think you are 'off the idle circuit', a bit too high, so the mixture screw is adding fuel that must be consumed by the middle circuit... you have to have high idle rpm to keep it running as it will be too rich. BLack spark plug indicates too rich.
tom

[lupo76]

Had a smimilar problem when installed my 47mm BBK. Stock Airbox without restrictor and stock Exhaust.

I have a mainjet of 80 now ans still runs a bit too rich when under load . Stand revs good.

[tomhyyt]

May 12, 2019 13:38:12 GMT -5 90GTVert said:

Sorry I'm late. Been busy.

It sounds basic compared to all of things you've listed, but have you tried adjusting the mixture screw once it's warm to see if you can improve it? How about something as simple as partially blocking the airbox inlet? That will richen it up. Would need to be a small step, like a piece of electrical tape just barely covering part of the opening. That could tell you if it wants more or less fuel based on how it reacts. Maybe not a bad idea to make sure there are no vacuum leaks as well.

Thanks !

 I did try to adjust the mixture screw in small steps and used the basic available help how to do that. With the bigger 34 plot jet  had to adjust it almost fully closed before rev rising started to stop. Changed to 32 size plot jet and still the same. Then dropped pilot jet size to 30 and after that I had to open it up 3 whole turns and a bit more so that seems to be too small jet. Oh and yes i did drive a couple kilometers to warm it up first. As now I haven't got any better results tinkering with the mixture screw

And what comes to the air inlet blocking I tried that too. First I took the whole airbox out to see if it has a big effect but that did not change a lot. Tried taping the inlet 3/4 fshut and 1/2 shut after that. No difference to idle but not very driveable WOT loss off power).

I did change the intake manifold and gaskets a before rebuilding the engine and used pilot spray to see if there was intake leaks but could not find any. 

I have some time on weekend to go tinkering with it so I will do these tests again and write the results up and see if they make what kind of difference. Also I did order a hopefully 20mm carburetor and jet kit to see if the problem lies there and to get a better jet kit at the same time. I'll report back after the weekend. Happily this is not in daily use so no harm to us at the moment. 

[tomhyyt]

May 12, 2019 17:47:55 GMT -5 GrumpyUnk said:

I'd start by lowering the idle rpm, a bit, and adjusting the idle mixture screw. I think going leaner(CW turn if idle jet adjust). If the idle rpms increase as you lean it out, lower the rpms a bit more, adjust, etc.
I think you are 'off the idle circuit', a bit too high, so the mixture screw is adding fuel that must be consumed by the middle circuit... you have to have high idle rpm to keep it running as it will be too rich. BLack spark plug indicates too rich.
tom

Thank you. I will try this on weekend too. Thought I did adjust the idle way down too. Right now the idle rpm rises when I open the mixture screw. But  I shall see how it goes on weekend. 

[tomhyyt]

May 13, 2019 5:11:16 GMT -5 lupo76 said:

Had a smimilar problem when installed my 47mm BBK. Stück Aitbox without restrictor and stock Exhaust.

I have a mainjet of 80 now ans still runs a bit too rich when under load . Stand revs good.

 I might have to try it with foam filter too. Happily I have one on stock already so I shall see if it makes any different. I would like to use the stock arbox. This is btw a bit different than the usual style.It's the one that has a rear "hugger" style mudflap in it at the same time.  I've took the outer restrictor out and left the one on the intake side on and that didn't make any dfference (both 20mm). 


Oh and for all. The model of ths thng s BT49QT-20B3 has some small dfferences to the usual 2 valve ones lke the airbox and the exhaust that looks like a 2T one. Otherwse the same and as sad has 2 valve head in now. 

Airbox like this:




Exhaust like this:

[tomhyyt]

Oh and pics of the machine itself for attention. This is now otherwse in good condition and has some small goodies in it that it would be a waste not to get it working.

[lupo76]

To be honest, it´s a very good lookin´ scoot in my opinion.
Don´t like it´s exhaust though, please mount a different one, one that looks like a 4T exhaust

The rest of that scoot looks pretty cool

[tomhyyt]

May 16, 2019 1:30:40 GMT -5 lupo76 said:

To be honest, it´s a very good lookin´ scoot in my opinion.
Don´t like it´s exhaust though, please mount a different one, one that looks like a 4T exhaust

The rest of that scoot looks pretty cool

Exhaust will probably be the next mod I will do. Maybe just a stock replacement with something that looks more 4t one.

Trouble is I don't want it to be loud and exhausts seems to be bit on expensive side. So I will just make myself something up whe I have time.

[lupo76]

Don´t make yourself crazy. Normal 139QMB exhausts go in ebay here for around 35 USD... And as far as I know, a stock exhaust is even good enough for 50mm BBK...

If you want some race exhaust, it´s more expensive. The asian made sport style kits go here for around 50 USD.

I bet you´ll find some.

[tomhyyt]

May 17, 2019 4:44:54 GMT -5 lupo76 said:

Don´t make yourself crazy. Normal 139QMB exhausts go in ebay here for around 35 USD... And as far as I know, a stock exhaust is even good enough for 50mm BBK...

If you want some race exhaust, it´s more expensive. The asian made sport style kits go here for around 50 USD.

I bet you´ll find some.

Around 60€ delivered is rhe cheapest I could find at the moment. That's almost 70usd and is a bit too much at the moment since my funding goes mostly to my motorcycle ar the moment Time will tell what I'll get but the one on is too much. Actually I tried it without the small muffler and it made no difference soundwise so the big black chamber has to have baffles in it too.

[GrumpyUnk]

Well, it looks good.. if you are into looking as if you run a 2T. My opinion is that factory mufflers work fine, not too sure if you get much for running the equivalent of a straight pipe except a lot of noise.
I did hear one muffler on youtube that I though sounded real nice, but cannot find it anymore... actually lost it the same day and could not find when I tried.
That Titan looks nice, as if they actually cared to make something that didn' shout with all sorts of cartoons, sayings, and images pasted on. (My Pink Pony model soon to be announced. Complete with pink horse-tail to attach to the package carrier........) It's a scooter. What. EVer. Have fun.
tom

[tomhyyt]

So I spent some time with this thing yesterday. Took the carburetor out and changed two sizes bigger pilot and 90 size main, jets into it just for test. Starts with electric start only with pumoing gas on fully. Will sray alive some time and after 5 mins want's to die even from slight twisting of gas to wot and smokes a lot so indication if too rich conditions. Just left it with so as it seems I just, have to wait for the new carb to arrive and tune it after that. Changed the oil to 15w40 since the weather has warmed uo and the oil must, have had petrol in it from all this trying to tune it. Once again tried to tune it with foam filter too but no difference at all. Left it so at the moment. I also notixed that I need to change engine bushings so ordered them and the tool to do it also. This thing is starting to be otherwise very good nick but the darn idle problem still excists. It is hopefully cured with new carb and proper trial and error tuning with it. I'm positive that it is just some small thing that i have not noticed or just don't know how to do something properly with these.

[tomhyyt]

May 18, 2019 10:58:50 GMT -5 GrumpyUnk said:

Well, it looks good.. if you are into looking as if you run a 2T. My opinion is that factory mufflers work fine, not too sure if you get much for running the equivalent of a straight pipe except a lot of noise.
I did hear one muffler on youtube that I though sounded real nice, but cannot find it anymore... actually lost it the same day and could not find when I tried.
That Titan looks nice, as if they actually cared to make something that didn' shout with all sorts of cartoons, sayings, and images pasted on. (My Pink Pony model soon to be announced. Complete with pink horse-tail to attach to the package carrier........) It's a scooter. What. EVer. Have fun.
tom

Oh and what comes to this models outlook it is almost srtaight copy from aprilia sr50. Everything else is oretty much the same but the engine.