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deadlyvirus
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Slow scooter
« Thread Started on Feb 2, 2010, 3:52am »
Hey guys I am having some major issues with my scooter here. I bought a 2008 baja sc50. It has the QMB139 engine. I cannot get the thing over 20mph even on a flat straight. Up hills I can only run 5-8mph.

I have read forum after forum, whether it is here or everywhere else on the internet, and most people are at least achieving 30mph stock. I want to get all the bore kits, re-jet, etc. but first I want to figure out what's causing this scoot to only go 20.

First thing I want to try is a CDI rev unlimiter and will want to put a tach on it. Do you think this would give me any performance? Or what would you guys suggest? Any help would be appreciated.
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Re: Slow scooter
« Reply #1 on Feb 2, 2010, 4:31am »
It may have been sitting for a while before you got it (how many miles on it?). That means the carb, fuel tank, filter, and even the vacuum shut-off valve.......all could be gummed up, restricting fuel flow. I'd start there.

If it will rev high while on the center stand (just for an instant or so), that's a good sign, but everything can still be gummed up. ???
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Re: Slow scooter
« Reply #2 on Feb 2, 2010, 5:27am »
It was brand new when I got it. Had about 2 miles total on it. I rode it as it is for a little while around town but I just had to put it in the garage for awhile until I can get this figured out.

It seems to rev high on the stand on a constant basis. There is a slight bog when I first turn the throttle but other than that it seems pretty smooth. FYI I'm about 170lbs on a bad day.
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Re: Slow scooter
« Reply #3 on Feb 2, 2010, 5:59am »
Do a valve adjustment too. Set them both at .004.
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Re: Slow scooter
« Reply #4 on Feb 2, 2010, 6:56am »
Light as you are, it should be able to do 35-40 easy, unless it has been restricted. Even then, you have a definite problem.

That slight "bog" when you suddenly open the throttle.....indicates a lean condition, perhaps caused by a varnish problem. I'd try running some Seafoam in the gas for a tankful or more, before going further. (from most auto parts places). Other fuel system cleaners may work as well.

These carbs do have a little accelerator pump built into the carb linkage, which is supposed to reduce any "bogs", but there could be other issues. They almost always come jetted lean from the factory.....both mine came with #80 main jets. A bigger jet will help out. Around #88 seems a common upgrade/size.
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Re: Slow scooter
« Reply #5 on Feb 2, 2010, 10:29am »
my scooter wasn't new, but I also had issues with it going way to slow. I changed the fuel / vac. lines but it was still doing the same thing until I changed the vacuum petcock then it was great. I was told that some of them even new will mess up.
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Re: Slow scooter
« Reply #6 on Feb 2, 2010, 12:19pm »

Feb 2, 2010, 5:27am, deadlyvirus wrote:
It was brand new when I got it.

Was the engine "shipping oil" changed . . related thread?


Feb 2, 2010, 3:52am, deadlyvirus wrote:
I want to get all the bore kits, re-jet, etc.

Why did you purchase a scooter that was NOT suitable for your performance expectations?
« Last Edit: Feb 2, 2010, 12:34pm by tortoise »Link to Post - Back to Top  IP: Logged
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Re: Slow scooter
« Reply #7 on Feb 2, 2010, 2:54pm »
If thats a Baja Sc50 from Pep Boys, I've heard those are complete dogs on performance. There are some threads here and on a couple of other Scooter forums, where people complain they can't get a decent performance from an SC50. I believe I read somewhere that they have a slightly different variator which may be your problem. But maybe some other Dawgs can chime in on this, but this is not the first time I've heard about this on a SC50 Scooter.
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Re: Slow scooter
« Reply #8 on Feb 2, 2010, 7:07pm »
It is the sc50 and I've heard they are a dog on performance as-well but there are others out there that are getting decent performance. I'm not expecting a power horse here but I know there can be something I can do.

I did adjust the valves to .004. There was no spacing in them at all. One problem I ran into was the bottom valve, or I should say the bottom rocker arm, was so tight I couldn't really adjust it at all. After a fight and some nut loosing I was able to finally adjust it. But now I am getting no compression. My kick start is really easy to move and won't build up enough compression to turn the motor over.

Any ideas?
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Re: Slow scooter
« Reply #9 on Feb 2, 2010, 7:43pm »
I suggest you check your valves again. It appears that one of the valves is staying open and not closing on the compression stroke. Now since you mention the bottom rocer arm was so tight that you couldn't adjust it I would suspect that valve. It could be getting stuck and not closing. Open it up again and turn it over and see how the valves move could be a broken spring on that valve.
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Re: Slow scooter
« Reply #10 on Feb 2, 2010, 11:01pm »
Take a look at the engine code under the kickstarter. If it says 139QMB-3 then your pretty much stuck with what you have. These engines have a 2 stroke style transmission mated to a 4 stroke engine and they are total dogs.

Inflate your tires.
Check your belt and rollers for wear
Adjust your valves to .003 intake .004 exhaust
Oil Change
Gear Oil Change
Spark Plug

Dont do anything else performance wise as it wont help.

Link to the valve adjustment.

http://enviromoto.proboards.com/index.cgi?board=howto&action=display&thread=14
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Re: Slow scooter
« Reply #11 on Feb 2, 2010, 11:45pm »
I have a 2008 Baja SC50 it is no dog on performance. A broken valve spring would make the valve loose and your engine would probably self destruct I suspect on adjustment you aren't TDC. But I will tell you one thing the carb is very cheaply built, I mean the drain screw isn't even built into the carb, it's a remote drain on the end of the hose and on mine the accelerator pump leaked. But the easiest solution would be to open up your carb and perform a thorough manual clean, meaning remove the jets and remove the slide assembly and spray carb cleaner through the jets and blast the bowl area clean. Don't spray the diaphragm, if it feels stiff a little wd-40 and let it soak into the rubber and it'll soften up. Or you could just buy a new better carb which is what I did. Way easier since half the screws on my original were seized. And still with a brand new carb manually disassemble and clean it to remove the gooey corrosion inhibitor it ships with. I'm betting dollars for donuts it's your carb. It was my problem and mine initially behaved like that when I got them. I think when people get them they have no prep work since they're from Pepboys. They perform like dogs because of it and they set the bike aside and leave the bike alone which compounds the problem until they sell it. I'm sure a little work and you'll hit 40 mph easy. After all it still is a GY6 under there, no different than everyone elses. If you've never cleaned the carb out before it's a good idea to do it since I know they don't do it at Pepboys.

Another good idea is to check your air filter. I notice my foam bowed in some. Mine didn't have any support frame. Maybe yours got sucked partially into the tube. I simply glued some screen I got at Lowes around the filter support so the foam doesn't bow in.
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Re: Slow scooter
« Reply #12 on Feb 3, 2010, 12:00pm »
There are several SC50 versions out there the 139QMB-3 is a different beast all together. Go look at your engine code and tell us which model you have querty.
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Re: Slow scooter
« Reply #13 on Feb 3, 2010, 3:42pm »
All I see stamped on the code under the kick starter is HS1P39QMA the other number is just a serial number. Anyone know what kind of engine and transmission type is fitted then?
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Re: Slow scooter
« Reply #14 on Feb 3, 2010, 4:06pm »
Thats pretty much your standard 50cc GY6
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Re: Slow scooter
« Reply #15 on Feb 3, 2010, 8:27pm »
I have the same stamp as qwerty. So apparently we have the standard gy6 engine.
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Re: Slow scooter
« Reply #16 on Feb 4, 2010, 1:23am »
Are you still getting no compression?

If you're still not getting any compression I'd say you need to check the valve adjustment again.

The last thing I read was that after you adjusted the valves you had no compression. If that's the case, odds are the valve adjustment was done wrong.

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Re: Slow scooter
« Reply #17 on Feb 4, 2010, 3:05am »
Hey sorry I wasn't able to work on it today but I'm off tomorrow so I will be jumping on it to see if I can get it going. My guess was the adjustment was done wrong. I'm no mechanic but I found some great info on how to do it from this site plus I have the repair manual. I can follow directions but I still probably did it wrong.

My biggest concern is the fact that one of the rocker arms makes it pretty much impossible to do an adjustment on one of the valves. What I did the last time was I had to loosen the brace on the side of the non movable arm but I know I lost the adjustment once I tightened the brace back up.

My guess is the valve spring is bad but like I said I'm no mechanic. I just found it weird that that rocker arm had 0 play in it at all. I couldn't move it in the slightest to do the adjustment. I was able to adjust the other valve no problem.
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Re: Slow scooter
« Reply #18 on Feb 4, 2010, 6:29pm »
sounds like it wasn't set a TDC when you adjusted. As before mentioned the valve spring might be toast.
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Re: Slow scooter
« Reply #19 on Feb 4, 2010, 6:31pm »
Alright guys I got the valves adjusted no problems today and it started right up. compression was great and idles smooth. However, now when I gas it the scooter will die.

After getting into the carb a bit to check for cracks and what not there is something I did notice. In the vacuum chamber the diaphragm is in great shape but I noticed there is no fuel pin. Is this normal in these small scoots? Or did they just forget to put in a fuel pin? I'm guessing the latter and would explain the engine cutting out at any throttle at all. Because then wouldn't it just flooding itself out?
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Re: Slow scooter
« Reply #20 on Feb 4, 2010, 6:47pm »
great news it's not the spring. Yes it's supposed to have one. You might want to pick one up.
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Re: Slow scooter
« Reply #21 on Feb 4, 2010, 7:07pm »
I thought so. Damn Chinese manufacturers. Any idea at all where I could pick one up? I have been googling the hell out of it and can't find one for sale.
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Re: Slow scooter
« Reply #22 on Feb 4, 2010, 10:37pm »

Feb 4, 2010, 7:07pm, deadlyvirus wrote:
I thought so. Damn Chinese manufacturers. Any idea at all where I could pick one up? I have been googling the hell out of it and can't find one for sale.

Here you go.
http://www.partsforscooters.com/Carburetor_needle_set_qmb139?sc=9&category=468
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Re: Slow scooter
« Reply #23 on Feb 4, 2010, 11:51pm »
I figured it was the valves after you said it didn't have any compression after they were adjusted.

And about that needle - I don't know how it went down the road at all!

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Re: Slow scooter
« Reply #24 on Feb 5, 2010, 2:19am »
Man you guys are awesome. I wouldn't have been able to even get this far without ya'lls help.

I appreciate the link for the needle. I searched up and down for that damn needle and couldn't find anything. I'm going to hit some scooter stores here in town to see if they have it.

I don't know how it made it down the road either. It really baffles me. My only guess is with the valves being as they were not adjusted at all then it didn't have that much of a demand for fuel. But then again it should have still died when going down the road. I guess that would explain my initial bog when I turned the throttle to begin with. Go figure.

One other thing. Would the high performance CDI box really give me anything or would I just be wasting my money? I have read a ton of stuff on this forum about it. Some people say it would help, others would say it's a waste. I'm more looking for uphill power, at least just a slight increase. I know I could change the rollers but that seems more of a pain in the arse.
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Re: Slow scooter
« Reply #25 on Feb 5, 2010, 2:31am »
I tryed a no rev CDI and really didn't see no improvement.
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Re: Slow scooter
« Reply #26 on Feb 5, 2010, 5:00am »
Are you saying the needle (what you call a pin) was completely missing? That's a new one.....

That needle has a "C" clip on it......make sure the one you get has that clip. It also has a little plate and a screw that hold the needle snugly to the slider. All those parts are necessary, so be careful what you're buying. A complete new carb can be bought for $35-50 here and there. Local dealers almost never have stuff like this new on the shelf.

A wrecked scooter could be robbed for this part, as long as it's a 50cc GY6.

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Re: Slow scooter
« Reply #27 on Feb 5, 2010, 8:00am »
Changing the rollers isn't that big a deal. I would suggest 7g Dr Pulley sliders and a NCY variator from Enviromoto. They really keep you in the powerband to help on hills!
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Re: Slow scooter
« Reply #28 on Feb 5, 2010, 8:44am »
I would get the engine running right before I did any other modifications.

I think after that the best money would be spent on lighter rollers like Goldcoast said, 6.5 or 7. Or, better yet, a performance variator and lighter sliders instead of rollers. (You have to pull the variator to install the rollers, so it's already off, not any more trouble to change it.)

There are a lot of variator mod threads here, take some time and read a few of them and see why it's a good idea.

But first, get what you already have running right and tuned.

So you're saying your needle is gone, like in this picture?
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You'll need the clip and fastener too, if you can't get that I'm with Bluefront - get another carb.
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Good luck!

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Re: Slow scooter
« Reply #29 on Feb 5, 2010, 3:47pm »
yes that needle and clip is gone. there are two ways as to the reason it's gone. one the manufacturer didn't put one in or the place I bought the scooter brand new from forgot to put it back in when they put the scoot together and tried to start it. I'm going with the latter.

That really sucks that I will have to buy a new carb. I was afraid of that when I was looking for the needle itself. I didn't know it needed the clamp aswell. I thought the needle would just sit down in the diaphragm with the white plastic float on top of it and the spring over it all. Guess I was way wrong lol. It would probably be easier to just buy a new carb and install it then searching up and down for the parts I need.

I appreciate all the tips and helping me out. You don't know how much you guys have helped. Once I get the parts or the new carb I will see how she runs. Hopefully it will run like it's supposed to then I can jump on installing a variator and sliders like you all recommended.

Thanks again you guys.
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