50cc Scooter won't rev up, Keihin PD19J-9 carb

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  #1  
Old 07-28-12, 01:23 PM
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50cc Scooter won't rev up, Keihin PD19J-9 carb

This is a brand new 50cc scooter (Chinese).
The carb looks like this:
https://www.dropbox.com/s/zkxdw73kdjdylol/Carb.png

I can get it to start and idle but
- opening the throttle makes is cough and miss,
- and it doesn't rev up,
- and it doesn't get better after warmup

Out of the box, it wouldn't idle (just died), but with careful throttle control I could start it and was able to ride for 5 minutes, coaxing the rpm's up to cruising speed. I increased the idle stop screw until it would keep running.
- after a while, it would not rev up at all, just would cough and miss.
- "choking" the air intake makes it die, leading me to believe it may be running rich
- there aren't any idle or high speed jets to adjust, only the idle stop.
- main jet #82, idle jet #33
- passages and jets appear clean and clear
- main CV needle has only one clip position, nothing to adjust there
- CV slide moves by hand, or if I blow into the vent above the carb's throat
- lots of fuel in the bowl, vent line drains fuel when carb is tilted over
- accelerator pump works (I see the jet of fuel)
- when I open the throttle, the slide jumps up and down a little, but never rises much.
- when I see the slide rise, I can also see fuel come up out of the main jet behind the slide needle.
- maybe the auto choke is not closing? But problem is present warm or hot.

What can I test on a CV carb (I'm a CV noobie)?
- hand-choking?
- what else?

What can I adjust?
- jet sizes
- idle stop screw
- what else?

Here's a link to a video of the scooter idling, and dying when the throttle is opened: https://www.dropbox.com/s/rovxuufpnl...07K%20Idle.MOV


Thanks, /John
 
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Old 07-28-12, 06:09 PM
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More test results

The automatic choke seems to be operating normally. It is getting hot after idling for 5 minutes and when removed from the carb it is definitely extended, perhaps a 10th of an inch.

The spark plug is soot black, indicating richness.

Squirting gas or starter fluid into the carb makes the engine die.

Holding the slide open does not affect idle and does not help the engine to rev when opening the throttle (it will still die).
 
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Old 07-28-12, 07:19 PM
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Can't you return it? It is likely you bought a lemon. Trying to fix something brand new that doesn't work doesn't make any sense.
 
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Old 07-29-12, 12:16 PM
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Return?

Ray,

Difficult to return as it is a shipment from east coast to west coast of a 220 lb machine. They've offered to replace the carb, but it's currently out of stock and back ordered to China.

Seems like a carb problem, not really a lemon. I've got the following choices, I believe:
a) wait for a new carb, unknown delivery time.
b) take it to a local shop for repair
c) continue work - probably by re-jetting the carb.

/John
 
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Old 07-29-12, 01:00 PM
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I've run into a couple Chinese carbs that were tossed in the garbage. Simply very poor manufacturing quality and there was no way to get them running right. They did good enough to get the engine to ile/run but little more. In one case I replaced it with a Mikuni which made the engine run perfectly. Unfortunately the Mikuni was four times the cost of the Chinese equivalent (if you can call it an equivalent).

I hate to bash Chinese products because they can make high quality stuff. If just seems that when you get to the low end (cheap) it's really the luck of the draw. Good ones run well but once in a while you find one that makes you wonder about their quality control.
 
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Old 07-29-12, 02:23 PM
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I wasn't going to mention the "C" word and quality in the same sentence but I agree which is why I said return it. The seller needs to pay shipping costs not you. Hopefully you used a credit card. If so and the seller is uncooperative file a complaint with your credit card company. You may have already risked your ability to return it and voided any warranty by trying to fix it. I would suggest quit while you have a legal leg to stand on.
 
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Old 07-29-12, 07:11 PM
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Japanese, not Chinese

The carb, as stated in the original post is a Keihin (Japanese). Thought I'd clear that up.

Thanks for the advice.
/John
 
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Old 07-29-12, 07:29 PM
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The carb, as stated in the original post is a Keihin (Japanese).
Or a counterfeit.

................
 
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Old 08-04-12, 03:32 PM
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Yes, it could be counterfeit. I don't know if I would be able to tell.

I ordered another carb and some jets. New carb installed, did *exactly* the same as the old. So I re-jetted the original carb for leaner (82 down to 78) based upon the black sparkplug and re-installed it. Same results.

Some observations:
- When I open the throttle slowly, I see a lot of slide flutter as the engine fires/misfires. Clearly the vacuum is very unstable.
- As the slide rises, I can see a spurt of fuel from the main jet, then the engine misfires and the slide goes back down. This is fast - maybe 10 times/sec.
- The spurt of fuel is not well atomized or dispersed. It's more like the accelerator pump's jet.
- If I hold the slide wide open and then start working the throttle, it still misfires like before - so it's not starved for air?
- If I hold the slide closed and then start working the throttle, same result.

Could valve timing cause this? I'm trying to think of non-carb issues that may be causing it.
 
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Old 08-04-12, 04:38 PM
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How much money and time are you willing to spend to fix something that was broke when you bought it. As I suggested before tell the seller you want to return it at their expense and expect a full refund. If they don't agree turn it over to your credit card company. If you didn't pay by CC then maybe time to sell it for scrap or replace the engine.
 
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