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Post by willylaage on Aug 25, 2011 13:37:54 GMT -5
I just installed a new ncy 50 mm kit and performance cylinder as well as performance cam and I cannot figure out why it wont run properly.
it will start just fine and idle decently well but when I give it a little throttle it starts to rev up and then stops and repeats pretty much surging. I have checked the fuel line and and tried a .96 and .98 jet. the scooter is a 08' colt gy6 139qmb.
I'm pretty sure it is an issue with the carb but don't know as much as i would like about them to just tear it apart and rebuild it. thoughts?
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Post by 90GTVert on Aug 25, 2011 14:36:43 GMT -5
:welcome3:
Is the spark plug black and/or wet? Sounds like it might be too rich.
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Post by willylaage on Aug 25, 2011 16:44:41 GMT -5
yeah its pretty black
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Post by 90GTVert on Aug 25, 2011 17:24:31 GMT -5
Oh yeah. Try a smaller main jet.
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Post by willylaage on Aug 26, 2011 9:29:45 GMT -5
I put in an .88 main jet. It runs better but still wont maintain rpm (surges) I think I need to adjust the mixture and possibly put in a different jet but have run out of time until sunday. Any more pointers on the direction I should take I'm pretty new to tuning carbs my idea of tuning is hooking up a computer and tweaking ratios till i like the curve.
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Post by 90GTVert on Aug 26, 2011 10:19:26 GMT -5
Tuning a carb is kinda the same trial and error as there is for tuning an EFI setup, just with a lot more grease under your fingernails. Check the spark plug for condition and color and tune for best power. The 4 strokes are pretty forgiving, so just start by tuning for a good feel. Once you think it feels right, put a new plug in and go for a ride staying around WOT as much as you can safely then check the plug color. If it's white after a bit of riding you will probably want to go a step richer. For the 4T if it's tan you should be good to go. Tune for WOT first and then adjust the rest.
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Post by venusproject on Sept 27, 2011 13:58:43 GMT -5
if you're running stock air box and stock exhaust you should be about 80 or 82 main the pilot should be around 38. You might want to get a carb with an accelerator pump.
This should be a good starting point check the plugs though cruising 1/4 throttle hit the kill button checks the pilot full throttle then kill checks the main.
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theo547
Scoot Enthusiast
Posts: 497
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Post by theo547 on Sept 27, 2011 21:03:26 GMT -5
I dont think Venusproject noticed this is a 50mm bore - 88-90 is about where you want to be. I had alot of trouble with mine using a free flow air filter... lots of surging.. went back to stock\derestricted airbox with a extra 1 inch hole cut out in the back off it and it runs pretty good with my 90 main in it.
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theo547
Scoot Enthusiast
Posts: 497
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Post by theo547 on Sept 27, 2011 21:05:25 GMT -5
Been a while sense i have thought about it.. i also had alot of surging issues before i put on my free flow exhaust. so maybe you want to try looking up how to mod your stock exhaust or buy a new one.
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Post by venusproject on Sept 28, 2011 2:27:15 GMT -5
I dont think Venusproject noticed this is a 50mm bore - 88-90 is about where you want to be. I had alot of trouble with mine using a free flow air filter... lots of surging.. went back to stock\derestricted airbox with a extra 1 inch hole cut out in the back off it and it runs pretty good with my 90 main in it. I built a 47mm kit with ported head with a 20mm with accelerator pump running 35 and 78 main running through the stock air box untouched and stock exhaust the bike buried my peds 55mph speedo up and down hill. I cut off the front snorkle of the air box and I had to jump to 85 main and the bike felt good but barely did 45. So I bought a stock box on ebay. My next project was a Ruima 52mm cylinder with ported NCY 52mm race head and cam. Bike felt lean, so I upped the pilot to 38 and main to 85 I put on a typical glass packed muffler had to up the main to about 88 but the bike lost a lot of off the line torque. So on went the stock pipe. This set up could lift the front tire a little on take off and buried the needle a lot faster. I think in Hawaii motors run a little richer so maybe one size up with untouched air box. Sorry I know I'm new to 49ccScoot so you can just tell me STFU and I'll understand.....  |
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Post by reveeen on Sept 28, 2011 5:03:29 GMT -5
Being just a foolish old man:
I get to wondering, because I know that adding a long(er) duration camshaft to an engine messes with the engine vacuum, the way in which a "vacuum carburetor" is supposed to behave in the absence of "normal" engine vacuum?
I also wonder too: how a vacuum petcock (or fuel pump) is also supposed to work?
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Post by lackthereof on Sept 28, 2011 14:20:59 GMT -5
Being just a foolish old man: I get to wondering, because I know that adding a long(er) duration camshaft to an engine messes with the engine vacuum, the way in which a "vacuum carburetor" is supposed to behave in the absence of "normal" engine vacuum? I also wonder too: how a vacuum petcock (or fuel pump) is also supposed to work? I doubt that it would have a huge impact. The carburetor slide will open so long as there's basically any vacuum at all. These carburetors basically work by keeping the slide closed just enough to maintain the small amount of vacuum (and thus air velocity) necessary to keep the carb mixing fuel properly. And from what I've seen on mine, it goes to full open very quickly and stays there most of the time. The fuel valve is the same way, it opens when there's basically any vacuum at all. "normal" vacuum or not, if the engine is able to run there's always going to be enough vacuum to keep these working right. Fuel injected engines, on the other hand, regularly experience zero vacuum under heavy load, and so our vacuum petcock would be useless. But those engines don't have to worry about leaky inlet vales or overflowing float bowls, so they can just leave the fuel on all the time. |
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Post by reveeen on Sept 28, 2011 14:39:19 GMT -5
Yes but.......... by going back to the stock air box you are restricting the air flow causing a vacuum.
Without enough vacuum you will encounter strange vacuum carburetor performance unless you have compensated by changing/fooling with the diaphragm spring.
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Post by lackthereof on Sept 28, 2011 16:03:31 GMT -5
a-ha, but the "open air" side of the slide diaphragm is actually vented inside the mouth of the carburetor, not to the true outside air pressure. So by sheer force of mechanical simplicity, it only "sees" the vacuum difference between the engine and air filter, and ends up still working properly.
You still have to make normal jetting or needle changes, though.
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