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Post by pabickwermert on Oct 5, 2008 17:27:22 GMT -5
OK, Here is the skinny. 2008 Roketa with 350 miles. @ 250 miles it started being hard to start and does not like to idle for 30-45 seconds before stalling. Here is what I think I know. Spark - yes - blue and strong fuel - plenty in bowl once started, it runs fine, just does not like to start or idle fuel petcock working fine Carb is spotless. I took it apart to make sure all passages were clear. compression is 145 pounds air filter clean no cracks or leaks in intake or airbox replaced all fuel lines, fuel filter, vacuum lines tried another cdi (american racing with different timing curve) This thing looks great but it is driving me nuts because when I stop it dies and then does not want to restart. Any ideas on what to try next? Thanks everyone for the assistance. Paul
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Post by harrywr2 on Oct 5, 2008 19:00:59 GMT -5
Adjust Idle mixture screw/speed. At cold idle the wheel should turn a little..when warmed up it should not turn at all when on the center stand. Start at 2 1/2 turns out from seated seated. This time of year 2 1/2 turns should be pretty close to right. Cooler less humid air makes the bike run a bit leaner. Adjust when the engine is warm.
Adjust the valves/rocker arms.(Normal maintenance would be at 600 miles) 0.002" clearance set at top dead center on a 50cc. (Engine stone cold)
If you do a search you will find 'valve adjustment'...not that hard. 4 8 mm bolts holding the rocker valve cover on then a matter of finding top dead center...loosing two lock nuts..adjust the gap with a feeler gauge..tighten up lock nuts...recheck...put valve cover back on. 10 minute job if you can get at them without taking off too much plastic.
Both are definite causes of rough idle/no idle..and both are part of scheduled maintenance.
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Post by Enviromoto on Oct 6, 2008 20:20:14 GMT -5
The valve lash you need for the QMB is .004mm intake and exhaust, dont go by the book that is way to tight. Trust me!
For good measure replace the spark plug and if that dosent work replace the coil.
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Post by harrywr2 on Oct 7, 2008 17:01:19 GMT -5
The valve lash you need for the QMB is .004 in intake and exhaust, dont go by the book that is way to tight. Trust me! For good measure replace the spark plug and if that dosent work replace the coil. The book I have been going by is the Official $12 Scootdawg Owners manual.(page 21)
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Post by pedez2000 on Oct 8, 2008 1:44:08 GMT -5
could be slow/pilot jet too. try the valves first.
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Post by Enviromoto on Oct 8, 2008 8:30:15 GMT -5
The valve lash you need for the QMB is .004 in intake and exhaust, dont go by the book that is way to tight. Trust me! For good measure replace the spark plug and if that dosent work replace the coil. The book I have been going by is the Official $12 Scootdawg Owners manual.(page 21) Lee was going off of the free GY6 139QMB manual specs, they are wrong. That manual was produced a couple years ago and this new knowledge came out in the last year due to Indy50 and myself trying different setups to see what works the best. .004 intake and exhaust should be added to the manual to keep it current.
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Post by indy50 on Oct 8, 2008 15:57:46 GMT -5
The book I have been going by is the Official $12 Scootdawg Owners manual.(page 21) Lee was going off of the free GY6 139QMB manual specs, they are wrong. That manual was produced a couple years ago and this new knowledge came out in the last year due to Skipp and myself trying different setups to see what works the best. .004 intake and exhaust should be added to the manual to keep it current. Yup, .002 way 2 tight. on a stock qmb .004 both is good mmmmkay ;D I set my 82cc at .008 int & .010 exh
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Post by tortoise on Oct 8, 2008 18:43:21 GMT -5
For what it's worth . . the valve clearance specification for the similar Kymco Agility 50cc engine is .04mm cold (.00157 inches) . . source (part 1 file).
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Post by harrywr2 on Oct 9, 2008 15:42:58 GMT -5
The engine plate on mine says 0.06 mm for Valve Latch(actual spelling on the plate)...(0.023 inches)
Just as an example A 2005 400cc Burgman has a listed compression ratio of 10.2, 2007- 11.5, 2009- 10.6. Obviously Suzuki is changing camshafts every other year. Changing camshafts changes valve timing..changing valve timing changes valve lash.
If I go to the JM Star website...I can find a range of 49cc(EPA approved) 4 stroke engines produced by the same manufacturer that produce anywhere from 2.4 horsepower to 3.35 horsepower...with max torque occurring anywhere from 4,500 rpms to 7,000 rpms.
Something must be different...probably valve timing and camshaft design.
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Post by meitianman on Oct 9, 2008 19:47:18 GMT -5
bore, stroke, and the cam shaft timings can vary your horsepower/torque and where it peaks in a big way. generally the shorter stroke engines "stroke shorter then the bore is wide" will make thier power at much higher rpms then strokers. f1 cars are a prime example of this, those engines have almost no power till they start getting above 10k. ive thought about taking my stock cam to a machine shop and seeing if i can get them to grind it for more lift which would lower my peak hp/tq rpm some. anyone run a high performance cam on a stock engine?
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