carburetor idle mixture adjustment interval

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I had 2 previous carburetted bikes. A Chinese made Lifan LF-125 bought brand new back in 2008 and a used 1993 Ninja 500 bought in 2009. Both bikes didn't need adjustment to the idle speed or the idle mixture screw. They all ran alright when warmed up. I got rid of them after riding each for a year. I now bought a used 2008 model carbureted PGO G-MAX 200 scooter. The engine is an air cooled single cylinder. This bike often need adjustment to the idle mixture screw. I bought this bike in winter. When I first rode it in temperatures of 10 Celsius, it has a slight hesitation when accelerating normally from a stop. The hesitation would be worse if I twist it real quick to 40%. When the weather got warmer, the hesitation went so bad that it's un-ridable when warmed up. It would idle OK, but when you twist the throttle, no matter how slow/fast you twist, the engine will die. That's when I decided to adjust the idle mixture screw, which when i turn counter-clockwise (leaner), the RPM went up, then I adjusted the idle speed down to factory spec 1700 rpm, then i adjusted the mixture screw again until the RPM is highest, then adjusted the idle speed, etc... At the end I turned counter clockwise 2 full turns. After that, the idle and the low RPM operation (from 1.7k to 4k rpm) was so much smoother and controllable. Through out the manufacturer OCI of 1000km, the engine will run leaner and leaner. For example, when i was 400km into the OCI. From idle, when I slowly twist the throttle a little, you can hear the engine bogging down / misfiring and wanting to stall. If I get the bike moving at 15mph (throttle 10%), and I slowly accelerate, the bike will bog down for 1 second, then takes off. If I twist the throttle quickly, the engine would respond immediately with no bogging down. Only slowly twisting will cause the bogging down. I solved it by turning mixture screw clockwise (richer). If i was 700km into the OCI, again, i have to turn clockwise (make it richer) of the mixture screw. When I finally changed the oil, I will need to reset the mixture screw (turn counter clockwise to make it leaner) for it to run properly. Engine temperature also seem to play a role too. When the bike is ran within town, low speeds, it seem to need richer idle mixture. But if bike was ran on highway 60mph (90% throttle 7k rpm) for at least 30 minutes, it seemed the leaner idle mixture is OK. I am a bit tired of taking the seat off to adjust the idle mixture. A bike requiring such frequent adjustment, Is this normal?
 
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Is the screw working loose with vibration? Loctite can stop that.. The jet shouldn't erode hardly at all over a year of 'hobby' use so I'd find out what is changing, and stop that change rom occurring.
 

jon_a_127

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I ride it to work everyday, totaling 43 miles per day. I don't think the screw got loose, because everytime I take off the seat to look at the mixture screw, the screw would be the same way as last time I adjusted it. (the flat head screw was angled the same way the last time I adjusted it.) Would a vacuum leak cause such problem? I think there's a hairline crack at the inlet manifold.
 
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A vac leak will cause a lean condition but it should've there all the time. You said its ok inn the cold but gets worse as the weather warms...that would suggest too rich.
 
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Can you find the factory setting for the idle screws? Normal mixture screw setting is about 1-1.5 turns from the fully seated position. You will know the mixture is right when the throttle application, from idle speed is instant with no hesitation, bogging down etc. Also, once revved up and with the throttle shut, the RPMs should come down fairly rapidly. If they hang and come down slowly, you need to readjust. Edit: All adjustments must be done with the engine fully warmed up and at operating temperature. Full choke should be used for cold starts.
 
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jon_a_127

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I ride around town for 20 mins before adjusting. I didn't want to get it too hot such as riding highway, because if i adjust it with the engine super hot, the idle mixture will become too lean for slow town traffic as the engine wont be as hot during slow town traffic. so i am trying to find a balance The carb has an auto choke. Every time when i adjust the mixture, half a turn is all it needs to run properly again. Old oil and new oil affects the idle mixture the engine would take. If i am about to change the oil, i ride it at least 15 mins , then change the oil. After putting new oil in, i ride again, immediately the idling rpm will be lower and along with the hesitation from idle rpm no matter how slow/fast the throttle is twisted. I will need to counter clockwise (to go leaner) on the idle mixture to solve it (half turn to 1 full turn). Up the rpm range 5k+rpm, i dont feel any hesitation/stumble whether pre or post oil change. just more power and smoother engine sound with new oil. I dont know if my work commute stresses the engine too much as my commute is 60% highway, 20% mountain, 20% town. The little 200cc engine needs 90% throttle to maintain 62mph. I never did oil analysis on this bike, but every oil change, the oil came out very dark like black color
 
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I am thinking you are getting fouling on the mixture adjustment screw tip. Or the orifice it resides near. Try this, before you readjust, next time: Count the turns in necessary for the screw to seat (Gently! Very gently!. Then remove the screw, and clean both needle end and the hole it came out of with a spray can of carb cleaner. Start over with the adjustment, from the previous setting. If you find a black buildup on screw's needle end, it may be from residue from crummy fuel, or it might be from oil making the trip through the crankcase ventilation. If it is from fuel, an additive (Like Techron) may fix it. If it is from oil, make certain you are not overfilling, thus flooding the PCV system.
 
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Originally Posted By: jon_a_127
Would a vacuum leak cause such problem? I think there's a hairline crack at the inlet manifold.
Absolutely, it could be temperature dependent, too. The crack could get wider or narrower when heated. If the inlet manifold is cracked, it needs to be replaced immediately, as a vacuum leak could cause overheating and seizure of the engine. Also, idle mixture depends on the cleanliness of the air filter. If you are replacing or washing your air filter at the same time as when you change your oil, that would explain the need for mixture adjustments coinciding with oil changes.
 
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You don't do the whole lot of popping the valve cover, re-torquing the cylinder head bolts and nuts, and measure and adjust the valves, before worrying about the carburetor adjustment? On my old airhead I found I rarely need to adjust the carb, but I frequently need to adjust valves, and I can't get those right until I re-torque the cylinder head bolts. But, before I knew the correct sequence I used to tweak the carb a lot and scratch my head. Now, when it isn't running right I don't touch the carb, I measure the valves.
 

jon_a_127

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My bike had a suspicious milege of 12000km (7450miles) on the clock when i got it back in Feb 2015 (i'm the 5th owner). It didn't have any service history. The under chassis and the body work looked older than the milege. So it's possible that it's overdue for valve clearance check.(valve adjustment and valve clearance is the same thing right?) The carb is a Keihin CVK. I don't have the time, space, manual to take the valve cover off to check the valves. The most DIY job i do is change the oil, air/oil filter. I took my bike to a "Taiwanese scooter specialist" for a service. I told the mechanic about this strange problem. At the end, I had my final drive gear oil, CVT belt, inlet manifold and Carburetor slide spring changed. (Number 3 from the below image borrowed from Keef's bonnie page) He didn't explain how changing the spring will solve the "old vs new oil" idle mixture problem. I got charged in Hong Kong Dollars, CVT belt $680 + inlet manifold & slide spring $1000 + gear oil $80 = $1760HKD (225USD) Since I am 650km into the 1000km OCI, very soon i'll do an oil change, and I'll know if i need to adjust the idle mixture again.
 
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