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Post by apk5009 on Oct 1, 2014 8:06:46 GMT -5
Just installed my 70cc kit on my scooter after my wrist pin bearing failed and shredded my stock cylinder/piston. I've spend the last 5 days since the install taking it easy, driving the scooter around town to break-in the engine. At 1/2 throttle the scooter ran like a champ. Smoother takeoff, great acceleration, no hesitation. Last night I decided I was finally comfortable opening the new kit up to see what it could do. After a few seconds at WOT the engine started to sputter and bog down and hills, forget it. Bogs down almost immediately. If I lay off the throttle I can get the engine to run smooth again, but inability to get up to full throttle is obviously a problem.
My stock 19mm carb had a #70 main jet on it prior to the 70cc upgrade. I installed a #90 main jet when I upgraded. My gut is telling me that the main jet was maybe too big and I don't notice the rich condition until WOT. I have some new plugs on the way so I can do some plug chops, but I was hoping someone on here may have some input to aid this process.
I have a main jet kit with jets from #70 to #110, increments of 5. Thanks in advance!
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Post by 90GTVert on Oct 1, 2014 9:10:22 GMT -5
I agree with your thought that it's rich. Try an 85 main jet and see how it does. If it improves try an 80. If it gets worse try a 95. For best results, try making test passes over a set distance in similar conditions (attempt to have a level playing field for each test) and record the results with each jet. Do it both directions if you aren't using a loop of some sort as your test spot so you can cancel out variables a bit more. Write down speed and RPM and any observations that you feel are relevant and you should see a pretty clear pattern of which jets work best emerging. That way is easier and cheaper than cutting up a bunch of spark plugs. Oh, and if you find that a couple of main jets seem to work the best, go with the larger one to stay on the richer safer side. Plus, jetting tends to need to increase slightly as colder weather approaches.
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