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Post by jdyork on Jul 13, 2019 15:34:59 GMT -5
Hi all, This is my first post on this forum. I have a 139QMB (50cc) scooter. Last month I installed a 72CC kit, stock head, valves, exhaust, carb, air cleaner, CDI, Coil, Etc. NGK plug. Ran new for first couple of weeks, then started to miss throughout rev range. I've done the following with no improvement. Replaced carb with multiple carbs, no change, including new carbs. Rejetted from high 70's up to 90's and no improvement just got worse with the largest of jets. replaced coil, fuel line, all vacuum lines, air filter, exhaust gasket, manually pulled vacuum on fuel petcock to rule out too little vacuum to petcock. Replaced intake manifold gasket.
Swapped out gas.
I'm going to pull the motor appart again tonight and start from scratch.
acts very much like carb problem but again, I've swapped out 4 including 2 new.
Super frustrating as it ran new and then developed the miss, which has gotten worse.
Could this possibly be a head gasket? She starts first kick and idles perfect. I'm also wondering if exhaust restriction but can't imagine how it developed the restriction. Has 1300 miles. any and all help is appreciated. Also, when I pulled vacuum on petcock fuel flowed freely into bucket. Thanks all, Joel
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Post by GrumpyUnk on Jul 14, 2019 10:30:27 GMT -5
Could be you should check the valve clearance. Sometimes the original setting can be 'off' a bit when re-assemling, and change as the engine runs a while. That could cause misfire. Pull the sparkplug and check for deposits and gap, cracks in the insulator(ceramic inner & outer portions) which can cause misfire under load, as can a misadjusted gap. Less likely are CDI, coil, spark pickup, and stator AC power for CDI winding. Have you checked that you do not have any loose wires, or connectors? Intermittent spark can be caused by loose or intermittent connections. tom
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Post by jdyork on Jul 15, 2019 9:56:49 GMT -5
Thank you grumpy. I installed new top end yesterday and ran same I installed old carb and she ran better but low on power. I will install brand new carb and see what that does. I’m also going to swap out exhaust as it feels a bit like I may have restriction
So odd that she ran perfect and then started to miss
I’ve put in new plug and double checked wiring and the coils is new
I’ve also swapped out cdi
Grrrr frustrating Thanks again Joel
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Post by jdyork on Jul 20, 2019 4:10:29 GMT -5
Ok, so I walk into the garage and straighten up things and see the two guide collars from the studs for the head on the bench. I pull motor apart again, install new top end again and brand new carb. Put back together and she runs new. I’m thinking a couple of back to back bad carbs may have been my problem. Thanks all, Joel
Currently have a gofit 72cc kit and carb. I’ve had good luck with them so far. And super inexpensive.
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