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Old 13th May 2015, 21:31
8 Valve Ed 8 Valve Ed is offline
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Hi, to run (properly) the shut off valve needs 12 Volts.

There should only be two adjustments, the mixture and the throttle stop to adjust the tickover. Set the tickover throttle stop so the engine runs as slowly as possible then adjust the mixture so it runs sweetly at tickover. The mixture screw only adjusts the slow running jet, not the main jet(s). That can only be adjusted by re-jetting the carb which requires a dyno and somebody who knows what they are doing.

Adjusting the slow running mixture one way should make it richer and slow the engine, turning the other way should make it weaker and speed it up. You will have to adjust the tickover as you adjust the mixture to keep a 'nice' slow tickover, maybe 600-800 rpm. Don't set the engine too weak, err on the rich side for safety or you may create an overheating problem.

If it still runs like a dog, especially when you rev it up, the most likely reason is crap in the air and fuel passages and/or crap in the air/fuel jets. That calls for stripping the carb and cleaning with rattle can carb cleaner or an ultrasonic cleaner then blowing out with compressed air.

Another possibility is the choke is stuck on. But judging from the external appearance of that carb it could be a combination of internal crap and bad adjustment. The PO probably though adjusting it might fix the running, so that to me suggest the need to look inside. Water in a passageway is yet another possibility.

... Another thought is the accelerator pump, it may not be working, or it's dribbling after the acceleration pump has stopped. Maybe the non return valve is stuck.

Last edited by 8 Valve Ed; 13th May 2015 at 21:36.. Reason: Added comment.
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