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Old 04-20-2009, 06:59 PM   #1
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Default CM 400 Carb help

Ok thanks to this forum I found the Z1 Enterprises site and got some parts for the carbs. Float needle valves and they fixed the over flow problem. Now it starts great and idles like a new one. It falls off real bad above idle but at full throttle runs great. Since I don't have a lot of experience with CV carbs I was wondering if the vacuum boot's in the top half could cause this rude behavior. I am pretty good usually with carbs, re-building etc. I thought I had the jets all cleaned and the CV valves open freely and fall with a little thump just like they are supposed to. I didn't take the top end apart because the valves seemed to move so freely but now I am wondering if one or both of the diaphrams might be bad.

Any thoughts on midrange power loss?
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Old 04-20-2009, 08:21 PM   #2
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It could be that, hard to diagnose these things over teh Interwebs. Could be a synchronization problem, could be a bent needle, or worn main-jet/needle-guide, or plugged emulsion-tube orifices..............

It's easy to check the diaphragms, though - merely take the 4-5 screws out of the top cover and remove it, taking care not to lose the spring. The 'phragm may be slightly "stuck" to the carb body, loosen it and the slide should come out easily. Any obvious tears or cracks means they're bad. Hold the rubber up to a light and gently stretch it - any tiny holes are a vacuum-leak. If you're on a budget, some RTV dabbed-into a pinhole and allowed to cure will temporarily fix it 'til you obtain new slides.

Clean the slide itself of any gunk, as well as the slide bore. Look for galling or excessively-deep scratches on the slide and bore walls. There will almost-certainly be some scratching. Careful, don't bend the needle. If there is any dry-film-type coating on the slide, take care not to rub it off. Also - avoid getting carb-cleaner on the diaphragm rubber - it will cause it to stretch and swell, making it hard to get back in place upon reassembly.

If all is good, reassembly is the reverse - take care to properly orient the diaphragm, there's usually only one way they will fit in place, it should be obvious. Don't forget the spring (don't ask me why I'm reminding you!).
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Last edited by The_AirHawk : 04-20-2009 at 08:26 PM.
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