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Old 03-23-2011, 05:19 AM   #1 (permalink)
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Change to a single carb on twin? cm400.

I am looking at a basket case Honda cm 400. The carbs are off of it, witch is never a good sign. The Chain is also off of it. I believe all the parts come with it. What do you all think of building a manifold out of pvc, and epoxy, and mounting 1 carb instead of 2. It would probably have to be bigger than the ones that come on it. While I am at it I would change the gearing to lower rpm's.

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Old 03-23-2011, 06:46 AM   #2 (permalink)
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I think your manifold would fall apart, leaving you stranded. Also, getting the mixture distribution correct is harder than it looks.
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Old 03-23-2011, 08:02 AM   #3 (permalink)
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It might be possible to find a single-carb manifold available from a similar model. If not then you might be able to convert one by fabricating an adaptor plate or by some other such method. These will all be made of iron or steel, PVC doesn't sound like a great idea at all.

To raise the gearing, you'll want to fit a larger front sprocket and/or a smaller rear sprocket.
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Old 03-23-2011, 10:48 AM   #4 (permalink)
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First off, does the engine turn over and does it have compression? if it does then it is not so much a basket case as it is a motorcycle that someone was doing a tune up on and quit.
A single carburetor is going to be harder to tune and is not going to give you any positive results.
So with what you have the first step is going to be to take the top domes off the carbs and make sure that the vacuum diaphragms are intact, these cost $140 each if you have to buy a new one so be careful! if those are intact and nothing on the carbs looks to be broken then order up a rebuild kit, they will cost you between $20 and $35 depending but include new gaskets, jets and needle valve, you'll have to look up what the adjustment screws all need to be set to but after you put the carbs back together they should treat you well for years, at least until you leave some gas in it over the winter to go bad, then you have to rebuild them again.
I've built new intake manifolds out of copper pipe soldered together, do not use plastic unless you want to set your motorcycle on fire, the only time you should use plastic or rubber is if it is labeled as being safe for contact with gasoline.
If you do want to play around with the intake tuning, building long straight intake runners between the carbs and the air cleaner can help with improving mileage, you can also get pre made foam air cleaners or bulk foam that you cut and sew to build your own air cleaners, in all the motorcycles that I have owned and worked on I like foam air cleaners the best, they trap the most dirt and they provide good air flow, to clean them you just use soap and water then oil them with high tack air filter oil.
Once you get the motorcycle back together and working then start looking at improving other parts of it, tires dry out with time and old dry tires don't grip as well, I find it well worth it for my own safty to buy good tires, if you can find touring tires that will fit they tend to be lower rolling resistance and are good all weather tires that will be safe and give decent MPG.
If you put synthetic oil in the engine remember that you have a wet clutch, the clutch is in the engine oil, so synthetic oil that is designed for motorcycles is a good idea or you might destroy the clutch.
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Old 03-23-2011, 02:06 PM   #5 (permalink)
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Thanks for the replies. I see pvc is out and copper may work. I guess I would keep dual carbs. I have not checked bike out yet so I don't know about compression. If I buy it I would go through the whole thing. It may be only $100 or so, the guy just wants it gone. I would budget around $700 for repairs to be safe. I would have the work done do to my lack of time.

Thanks for your input.
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Old 03-24-2011, 11:07 PM   #6 (permalink)
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I am going to offer an opinion that is possible and fairly easy to do with basic hand tools. I wouldn't spend more than 100 dollars on the bike that you are looking at.

I have a pvc and carbon manifold holding a single carb on my Yamaha 700. It works fine. and gets about 50mpg. It is about 3 years old. I am sure you could do it that way if you wanted to.

I used a single stock carb the first year but switched to a dirt bike carb later when I had issues with my remaining carb.

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Old 03-25-2011, 09:04 AM   #7 (permalink)
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Nice thing about pvc is that it is pretty soft and easy to work with. I built my manifold in place and then coated the interior with KBS tank sealer.



I am running a 34 mm carb at present the stock was 40mm. At 80 mph I am 1/8th throttle. I am sure that the bike would go 80 with a 20mm carb. Probably with more economy.



A simple Y shaped manifold made with some PVC 1 1/4" sink drain would be about the right diameter for your 200 cc cylinders. Just butt glue the parts to make the shape that you want. Then take the assembly and spray glue the reinforcement material to the plastic pipe wet it out with epoxy and the wrap the thing with strips of polyester or rip stop nylon cloth. The cloth can be peeled off once cured. You don't have to use carbon fiber regular fiberglass will work.

All that said if you get the bike to run stock you will have a fairly well engineered bike that should save you some gas.
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Old 03-26-2011, 05:44 AM   #8 (permalink)
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I have no idea about the carbs or manifold, but that model is lovely. My dad had one (CM400E) in the early 90s that was a dream to ride around town on. Sure wish 400 twins hadn`t gone the way of the dinosaurs.
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Old 04-12-2011, 04:16 PM   #9 (permalink)
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The cm400 is a great bike and get about 60mpg stock. A single carb will not need to be syncronized and balanced like twin carbs would. FYI the cm200 twinstar used a single carb and the cb200 used twin carbs. Pretty much the same engine.
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Old 04-14-2011, 07:33 PM   #10 (permalink)
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you can change to a single carb, but it wont get you anymore MPG than if you just tweeked the stock carbs down a little. plus, it will be a booger to get lined out just right. you might actually make the mileage number worse.

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