I'm having some trouble with my CM400A (think it's carb related)

hiptip

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Howdy, new poster here.

Bought the bike about a month ago (my first bike) and up until today it's been running like a dream. Today I ran into a weird problem. Once I hit about 20mph, the bike has trouble accelerating any faster. I can sort of coax it if I take it really slow, otherwise the engine really growls and it sounds like something isn't right. It then stalled shortly after once I brought it to an idle. This problem kept up for a bit until eventually the bike could hardly maintain 10mph.

I'm guessing this is maybe a fuel line issue? I'm not really sure though, I don't have much experience diagnosing mechanical problems.

The bike also really sputters in idle when it's not stalling.

Update: as of today it really struggles to stay running when the choke is closed and it sounds like maybe only one cynlinder is firing.
 
I can't offer you much on the SOHC 400 series, but I can tell you that any engine will struggle when the choke is closed... so just so we're all working with the same terminology - the choke is closed when you're trying to start the engine cold. Closing the choke causes the engine to draw harder on the carb and pull additional fuel into the cold engine while reducing the air flow at the same time. When the engine warms up, you fully open the choke and I'm sure that's the position you were thinking of when you mentioned it.
 
Also, check back to your introduction post, I gave you a link to some stuff to help you around the forum.
 
Moved to SOHC/Fuel.

Close your petcock (it should always be closed when the engine is not running).

Disconnect the line from the petcock to the carburetor.

Place the loose end of the fuel line in a suitable container and open the petcock. Do you have a good steady fuel flow?

With the fuel line re-connected, open the petcock for about a minute, then close it again. Open the drain screws on the bottom of each fuel bowl and measure the amount of fuel that comes out.

Let us know the results.
 
This does sound carb related but I would check the coil and plug end caps for resistance first with your VOM, volt-ohm meter.
Plug caps unscrew from the wires and should read @5,000 ohms
One probe in each plug wire while the caps are off. Should read between 7,200 and 8,800 ohms
The primary side is the small plug with a Yellow and Green wire. That should read less than 1 ohm.
You should be using NGK D8EA sparkplugs.
Do a compression test with both plugs removed. Throttle held wide open and cranked until the gauge stops rising. Spec is 185 psi +/-14 psi
From there if everything checks out good you will need to read the sticky posts in the Fuel section on rebuilding these carbs.
 
Some bad news. I attempted to remove the carb and as I was taking it out, both rubber boots (on the non-intake side rear-side) sort of crumbled and are seemingly irrecoverable. The good news is that I may have found the problem, the bad news is that I've seemingly made it much much worse. The intake boots are bolted on but its not clear to me how the rear boots are held in place.


video of the damage: https://youtu.be/hRxFIXLFBp0
 
The air box boots are glued into the air box and you simply push hard enough to break the seal. Finding replacements is problematic. They only cam with the air box. I'm not sure but the may be specific for the A model. Post the OD of the carb where the boot fits and I can check the manual boots
 
I actually just found a replacement airbox for pretty cheap so I went ahead and bought that -- should be coming on Thursday. In the meantime, I'm going to continue taking the carb out and inspecting the float and jets. Anything else I should keep an eye on when inspecting the carb?

Just a note -- the bike had been sitting out in the rain for about a month. This may have led to some of the carb problems, I'm not sure. Also when I started it today before taking it apart, it was revving pretty high when idling with the choke on.
 
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