Single carb conversion... again

[Tim H]

Hi y'all,
I'm new to the forum and new to motorcycles. I just bought a 1982 XS400 Maxim for more than it was worth (what can I say, I was blinded by youthful enthusiasm) and I'm thinking about converting it to a single carb. A little bit of research showed that it's been done but it'll make one cylinder run lean and may cause blowback through the carb.
I'm still a little confused as to what causes this. I understand that the intake valve closes on one cylinder, causing a little bit of blowback and some pulsation that messes with the mixture... but my question is, how? How does this throw off the mixture? Would anti-reversion chambers/plates help by any chance?
Thanks in advance,
Tim

 

[BBS360]

Your interpretation of the cause of the imbalance is not correct. Nothing to do with the valve themselves but rather when they open and close.
Anti reversion chambers would do nothing for the problem.

The issue is the 180° crankshaft.
For single overhead cam bikes at least, the left cylinder intake valve opens at 30° before top dead center and closes at 70° after bottom dead center.
By my mental math that means the left cylinder intake valve is open for 280°.
Since the right cylinder starts its intake cycle 180° after the left cylinder there is an overlap of 100° of crankshaft rotation where both cylinders are sucking fuel from the carbs.
No problem when the cylinders have individual carbs but when they share a single one you can't deliver fuel evenly.

Works fine on bikes like the XS650 that have 360° timing with no intake overlap.

 

[Tim H]

Ah, I'd been fed some misinformation on a different website. Thanks for the input, I guess I'll have to put that idea on the shelf.
ATB,
Tim

 

[NewHavenMike]

Why are you thinking of doing a single carb anyways? Id rather just get an SR400 or something similar.

 

[Jeff0133]

Anytime you have intake overlap there will be issues...or tradeoffs if you will.
Why one carb anyway?...might as well keep dual carbs but go to flatslides instead...
That's what i am planning for mine anyway...once my ongoing project is done