Non V Max Carb Problem

[Icepick]

I'm working on a buddies bike, ('78 XS 400 sitting for about 9 years,-it started out with just a tune-up, clean carbs etc...)
So after cleaning carbs, new points, coils, air filters, adjusting valves, the right cyl. is backfiring BOTH thru the carb. and out the exhaust!
Everything in the carbs look fine, the floats are set exactly per the manual.
Valves, points, timing are adjusted per the manual.
was able to sync the carbs, but still the backfiring.
I did notice that on the offending cyl. when I had the cap off of the port where you hook up the sync tool and the engine running, the engine ran fine, NO backfiring, as soon as I put my finger over the port, backfiring again.
I would appreciate any help with this issue.
Thanks

 

[vmaximum]

Make sure to go through everything with canned air or something to check for vaccum leaks. I had that problem with an old virago

 

[naughtyG]

Ya I reckon you defo have something clogged in that right carb somewhere if the vacuum port changes how it runs, I think.

Maybe Mark (maleko) will know better..

 

[Icepick]

cool took vmaximum's advice, no backfiring now when i'm riding it and pull the clutch in, the engine starts revving up, any ideas?
Thanks to all...

 

[naughtyG]

cool took vmaximum's advice, no backfiring now when i'm riding it and pull the clutch in, the engine starts revving up, any ideas?
Thanks to all...

Well that's normal, isn't it? Unless you let go of the throttle the motor will rev up when you pull the clutch lever.. :ummm:

 

[Icepick]

yea, i forgot to mention that, release throttle, pull clutch, engine stays at rpm's or rev's up.

 

[Redbone]

Is it possible that the choke or throttle linkage is sticking?

 

[Icepick]

doesn't look like it, it's like the slides are sticking but everything is operating like it's supposed to, (except for the RPM revving problem)

 

[Icepick]

could it be a timing problem that is missed?

 

[naughtyG]

Did you sync your carbs?

I'll bet they are out of sync, and as a result, the throttle won't go back properly - I've seen that happen many times when people play with the sync screws without a vacuum gauge attached.

 

[Icepick]

got it! they were synced, but i had the idle too high then synced the 2nd carb, all is well