Topic: CB650 running issue

[Bikeriderz]

Howdy, guys. 

I recently acquired an '81 CB650, that I'm trying to turn into a Cafe Racer style bike.  I got it all together this weekend and put the freshly rebuilt carbs in.   Bike ran fine up to 1/4 throttle or so.   Plenty of power around the neighborhood, but once I got out on the open road it would bog down.  Then the bike wouldn't restart.   Pulled the plugs, black as sin, so I pulled the carbs.   
I have a set of junkyard carbs, so I pulled the 120 main jets out of the fresh carbs and installed the 100 jets out of the donor carbs, put it together and tested it out.  now I have full throttle, but going down the road it stutters at high throttle.  Plugs look good.   Any ideas? 

Cylinder one is a lot cooler than the others, using an infrared temp gun. not sure why or if it matters...

[sinister902]

120 mains area absolutely too big. 100's are stock so you're good there....

but you should check/adjust valve clearances and then sync your carbs to eliminate any "flat spots" or hiccups in the revs

[Bikeriderz]

Supposedly they were when I got them from the shop.   But then again it had those mondo jets in there.   
Would syncing help with the stutter at high revs?  And how hard are these motors to adjust valves?  We talking adjuster nuts or shims?
it feels like its dropping a cylinder or two.   I'm thinking maybe it's fuel starving.  I'll check my inline filter and my line for kinks when I get home. 

[sinister902]

sync would in fact help with stumbles. valves have adjusters so not difficult. if you think it's dropping a cylinder dowble check float height and make sure the mechanical timing advance is functioning properly.

[Bikeriderz]

Float height was close to even, but I don't see a way to adjust it, they're plastic and one piece. 

I'll have to look up the timing advance and find out what I have to do there. 

[sinister902]

bsaically use a timing light and verify it's hitting the F mark at idle and that at about 3-4k rpms it's hitting between T and F.

[cb650]

120's are stock for 81     90 for 79/80

[Bikeriderz]

10-4.   I'll grab a timing light and check.   Thanks for the help, guys. 

[Bikeriderz]

Well, I forgot the timing light.  Ugh. 

I pulled the wire on Cyl 1 (the cold one) and ran it a little bit. Pulled the plug- wet.  So I'm definitely getting fuel.   Then I pressure tested it.   90 pounds.   Tested Cyl 4 at 150.  Added oil to Cyl 1 and pressure came up to 150.   So I guess I've got a rings problem there.

Took it for a ride to see if my temporarily increased compression helped anything.  No dice.  Still stuttered and still tested cold.   

I read elsewhere (on here, I think?) that the stuttering issue might be cleared up by removing the side cover or air box cover. Tried that next and voila!  No stuttering, bike runs great.   Air filter is pretty dirty. I'll try to find one.

[sinister902]

90psi is definitely low, but also will definitely run....I'd say enjoy the season and think about a top end rebuild over next winter.

[Bikeriderz]

My thoughts exactly.