CB550 Cylinder number 3 not firing

bergers

Member
Hey everyone, I've been working on my bike on and off for the past year and have replaced a lot of parts and checked over stuff while going through it. The list is as follows:

NEW
carb kit for all 4 (floats, float valves, jets, seals, o-rings, etc.)
fuel lines
coils
spark plugs/wires
battery

CHECKED
carbs hydrodipped and cleaned thoroughly
bench synced(used something like an eighth inch drill bit under the slides and brought them to all the same height)
intake/exh valve lash
compression (bike cold, between 125 and 140psi for all 4)
point gap
spark(ok/strong on all 4)

I ran the bike off of a small temporary tank, it starts fairly easily with a shot of starting fluid, once warm it starts without it. The problem is cylinder number 3 is dead cold, I can put my hand the exhaust header at the head for number 3 while If I did for the others I would get a nice 2nd degree burn. A bike needs 4 things to run; compression, air, fuel, and spark. I know I have spark, I have compression and air, so I assume at this point it's some elusive fuel issue. All bowls have fuel(some overflow periodically), so I'm not sure if it's a float height issue or a carb body needs to be replaced. Any help is appreciated
 

coyote13

New Member
Is there an accelerator pump on #3? If there is, might check that the diaphragm doesn't have any tears or pinholes and isn't pinched. Also make sure the slide is synched correctly with the other three carbs, I had a lock nut that kept backing off on my CB750 and chased my tail on it for quite some time before I realized the slide was wide open all the time
 

bergers

Member
coyote13 said:
Is there an accelerator pump on #3? If there is, might check that the diaphragm doesn't have any tears or pinholes and isn't pinched. Also make sure the slide is synched correctly with the other three carbs, I had a lock nut that kept backing off on my CB750 and chased my tail on it for quite some time before I realized the slide was wide open all the time
Nah its a 74 550, so no diaphragm. And I corrected the original post, I meant to say I synced all of the slides to each other with the drill bit, not the butterflies. I would think that should at least put me in the ballpark of having them synced.
 

cxman

Active Member
DTT SUPPORTER
squirt a bit of starting fluid in #3 carb when the bike is running

does it pick up? if it does then you know the slow jet circuit or ? sync? is whacky

if it does not swap the plug with another cylinder or a new one restart
 

bergers

Member
cxman said:
squirt a bit of starting fluid in #3 carb when the bike is running

does it pick up? if it does then you know the slow jet circuit or ? sync? is whacky

if it does not swap the plug with another cylinder or a new one restart
That's a good idea, I'll try to do that tomorrow if it's not raining.
 

coyote13

New Member
Suppose it could also be carb -> head vacuum issue. Check to make sure that carb is fitted properly in the insulator boot and that the boot doesn't have any cracks.
 

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