Topic: Vacuume sync

[Jimbo]

Hi guys

When performing a vacuume sync is there any particular order that cylinders need to be adjusted in ?

Thanks in advance

If you've never owned one you'll never understand

CB400F

[Flyin900]

Since all four carbs are adjustable I just adjusted the two carbs that we're off from the other two. Two were very close to each other and the two that were off were reasonalbly close and ended up with all four bang on as a match.

On later bikes in the 80's they have the #2 carb as non adjustable and used as the reference carb, so on that set up there is a adjustment sequence that needs to be followed.

[70CB750]

I usually adjust all four, going from one to another, up or down, till they all show the same. 

[flybox1]

Since all four carbs are adjustable I just adjusted the two carbs that we're off from the other two. Two were very close to each other and the two that were off were reasonably close and ended up with all four bang on as a match.

On later bikes in the 70's & 80's they have the #2 carb as non adjustable and used as the reference carb, so on that set up there is a adjustment sequence that needs to be followed all other carbs need to be set to match #2.

FTFY

[Jimbo]

Lol that's that then

The only reason i ask is because when i pulled the carbs off the other day i noticed 1,4 throttle slides were up all the way and 2,3 were were closed slightly, would the slides being wide open stop the cylinders from firing (taking in to much air thus not igniting and running super lean) as iv been having issues with 1,4 not firing properly (getting hot) im wondering if my vacuum sync is completely off hence not firing all cylinders.

[Scott S]

 Up all the way? As in WOT (Wide Open Throttle)?

[Jimbo]

Up all the way? As in WOT (Wide Open Throttle)?

Yes..

[Scott S]

 Then, yes, that's definitely a problem. Can you post some pics of the carbs? Throats from both sides, intake and engine. The linkage arm and an overall view from the top that shows how it's all put together.

[Jimbo]

Yes i can do that, The only issue is iv now reset the cabs back to normal so all slides are back in the same position. i can return them to how they were and snap a pic for you.

[Scott S]

 No, that's not necessary. What was holding them up?

[Jimbo]

Now i come to think of it im not entirely sure, Majority of the screws were screwed down all the way so something may of been stopping 1,4 from returning to normal, i will bench sync the throttle slides tonight and do another vacuum sync but i will make sure im only doing small adjustments.

[flybox1]

Before you bench sync your slides, turn all vac sync screws to the middle of their adjustment range (then bench sync them)
This will help ensure you dont run out of adjustment range once you start your vac sync.

[Jimbo]

Hey guys iv got the vacuume screws screwed all the way down but there is a difference in slide height ?? It's the same for the other side of the carbs. Cylinders 2,3 have a slight gap at the bottom where as 1,4 close completely. Is this normal on the cb400f carbs or do you think there is an issue???

If you've never owned one you'll never understand

CB400F

[Jimbo]

The pic is of cylinder 3,4

If you've never owned one you'll never understand

CB400F

[flybox1]

Yes, its an issue.
do you have your slides installed backwards on a few of those?  the cutout should be facing the airbox. #3 looks correct.

[Jimbo]

Yes, its an issue.
do you have your slides installed backwards on a few of those?  the cutout should be facing the airbox. #3 looks correct.

Fixed....

Thanks Fly. Yet again you saved me a lot of head scratching and cursing !

[flybox1]

the pictures really helped