Topic: Bench syncing carburettors cb750k6

[paulreade]

Hi guys.
Just inspecting the carbs before reinstalation. I have no 4 cylinder which is rich. I have looked at the slides and note that no 4 is slightly higher than the others, I assume therefore this is possibly why it's a tad rich ?. I will use the torch method to set, Q: is the adj made by the adj on the top of the carb.

Second point , my choke slides do not appear to move in sync. Nor do the slides on 2/3/4 retract fully ? When the choke is open. Is this critical ( should all slides move in sync and open / close together for perfect operation ??)
Is there a paper / video available to watch ?
Regards paul

[BenelliSEI]

Yes, at the top. Not sure what the “torch method” is? You can adjust the choke slides with the linkages but those look fine.

I use a clean 3/16 - 1/4” drill bit as my standard. With the idle screw, set #2 so the shank just slides in between the slide and the bottom of the bore. Removing the idle screw spring makes that easier. Then, set the other three to match.

I’m sure you’ll get lots of feedback here!

[DaveBarbier]

I can only assume that you’re from the UK and a torch is a flashlight. Using the flashlight method is aiming the light at you from the back of the carbs and adjusting the slides so you see the same sliver of light.

I have also used the drill bit method. 1/8” is the preferred. I think 1/4” would set the slides too high. I like the idea of using feel instead of my eyes. Like a feeler gauge for tappets.

I don’t know these carbs but the adjustment would be at the top and sometimes there’s a non adjustable carb which you set all the others to.

After the bench sync it’s a good idea to vacuum sync them to get an even smoother running engine. But a bench sync is close enough for rock and roll.

I would also look elsewhere for your rich running condition. I don’t think richness is really affected by out of sync carbs. Maybe you want to go into more detail on how you’re determining the rich cylinder.

[setdog]

the slides are adjusted using the nuts on the top of the carb.

the choke flaps are linked.  the links are threaded together and can be adjusted.
if old they can be stuck together pretty tightly and if you twist too hard they can break.
you have to remove the little tiny cotter pins and washers and those little pins can be broken easily
i bought a stack which i keep in a drawer for later use.
i try to get my flippy flaps working as well as i can but they are kind of annoying to get just right.



torch (flashlight?) method i assume is basically using a light to know when there is a gap

[Bodi]

The choke plates never sync perfectly, at least I have never seen it. The linkages have adjustability, between each body pair there's a fiddly thing to allow adjustment. Work from the end where the choke lever or mechanism is.
I try to get them all opening flat to airflow when fully open, and let them sit wherever that puts them with choke fully on. They don't need to seal tight to have max choke ... the enrichening effect will be very strong even if not all are fully closed tight. I prefer to have the carb airflow restricted as little as possible when choke is off, plates horizontal.

[bryanj]

If using a drill bit to sync 1/8 is too big you need a maximum of 1/16

[DaveBarbier]

FAQ says 1/8”. 1/16” would work just fine though. I actually think I used a 1/16” when I did mine. Can’t remember. Either way, I think doing the drill bit method is easier and more accurate than the flashlight method.

And to reiterate, vacuum syncing is normally done after a bench sync to get it more finely tuned. Hope delta doesn’t see this, he’ll go on one of his rants again.

http://forums.sohc4.net/index.php?topic=5410.0

[paulreade]

using the drill bit method - is the bit inserted on the inlet or the outlet ( outlet being closest to the barrels). Pardon my ignorance but new to all this.
Yes from uk.
Again thanks for the time to reply !
Regards
Paul

[PeWe]

Inlet side easier since outer side has an opening.
I used drillbit, later checked the entire rack to daylight,  raised the throttles with idle screw to see if all got light shining thru at the same height. The vacuum might differ anyway.

I replaced the CB750 carb tops rubber seals last time since the old rubbers were worn and cracking dry. This to ensure no leak, if leaking here same amount of air slipping thru
Do not let the throttles to snap hard on the drill bit. Throttles will be damaged and get burrs affecting the smoothness when throttles should move, especially back down. Do not ask me how I know

CB750 K6 spring is a real pain, way too hard. I bought a very expensive replacement softer spring  from Webike Japan.  The last 1/5 throttle lift will bend the entire rack/throttle pins. This last lift that will not do much or anything extra need the hard spring to move back when releasing the throttle grip.

[paulreade]

Thanks all, for the input. Will give the 1/16th drill bit a go.
Paul

[paulreade]

1/16th worked fine. Chokes all adj also. Proof I guess will be on startup. Vacuum sync an option at a later stage when a fan is obtained.
Regards
Paul

[PeWe]

Bike will run OK, if running uneven verify ignition which most likely is the root cause.
If stock points make sure both point gaps are equally set. Only way is with Dwell meter. I use a cheap auto multimeter with dwell.
Stock ignition might advance too early due to worn old springs. Tighten them will help to get correct timing at idle and at full advance

[paulreade]

Cheers guys
Appreciated

[paulreade]

Update: bench sync done, static timing and points reset. New oil and filter and timing chain adj. the 'clattering' I suffered before has all but gone and the engine runs like a sowing machine. No 4 carb was slightly out so I guess the sync has sorted the noisy clatter from the engine. Very pleased with the outcome. Cheers guys.
Regards
Paul