Author Topic: Cb750k 6 carb bench sync  (Read 365 times)

Offline Paulreade

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Cb750k 6 carb bench sync
« on: May 05, 2019, 11:38:37 AM »
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
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline Simon46

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Re: Cb750k 6 carb bench sync
« Reply #1 on: May 05, 2019, 11:59:22 AM »
Sorry when I saw the post  thought your 750 had 6 carbs. Oops
1978 Cb750k8 US Import

Offline K2-K6

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Re: Cb750k 6 carb bench sync
« Reply #2 on: May 05, 2019, 01:21:09 PM »
Bryan posted a really useful concise method to sync these carbs.

http://www.sohc.co.uk/index.php/topic,17769.0.html

Completed diligently it'll get easily within tolerance of correct setup.

The choke doors,  not real accuracy required to work as choke but should all clear the main venturi when retracted fully. Make sure handle is in normal/closed run position, adjust linkage to get them all clear of venturi. 

The choke has means of bleeding air if required into the engine so they'll do that irrespective of their ultimate restricted position to start the engine.  Don't worry if there is positional error at that point.

Offline Paulreade

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Re: Cb750k 6 carb bench sync
« Reply #3 on: May 06, 2019, 08:31:53 AM »
I shall have a look , many thanks .
Paul
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline Bryanj

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Re: Cb750k 6 carb bench sync
« Reply #4 on: May 06, 2019, 12:54:45 PM »
I notice you have the same post on the US forum. As i said over there use a 1/16 drill not 1/8, which the yanks seen to use, as the smaller the drill the less chance of having the slides open too far for the idle screw to lower properly!

Offline Trigger

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Re: Cb750k 6 carb bench sync
« Reply #5 on: May 06, 2019, 02:18:27 PM »
Yep, the yanks always know best  ??? Even though i have hundreds of questions from them each week  :o

Offline Paulreade

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Re: Cb750k 6 carb bench sync
« Reply #6 on: May 06, 2019, 03:57:56 PM »
Yep concensus seems to be 1/16 th drill bit. Have just ' bench synced' the slides and chokes as no 4 was slightly out of kilter.
I like to post on both sites as it's interesting getting feedback from owners, whatever the background. I think the secret is to consider fully the responses and act accordingly. Having never 'tinkered ' or 'fettled ' a bike since the 70's it's an interesting learning curve for a 'diy' mechanic. So far though I haven't broken anything and the bike is showing gradual improvement.

By the way graham, took the engine to Des and he managed to retap the threads without a time sert so fingers crossed for a rebuild ( partially there) and assuming I manage to get it back together the same way it came apart it hopefully will fire up. If not I suspect I may have to post again with list of Q's !!!!!
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline Trigger

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Re: Cb750k 6 carb bench sync
« Reply #7 on: May 07, 2019, 06:56:40 AM »
That saved you a long trip to the other side of the country  ::)

Offline Paulreade

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Re: Cb750k 6 carb bench sync
« Reply #8 on: May 08, 2019, 08:38:58 PM »
Agreed, stuck a powerful suction Hoover to the exhaust stub with inlet valve closed and exhaust open, left this running for a few minutes and didn't get anything out apart from a few black bits ( very small). So hopefully all clear. If it fires up I'll fire up on 1/3/4 and leave plug out of 2 for a few revolutions. Fingers crossed !!
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline Trigger

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Re: Cb750k 6 carb bench sync
« Reply #9 on: May 08, 2019, 09:15:45 PM »
If any small bits left on top of the piston ( very unlikely ) Start the bike up with all 4 plugs in but,not the exhaust of the cylinder that has been re tapped. You need pressure from that cylinder to throw anything out with a quick blip of the throttle. 
« Last Edit: May 08, 2019, 09:18:44 PM by Trigger »

Offline Oddjob

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Re: Cb750k 6 carb bench sync
« Reply #10 on: May 08, 2019, 10:15:54 PM »
Not sure if I've mentioned this before Paul but when you get it running I can pop over with my vac gauges if it helps.
Don't play stupid with me, I'm better at it

Offline Paulreade

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Re: Cb750k 6 carb bench sync
« Reply #11 on: May 11, 2019, 09:21:03 AM »
Appreciated guys. Thanks
Paul
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline Paulreade

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Re: Cb750k 6 carb bench sync
« Reply #12 on: May 12, 2019, 09:01:04 PM »
By way of an update, followed everyone's advice, appreciated as ever.
Bench synced carbs, adj timing chain and reset the points etc.
Started her up and she fired first time. Ran her today and the knocking I had which was reduced when I replaced the clutch has now all but gone. It literally sounds like a sowing machine. It must have been the carbs slightly out of balance (no4). Quite amazed how smooth the engine is now.
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline K2-K6

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Re: Cb750k 6 carb bench sync
« Reply #13 on: May 14, 2019, 09:30:44 AM »
Interesting progress and all sounds good.

The elegant simplicity of that bench syncing method,  without special tools and using the same drill bit, really does take out all the variables of the carb operating mech.

Certainly if they are off the bike I feel it's the most accurate way of setting them. In service, installed and running, then vac guage method is viable just from a convenience point of view,  but it holds no greater accuracy than diligent setup.

From discussion we've had before, it seems like they originally arrived at production line set like that and ran perfectly.

Honda's "icing on the cake" in setup was to adjust the idle air screw to equalise the tickover and slow running. Described in their original manuals,  they accept up to 1/8 turn plus or minus on the air screw setting to calibrate each idle circuit. 

The main syncing is after all a method of getting parity to each cylinder for air volume under maximum load, smooth tickover in a consequential byproduct of that. It's not the principal reason as generally viewed in so many discussion of multiple carb sets.