Topic: Bench Synching Carbs - 750 K2/K4

[The Lone Builder]

I put the bike back together after replacing valve guides etc.

If fired up easily, with good oil pressure - not surprisingly since I had cranked it to exhaustion beforehand, and then checked using the starter.

Anyway, the problem was the idle, in that there was none! The engine raced to about 3 500 rpm, with the idle screw fully out, so I took the carbs off to recheck the synchronisation.

With them off I checked and reset the float levels, just to be sure.I also read many of the posts to make sure I was doing things right.

So the first thing is that the setting is done from the INTAKE side? Is this correct? If so then that was my mistake the last time.

The procedure I followed was:
     1.  With the idle screw out, I lowered all the slides as far as possible.
     2.  I set the lock nuts on the rods so the rod and nut was just past flush.
     3.  I set No 2 slide – still on the inlet side – using the idle screw and locked this in place. I used a 3,5mm (slightly >⅛" drill bit.
     4.  I adjusted the other three slides using the adjuster nuts and locked them down also.
     5.  I operated the slides a couple of times and rechecked the gaps, adjusting No 4 which was a little low.

So I just want someone (or two) to confirm that I have done it right - especially the inlet side operation.

[BenelliSEI]

Below is a “copy and paste” from an earlier post I wrote in my K6 Project file. I do measure from the intake side (the side the air box connects too). The critical piece is getting number 2 to close down correctly. The rest just get set to match. I use a larger drill bit as I think it’s easier to “feel” the drag in the throat of the carb. Secondly, make sure you don’t have an air leak somewhere. The symptoms are the same as a carb hung open.

“I usually prep a long shank, 1/4 drill bit by making sure the shank is smooth and burr free. I stick it in my drill press backwards and dress the shank with some Emory cloth so it is really smooth and polished.

I start by removing the idle screw/spring and park the spring off to one side. Screw the idle screw in until it touches the operating bridge and add a few turns to the screw. Adjust #2 carb so the slide just barely lifts off the bottom of the carb throat. Lightly lock down the top nut, without moving the base nut/thimble.

Next I crank the idle screw up until I can just slide in the shank end of the 1/4” bit, between the belly of the #2 carb throat and the bottom of the slide. Adjust it so the shank must be flat and centred to just drag in and out. Now using the top adjusters on the other three carbs, set them to the same height….. Lightly lock the top nuts and done. The rubber dust covers on the top adjusters are a pain, but I like the look and they keep the sliding contact faces clean.

Remember to put the spring and washer back in the idle screw! I always check the sync with my old Motion Pro sticks, once the bike is running. I’m often surprised how close this mechanical routine gets it”.

[The Lone Builder]

That sounds good to me. I'll put them on now and see how it goes.

Last time I did this - yes I did it properly before  , I also checked with gauges and it was also close enough for government work! 

[BenelliSEI]

Good luck! Are you headed to North America this year?

[The Lone Builder]

Yup!

I land in Halifax on 16 April - provided all goes well this afternoon

🥶 from what I see.

[BenelliSEI]

Got my Ontario address in your Trip notes? John D.

[The Lone Builder]

I do.

Was looking at routing options earlier.

Good news is, carbs are on and bike running. Just in time for the rain!

[BenelliSEI]

I do.

Was looking at routing options earlier.

Good news is, carbs are on and bike running. Just in time for the rain!

Excellent!

[HondaMan]

Wow, I'll your biceps are packing your shirt sleeves by now?
I hate putting these carbs on...

[The Lone Builder]

Ha, they're tough enough to get on!

I take the effort out by using a ratchet strap carefully positioned.

https://youtube.com/shorts/WCRFD86XlfM?feature=share

[pjlogue]

Ha, they're tough enough to get on!

I take the effort out by using a ratchet strap carefully positioned.

https://youtube.com/shorts/WCRFD86XlfM?feature=share

  Yes and a slight bit of high vacuum grease does the job.  When I first did my restoration on my'76, 750 I tried  to synch the carbs.  No go until i realized the idle stop screw had to be backed out all the way and then put a 1/16" in drill bit in each carb slide and adjust the top nut and lock nut to ~2 threads showing above the lock nut.  Firm the lock nut up and then do a carb sync after the engine is fully warmed up.  The idle adjust nut is adjusted as you do the carb sync to maintain proper idle rpm..

-P.

[M 750K6]

If it's really difficult getting the carbs back on, check the rubbers on the engine side are properly lined up. Something like a flat 4" x 2", checking each rubber sits square against it. Hairdryer on the rubbers and they become flexible, then the carbs press home without a struggle.

[timtune]

Great tip with the ratchet straps there Lone Rider.

Question why do you starty the synch with carb #2? I started with #1 is that wrong??

[BenelliSEI]

Great tip with the ratchet straps there Lone Rider.

Question why do you starty the synch with carb #2? I started with #1 is that wrong??

I use #2 and lock it down. The two inner carbs are more awkward to adjust than the two outers, so this means I only work with numbers 1, 3, and 4 when using the manometer. But, really makes no difference.

[The Lone Builder]

Great tip with the ratchet straps there Lone Rider.

Question why do you starty the synch with carb #2? I started with #1 is that wrong??

I believe on older models, No 2 was set and had no adjustment, so you had to start there.

Perhaps it is now just tradition, but some – I think I read it on a post by HondaMan – say because of teh location of the return spring, No 2 slide returns to the set point more reliably than the others.

But as BenelliSEI says above, I don't think it makes any difference, provided No 2 can be adjusted.