Author Topic: Vacuum Synching CB 750 Carbs. (Read 4122 times)

rddcw · « **on:** February 26, 2016, 07:54:55 PM »

I'm about to Vacuum Synch my Carbs. off my 74' CB 750 K4 in a few days. I've remounted them after thoroughly cleaning and checking all mechanical adjustments including bench synch and float set. Have started the bike after I remounted the carbs. and seems to idle fairly well and respond to throttle but appears to come back to idle maybe a little slow?

Have purchased an individual 4 gauge carb. synch kit and am awaiting a fuel Y-splitter to plum off of the external fuel container that came in the kit to the carbs.

Trying to do all my Homework before going thru this process and would appreciate Any Tips or Procedures to follow. I thought I read there is one carb. that you make reference to in setting the vacuum of the other 3 carbs. to it ? Is that right ?

I'm sure there is a lot of you Tech Guys that have this procedure down pat.

As I said, would Greatly Appreciate any Help

MoMo · « **Reply #1 on:** February 26, 2016, 08:26:50 PM »

That one carb you sync to is for later models

BobbyR · « **Reply #2 on:** February 26, 2016, 08:28:27 PM »

I would suggest you download the shop manual for your bike. There is one carb that is used as the reference carb. The other three should be adjusted to pull the same vacuum as that carb irregardless of the reading on that carb, the idea is having all carbs pulling the same vacuum. One tip is to calibrate your gauges. There is an adjustment screw on the gauge face. To do this you get the bike warmed up. Carb 1 and 4 are easiest to do this with. Plug each gauge into that carb and use the screw to have each guage read the same. Then you can plug the gauges into the carbs.

I have a 78K so my reference carb is #2 and is not adjustable. It may be different with the 74. The manuals are here http://forums.sohc4.net/index.php/topic,17788.0.html

Mr Freeze · « **Reply #3 on:** February 26, 2016, 11:52:58 PM »

Basically since you've bench synced your carburetors beforehand, they should all in the ball park of one another. Set up a fan to blow air across the motor and start up the bike and let it get fully warmed up. Then shut it down and hook up the gauges with the vacuum tubes. Start again is see how close they all are. You are just looking to get them all to match as closely as you can with the slide adjusters on the top of the carbs. If everything was close to begin with there shouldn't be a lot of adjustment that needs to be made. It's a fairly easy process.

PeWe · « **Reply #4 on:** February 27, 2016, 12:39:43 AM »

Then make sure that the points are perfectly set too for a smooth runner.
http://forums.sohc4.net/index.php/topic,147444.msg1678696.html#msg1678696

rddcw · « **Reply #5 on:** February 27, 2016, 04:03:38 AM »

On thing I notice when I started the bike and let it idle, there seem to be a lot what appeared to be cam chain noise or ? This quieted down considerably when I pulled in the clutch lever?

I did adjust the cam chain per Factory Manual which I have.

Also, I'm going back and check the valve adjusted. I may have adjusted them loose from mis interpreting the valve clearance measurement? Intake at .002 thousands and .003 thousands on the exhaust. Is this correct?

But if the valves were little loose or the cam chain was worn, why does pulling in the clutch lever really quiet things up as much?

70CB750 · « **Reply #6 on:** February 27, 2016, 04:38:18 AM »

It could be the primary chain, at least that's what I seem to remember somebody told the pull clutch test will prove. In my case it made no difference, thats why I did not follow up on it.

But do the vacuum synch first, many weird noises go away as soon as all carburetors are lined up.

rddcw · « **Reply #7 on:** February 27, 2016, 04:46:09 AM »

Thanks, I plan on doing that first.

Is my interpretation of the valve measurement clearance correct?

70CB750 · « **Reply #8 on:** February 27, 2016, 04:59:52 AM »

I adjust valves to Hondaman recommendation - 0.003" and 0.004" for the exhaust. Makes the "sewing" machine louder but suppose to be good for longevity.

I believe you are ok, according to the shop manual.

BTW if you dont have shop manual, download it here:

http://forums.sohc4.net/index.php/topic,17788.0.html

MoMo · « **Reply #9 on:** February 27, 2016, 05:25:53 AM »

Sequence should be: valves, cam chain, timing and lastly carb sync. I've always set the valves to Honda's specs, which is 002 in and 003 ex...Larry

Johnie · « **Reply #10 on:** February 27, 2016, 06:05:13 AM »

By all means do put a fan on the front of the motor when doing the sync. I actually use 2 fans with one on either side of the front tire. You certainly do not need to overheat that motor. The K4 does not have a slave carb, so you will be adjusting all 4 carbs. Keep in mind when you adjust one carb it will affect the others. It can be a tedious process, but take your time. You want your idle screw out so that when you get the carbs idling nice and slow you can use that idle screw to bring it back to the RPM idle you like. Any chance you did a bench sync after the carb rebuild?

BobbyR · « **Reply #11 on:** February 27, 2016, 07:40:28 AM »

Quote from: rddcw on February 27, 2016, 04:03:38 AM

On thing I notice when I started the bike and let it idle, there seem to be a lot what appeared to be cam chain noise or ? This quieted down considerably when I pulled in the clutch lever?

I did adjust the cam chain per Factory Manual which I have.

Also, I'm going back and check the valve adjusted. I may have adjusted them loose from mis interpreting the valve clearance measurement? Intake at .002 thousands and .003 thousands on the exhaust. Is this correct?

But if the valves were little loose or the cam chain was worn, why does pulling in the clutch lever really quiet things up as much?

What you are hearing is clutch rattle which is universal to these bikes. You can do a search on clutch rattle and see all of the attempts made over the years to eliminate it. Carb synch and being in tune does help.

The test to see if it is clutch rattle and not a timing chain is to pull in the clutch, if the sound disappears it is nothing but clutch rattle. This you have done, so you can relax.

rddcw · « **Reply #12 on:** February 27, 2016, 03:26:17 PM »

Johnie, I can get the bike to probably idle less than 1000 RPM but seems to run better around 1200 or 1300 RPM. Are you recommending when performing the Vacuum synch to have the idling RPM below 1000 by back the idling screw out so far as to barely have the bike idle at all?

BobbyR, Appreciate the input on the Clutch Rattle I'm hearing. Makes me feel a lot better

I understand having the valve clearances a little looser, but will re-check to Factory Spec's.

70CB750 · « **Reply #13 on:** February 27, 2016, 04:02:44 PM »

Lot of bikes idle better around 1200 - 1300, set it where you like it the most.

MoMo · « **Reply #14 on:** February 27, 2016, 04:14:16 PM »

Quote from: 70CB750 on February 27, 2016, 04:02:44 PM

Lot of bikes idle better around 1200 - 1300, set it where you like it the most.

Back in the 70's every 750 our shops worked on was set to 1200 rpm

Johnie · « **Reply #15 on:** February 27, 2016, 08:13:35 PM »

Yes, try to set that sync below 1,000 with the idle screw out. When you get the sync spot on then go ahead and use the idle screw to get that idle you want. I find my 750's also idle best 1200 - 1300.

rddcw · « **Reply #16 on:** March 02, 2016, 10:43:03 AM »

Well I worked at setting the carbs. yesterday and today. Think I got them fairly even by the gauges. Only thing that questions me a little is that # 3 carb. threaded adjusted rod above the lock nut and adjuster is about 5 threads showing and the other 3 carbs. have about 2 threads showing? But bike appears to respond to throttle well and idles decent, so don't know $:-\$

Now I like to work on the clutch rattle which disappears after you pull in the clutch lever. Is there a decent fix that most of you have found that works ?

evanphi · « **Reply #17 on:** March 02, 2016, 10:48:21 AM »

Quote from: rddcw on March 02, 2016, 10:43:03 AM

Well I worked at setting the carbs. yesterday and today. Think I got them fairly even by the gauges. Only thing that questions me a little is that # 3 carb. threaded adjusted rod above the lock nut and adjuster is about 5 threads showing and the other 3 carbs. have about 2 threads showing? But bike appears to respond to throttle well and idles decent, so don't know $:-\$

go by what your gauges read. so long as you have some threads showing above the locknut, you're safe.

Quote

Now I like to work on the clutch rattle which disappears after you pull in the clutch lever. Is there a decent fix that most of you have found that works ?

you'll go bonkers trying to eliminate it. one day after syncing your carbs it'll be a nice quiet bike. the perfect sewing machine! Then you'll take the carbs apart for something without even touching the adjusting screws and you'll put it all back together and it'll sound like a bucket of bolts again. Ask me how I know this...

So no. There's nothing except a perfectly perfect sync to eliminate that noise.

flybox1 · « **Reply #18 on:** March 02, 2016, 11:10:14 AM »

Quote from: rddcw on March 02, 2016, 10:43:03 AM

Well I worked at setting the carbs. yesterday and today. Think I got them fairly even by the gauges. Only thing that questions me a little is that # 3 carb. threaded adjusted rod above the lock nut and adjuster is about 5 threads showing and the other 3 carbs. have about 2 threads showing? But bike appears to respond to throttle well and idles decent, so don't know $:-\$
In most cases they will be different. Ignore it.

Now I like to work on the clutch rattle which disappears after you pull in the clutch lever. Is there a decent fix that most of you have found that works ?

an oil high in zinc and low in detergents, good clutch plates, hondamans clutch basket oil hole mod, and proper adjustment.

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Author Topic: Vacuum Synching CB 750 Carbs. (Read 4122 times)