Topic: How exact does a vacuum sync need to be? '76 CB550

[DustyRags]

So I just did a carb sync after a top-end rebuild  (new gaskets all around, rebuild oil pump, rebuilt head, etc. Details: http://forums.sohc4.net/index.php?topic=110973), and got it pretty close. I think.

I used a home made oil-in-the-plastic-tub manometer (shoulda grabbed a pic, but my phone is misbehaving which is a whole other source of frustration at the moment), and got it all synced to where all four columns of oil were within an inch or so of each other, and not moving. Is that close enough, or do they need to be exactly the same?

Also: that didn't eliminate all the roughness and knock, so I think (another) session with the timing light is next. BUT I noticed my #3 spark plug cap was arcing to the engine block. Can I expect a new cap (or set of caps) to fix the roughness? Or is this a try-it-and-see approach?

Before I started futzing with it, I had a great ride from Oakland up to Berkeley. I'm a wet-behind-the-ears newby rider, so that was pretty awesome. First time merging in freeway traffic, and second freeway trip at all. Kinda annoyed it's acting up again...

Thanks for any help!

[WarwickE36]

Arching to the block means misfire.  Misfire= rough running.  Get the new caps! I bet it runs like a champ.  When you get the new caps, re-do you're 3,000 tune up and you'll be in great shape.  Not to mention arcing sounds pretty loud at idle, that is added noise on top of the clatter from the cam chain, and primary chain of the motor misfiring and being out of sync.  Also, it wouldn't hurt to check your primary chain tension next oil change, it does require pulling the oil pan, but it can save you a lot of headache in the future..


Modified to mention, carb sync should be the last thing you do after you fix the leaky caps.

[seanbarney41]

I made my own manometer too, and it was very hard to use although I did successfully use it on a cb400f, other bikes I could never get it to stop sucking the oil into the vacuum port...I think these designs are extremely sensitive compared to commercially available sets and that is what makes them tough to use...so if you got everything as close as you say, I'd say your good.  I've also found that if you do a good, careful job on the static sync, the vacuum sync will be very close and only noticeably affect running conditions at idle, that is, if your top end is in reasonable nic...I would love to see/read the details of your manometer set up, if you do a search on here, you will find a lot of variations.  I can wait for pics until you get your phone sorted

+1 WarwickE36

[WarwickE36]

I manned up and bought a 100 dollar unit.  Its both good and bad.  They unit wasnt calibrated perfectly, so I had to dial it in a bit to 0 all 4 gauges out.  The good part was the orifice is adjustable do you can stabilize the needle pulses down to a very very minimal amount.  I got mine set perfectly, but still thought to motor was noisy.  I finally realized these bikes aren't silent, there will be some chatter.  The biggest improvement I made was by rebuilding the motor , My primary chain was loose and it lets the poor sync take on a mind of its own.  If you search you can find videos I took of the slack before and after the rebuild, its a huge difference.   Hope this helps.

[Stev-o]

+1 get new spark plug caps. And clip 1/4" off the wires before you install them for a good connection.

[DustyRags]

Cool, thanks guys, that's about in line with what I thought- good to know I was about right. I was seriously frustrated this afternoon, what with the rough idle and another problem. I'll pick up the caps and get everything dialed in again.

Warwick- phone's sorted (I hope... I thought that several times already), so expect pics at some point. It's nothing fancy- 1/4 inch vinyl tubing (about 6 feet long each) connected with 2 tees (sort of like this: TT) with each of the four ends plugged into a tub. The end is laid to the base of a board, and the tubes laid up along it, with holes drilled and wire snugging the tubes to the board every 6 inches. I attached three of the four tubes, disconnected the fourth, dangled one end into a bottle of oil, sucked the other end like a porn star, then slipped it on and wired it down. Let the whole thing settle out about 24 hours, and plugged it into some brass carb sync adapters I picked up off Amazon, and away it went!

I need to check my primary chain next time, for sure- I poked at it a bit, but didn't seem to be causing any slap earlier, but I was too terrified to really get into it on the top end rebuild. I'll check it out next oil change. Can that be replaced without doing a yank and crack on the engine?

Sean- It did seem very sensitive indeed- just snugging down the lock nuts changed things, even if the orientation of the screw didn't change. Eventually I got all the nuts partly snugged down to the point where they were pulling the screws against the top thread (rather than letting it sit on the bottom one), adjusted it, and then snugged them all down to an even tension, and that gave me an accurate reading. It was pretty close to begin with, so I think the bench sync the PO did held up pretty well through the rebuild.

Thanks for the info!

[DustyRags]

Warwick- I used 20 w 50 oil in my homebrew beast, and it displays very little pulsing, if any. Just a nice, slow, thick, even motion. But once it gets going, it can take a second to react to any changes, because the oil moves so slowly. Seemed pretty accurate, but at this point I'm not gonna give it a suck just to test 

[WarwickE36]

Another thing I noticed syncing my carbs was that the linkages that actuate throttle movement on each carb were worn.  I can't help but think if these were tighter it would have made the whole process easier and more accurate.  I dont know if these are replaceable (doubtful), but if I ever own another 550 I would highly consider trying to rectify that problem. 

[DustyRags]

There definitely seemed to be some play in it, but I suspect there's some sort of force acting on those that'll keep them in line with each other.

And by "suspect" I mean "dearly hope because I want to stop wrenching and learn to ride!" and by "some force" I mean "black magic."

[WarwickE36]

Well when you twist the throttle the slack is taken out of the system but when you're doing the sync and at idle there is slack in the system.  Maybe Im wrong but my theory is that depending on the amount of free play in the linkage the sync could change between idle and twisted throttle.  Once the throttle is twisted and the slack is out, it should remain the same, but if set at idle with no slack it may be out of sync.  Does that make sense? 

[DustyRags]

Yeah, that makes sense. I guess that's why they say to sync it at idle and then check it around 3,000 rpm.

Been looking into chain slack- my bike's got just a tick over 19,000 miles on it, so I suspect the primary chain will be fine. Will check next oil change since I'll be dropping the pan to check for metal shavings anyhow.

[WarwickE36]

Mine had 30,000k and was wearing into the oil galley fairly significantly. 

here are my vids...


Before
CB550 Split Case Primary Chain Wear 

After
VID 20110904 164323

[DustyRags]

OK, cool, that's helpful. I'll dive in there next oil change. Thanks!

[crazypj]

Couple of things you need to do when synching carbs
'Blip' the throttle several times after each adjustment, allows any wear to 'settle in' and give a more accurate reading.
Use thinner oil in hime made manometers and fit a restrictor in the vacuum line, 1/16" hole should be fine (set of 85~100 main jets?)
If you still get too much 'bounce' fit smaller hole restrictor

[DustyRags]

Thanks, PJ! I've been blipping the throttle, seems to make a difference. It took me a while to figure out that just the act of pushing down on the screws messes with things, but once I got that sorted it wasn't too bad.

Wouldn't a thicker oil with no restrictors have a similar effect as a thinner oil with restrictors? I guess it would react faster, which would be nice, but for a $10 setup, this seemed to work fine

[Deltarider]

I have this feeling there's a lot of echoing (an internet disease) going on again about worn primary chains, so, if you'll forgive me, I'll echo myself.

The primary drive chain in my CB500 has done over 130.000 kms and is still good. Dunno what I've done wrong. All these years I've (ab)used the bike the way Soichiro intended. Certainly not around town. Could that be it?
(Touch wood)

[DustyRags]

Could be it. We'll see, I poked at it a while back and will check it again next oil change. In the meantime, new plugs and hopefully that'll help settle the roughness. Then check the points, re-re-set the timing, and re-re-sync the carbs. And hopefully that'll be it for a while. I'm really ready to ride.

[Bob Wessner]

Well when you twist the throttle the slack is taken out of the system but when you're doing the sync and at idle there is slack in the system.  Maybe Im wrong but my theory is that depending on the amount of free play in the linkage the sync could change between idle and twisted throttle.  Once the throttle is twisted and the slack is out, it should remain the same, but if set at idle with no slack it may be out of sync.  Does that make sense?

This certainly makes some sense re: the original carbs on the K0 with four separate cables. I would think less so with later carbs linked via a common shaft??

[WarwickE36]

I have this feeling there's a lot of echoing (an internet disease) going on again about worn primary chains, so, if you'll forgive me, I'll echo myself.

The primary drive chain in my CB500 has done over 130.000 kms and is still good. Dunno what I've done wrong. All these years I've (ab)used the bike the way Soichiro intended. Certainly not around town. Could that be it?
(Touch wood)

Call it what you want to call it... My evidence was in my videos.  Granted I have no idea what the 30k history was of my bike.  I only put 1,500 on it before rebuilding it.  But, the fact of the mater was, there was case damage at 30k on a 550 with a loose primary chain. 

I would imagine proper maintenance would lengthen the life of primary chains.  I cannot attest to that as I haven't owned one of these from fresh-failure.  But, since many of us here have purchased our bikes from barn yards and whatnot I find it important to mention it makes sense to take a peak doing an end of the year oil change.  Winter is a great time to rebuild a motor instead of finding out the hard way your primary was eating into your oil galley all summer and now you have 0 oil pressure and need a new motor. 

I realize in this case the bike in question probably doesn't need a primary chain at this point, but you never know.  I didn't think I needed one either.   Just hoping to pass on important information that MAY or MAY NOT save someone from a costly and frustrating situation.

[brewsky]

So I just did a carb sync after a top-end rebuild  .........and got it pretty close. I think.

I used a home made oil-in-the-plastic-tub manometer (shoulda grabbed a pic, but my phone is misbehaving which is a whole other source of frustration at the moment), and got it all synced to where all four columns of oil were within an inch or so of each other, and not moving. Is that close enough, or do they need to be exactly the same?

That should be way closer than close enough.
The Service manual for the 750's (don't have the 550 spec) says within 1.6" of HG is the tolerance, so 1" of oil should be WAY closer.

[Toxic]

Thanks for posting the before and after vids.
There is no point in dropping the pan and looking at my primary chain if I don't have a reference to how tight it should be.

Thanks man

[DustyRags]

Cool, thanks Brewski! I've got both the Honda and the Clymer manual, but my Honda's a printout I screwed up the page sequence on (printed them in reverse sequence twice, so the pages go 3-4-1-2-7-8-5-6 or something stupid like that) and I never use it. Clymer didn't have the specs that I saw. I figured it was fine, but once I get rolling on something 99% ain't good enough

[crazypj]

Well when you twist the throttle the slack is taken out of the system but when you're doing the sync and at idle there is slack in the system.  Maybe Im wrong but my theory is that depending on the amount of free play in the linkage the sync could change between idle and twisted throttle.  Once the throttle is twisted and the slack is out, it should remain the same, but if set at idle with no slack it may be out of sync.  Does that make sense?

This certainly makes some sense re: the original carbs on the K0 with four separate cables. I would think less so with later carbs linked via a common shaft??

Don't know about everyone else but I always check again after everything is tight and slowly open throttle to around 4,000 rpm.
If everything is as it should be (timing, valve adjustment, etc) all 4 should be equal all the time.
The K0 CB750 never stays in sync from one throttle opening to the next, but, they do usually stay within the Honda spec of <5cm/Hg.
If they get further out, you usually need new throttle cable (s). (1:4)
Sometimes running light oil through will work on the K0

[lucky]

I have a solution to your problems.
Its easy to do ,reliable, and simple.

There are two posts right now  complaining of syncing problems and both of you have home made tools. Do you see a pattern?

BTW.... on both posts they are looking for a solution other than the tool they built.

[DustyRags]

Awesome, you're giving away gauges? Sweet! Where do I enter to win?

Seriously, though- re-read my post. My tool, as far as I can tell, worked fine (hehe, that's what HE said!), but I'm new to bikes in general- this is my first bike, I've had it since August, and have never done this before. It's not a question about the tool, it's a question about me.

You don't have to answer questions if you don't want to. Nobody's making you. If it bothers you, just close the window.