Topic: Carb sync

[oldbiker]

Morgan Carbtune sell all spares.  www.carbtune.com

[ride_a_honda]

Hello, just thought I'd let you all know that I picked up a set from my local Honda dealer last year.

Now, I can't remember  exactly how much they cost, but I know that I didn't go broke buying them.

I have answered another thread with the same info, and as soon as I find my receipt for this item,
I'll let you all know how much I spent.

Cheers,
Lou.

[Quail "Owner of the comfortable k8"]

ok here goes:  I used my gauge that I used for my Triumph, Austins, and MG's Stromburg Carbs, to balance my cb750k8 carbs.  It goes over the throat and you dial it down until you get a reading on #2 then you put it on the others until you get the same reading.  I did this and they were way out so I adjusted the only adjustment I have at the top of # 1,3,4.  Now they all read the same.  Before #1 was the only one doing anything at idle.  It has more power now but will bog down if I really crank it fast.  The timing was advanced slightly and it does run better advanved with no pinging.

My questions:  How do I know #2 is working right?  It is some what better but on a slow accel or holding a speed ate 2000-3000 rpm
I feel the engine sputtering slightly.  If I run it a 3200 and up it feels good but I think it has a little sputter in the background ( if you know what I mean)  I am not looking for a guess (not to be rude) some of you have been through all of this before and if you want to call me 717-776-6440 collect I would love to hear from you.

[cben750f0]

g'day to the powers that be, i have a few articles from the late 60's, and 70's on the CB750... also an article on the race kit for the CR750.. i would like to post this, but am unsure of copy write and such... the CR article is for eldar, though it think everyone would love it... let me know if i can post them here...peace

[SteveD CB500F]

See Glenn's response to a similar copyright questions here:

http://www.sohc4.us/forums/index.php?topic=2414.0

A members only section that is not viewable by the general public should suffice - he hasn't done it yet though...

Glenn?

[Glenn Stauffer]

As far as copyright is concerned with respect to old magazine articles, I tend to take a loose view of the issue.  We're not denying anyone profits from their works since most of these articles are not available for sale anymore - many times the magazines no longer exist and the articles themselves are not available in reprints.  I have lots of articles from old magazines posted on the website and have for many years.  No one has ever approached me to remove any of this material.

You can see the article index for the 750's here: http://sohc4.net/?q=node/87

As for a member's only section, I was hoping to do that for the service and owners manuals.  Unfortunately, right now, this forum software won't handle the scans of the manuals since they are too big.  Still working on a good solution there.

--Glenn

[Glenn Stauffer]

As a follow-up to this regarding the best format for old magazine articles.  If you can scan them, do it to a pdf file.  I will take that and convert it to a standard html file so the article is easier to read online and takes up much less space on the server.  If you know computers and have an OCR software application that works with your scanner, that is what I use to extract the text and images from the articles.  I only put articles up in the original format (as a pdf file) if the formatting really matters.

--Glenn

[Midlife750]

Hi all,

    I have a problem getting my carbs in synch on my 76 CB750F...The carbs are the later type with the rectangular shaped tops.  It appears all the slides are down as far as they can go, but the main idle screw (the big knurled wheel on the right, as viewed from the top...) is backed out the whole way and the idle will not drop any more. The two cables have enough slack in them to see that they are not holding it open in any way either. Someone told me the carbs main shaft may not be together right... I got the bike and rebuilt the rack of carbs, but did not bust them down to individual carbs. I seen a 10mm locknut on the top with an allen wrench opening in the top near the center carb, where the assembly is the cables attach to... is this the set screw that allows the carbs to be set together? Also, I'm having intermittant bowl/overflow leaks from all 4 of them... very strange. The timing chain, points and timing adjustment are good and right on the mark with a timing light. It seems to be a hard starting bike too... I had to use the pod type filters since the original air box was missing when I got it. Some have suggested they are not restrictive enough to create a strong vacuum signal to pull the fuel up the tubes. Should I switch to the old style, round tops carbs?? I'm goint to remove the rack and have another look at the linkage...Any answers, pointers or tips you may have would be great! I'd like to get this bike on the road.. if only at least for the few weeks left of summer.

Mike

[bowhunter]

Hi all,

    I have a problem getting my carbs in synch on my 76 CB750F...The carbs are the later type with the rectangular shaped tops.  It appears all the slides are down as far as they can go, but the main idle screw (the big knurled wheel on the right, as viewed from the top...) is backed out the whole way and the idle will not drop any more. The two cables have enough slack in them to see that they are not holding it open in any way either. Someone told me the carbs main shaft may not be together right... I got the bike and rebuilt the rack of carbs, but did not bust them down to individual carbs. I seen a 10mm locknut on the top with an allen wrench opening in the top near the center carb, where the assembly is the cables attach to... is this the set screw that allows the carbs to be set together? Also, I'm having intermittant bowl/overflow leaks from all 4 of them... very strange. The timing chain, points and timing adjustment are good and right on the mark with a timing light. It seems to be a hard starting bike too... I had to use the pod type filters since the original air box was missing when I got it. Some have suggested they are not restrictive enough to create a strong vacuum signal to pull the fuel up the tubes. Should I switch to the old style, round tops carbs?? I'm goint to remove the rack and have another look at the linkage...Any answers, pointers or tips you may have would be great! I'd like to get this bike on the road.. if only at least for the few weeks left of summer.

Mike

The 10 mm nut is the locknut for the high speed idle. The screw inside  the locknut sets the high speed idle when the choke is closed. If that screw is making contact with the plate it rides on, that would cause the high idle. Your carbs are from a 77 or 78. Loosen the locknut, and back off the screw completely. The tank will have to be off to do it right. With the screw backed off, set
your idle to 1100 RPM or so on the main idle screw with the engine warm. When everything is running smoothlly, shut off the engine, pull out the choke, then screw in the set screw so it just touches the cam it rides on. After it touches the cam, add another turn and a half or two. On full choke, and a cold engine, it should idle between 2 and 3 grand. You may have to do this a couple times over a couple days to get it right. DAMHIKT.

    As far as the leaking overflows. more than likely caused by rust clogging the float valves. It doesn't take much.Most of the times that it's happened to me, I never found the crud that caused the problem! Install an in-line fuel filter from the auto parts store. The so-called filter in your tank is likely gone, or broken. Mine was(gone). While you're cleaning your float valves. double check your float level. Check the manual. I believe it's about 14 mm with the valve closed and just touching the float needle. It's pretty critical on the SOHC4's Affects almost everything on the carburation of the machines.

Bowhunte

[motodude]

OK, I've finally, finally completed my two-year resto of my 350F and everything is back together.  All that's left is to sync the carbies.  My question is, should I do this with the airbox ON or OFF?   

now I gotta go locate a synching  gauge set somewhere....

thanks!

[Buffo]

Do all of your carb adjustments with the airbox on.

[SteveD CB500F]

Get a Morgan Carbtune II - they're the business.  They appear on eBay all the time if you don't want a new one.

Search this forum for "Carbtune" to see other's views on these vs guages.

[MikeDeB]

Get a Morgan Carbtune II - they're the business.  They appear on eBay all the time if you don't want a new one.

Search this forum for "Carbtune" to see other's views on these vs guages.

+1 on the Morgan Carbtune II.  I have one and love it.   

[sradja]

I am syncing my friends 1977 cb550 carbs. I am syncing 1,3,4 to the number 2 carb. I am using a merc synchronizer and the number 1 carb is not giving any reading at all. The tool is working fine...I just did my 750 2 days ago. I unscrewed the bolt at the bottom of the bowl to see if there was fuel and fuel began to leak out of the over flow tube. My friend assured me he had cleaned the jets, etc. Should I take his word for it or can there be something else causing this problem of no reading for number 1 carb.

[Bob Wessner]

Have you tried switching the offending vacuum line to a different carb to see what happens? Any chance, if there is a restrictor in the vacuum line it has become blocked? Just a couple of off the wall thoughts.

[TwoTired]

The vacuum is caused by the intake stroke of the cylinder.
No vacuum reading can be caused by the cylinder not sucking due to a mechanical issue with valves or pistons, the slide in that carb is stuck wide open, or a malfunction of the test aparatus.

Cheers,

[Jonesy]

Bob Wessner was kind to lend me his Carbtune to try out on my bike. I really like it. Thanks, Bob!

However, when balancing the carbs I noticed some things. I have the 750 K1-on carb setup (All mounted on the common linkage) I'd get it synced, but every time I'd blip the throttle it'd read a little different. If I gently pressed on the adjusters the reading would change, even though the nut was tightened. Could the springs in the adjustment collars have weakened or gummed-up? Could the plastic parts be worn where the linkage acts on them? Also, when it's balanced and "set", after I put the little dust boots back over the adjusters, it reads out of sync. I remove them and it's back in sync. I checked and the boots aren't being pinched by the carb linkages. Any ideas about this wierd behavior?

[Cvillechopper]

Have you rebuilt the carbs?  Sounds to me like you've got some O-rings letting air get by that shouldn't be, but I'm far from a carb guy.  Bob W. will know

[Jonesy]

Yup. All new gaskets, carb tops, etc. No stone was left unturned!

[heffay]

dp~
after the blip of the throttle do they change to a consistent unsync meaning, they are synched... blip the throttle... 1 same 2 same 3 same 4 a bit out of synch... OR...
blip the throttle... and it all goes haywire... differently every time?

if it is the latter... i would say that the test guage was not made for what you are trying to do.

if it is the first... you f 'd up somewhere

p.s. your boots leak

[Bob Wessner]

Dan,

Very strange indeed. Not much help since I have never seen or worked on later carbs. Your comment about the sync changing after you slipped the boots down has me curious. I never checked mine again after doing that...hmm?

[Bob Wessner]

"Also, when it's balanced and "set", after I put the little dust boots back over the adjusters, it reads out of sync. I remove them and it's back in sync."

Was thinking about this some more and trying to translate to the older carbs on my bike. Just curious, when you put the boots on and you say they were out of sync again, were they out of sync with a higher vacuum or were the readings all over, such as some higher, some lower than before putting the boots back on?

[Jonesy]

dp~
after the blip of the throttle do they change to a consistent unsync meaning, they are synched... blip the throttle... 1 same 2 same 3 same 4 a bit out of synch... OR...
blip the throttle... and it all goes haywire... differently every time?

if it is the latter... i would say that the test guage was not made for what you are trying to do.

if it is the first... you f 'd up somewhere

p.s. your boots leak

Heff- After a blip, one carb will kind of wander a tiny bit. I think the Morgan Carbtune was meant for carb syncing....

By boots, you mean manifold boots? Well, they're brand new with new clamps. Spraying stuff on them doesn't change any running quality.

"Also, when it's balanced and "set", after I put the little dust boots back over the adjusters, it reads out of sync. I remove them and it's back in sync."

Was thinking about this some more and trying to translate to the older carbs on my bike. Just curious, when you put the boots on and you say they were out of sync again, were they out of sync with a higher vacuum or were the readings all over, such as some higher, some lower than before putting the boots back on?

Bob, the later carbs have a boxy rubber cover that fits over the end of the carb slide rods. Over time they harden, split and fall off. Lots of bike have them missing. I'm wondering if there is some slop in the adjustment nuts. They are split into two pieces, and the pieces are spring-loaded to tightly grasp the throttle rack, but allow for movement. I'm wondering of the new, stretchy dust boots are pushing the adjusters tighter against the throttle rack, changing the throttle slide position. Put the boot on and the reading would climb up higher than the others.

[jsaab2748]

If you take the top off a carb and pull the throttle sleeve/adjuater rod assy out, you will notice a clip and a spring on the bottom of the rod (where those two screws are). The clip has sort of a forked clam shell shape, and has a spring sandwiched  between it's halves. Make certain that the spring has a slight preload on it, (by bending the top half of the clip down slightly, and paralell to the bottom half, if necessary) If the spring is not preloaded (loose inside the clip) and the clip has been bent up somehow, there will be "SLOP" in the throttle rod assy, I.E. the rod moves a little before the sleeve begins to move. This may cause what you're describing. I've seen the clips get bent from previous owners trying to force the throttle open when the carbs are stuck. I believe the correct name for the rod/spring/clip is the "lifter assembly"(?) and the spring and clip act as a dampner. A shadetree explanation I know, Hope I didn't confuse

[jsaab2748]

Just noticed your diagram,the clip i referred to is called the needle set plate. See if it's bent upward.