Author Topic: Help in syncing / adjusting carbs (Read 8361 times)

Kyytsis · « **on:** July 29, 2012, 10:14:09 PM »

Hi,

I recently bought a CB750 K7 which has been built in cafe style. It has pods on the original Keihin carbs.

The seller didn't adjust the carbs and when I got it it didn't have the screws in the sync ports and it ran bad. Now I tried to sync the carbs with a U-tube manometer but it is so far off it can not be done. Running the bike for just some seconds sucks the oil from the meter into the engine. Trying to adjust by listening to the engine gave mixed results - very high idle and banging noises you really don't want from an engine.

Any advice; if I remove the carbs from the bike for cleaning etc. can I sync them off-bike - at least get them close? Could I just measure the fixed settings of the fixed #2 and then adjust the others to that?

What settings would you recommend for the carbs, I don't know what jets there are, but assuming they are stock. Float height, needle...?

Never taken carburetors apart but it seems there is finite number on moving parts

Thanks!
Ville

ncstatecamp · « **Reply #1 on:** July 29, 2012, 10:25:29 PM »

I was scared the first time I took my carbs apart as well. Just for the first time only disassemble 1 so you can look at the others for reference. By the third you'll have it memorized, not really THAT many parts. You what to buy a tub of Chem-dip though, and if you don't have it a dremel. As for syncing I haven't done mine yet but I most likely will attempt to bench sync mine, top result on google had a good result last time I checked. One reason you may have had such a hard time was the pods. Yesterday I was talking to a Honda mechanic and he said he could sync and tune an air boxed carb in 45ish minutes but a pods'd carb anywhere from an hour to 6...

lucky · « **Reply #2 on:** July 30, 2012, 07:33:28 AM »

With no screws in the sync ports the engine would be running so lean from those massive air leaks it probably would not run. And yes it would make banging and popping noises.
WOW.

That is a 1977 version so the carbs also have the accelerator pump and nozzle system.
To see if that works remove the pods and look into the intakes with a bright LED flashlight with the engine off. Turn the throttle a couple times and see if gas squirts out of the brass nozzle sticking up in the opening of the carb.
The gas squirts towards the inside of the engine.
All four of those must work.

Since they were missing the sync port screws who knows what else is wrong with the carbs.

With pod air cleaners you will need to rejet the carbs. more later....
You will need to tell us it it has stock exhaust pipes.

Kyytsis · « **Reply #3 on:** July 30, 2012, 11:53:12 AM »

I noticed the missing screws on the sync ports when I got the bike home and replaced those. The interesting thing is that it had some kind of idle still, a real bad one, but still... But that just goes to show how screwed the carb settings must be as it should not run.

I'll check the nozzles, thanks for the tip!

The bike has 4-1 downpipes and some aftermarket muffler. See picture below..

Help appreciated, thanks!

Ville

liPPy · « **Reply #4 on:** July 30, 2012, 12:33:43 PM »

Nice looking bike!

Before you start playing around with re-jetting at least do a bench sync. It's straightforward enough. Search the site for details.

And while you have the carbs off take a look at your jets and compare to this page to see if you are running stock or not: http://www.motorcycleproject.com/motorcycle/text/specs.html

flybox1 · « **Reply #5 on:** July 30, 2012, 02:10:35 PM »

agreed ^
need to know what jets are in there, as well as make note of the needle clip position, and fuel mixture screw.
small changes here have a HUGE effect on the running/tuning of your bike. come back and let us know what you find.
clean everything and do a bench sync while you're in there...

nice looking bike BTW

lucky · « **Reply #6 on:** July 30, 2012, 02:21:59 PM »

The bench sync is not the same as the earlier models.
On this 1977 version just make sure all the slides are sitting on the bottom.
You know all the way down. IF you take the top off of the carbs
#1,#3,and #4 are adjustable So take your 8 mm wrench and loosen the nut and turn the adjuster with your screwdriver until the slide is sitting down all the way you will be able to tell there is no tension on it when it is all the way down.

Since the bike has pods and 4 into 1 exhaust I would set the clip on the needles on the SECOND groove down from the top. Not on the first groove like a stock bike.
At least that 77 still had adjustable needles.

#120 main jet
# 40 idle jet or #42 if the #40 wont work.

lucky · « **Reply #7 on:** July 30, 2012, 02:32:24 PM »

It is strange that this bike looks like it owned by someone that looks like they know how to work on the bike but to remove the sync screws and leave them out?

I wonder if it really is was sync screws??

Something seems off.

bjbuchanan · « **Reply #8 on:** July 30, 2012, 06:13:39 PM »

^^^I agree with lucky's last post. Put up a pic of the screws you are adjusting or atleast a little bit better description. The bike seems very gone through for such a little detail to be missing (and painfully obvious at that)

Kyytsis · « **Reply #9 on:** July 30, 2012, 10:42:04 PM »

Thanks for all the advice again!

I agree, the bike looks great, but when digging deeper it is a collection of "not-so-working-solutions". Like the foot controls which flipped over when changing to a bigger gear or clipons hitting the tank. It is sort of a version one and I'm now refining it to version II which should be better. Lastly, the bike was built a few years ago but it had not been ridden more than few hundred miles. I've put more distance on it during the month I've had it already...

Here are some more photos; first just showing the pod setup and the latter showing the missing screw I have already replaced. Looking to do bench sync at the end of the week.

Cheers,
Ville

lucky · « **Reply #10 on:** July 31, 2012, 10:19:27 AM »

Good thing you saw that missing screw or that piston could be history.
Good thing it was not ridden much either.

Kyytsis · « **Reply #11 on:** July 31, 2012, 11:09:36 AM »

To be on the safe side; is there any easy way to check the pistons? Compression test?

Kyytsis · « **Reply #12 on:** August 02, 2012, 11:44:37 AM »

Quote from: lucky on July 30, 2012, 02:21:59 PM

The bench sync is not the same as the earlier models.
On this 1977 version just make sure all the slides are sitting on the bottom.
You know all the way down.

The carbs are now off the bike. Didn't do much to them yet but looking at them shows that the slides are not closed (not even close when the throttle is closed). All of them look pretty evenly open, but much more open than I'd guessed. I'll post a picture tomorrow when I get a chance. Is there any case that would make the #2 to also be incorrectly set, even when that slide is not adjustable...?

Quote from: lucky on July 30, 2012, 07:33:28 AM

That is a 1977 version so the carbs also have the accelerator pump and nozzle system.
To see if that works remove the pods and look into the intakes with a bright LED flashlight with the engine off. Turn the throttle a couple times and see if gas squirts out of the brass nozzle sticking up in the opening of the carb.
The gas squirts towards the inside of the engine.
All four of those must work.

They all work - that at least is good news.
I was little surprised when I had the carbs off. I had thought that the throttle controls the butterfly valve as well, but it seems that it is just connected to the choke? Or is there a mechanism that also operates the butterfly with the throttle I didn't notice? When doing a bench sync I'm looking a the slides, right? Do I need to sync the butterflies? When throttle is closed, what is the correct position of butterflies.. they say there is not a dum question.. $:-\$ I've worked mostly with bikes that do not have carburetors...

Thanks!
Ville

lucky · « **Reply #13 on:** August 02, 2012, 12:32:04 PM »

Quote from: Kyytsis on August 02, 2012, 11:44:37 AM

Quote from: lucky on July 30, 2012, 02:21:59 PM
The bench sync is not the same as the earlier models.
On this 1977 version just make sure all the slides are sitting on the bottom.
You know all the way down.

The carbs are now off the bike. Didn't do much to them yet but looking at them shows that the slides are not closed (not even close when the throttle is closed). All of them look pretty evenly open, but much more open than I'd guessed. I'll post a picture tomorrow when I get a chance. Is there any case that would make the #2 to also be incorrectly set, even when that slide is not adjustable...?

Quote from: lucky on July 30, 2012, 07:33:28 AM
That is a 1977 version so the carbs also have the accelerator pump and nozzle system.
To see if that works remove the pods and look into the intakes with a bright LED flashlight with the engine off. Turn the throttle a couple times and see if gas squirts out of the brass nozzle sticking up in the opening of the carb.
The gas squirts towards the inside of the engine.
All four of those must work.

They all work - that at least is good news.
I was little surprised when I had the carbs off. I had thought that the throttle controls the butterfly valve as well, but it seems that it is just connected to the choke? Or is there a mechanism that also operates the butterfly with the throttle I didn't notice? When doing a bench sync I'm looking a the slides, right? Do I need to sync the butterflies? When throttle is closed, what is the correct position of butterflies.. they say there is not a dum question.. $:-\$ I've worked mostly with bikes that do not have carburetors...

Thanks!
Ville

Since the #2 carb is the non adjustable slide carb the only thing that will keep the slide up is the IDLE screw.

Make sure it is backed off all the way.
If you have calipers you could measure the other slides the same as #2 this way.

Take off the top cover of all of the slides.
.007thousandths WILL make a difference. Measure carefully.

The using a vernier caliper depth measurement measure from the top edge of the slide to the top edge of the carb. Write that down.
Now get all of the other slides to match. by makeing that same measurement and adjust accordingly with that 8mm adjustment nut and the screw on each carb.
I will post pics.
Notice the 2nd photo down (the nut and adjuster screw)

The butterflies are chokes. Do not mess with them. Do not take the shafts and springs apart. Another member of this forum just took it apart and could not get them back together again.

I would not even do it unless I absolutely had to. No reason. Just clean and blow dry.

Click once on the photo for a larger version.

lucky · « **Reply #14 on:** August 02, 2012, 12:43:07 PM »

Leave the carbs together . Do not take them all apart. No reason too.

Kyytsis · « **Reply #15 on:** August 02, 2012, 12:59:59 PM »

Lucky, Thanks!

Ville

Kyytsis · « **Reply #16 on:** August 04, 2012, 03:12:51 AM »

Some progress last night...

I first removed the tops and did a bench sync with a drill bit. I did this by first putting the bit between the #2 slide and carb housing and then adjusted other slides to match that opening. They were clearly off, now they are aligned.

Next I removed the float bowls and was surprised how clean everything was.

I removed the floats, checked the float valves, and removed the jet and cleaned all the holes. The one in the center says 125 - is that the main jet? The top one says 20, that sounds really small if that is the slow jet, I thought it should be like #40?

I opened and cleaned the fuel screws and then set them to 2.5 turns from fully closed.

The next thing I'm guessing I should do is to remove the slides and the needle through the top of the carb? That means un-hooking them from the adjustment system...so I get to do the sync again. I looks quite simle, removing a spring and then pushing the connecting "rod" to the side and then lifting the slide needle assembly?

To which groove should I put the needle?

Thanks!
Ville

Kyytsis · « **Reply #17 on:** August 04, 2012, 03:37:36 AM »

Just realized that the slow jet is the one between the fuel screw and main jet. I didn't see an easy way to remove it (guessing it is pressed in?) - I did learn from this site that if you remove them using pliers you could have hard time putting them back?

I'm suspecting that there are some blocked holes in these, so they would need to be cleaned...

Next time I'll be in the garage WITH laptop. Reading this: http://forums.sohc4.net/index.php?topic=48858.0

and this:
http://honda-cb750-s.456789.n3.nabble.com/How-to-1978-CB750-slide-needle-shims-td3688760.html

Ville

bollingball · « **Reply #18 on:** August 04, 2012, 04:10:07 AM »

You really should pull the slow jets. Just twist and pull. You can wrap them with a piece of leather if you want to I don't. When you get them out clean out all the holes and the passage way that the jet was in. When you get the first one out you may see a witness mark just gently tap it back in I use a hardwood dowel and very small tack hammer. Don't forget to check the size. I don't have pods so listen to lucky for that.
Ken
PS here is a slow jet

bollingball · « **Reply #19 on:** August 04, 2012, 04:13:16 AM »

If you click on the photo you will see that taper. that is what it seat on when tapped back in.

Ken

dave500 · « **Reply #20 on:** August 04, 2012, 04:19:39 AM »

anyone really needs compressed air to truly clean carbs,some people dont have air and just knock rubbish from one place to another with the best intentions,even with wire you must use compressed air theres no other way.

Kyytsis · « **Reply #21 on:** August 04, 2012, 04:21:50 AM »

Thanks Ken, will do that.

The only compressed air I've access to is either in a can or in a pump...

dave500 · « **Reply #22 on:** August 04, 2012, 04:28:08 AM »

if you can manage take the bodies to a fuel station that has a compressed air tyre pump,if its one of them gay dial up digital things set it as high as it will go,youll need to make up an adaptor to let you blast air though or something,a can wont do the pressure,nor will blowing hard by mouth,,i think this is where guys end up pulling carbs over and again because they arent truely blown through,,its the only way full stop,or visit a local workshop with all the parts ready and they might let you use the air?

dave500 · « **Reply #23 on:** August 04, 2012, 04:43:51 AM »

i now wonder how many of the past carb problem threads relate to the fact that who ever didnt have handy compressed air?its rarely asked "do you have an air compressor?"we all just assume that we all have an air compressor?yeah i cleaned the carbs real good dosent mean #$%* in that sort of case?

Kyytsis · « **Reply #24 on:** August 04, 2012, 04:48:33 AM »

Good point. I'll ask around to find a compressor...

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Author Topic: Help in syncing / adjusting carbs (Read 8361 times)

bollingball

bollingball