Topic: Vacuum Test Carburation

[rddcw]

Took my 74' CB 750 K4 out for test ride this morning. Was idling well this morning. Set it at about 1200 rpm's. After it I rode for about a mile, the idling stuck at around 2500 rpm. Didn't want to return back to the original setting ?  So I rode the bike for about 30 miles or so, then returned home and re-set idling as before.

The only thing I noticed that when you would blip the throttle, it was now slow to return to idle. And as it was idling, it became quieter when you pulled in the clutch lever.

So without really knowing the state of tune that the bike is in, it probably is in need for a good tune-up even though the bike ran well during the test run.

I've rebuild carbs before and set them according to Factory Service Manual but have never purchased a good set of vacuum gauges.

So my question is without pulling the carbs. off first, would synchronizing the carbs. be the first step in resolving the idling problems?

I've see different sets of vacuum  gauges out there, some like the old mercury manometers or some with 4 individual gauges. Which one do you guys like best and use?

[mrbreeze]

Classic sign of carbs out of synch. You could synch them but may be a waste of time if you don't address the other tune up stuff first. The carb synch should be the last part of your tune up. I use the Morgan Carb Tune. It has metal rods in tubes instead of vac gauges or merc sticks.

[BobbyR]

There are some things to do before you synch the carbs.

Adjust the valves.
Adjust the points and timing.
New Plugs.
Then you can synch with gauges or sticks.

[cooldrum]

I ride a '74 K4 and I agree with those steps before carb sync. Adjust valves, replace points, condensers, plugs. Check timing and then sync.  You'll be amazed how your bike runs. It's easy to chase the source and not consider the other components that affect how your motor runs. good luck!

[rddcw]

I'll start Fresh from Step 1, but still like to know which Carb. Synch Tool that most of you guys use before I purchase a set?

[DH]

Sometimes the dust boots on top of the carb adjusters aren't seated in the groove provided, and
the lower portion of the boot gets caught under the throttle shaft, holding the throttle valve of the affected
carb open more than the others. When it happens, it has the same effect as an out of sync carb.
Make sure the boots are not torn, or mis shaped, and seated in their proper grooves

[rddcw]

I'll check for that too and appreciate the input.

Still waiting for consensus which Carb. Synch Tool to use?

[Davez134]

Morgan carb tune is great. I use this one http://www.amazon.com/Vacuum-Carburetor-Synchronizer-Carb-Gauge/dp/B004MSJ7E6

On the one I use you just have to make sure the damper valves are closed to restrict the needle movement. Whatever one you use, just make sure the rest of your tune up is complete first, as each step effects the next. There are other usual suspects for the symptoms you're describing. Carb synch does not have to be "exact" just very close, which either style gauge can help you accomplish.

[Stev-o]

I use Honda Factory gauges, pretty rare tool to find these days.

[flybox1]

 :PDitch the bottles and invest in (dial) vacuum gauges you can actually calibrate.

[hotdog]

I use Honda Factory gauges, pretty rare tool to find these days.

Yes they are Steve, tried to bid on a set on u.s. eBay once... Bidding went through the roof.

I have just ordered a set of Morgan carbtunes also after using gauges for a long time - hopefully they should be here soon.

Instagram - Lumpy Cam Motorcycles

FB - Lumpy Cam Motorcycles

[rddcw]

Dave,  I saw those Gauges on Amazon and ebay as well. There is a vender on ebay that also sell the gauge kit with an external fuel container w/shut-off valve which would be a nice feature to have.

I saw this Carb. Synch Tool also. Would this be the right tool to purchase or is it even necessary?

[oldhatt45]

rddcw,

Here are a few choices for Gauges.  I'm sure there are many others, but this is just a quick search.

http://www.ebay.com/itm/New-Vacuum-Carburetor-Synchronizer-carb-sync-Gauge-CB-GS-KZ-550-650-750-850-/380985091328

http://www.ebay.com/itm/Vacuum-Carburetor-Synchronizer-carb-sync-Gauge-Yamaha-XS-XJ-1100-1200-/380901309371

http://www.ebay.com/itm/Vacuum-Gauge-4-Carb-Carburetor-Carburetter-Synchronizer-Set-kit-Hoses-Extensions-/321695652853

http://www.ebay.com/itm/Fuel-Pressure-Carburetor-Vacuum-Synchronizer-Carb-Sync-Gauges-CB-KZ-550-750-650-/171928887016

Key things to look for in a Gauge set are the extension tubes that screw into the carbs.  The inner carbs require a longer extension than the outer carbs.  Also, since these carbs show more of a "pulsing" type of flow, they tend to cause the needles on the gauges to flutter, so you want gauges that have adjustable dampers that will allow you to dampen the flutter.  The damper could be as simple as an adjustable clamp.

Anyhow, have fun doing the sync'ing.  Once you get the idea, it will actually be fun.

Oh, and in case you don't know this, when you are doing the sync, you don't really care what  the absolute reading is.  You are interested in just getting all 4 carbs reading as close to the same as possible.

Hope this helps at least a little bit.

Charlie

[Stev-o]

Good advice Charlie, I can tell you are a Pro!

[oldhatt45]

Stev-o,

No, just usually the guy called in at the last minute to share the blame!!!!!     
And I wouldn't be half as knowledgeable without the help of all those on the forum that have gone before me!!!!
Especially guys like YOU!!!!!  And I mean that sincerely.

Knowledge not shared is not knowledge. 

Charlie

[rddcw]

Charlie,  This set from ebay looks nice and includes the fuel container. Priced at around $90.00 delivered. From the same vender as the first 2 choices you gave. I think the gauges may be better on the 2nd choice from looking at the pictures but don't know?

[whizzer]

The carb sync screwdriver / holder tool is worth the money, much easier than fiddling with Box end and screwdriver. Also helps if replace the brass tunes with permanent set that beg sealed with little rubber caps. Taking the screws off and screwing in tunes each time is asking for cross thread if not careful. Got both from Motion Pro. 

[oldhatt45]

rddcw,

The set you show a pic of looks good.  I see the brass stems so that's good.
Do not see the damper valves.  Also do not see the rubber tubing that goes between the brass tubes and the gauges.
The Auxiliary tank is a Definite YES!  I have the same exact one (bought separately.  LOL)

I would ask the seller those questions about the dampers and tubing. 
Other than that, I don't think you can go too far wrong with the one in the picture.

Charlie

[rddcw]

http://www.ebay.com/itm/380985151428?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Charlie,  This should be the link to the ebay listing.  So I'm not sure it is a complete kit?

[oldhatt45]

rddcw,

The link gives the description of what you are buying.  It includes the damping valves and the tubing for the gauges.

If it were me, I would have no problem purchasing this particular unit.
From the description it does include everything you need, including the Auxiliary tank with a shut off valve!

Charlie

[rddcw]

Thanks Charlie,  Purchased!  Look forward receiving it and get this bike running right 

[Davez134]

Dave,  I saw those Gauges on Amazon and ebay as well. There is a vender on ebay that also sell the gauge kit with an external fuel container w/shut-off valve which would be a nice feature to have.

I saw this Carb. Synch Tool also. Would this be the right tool to purchase or is it even necessary?

Sorry for the later response, just saw this. I have no idea what that is and have never used it, so I guess the answer is no it's not necessary haha

The synch kit you got will be good, I have the same gauges. One thing I did before synchronization was hook each gauge, one at a time obviously, to the same cylinder just to make sure they are all functioning properly and giving the same reading.

[rddcw]

Dave and Charlie,  I did see that the gauges register up to 38 hg but I only see on the gauge pictured in the listing at 30 hg, so not sure there?

The Factory Service manual says that the standard vacuum reading should be between 20-22cmHG, so I guess I should be all right?

[hotdog]

I use Honda Factory gauges, pretty rare tool to find these days.

Yes they are Steve, tried to bid on a set on u.s. eBay once... Bidding went through the roof.

I have just ordered a set of Morgan carbtunes also after using gauges for a long time - hopefully they should be here soon.

Instagram - Lumpy Cam Motorcycles

FB - Lumpy Cam Motorcycles

These arrived today, have not used them yet. Just posting pics for the O.P. and for anyone else that might be looking...

You will see the adaptors are plastic screw-ins and are quite short, this won't be a problem for me as I already have a couple sets of motion pro brass screw-ins that make accessing the inner two cylinders a price of cake.
As mentioned it would pay to have the longer screw-ins to make the whole carb-syncing experience as painless as possible.
Also the bag pic - the dampner used with the Morgan carb-tune is a piece of clear hard plastic that has a small orifice through it, not sure if you'll see it in the pic..? Needs to be chopped into 4 and one piece inserted in each vacumn hose close to the carb end.











Instagram - Lumpy Cam Motorcycles

FB - Lumpy Cam Motorcycles

[jonda500]

Dave and Charlie,  I did see that the gauges register up to 38 hg but I only see on the gauge pictured in the listing at 30 hg, so not sure there?

The Factory Service manual says that the standard vacuum reading should be between 20-22cmHG, so I guess I should be all right?

I have the same ones - but mine didn't come with a case or auxillary tank.

As mentioned, don't worry about getting the right number, ignore the factory specs - all that matters is that they are all set the same.

Before you start unscrew the clear plastic covers off the guages and adjust the little screw on each one as required to set them all pointing exactly the same (actual reading again doesn't matter - I set mine all to zero)

The plastic dampeners I inserted in the lines just below the guages (you need to cut the lines) - screw the plastic adjusters all the way in before trying to push the rubber hoses on or they can fall out and you can spend the next 2 hours looking for it!   Once you have the engine going these need to be screwed out and then back in just enough to eliminate most of but not all of the needle fluctuation- if the needle is not moving back and forth slightly it is not giving you a true reading!