Topic: Vacuum Test Carburation

[Davez134]

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?

Those are fine. Incriments are in inches as well, so convert that to cm, and you'll see there is plenty.

[rddcw]

Should of looked at the measurement on the gauges closer. So 20-22 cm would equal 7.87-8.66 inches, plenty of gauge reading.

[oldhatt45]

rddcw,

As the other guys have said, the absolute reading doesn't matter a lot.  Just so that all gauges are reading the same. 
Before you try to sync the carbs, connect each gauge to the same cylinder and make sure it reads the same as the other gauges, that way you are sure that the sync is really "sync'd".    Like jonda500 said, you can adjust the individual gauges when you are checking the calibration of the gauges.

Just take your time and it will come out right. 

Charlie

[strynboen]

and use a blower /ventilator to cool the engine..it often takes longer time to do ..first time trying it..and the engine runs hot in few minuts..

[rddcw]

Looking over my Factory Service Manual and reading the supplement to CB750K1-K4, these stock Carbs. on my CB750 K4 appear to have the synch adj. on the tops of the carbs. It made mention to make sure that the threaded rods are extending at least on thread above the lock nut. In my case looking at the outer carbs. 1 and 4, there is no rod length showing? And without removing the dust boots on the 2 inboard carbs, I'm assuming the same

Also, the clearance or gap between the top of the carb. and adjuster screw is 1.8-4.8mm which if my micrometers are held right, give me a measurement of 4.8mm ( #4 carb ) and 5.5mm ( #1 carb ). Of course this is when the throttle is closed.

So, if I'm to read these instructions correctly, does all of this have to be performed before you even start to such the carbs with vacuum gauges?

Just trying to do my homework/research before getting started.

[BobbyR]

One of the joys of working on these bikes is that they have had previous owners that had their own style of expertise.  In other words....trust nothing.

Make sure your slides are facing the correct way which is a common problem. On my K8 the #2 is  the reference carb and is not adjustable.

Once you get the guages hooked up you will see what is up. Putting a fan on the motor is a must. This can take longer than you would expect. Each time you ajust one carb the others change, you go back and forth until they are all even.
 

[rddcw]

The person I purchased the bike from was kind of a restorer who basically purchased the bikes and flipped them. He had purchased the bike from a mechanic and said was maintained well. But after receiving it, there was a lot of things I went over and re-done. But not unusual, I try to be fussy and like things nice.

The bike runs fairly well but appears to have a a carb. synch problem consequently requiring me to purchase the tools I need to check them

I'm just trying to understand the Shop Manual procedures first and see what has to checked and adjusted.

[mrbreeze]

One thing to remember is that if you don't mind doing the work every tool you buy will easily pay for itself as opposed to giving that money to pay for dealership labor that could be performed by a tech that doesn't really know much about older bikes. If you plan on keeping the bike for any amount of time the tools will be used again in the future. You may even make a few bucks on the side working on other people's machines.

[oldhatt45]

And to add to mrbreeze's statement. 
The Gauge set I have is 40+ years old.  Bought when I had my Honda CL450.
Point is, tools don't go bad.  LOL

Charlie

[Stev-o]

He had purchased the bike from a mechanic and said was maintained well. But after receiving it, there was a lot of things I went over and re-done.

Dont ever believe the seller!

[754]

2 things ..
 Hondaman has an excellent tip in his book... Run the slids all the way up and hold open.
 Then loosen the knurled nut on top of the carb, and let the top plate center itself , over the rod .
 Not doing so , or having a bent rod, can affect the ability to return to idle.
 
 The other, if carbs are all avways off the final setting zone.. Rod synching, may speed up the process.....getting them all pretty close.

[rddcw]

Charlie,  Brings back memories. When I first starting riding legally, I purchased a new Red and White 70' CB 350. Cost me $627.50. My Dad had a Silver 69' CL450 Scrambler.  Then in 72' he purchased a 69' CB750 from a School Teacher where we lived who never could get it to run right. Don't know if it was Sandcast or not, but Dad re-painted it Black and Blue not knowing the value of those bikes back then. I guess that's why I bought this 74' trying to recapture some of the memories when I was younger.

I'll try that Tip on centering the top plate over the rod. I'm assuming you mean hold open the throttle and while holding it open, loosen the knurled nut. Is that about it?

[Garage_guy_chris]

Well guess what i did this afternoon. I was messing around with the carb sync and my homemade ATF nanometer. For anyone curios here is a video i did. I know a lot of you guys prefer the manufactured solutions but these work quite nice. I made a real simple 2ch one for my CB450 years ago when i was a broke college student and thought i would try a 4ch one. 

http://vid3.photobucket.com/albums/y61/corolla_pilot/20151020164448_zpsznmydlt1.mp4

[754]

Yes,  ...and if you have a bent rod, it will cause problems.. Not common.
 Just another thing to check..

[rddcw]

Thanks 754, I'm assuming you loosen the knurled top of the carb. slightly while holding open the throttle and the tension on the slide and adjustment rod by holding open the throttle will re-center these internal components of the carb. ?

Even though there is no visible threads of the rods showing on the carbs above the lock nut, should I make this adjustment too per Service Manual or does that really matter?

That carb. synch tool that I posted earlier in this Post will definitely not work with my carbs. Oh well, should of the studied the manual a little closer and see where the synch adjustment was made then realized it wouldn't work. Maybe I can use this too somewhere else. Have a 77' KZ1000A I picked up over a year ago and haven't carb synched it yet.

[oldhatt45]

rddcw,

Why won't the carb sync tool work???
How about a picture or 2 of the carbs.  Also, which carbs have you got?
That sync tool should work.

Charlie

[rddcw]

Charlie,  Correct me if I'm wrong, but the Carbs. that are on my 74' CB 750 K4 are the Stock Keihin carbs. The Vacuum synch adjustment is at the Top of the carb. where you have a 10mm locknut and 17mm adjusting nut underneath.

The Carb. tool has an 8mm socket.

[754]

Rddcw, yes correct on centering on the pull rods .
 If there is a spec for thread showing above locknut, i would get it close to that.

[oldhatt45]

rddcw,

There are several different versions of carbs on the 750's.  Yes they were all Keihin's. 
If you look at the top of the any of the carbs, you'll see a flat area.  On that flat area there will be a stamping that has a few numbers and possibly letters on it.  THe carbs on my K6 are (stamped) 086a (just for reference).

OK.  That explains why I was surprised.    I thought you were referring to the gauge set.  Duh. 

Yup, that one probably won't work.  LOL  It's just as easy to use a 17mm wrench and a 10mm wrench as opposed to the "special tool".     

And, listen to 754.  He provided you some good info.  The manual gives the approximate amount of thread that should be exposed.

Charlie

[rddcw]

And Thanks Again to All for the Tech Tips and Support !!!   Should be getting the vacuum gauge kit next week in the mail and will start the project then.