Topic: maintenence question

[Tom.Walsh]

Hey I just pulled my carbs off and got them all cleaned up and now I'm waiting on a gasket kit.  I was wondering if therey was anything I could do to the bike with the carbs off to make it quicker to get the carbs synced and get back on the road.  Thanks alot.

[1timduke]

You could do a search for a bench sync thread.   It's relatively easy, but not a substitute for the real vacuum sync, it'll get you close though if the balance was off.

-Tim

[Alan F.]

Welcome Tom, what bike have you got there?  There are usually a ton of things to take care of before you synch your carbs (often it's the last step).

New plugs, plug wire caps, new points, set point gaps, igniting timing and dwell, set cam chain tension, set valve clearances....

After reassembling your carbs there are a few things like setting correct float height that you'll want to do before you'd want to break out the gauges or mercury stix...

A 'bench-synch' is pretty common to ballpark the carb slide positions in relation to the reference carb (one isn't adjustable, it's the one the other 3 are set to match)

Run a search, do tons of reading and you'll have a working knowledge of the processes involved in no time.

and please be sure to add the year and model of your bike to your signature line so someone doesn't ask every time you post a question...

Cheers, Good Luck, and welcome.
-Alan

[Johnie]

The FAQ area for carbs is great.  That is were this came from:

I need to sync my carbs but have no vaccum meter. How do I bench-sync?
When I rebuild a set of carbs, I always bench sync them before I re-intall them. Basically, I use a 1/8" drill bit as a gage. Here's how you do it: Adjust the idle screw (the main one) until the 1 carb outside of it is open enough to just let the 1/8" drill bit through. Then adjust the _sync_ screws on the other 3 carbs until the drill bit just fits them as well. You'll probably want to close the idle screw before you bolt them back on the bike, 1/8" openeing is pretty big. This isn't a perfect sync, as there are other things that will affect the airflow to a given cylinder, but it will get you pretty close.

On a side note...I use a paper clip as I found the 1/8th drill bit was to large.  And make sure you measure on the flat side of the slides and not the cut out area.

[OldSchool_IsCool]

Welcome Tom!

Here is a second method to sync carbs.  I find it less error prone.

To syncronize your carbs while they are off the bike, use the flashlight method.  Take all four caps off the tops of the carbs.  You see that #2 does not have and adjustment screw.  #2 is your reference carb.  Place a flashlight in the inlet throat of #2.  Now adjust your idle speed screw until you see the light fade to nothing on the outlet side.  Once you have that set, adjust carbs 1,3 & 4 so that they too are set to the point where the light fades to nothing.  You are now ready to reinstall the carb bank.
 

[Alan F.]

OOOH!   I like the flashlight method, why is this the first I have heard of it?

Good tip OS_IC!

[OldSchool_IsCool]

OOOH!   I like the flashlight method, why is this the first I have heard of it?

Good tip OS_IC!

TwoTired mentioned it in passing once, he is the inspiration.  He didn't describe the process, so I'm filling in some detail.  I've synced 2 banks this way so far and they were amazingly close to synced once I hooked up my vacuum guages.  That may also be due to being a bit OCD about valve lash adjustments.  I think what the flashlight method assures is that all four sliders "bottom out" identically rather then relying on a measured offset.

[JBMorse]

If you get really bored try the soda blasting idea in the carb FAQ section.  I did it to mine and it worked really well.  The carbs don't look great! 

[Alan F.]

TwoTired mentioned it in passing once, he is the inspiration.  He didn't describe the process, so I'm filling in some detail.  I've synced 2 banks this way so far and they were amazingly close to synced once I hooked up my vacuum guages.  That may also be due to being a bit OCD about valve lash adjustments.  I think what the flashlight method assures is that all four sliders "bottom out" identically rather then relying on a measured offset.

Very cool, maybe a little write up in the Tips and Tricks section would be easier for me to find later on...?

[OldSchool_IsCool]

Very cool, maybe a little write up in the Tips and Tricks section would be easier for me to find later on...?

Sounds like a plan!  Maybe with photos too.  But I would like to get TT's comments on the procedure first, just in case false assumptions were made.

Oh Lloyd!! 

[Tom.Walsh]

I have a 78 cb750.  And thanks for the info but I was also kind of curious on where to find the information on how to perform that maintenance.  I've been looking around and now that I have the processes I should do I might be able to find them.  Thanks again.

[TwoTired]

Very cool, maybe a little write up in the Tips and Tricks section would be easier for me to find later on...?

Sounds like a plan!  Maybe with photos too.  But I would like to get TT's comments on the procedure first, just in case false assumptions were made.

Oh Lloyd!! 

Yeah, I wrote it up a few times in various post responses.  But, it never made it to the FAQ.  It seems drill bits are more popular than light sources. 

What you described is very close to the method I've been using since the 90's.  Since I have a vacuum gauge set, I only need it to get close, and the light method does that just fine.  Even on the earlier carbs, you just pick the slide that is closest to the bottom of travel, and turn the main idle knob out till the light "just" shuts off. (newer carbs would use the non adjustable #2 slide as the reference.)  Then you leave the idle knob untouched while you adjust the other three carb's slide travel adjustment 'till the light *just* shuts off for each slide in turn.  This results in all the carb slides being quite close mechanically, and retains the ability the fully close each slide.  This is important with the earlier carbs as all the slide adjustments can be inline but skewed so the sides can't fully close.  Then the bike won't idle at a low enough speed, and you have to start over again.

With the light method, you can shut the slides off as well as the engine, so you just make a guess at turning the idle knob to open the slides some, so that the bike will start.  Then crank it back to correct idle speed, and do the vacuum sync.  It will be close.  And, it will be real close if each cylinder actually has equal volumetric efficiency.  But, that is what the vacuum sync is for, to compensate for individual cylinder differences in breathing ability.

In my early days of carb syncing, I got the same problem that many have had with the drill bit method.  All the slide adjustments got skewed so the bike would no longer get down to idle speed, regardless of idle knob position.  Had to start the sync process all over after giving each slide a full turn tweak, then pick one and adjust the others to that one.  I've never had the idle issue using the light method.

Lately, I've just been using the a little instrument bulb soldered to two wires and connected to a 12 V battery.  With the carbs intake side sitting on the bench, you just slip the bulb in the carb mouth on the bench.  It's the only light that can get through, that way, giving high contrast. And, both hands are free make any needed adjustments.

Thanks, for the credit, OldSchool_IsCool!

Cheers,