Hi everyone, I've been reading a lot about carb syncs, and I would like a more thorough understanding of what is happening. Just to make sure I'm doing everything correctly. I have a 1996 CBR600 F3.

I bought a twinmax carb synch, it can only analyze two carbs at a time. The hoses it came with do not fit snug around the motion pro carberator adapters. Will this be an issue?

The CBR manual says the third cylinder is the cylinder to be compared against. Does this mean the third cylinder will never need to be adjusted in any way?

By playing with the screws that adjust the carbs, is this the way we're able to make the bike run lean or rich?

Since I'm only comparing to one cylinder at a time, does it matter if the cylinder is currently in its intake stroke vs its exhaust stroke? Should all the cylinders be balanced to 0 when hitting the accelerator?

What is this carb synch measuring exactly?

Does the first adjustment screw adjust the first carb, second screw the second carb, and the third screw does the fourth carb?

Thanks for your time.

 

First off if I haven't said it already, welcome to the forum!

The hoses should fit snug into the adapters because a loose fit might leak and cause inaccurate readings.

I suppose it can be done with only two gauges however it might require some extra fiddling to get everything spot on.

No, synronizing the carbs does ot affect the air/fuel mixture, those screws are located in the bottom of the carbs and will require a special screwdriver with an angle gear and a special tool bit. The syncronizing screws are located more like between the carbs than in them.

The purpose of the carb syncing procedure is to get the same amount of "fill" into every cylinder, what you are adjusting precisely are the round brass colored "flaps" in the carb throats, and their relative position to their each other. The #3 cylinder does not need adjusting because it CANNOT be adjusted, it is the benchmark what the others are syncronized to.

Not sure what you mean by the stroke thing, when the carbs are syncronized the bike will be IDLING, otherwise you cannot do it, the gauges measure the airflow, or more precisely the vacuum pressure inside the carburettors and the intake throats. The bike should be warmed up enough to idle steady without the choke.


1=1, 2=2, and 3=4, BUT REMEMBER to FIRST adjust the second and only then the first because they are linked to each other, if you first adjust the #1, and then fiddle with the #2, the first one is no longer where it should be.

Hope this clears even something.

Oh! And when you're done, remember to remove the hose fittings and re-plug the holes or the bike will not run right due to the massive intake leak through the fittings.

 

Hey there,
You've asked a lot of good questions. I'll do my best to answer them.


With any engine that has multiple carburetors, you want them acting as one single unit. They need to deliver the proper fuel/air mixture to the cylinders at the same time. When you "sync" carbs, this is what you are doing, you are getting those multiple carbs to act as one. On our bikes this is accomplished by controlling the vacuum.


Having said that, a proper sync starts with the valves. If they aren't shimmed within tolerances, nothing will be right no matter what you do as they provide the starting point of the vacuum.


The process of syncing on our bikes is adjusting the butterfly valve on each carb so they draw air into the carbs so that the negative pressure will be equal. Due to the fact that no two parts are exactly the same and also due to the fact that parts wear, the physical position of the butterfly valves will be slightly different. It's the pressure you want to be equal.


When you make the tiniest tweak on one of your carbs, it directly affects the other three. Understanding that, while it may possible to use your twinmax set, I'm not sure how it would. You really need to see all four carbs at the same time. Somebody may chime in and tell you how to do it.


Since you are adjusting vacuum, you most likely have issues if your hoses don't have a totally snug fit. I'm guessing you be highly frustrated as you will be chasing your tail.


You are only comparing one cylinder at a time, and #3 is the master, but like I said, the smallest tweak on one affects the others. Depending who far off your carbs are, you may have to tweak them getting them in the ballpark, then adjust your idle screw on #3 to get you back down to your proper idle RPM, then start the whole process over.
The screw in between 1 and 2 adjusts 1. The screw in between 2 and 3 adjust 2. The screw in between 3 and 4 adjusts 4.


The actual number on the gauge doesn't matter. What's important is they are all the same. When you are finished, you want them the needle on each gauge, or the fluid if that's what you have to at the same level when idling and when you rev the engine.


Your engine does need to be warm to the point of the fan kicking on.


Also, if you have your carbs off the bike, it's best to start with a "bench sync" before you start the real sync. It's not required at all, it just makes the final sync easier.


The process is very much a back and forth process. Don't think you will be to turn a screw once and that will be it. You will be tweaking that screw multiple times.

 

both these dudes beat me to it with the solid explanations. might be worthwhile youtubing it to actually see and see someone talk you through whats actually happening. definitely more awkward just having the two bank vacuum sync. if its an analogue one remember to sync the needles together on the carb sync. they can get knocked out of alignment really easily.

anyway i've got an addition to make, its worthwhile leaving a length of silicone hose attached to the vacuum points on 2,3 & 4 cylinders, blanked off at one side to make future balances much easier. can't really do it with cylinder 1 as it has the easily accessible screw bit.

easy enough job, will take two or three times longer than expected but just take your time and don't get stressed with it.

 

I had nothing to offer...
So Ill leave this for inspiration.

 

Hey guys, thanks for the replies and welcome greetings.

So I started with synchronizing #3 and #2 cylinders. Then #3 and #1. Then #3 and #4. Went back to comparing #3 and #2 and it was WAY OFF.

So I started tweaking all of them again, just to end up with the same result. I know you guys said I'll have to do this multiple times. But I didn't expect it to be thrown off this much each time. Do you guys think this is normal and I'm progressing?

Also, the engine dies when I removed the carb synch machine. Is that normal? I was guessing because there's massive vacuum leaks since nothing was plugged in.

Also, since I'm only synchronizing two carbs at a time. Do you think I should plug up the other two carbs while synchronizing?

 

Very much so. You basically answered your own q. You'll never get them synced if you don't plug the ones you're not syncing cause now you are adjusting them to have a huge leak. A broken carb boot is enough to throw them off not to mention a hole in every throat. The system is very sensitive for leaks.

 

I think your other problem is that you are adjusting them all to #3, but it's not that simple.
One of those screws adjusts TWO valves at the same time. At least if it's like the F2 carb or my suzuki carb.
I know it sounds weird, but it's normal. For instance on the F2 carb, one of the screws on the outside adjusts just the valve for its carb, but the next one in adjusts the valve for it and the one next to it. But the other outside carb, next to the "base carb" it's screw only adjusts its carb.
So you need to know which screw is adjusting two valves at once.
If 3 is your base carb, then the screw for 4 likely only adjusts carb 4.
so screw 1 likely adjusts itself and screw 2 adjusts 1 and 2 at the same time.
So if you adjust 2 to the correct spot based on its correlation with 3, then you likely knocked 1 out of whack. 1 and 2 would need to be adjusted together, then adjust 2 to match 3.

Don't go by my numbers though, they are just for reference.
Handy step by step. At 11:18 he mentions how you balance 1 and 2 together, then 3 and 4 together, then the pairs together. Different bike, but same purpose I mentioned above.

 

Keeping everything airtight is an absolute must. It's going to take you a while to do this with your 2 gauge setup. Not saying it can't be done, but you're going to need lots of patience.
And a reminder, make very small tweaks each time.

 

Existenz_is is correct with what he is saying about being mindful about the carb linkages. But our bikes don't have that. On our bikes, there is only adjustment for each individual carb.