Carb sync just for idle?

[littleroot]

So this week I thought "what if and what would it cost" if I let a "respected" shop do the carb tune and valve adjust on Tom's 801100E because I don't want to screw it up and I don't yet own the tools. I got a couple recommendations so I call them up and talk to one of the mechanics. He asks if it is it idling OK and I say yeah, it idles fine but it is starting to vibrate a bit about 3k. Then he says, "then it doesn't need a carb sync if it idles OK because sync is only on the pilot jet, it is not used above 3k RPM. And you would know if the valves were out of adjustment because you'd notice performance issues before vibration issue."


I know when I watched RenoBruce use the carbtune to sync Tom's red bike, which before he did anything to it, it idled just fine, the bike revved a whole lot smoother after he did the sync, and he synced it with the revs at either 3k or 4k, I cannot remember. And it really did it take some vibration out of the bike.

So I'm confused.

I know this guy works on a lot of different bikes so maybe he is not not up on the particulars of the GS bikes.

He did tell me I should do the compression test before I do anything which was excellent advice as I had not thought of doing it because I am a newb/idiot and assumed the engine was perfect because the bike is beautiful.
But he said if one cylinder is weak then the other cylinders have to make up for it, or could many other things. So yeah, I'm going to do that as soon as I can get a 14mm to 12mm step-down for my compression gauge

Just wanted to share and if my friends here have to say.
Thanks

 

[gs850cafe]

if an when you do a carb sync you need in check valves, good clean air filter clean carbs.

the biggest notice i know of a carb sync is crusing down the highway and vibration...

my unsynced 850 vibrates me off the seat an like to "cough" every once in a while...bought a sync tool an now its fixed in that area

but a carb tool, learn it, post a local ad to sync carbs for 25 bucks....i made my money back and them some with my tool

 

[Agemax]

synching carbs will only have effect at idle and just over as you say.

 

[littleroot]

if an when you do a carb sync you need in check valves, good clean air filter clean carbs.

the biggest notice i know of a carb sync is crusing down the highway and vibration...

my unsynced 850 vibrates me off the seat an like to "cough" every once in a while...bought a sync tool an now its fixed in that area

but a carb tool, learn it, post a local ad to sync carbs for 25 bucks....i made my money back and them some with my tool

Now that sounds right to me. I was ready to pop $100 for the carbtune tool but I've found so many maintenance things to do on this bike I've already spent $600 just to be safe on the road

 

[BlackStar]

I guess it would depend on whether you sync'ed them at idle only or at some other RPM. They'll likely only be dead on where you did the Sync. If everything else is in a good state of tune, the sync. should be close at other throttle settings. The last time I did mine, I adjusted so they were good at around 3000 RPM. Now runs smoother than some of my modern FI bikes. I don't recall how they read at idle, probably pretty close. I was willing to sacrifice a little at idle, for a smoother engine going down the road.

 

[Agemax]

they should be set at idle, or slightly high idle. it is a mechanical setting so unless you have worn or sloppy slide linkages, or butterfly valves (cv carbs) then they will be accurately synched from idle to WOT

 

[gs850cafe]

Now that sounds right to me. I was ready to pop $100 for the carbtune tool but I've found so many maintenance things to do on this bike I've already spent $600 just to be safe on the road

IMO a carb sync isnt necessary as long as u have done a fairly well bench sync...i ran 1/2 last summer unsynced

i did it just cause i guess lol

 

[Steve]

The effects of mis-synchronized carbs will be most noticeable at idle. Here is why:

Let's assume, for a moment, that the carbs need to be open 3% to idle at the proper speed. (I have NO idea what the actual number is, this is just for this illustration.)

Let's also assume that carb #1 is open 3%, #2 is open 1%, #3 is open 5% and #4 is open 2%.
Since #2 is only open 1%, it's basically not doing much at all. #4 might be contributing a bit, but #3 is going to be doing all the work to keep the bike idling.

Since all the throttles open the same amount, let's move to half-throttle.

#1 will be open 53%, #2 is open 51%, #3 is open 55% and #4 is open 52%. The actual differences are the same, but the amount they are off compared to the actual opening is not that bad.

Now let's go to full throttle. #3 is going to be the one that tops out first and limits the rest.
#1 will be open 98%, #2 will be open 96%, #3 will be open 100% and #4 will be open 97%. Again, not enough difference to notice.

Your best synchronization results will be at, or just above, idle. The book actually calls for 1500-2000 rpm, that might be just to smoothe out the vacuum pulses a bit. I have always done it about 1200 rpm and had good results.


Now for some observations on other points you brought up:

1. Yes, the pilot jets are the only ones in use during the carb sync process, but that has nothing to do with the sync process. You are synchronizing the throttle openings, not the jet settings.

2. Technically, you can synchronize the carbs at any engine speed you choose, but, like I pointed out above, any differences will be most apparent at lower speeds.

3. The basics of a carb sync are the same for any bike. They also apply to modern bikes with fuel injection throttle bodies. I did the same basic process on my sister's Honda ST1300.

4. A compression test is seldom a bad idea, but it is no guarantee that all the cylinders will be capable of putting out the same power. You could have perfect compression numbers, but no power in one cylinder if your carb jets happen to be plugged. You would also get good vacuum numbers on the sync gauge for that cylinder with the plugged jet, so even that is no guarantee that there will be no vibration. You will have perfect vacuum balance, but no power in one cylinder, therefore you WILL have a vibration.

I did not notice it mentioned in this thread specifically, but you want to be sure to check your valve clearance BEFORE doing a carb sync. If you adjust the valves after synching the carbs, you will affect how much air that cylinder will breathe, so you will change the vacuum level, too.

.

 

[chuck hahn]

I dont have any of my factory manuals right here, but i recall anumber of around 1750 to 1900 as the idle speed ( set with the big knob ) as the preferred RPM.

Also note that as you adjust and get them more level, youll be readjusting the idle to keep it in the called for range.

And to add to the valve adjustments..it is done on a stone cold engine...always.

 

[Steve]

And to add to the valve adjustments..it is done on a stone cold engine...always.

Very true.

Don't even fire it up "just to see if it runs" or to bring it in from outside.
If you fire it up, plan on waiting several hours before checking the valves. :-\\\

.

 

[chuck hahn]

From the 1000E manual on BIKECLIFFS website. The number is relevant to all GSs I would assume..."Set the idle at 1,500 to 2,000 RPMs."