Are We All Balancing our Carbs Wrong

[Tweety]

I balance the carbs so they both read the same at 2.5 rpm and I never get carb farts and you can feel both cylinders pulling the same on the throttle .

I'm very interested to know why you choose to balance them specifically at 2500rpm?

Phil

sent from my 'phone

I always balance at 2500 and check at 5000... The longer you are from the point where you have balanced, the larger the error will be, so balancing at idle means you could be way of at full throttle... Ie, makes sense to go a bit higher, around where you normally rev the bike, but not as high as to be totally out at idle... The number is pretty arbitrary really, just as long as you do the same every time...

[Tweety]

As for the 20 mmHg... It's worth fiddling with, I'm about to go into the carbs anyways, so I might as well try it...

[Phil_H]

I balance the carbs so they both read the same at 2.5 rpm and I never get carb farts and you can feel both cylinders pulling the same on the throttle .

I'm very interested to know why you choose to balance them specifically at 2500rpm?

Phil

sent from my 'phone

I always balance at 2500 and check at 5000... The longer you are from the point where you have balanced, the larger the error will be, so balancing at idle means you could be way of at full throttle... Ie, makes sense to go a bit higher, around where you normally rev the bike, but not as high as to be totally out at idle... The number is pretty arbitrary really, just as long as you do the same every time...

I see some logic in what you say but it's not strictly correct.

As the carb butterfly opens, vacuum reduces. At wot there is virtually no vacuum, also, the wider the butterflies are open, the less effect of them being marginally out will be.

Having said that, my bike is balanced perfectly at tickover, if the revs are increased, the vacuum goes out by some margin - worst at around 2k rpm, by 3k rpm they are back in sync!

Phil


sent from my 'phone

[Tweety]

I see some logic in what you say but it's not strictly correct.

As the carb butterfly opens, vacuum reduces. At wot there is virtually no vacuum, also, the wider the butterflies are open, the less effect of them being marginally out will be.

Having said that, my bike is balanced perfectly at tickover, if the revs are increased, the vacuum goes out by some margin - worst at around 2k rpm, by 3k rpm they are back in sync!

Phil


sent from my 'phone

I never said it was strictly correct either... You said that...

What I said was that I'm setting them up to a point where they will drift a minimum for normal riding... Ie, I know very well that the more throttle opening, the less effect of vacuum, but with a starting point of 2500 rpm, they drift a minimal amount towards both ends ie idle and mid throttle at around 5000 (hence why I check 5000 as well as idle after setting at 2500) And if they are near as possible spot on at those three points, they will be as good as possible at WOT too, with the fading effect...

I'm not saying I know something no-one else knows... I'm just saying that this is what I have found to work, after about 100+ hours of dyno time on stock and tuned VTR engines... With the standard method you are using, I ended up with a "hole" in the middle of the power band, where the carbs drifted apart, usually right where I wanted the power, as I punched the throttle from cruising, or a standstill...

[8541Hawk]

Another thing to keep in mind is if you are running the stock stacks, the different lengths will make it a bit difficult if and when you try to sync at multiple RPMs

[VTRDark]

I think you have just hit the nail on the head there or hit the jackpot, so to speak. I bet all the 20mmhg difference is doing is accounting for the short and long stacks so if running two stacks the same length, billets for example, then one would want things balanced evenly. Now that really does make sense

(:-})

[kitsun]

Guys, re read the carb synchronisation bit in the factory manual. IMHO it should be 20mm Hg or 0.8 inch Hg difference, not that the rear has to be pulling more than the front.

Verbatim:-

Carburetor Vacuum Difference:
20mm Hg (0.8 inch)

no where does it say to make the rear cylinder have a higher vacuum, if the rear cylinder needs to pull harder than the front, it would be specifically stated as such on the factory manual.

kit

[kitsun]

Guys, re read the carb synchronisation bit in the factory manual. IMHO it should be 20mm Hg or 0.8 inch Hg difference, not that the rear has to be pulling more than the front.

Verbatim:-

Carburetor Vacuum Difference:
20mm Hg (0.8 inch)

no where does it say to make the rear cylinder have a higher vacuum, if the rear cylinder needs to pull harder than the front, it would be specifically stated as such on the factory manual.

kit

Also reading from the Carbtune instructions it says NOT to balance at high engine speeds. Balance at idle speeds. And the difference between the cylinders should be within 20mm Hg.

kit

[VTRDark]

So why would the manual even mention the front carb being the base carb which suggests they are to be different. Interesting that the Carbtune site mentions within 20mm Hg but I think that is a generalisation as most bikes (mainly IIL4s) would be the same balanced across the board evenly and would also have the same size stacks

However after looking at a Honda service manual I downloaded it states that the rear cylinder should be 20mm HG more.

I'd be interested to see what it says here. Is this the same style Honda UK manual we have links to in the Knowledgebase or a different kind?

(:-})

[kitsun]

So why would the manual even mention the front carb being the base carb which suggests they are to be different. Interesting that the Carbtune site mentions within 20mm Hg but I think that is a generalisation as most bikes (mainly IIL4s) would be the same balanced across the board evenly and would also have the same size stacks

However after looking at a Honda service manual I downloaded it states that the rear cylinder should be 20mm HG more.

I'd be interested to see what it says here. Is this the same style Honda UK manual we have links to in the Knowledgebase or a different kind?

(:-})

its the manual i downloaded from the link on this forum. so it has to be the same. service manual does not say anything about the rear cylinder, only that the front cylinder is the base carb i.e. the one that is not adjustable, while the rear cylinder is to be adjusted to within 20mmHg of the front. So if you are 0-20mm Hg is the range you have to aim for. more than 20mm Hg is and you need to sync your carbs

kit

[Flatline]

Surely someone like Roger Ditchfield will know about this?

[Tweety]

Guys, re read the carb synchronisation bit in the factory manual. IMHO it should be 20mm Hg or 0.8 inch Hg difference, not that the rear has to be pulling more than the front.

Verbatim:-

Carburetor Vacuum Difference:
20mm Hg (0.8 inch)

no where does it say to make the rear cylinder have a higher vacuum, if the rear cylinder needs to pull harder than the front, it would be specifically stated as such on the factory manual.

kit

Actually, I think you are dismissing all of this a bit too easy... And you are mixing and matching your sources to get the answer you prefer... Not what they are actually saying... On the other hand, I'm not going to claim they say something specific... I'm just going to look at what they really do say...

First... Very true the Carbtune manual states a few things... But, that is not specifically about the VTR's in any way what-so-ever... It's like said a very generalized manual on using the tool, nothing else... The reason for the recommendation to balance at idle is very easy... That's where the signal is the strongest, and most stable... So, it's a lot easier to balance it at idle... That is the one and only reason, not because it has any performance benefits, none what-so-ever...

When i balance at 2500 rpm, like stated I check at both idle and 5000 rpm, and at no time does my carbs differ more than 20 mm Hg at either point... So in reality, even with my method, I'm still following what was until now believed to be the spec stated in the manual... And I can without any doubt say that I have verified the claim that balancing at 2500 vs at idle, there is a performance benefit that can be seen on a dyno, and in running conditions, as the result is a smoother working carburetor at low to mid throttle...

So, unless you have another source, or a way to verify it, for me personally, I'm taking that argument out of the equation, as not valid... And besides, it's really a separate discussion anyways, to the one about a difference or not between the front and rear carbs...

Second it's true, it doesn't specifically say in the Honda manual that the rear cylinder should be +20mm Hg from the front... But on the other hand it does not say like you claim "within", either... Nowhere in that manual is that word used, or anything remotely resembling it, like "tolerance" or +/- or whatever... It states the front carb is the baseline, and it states that the difference should be 20 mm Hg. Not maximum, just difference... Just like it's possible to interpret the actual text to mean "within", even thought it's not correctly spelled out, it's entirely possible to interpret it the other way... Both are equally valid, based on the text and the language alone...

Third... We all know that the VTR's are from the factory set up with different sized jets in the different carbs, and the generally accepted theory is to compensate for the higher running temps of the rear cylinder, due to being in line behind the front, getting less cooling... And also, that temp difference is documented as fact... The well known, and accepted reason for the two lengths of stacks are to spread the power band across the rev range, with one working better at low RPM's the other better at high, giving a decent happy medium with one of each...

So, if the manual does not say maximum, tolerance, or whatever, it's at least feasible that the factory setting is with the rear running at +20 mm Hg, to further compensate for temps, jets, and different stacks... I'm not saying it does say that... I'm saying that the manual is unclear enough that both interpretations are equally correct in terms of language alone, and that it makes sort of sense, going in line with the other differences...

Now, I'm running two VTR engines, both heavily modified, and both with equal length stacks and wildly different jetting to stock... So my bikes wont really be a true test to the theory, but all the same, I'm going to try it, since that's the only way of finding out... I'll see if I can entice one of the locals to let me try it out on his bone stock bike as well... Regardless of the outcome, resetting to an even balance is a 5 minute job, so it's worth the experiment...

[Hvornum]

Well, i actually got the carbs on the bench while refitting a new chain tensioner..
Want me to give it a go? Would need a guide tho on which are the vacuum hoses

Besides the obvious gas blasts (which sounds absolutely amazing when gearing down and letting go of the throttle) I have a few theories I'd like to test, one being that the previous owner made all the carburetor settings during winter which has a huge humidity difference where i live.

Tried to find a decent guide here btw and on the swedish forum but no luck, any one bookmarked one?

[sirch345]

Well, i actually got the carbs on the bench while refitting a new chain tensioner..
Want me to give it a go? Would need a guide tho on which are the vacuum hoses

Besides the obvious gas blasts (which sounds absolutely amazing when gearing down and letting go of the throttle) I have a few theories I'd like to test, one being that the previous owner made all the carburetor settings during winter which has a huge humidity difference where i live.

Tried to find a decent guide here btw and on the swedish forum but no luck, any one bookmarked one?

I'm not sure if this is what you're looking for, anyway take a look if you've not already:-
http://www.vtr1000.org/phpBB3/viewtopic ... 31&t=14214

Chris.

[Hvornum]

Well, i actually got the carbs on the bench while refitting a new chain tensioner..
Want me to give it a go? Would need a guide tho on which are the vacuum hoses

Besides the obvious gas blasts (which sounds absolutely amazing when gearing down and letting go of the throttle) I have a few theories I'd like to test, one being that the previous owner made all the carburetor settings during winter which has a huge humidity difference where i live.

Tried to find a decent guide here btw and on the swedish forum but no luck, any one bookmarked one?

I'm not sure if this is what you're looking for, anyway take a look if you've not already:-
http://www.vtr1000.org/phpBB3/viewtopic ... 31&t=14214

Chris.

Wooha, it's like a bible of sorts..
Thank you! One or more pictures of where everything sits would have been perfect as a reminder next time I'll go about fiddling with the carburetors.. but this is perfect for now!