Are We All Balancing our Carbs Wrong

[jchesshyre]

Bit of an old thread resurrection here, but I've been reading this with interest along with some associated postings elsewhere on the 'net, as it does seem to make some sense and the manual is definitely ambiguous. I tried adjusting my carbs with the rear one reading 20 mmHg higher than the front, and although it was a subtle change it was definitely not an improvement on adjusting them to read the same.

Then, I thought of seeing what the Honda Common Service manual has to say about carb balancing and I think it gives us a pretty definite answer as to what the instructions/specs in the VTR1000 service manual mean, with the pertinent phrase being 'below the specification'.

[VTRDark]

Thanks for posting that. Yes things are definitely smoother when equal.

[kenmoore]

I have found that when my carbs are out of sync the bike vibrates at around 4 a 4500 revs in top gear.

I use this as an indication that it's time to do a sync.

I find that the vibration is lessened when there is a slight variance between front and rear.

I have a far from standard setup and won't bore you with it.

In all other respects the bike performs flawlessly at all throttle openings hot or cold.

I can't explain why it's better with a slight variance but it just runs and feels better and carb farts, cutting out are non existent.

What I'm trying to say is that with all the variables , do whatever makes your bike run the best.

I go mostly by sound, then check the sync to see if I am close and every time I do that I get the best sound and performance when there is a variance between front and rear. I replaced all inlet boots, new clamps and vac lines thinking that they had something to do with the bike running better with a variance and that made no difference. Still better with a slight variance. Another quandary is that I have equal length H Power velocity stacks and a Bernie Morgan modded air filter.

I work on a bike with a Ducati 900ss motor in it that has seperate Dellorto PHM40s fitted and it runs better with a slight variance as well.

I agonised over this many times and have come to accept that a slight variance is the best for me.

[VTRDark]

I have a far from standard setup and won't bore you with it.

Just dont mention the electric water pump

Your probably right going by sound and feel. If it runs smooth then it runs smooth, simple as that. It's a funny one though isn't it. The stacks really do make a difference where one has two longs as in H Power/HRC or one short and one long like OMs or Mori's. Then there's the jetting to consider with the front and rear being different. Too many variables involved.

[RedStormJ]

I just gotta throw my 02 in.

Back af Foreign and sports car we always did a few tricks setting up webbers. A pain in the A. After they were set it was like magic. Anywho here is what we did.

We dialed them down to zero and opened up the main screw to 1/4 turn. Then we would see if there was life. Usually just one carb was correct so we would fiddle with the screw on the other side. If both pipes got warm we would actually start tuning the carbs.

We would go by ten minute intervals max. to set the engine up so it would start and idle nice. we would listen for the engine to be balanced and temp out the exhaust header with an IR thermometer.

After it idled good we would usually knock back the timing some and start adjust a bit until it started nice and smooth and the engine idled good.

Then We would warm the engine to temp and leave it for a while. then we would adjust the crabs mixture screws again until it was nice and snappy.
Good temp across the pipes good start and a nice snap with no stutter and then only then the vaccum gage would come out and if the carbs were close we would call it a day.

I set my bike by ear cold went for a ride and adjusted it again a bit hot. I dont use my choke anymore and its colder than a witches titty for summer. 17C. typhoon season...
I gently crack my throttle and give it a spin. On a cold day it needs that slight crack to stay going. after about 5 seconds I give it a burst and let it warm up. I have been to three shops for an opinion and they all told me dont adjust it. "That is the best running vtr we have ever seen," That came from a shop that has a few VTR's for rent.

Now some opinion backed by science. Every crab is different. IF you could magically plop down on a dyno every morning that vacuum gage would mostly collect dust. The front and rear are indeed different. I do believe my front cyl is a bit fat and the rear is a bit lean. But once the trottle is opened the bike picks up hard. I believe the design does not allow for normal procedure to be used.IE you cant just set them to 50 50 and call it a day. just look at the stock stacks and the posistion of the front cyl.... Also the rear pipe is bent to hell to compensate for length where the front has a smooth bend. 2 afr meters would be the only way to really know but an old trick is to check the plugs. When I did 2 stroke sled's we always pulled the plugs whiles setting the carburettor's.
And if anyone has anything negative to say about it. My vtr can sit in traffic for a long time without overheating even though It did nearly blow my marbles off after an hour at idle. Try it and if anyone has a dyno let me know if they saw a difference.

[MacV2]

My vtr can sit in traffic for a long time without overheating even though It did nearly blow my marbles off after an hour at idle. Try it and if anyone has a dyno let me know if they saw a difference.

My opinionon this is don't blow your marbles off...Mine are too precious to me...Don't see much action these days but fun when they do...

[jchesshyre]

Excuse my ignorance, but I don't understand how the exhaust headers and even the length of the velocity stacks affect the relative vacuum pulled by each piston when on the intake stroke at idle. Surely this is only a function of the pilot screw setting and the throttle position (i.e. synch screw setting)?

[AMCQ46]

My vtr can sit in traffic for a long time without overheating even though It did nearly blow my marbles off after an hour at idle. Try it and if anyone has a dyno let me know if they saw a difference.

My opinionon this is don't blow your marbles off...Mine are too precious to me...Don't see much action these days but fun when they do...

So mac, did you shag frank again? Or the gay duck?

[sirch345]

Some interesting reads here on carb balancing

Chris.

[VTRDark]

Good post Redstorm.

Excuse my ignorance, but I don't understand how the exhaust headers and even the length of the velocity stacks affect the relative vacuum pulled by each piston when on the intake stroke at idle

I don't know how to explain this but have a look at the difference the stacks make when you change to two short or two long or even one long one short and then take into consideration the airflow. The vacuum level will change on your gauge with each configuration. With two long or two short it will either rise or drop on both cylinders and with one short and one long one of them will either drop or rise so they are uneven.

As for the headers they are set to a pre-set length to maximise the scavenging of wasted fuel back into the cylinder for a second go as it gets sucked back in on the overlap. Think of it like a pair of lungs breathing. The exhaust does not only blow out but it pulses out and in at the speed of sound according to pressure differentials from high and low pressure. This is what tapered pipes are all about as they smooth out / control the flow at each end. Then there's temperature difference between the front and rear to take into consideration as temperature affects pressure.

[jchesshyre]

Is it possible that the different length velocity stacks are to compensate for the different length and shape exhaust headers, and so the effects of both these differences should more or less cancel out when it comes to vacuum measurements at the inlet?

[jchesshyre]

Just another thought I had about this...I was thinking about it the other night as I was falling asleep (yes I am that sad) and it occurred to me that if you adjust the carb sync screw so that the vacuum in each inlet tract is the same at idle then by doing so you are in fact compensating for the differences between the two carb's pilot settings and velocity stacks. In other words, the different dimensions of the intakes etc mean that the vacuum drawn would naturally be different if the throttle butterflies' positions were physically identical, so by adjust the vacuum to read the same you are in fact compensating for this inherent difference. This is all assuming of course that the ideal is for both cylinders to be drawing the same vacuum despite differences in the intake tracts, which is an assumption that I'm not knowledgeable enough to make with certainty. But it seems logical to me that even if the engine is set up so that the mixture is slightly different for each cylinder, you'd still want both pistons drawing an equal vacuum in order to be physically balanced.

Does this make any sense at all...? It's just a different way of looking at it which in my half-asleep state felt like a eureka moment

[VTRDark]

I'm liking you way of thinking. You can join the club of bedtime thoughts Mr Mon is another one that thinks such things in depth while trying to get to sleep.

Did you see the following:
http://www.vtr1000.org/phpBB3/viewtopic.php?f=9&t=39969

[RedStormJ]

Just wanted to add to the reply by Carl on exhaust length and stack height.
You would think it wouldnt.... You might wish it wouldnt. Then you crack open a copy of exehaust theory I lost mine years ago but you have to think of all the space rocket physics stuff.

1. Exhaust pulse. Think a smoke ring coming out of a pipe. How did it form? The pulse of the smokers breath. If we could see the movement of exhaust gas slowed down we would see the gases not moving just out but back and fourth. vibrating PULSING. The pulses are caused by the explosion in the cylinder along with valve overlap. Hold that thought.

Think about a two stroke. How does it make power? Tuned pipes help a ton. The gas expands slows then rockets out a tiny hole on the other side. Why is it so small? So the fuel does not get sucked out of the combustion chamber. the large diamiter of the pipe allows the gas pulse to slow enough to stay in the combustion chamber long enough for the piston to seal the window and ignite the fuel while keeping a slight ammount of spent gas in the combustion chamber. large middle slower gasses skinny middle faster gasses more unburnt fuel.

Back to 4 stroke.

2. Duration. the valves most unfortunately can not open at just the right moment expel the exhaust gas an close at the best moment to keep any vaccum from building up in the chamber. Nor would you want it to. The valve has to open a bit early and close before the piston smacks it into a new j shape. same goes for the intake it cant just let air in at the best moment then shut at maximum efficency.

The theory is that the exehaust when set up just right uses the cylinders exehaust to pull on eachother IE scavange. Making a vaccum during open valve. This would allow the gasses to come out effortlessly. However!! This would also mean that during valve overlap a vaccum would assist the intake charge in coming into the engine. And thus changing the vaccum pulled.

If yall understand that cool if not ill use the two person method.

Imagine a high class gal. shes trying to suck a tapioca ball through a straw. Then another high class gal is holding her thumb partially over the other end of the straw. Now the gal signals the other gal to close off the straw with her thumb she does and when the moments right pop the tapioca ball flys into the gals mouth as the others thumb comes off the straw.

[VTRDark]

Some exhaust theory links.
Exhaust and intake tuning

How To Calculate Muffler Size and Exhaust Pipe Diameter

Exhaust System Theory 101

Exhaust System Technology

This is worth a read as well.
http://www.rc51.org/exhaust.htm

http://speedtalk.com/forum/viewtopic.php?f=1&t=1435

All very interesting reading.