Something new, balancing carbs?

[John Orchard]

ok I pulled the carbs apart on a used bike I just bought, just see what I have to deal with, the butterflies looked out of kilter with each other, and surely were after I played with them lol, anyways I put them all back together; ran like a dog (no surprise there ;-) ). Having not had to tune a multi cylinder for over 10 years (I've been racing vintage motocross), I gave my vac gauges away, a new set is in the mail.

So I can't sleep, have to still play with the bloody bike, so I rip the airbox off, and with the fuel tank pulled up at the front with a pair of tie-downs anchored to the subframe, I start the bike.

Now for the curious part, with the airbox off and the bike running rough, I'm getting and occasional spit-back through only the front cylinder carb, so I start turning the carb synchronizing screw, and it starts to run really smooth, so I keep turning the screw in the same direction, the bike starts running rough again and starts spitting-back through the rear cyl carb ! So I backed the screw off to the mid way part where it is running sweet.

Now all this is within 180 degrees of screw rotation, now as far as I am concerned 'those carbs are synch'd' and, without using any vac gauges! I'm 60, been building & tuning racing motorcycles for over 45 years, never have I thought to balance a pair of carbs in such a manner.

What say you chaps?

[Commando77]

Perfectly understandible for someone used to synching Amals. I use carbtune but by ear gets you very close.

[tony.mon]

With old style Amals you can hear them snap shut and tune to shut together, by ear. Easier with two cylinders than triples or fours!
Can't do that with cv carbs, so a vac guage is best.

[John Orchard]

This aint by sound, this by sight :-)

[Commando77]

Got it

[8541Hawk]

These bike do tend to run a bit different with the air box removed....for me, it has always been best to sync the carbs with everything installed.

[John Orchard]

These bike do tend to run a bit different with the air box removed....for me, it has always been best to sync the carbs with everything installed.

Yep agreed, but l'm surprised how close l got it without any gauges :-)

[fabiostar]

iv a few sets of carbs here and one of them just will not balance no matter what i do to them, the set im using took 2 mins with the gauges, they can be picky things these carbs

[buzzer]

for years I had a set of 6 vacuum gauges that I used to balance cars and bike carbs... I was never happy that I could often get a better tickover by ear... then one day I was balancing a set of triple Weber carbs on an Aston Martin... I got all six gauges steady on the same reading... but the tickover was still patchy. Then my mate came along and connected up his six mercury tubes... PERFECT tickover!

So then I connected all six of my gauges to a single vacuum source, and guess what, they all read different! some were really out .

I then started using just one gauge to balance on. here is a picture below of how it works... its a single gauge, and 5 valves (I only do 4 cyl these days!) the valve for the single gauge is used purely for damping the needle on the gauge. By opening the other valves individually, one at a time, you can note the reading on each cylinder. this is super accurate as there is no gauge calibration involved. it works a treat.

the little valves are fish tank valves... VERY cheap on Ebay.

of course mercury gauges are really accurate, but dangerous, fragile, and expensive. and the more expensive vacuum gauges that you can adjust, still go out of calibration. I find the single gauge works well. dont start me on the rods in the tube type gauges!

[tony.mon]

It's got to be more accurate using a single guage, good thinking.

[AMCQ46]

i like that solution

i use a ball bearing in a circular tube, and attach each end of the tube to each cylinder. if one is sucking more than the other the bal won't sit at the bottom.... adjust till it does. i have a 2nd tube and ball sitting above the working one on the same radius of curve, it is not connected to anything, but give you the datum for where the ball will sit under gravity... .. I will admit that I bought it rather than made it, but it was the lack of calibration problems that sold it to me

[kenmoore]

Carbs on these bikes are a delicate subject.

One thing I would have to say is that never ever remove the plastic plate underneath the carbs, it's right at the front in the V of the frame.

Secondly ,

Once the carbs are synced and performing as intended leave them the wombles alone. I know only too well the foibles of changes and after much fiddling arrived at a setting that worked .

I have remote vac lines and check the sync about every 10 rides.

I think my bike runs a bit rich sometimes but a good flogging fixes that in the short term.

I use 2 seperate vac gauges and always bias the settings to the rear.

I use lead additive in my fuel and tend to gauge how the beast is running by the colour of the exhaust. Old school I know but I am comfortable with it.

At the end of the day I say ride it and see, you can always change it !

[sirch345]

i like that solution

i use a ball bearing in a circular tube, and attach each end of the tube to each cylinder. if one is sucking more than the other the bal won't sit at the bottom.... adjust till it does. i have a 2nd tube and ball sitting above the working one on the same radius of curve, it is not connected to anything, but give you the datum for where the ball will sit under gravity... .. I will admit that I bought it rather than made it, but it was the lack of calibration problems that sold it to me

I use the same type of carb balancer for the same reasons

A good solution by Buzzer I agree

Chris.

[John Orchard]

Well further to my original post, the 'spit back' procedure that helped me originally only worked when l was a touch lean on the pilots (45's), since l changed to 48's the strange altetnating spit-back stopped.

Prior to receiving my vac gauges (after fitting the 48's) l set the butterflies with a piece of 1mm stainless wire trapped in each throttle body, l adjusted the synch until l had equal drag as l pulled the wire through each throttle body. The next day l received my gauges and check them ... spot on!

Yeh maybe l fluked it, l would never suggest this was an accurate way to balance ones carbs, but, if you need to ride and don't have gauges, it's worth a try.

[tony.mon]

It would work well with slide carbs, never tried it with cv, though.
Maybe you can set the ignition timing with a fag paper, too!