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Balancing carbs

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Ayep
Rossi
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Location : Paris

Balancing carbs

Post by Ayep on Mon 20 Apr 2015, 10:17 am

Hi all,

I had a go at balancing the carbs yesterday, after cleaning out the air filter, and noticed something strange. "Ear" reading and gauge readings were incoherent.

I build my own carb balancing apparatus, using aquarium tubing and oil in a "U" shaped accessory. Lengths of tubing are equal on either side. I could seem to find any air leaks in my setup (the connections to the engine). However, when I balance the carbs using my apparatus, if following that I unscrew the carb balancing screw, the engine revs go up and the engine sounds happier. Yet, this is in contradiction to what seems to be a gauge reading.

Normal behaviour? Or is my device flawed?
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eternally_troubled
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Re: Balancing carbs

Post by eternally_troubled on Mon 20 Apr 2015, 1:29 pm

Ah I half replied to this in your other thread - I hadn't seen this one!

I have had this problem too - I don't think it is your gauges (unless you have a hole you didn't know about).

When I get this effect I use the gauges to get the carbs mainly in sync and then adjusted by ear.  The gauges are good for finding the right point to start adjusting by ear.

On my bike the difference was quite slight - an eighth or sixteenth of a turn of the screw, is it similarly small on yours?

I also made sure that I was revving the engine slightly and then let it return to idle between readings - this should settle the linkage between the throttle butterflies and ensure you are getting the right reading.

In addition to all of that (now I remember) I also took some notice of what the readings were doing just as I revved the bike away from idle, so in the end the setting was a combination of three things:

1. the gauges reading at idle

2. ear

3. what happens to the readings as I (very) slightly open the throttle
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Ayep
Rossi
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Location : Paris

Re: Balancing carbs

Post by Ayep on Mon 20 Apr 2015, 3:23 pm

Well, unfortunately, on my bike the difference seems huge. We are talking in roughly a whole screw turn, possibly more. The further I unscrewed, the higher the revs... And the better sounded the engine. 

Seems strange. Either I don't get something, or my setup is bad. It's a home-made kit, i'll send a pic when I get a chance, but it's simple enough and should work. Yet, I see no reason why the revs change so much when I unscrew the screw...

yep, I give it a good bit of throttling between each screw fiddle, to make sure all is coherent there.
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eternally_troubled
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Re: Balancing carbs

Post by eternally_troubled on Tue 21 Apr 2015, 2:04 pm

Your gauges might be fine - have you checked the carb rubbers for splits? If there's any air getting
in it will screw things up.

Another thing crosses my mind: as you twist that screw it adjusts the offset between the left and right carbs, so one or other carb butterfly is going to be opening more as you turn the screw, allowing more air/fuel into one cylinder. Could this cause the increase in engine speed you are seeing? (i.e. are you better setting it as the point with the same readings and the lower speed).
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Beresford
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Re: Balancing carbs

Post by Beresford on Tue 21 Apr 2015, 3:13 pm

@eternally_troubled wrote:Another thing crosses my mind:  as you twist that screw it adjusts the offset between the left and right carbs, so one or other carb butterfly is going to be opening more as you turn the screw, allowing more air/fuel into one cylinder.


This is a similar arrangement to that used on the old twin SU carbs used on MG Midgets and Mini Coopers. The point of correct balance was when the tick-over was lowest, all else being equal.
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Ayep
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Re: Balancing carbs

Post by Ayep on Tue 21 Apr 2015, 8:47 pm

Ah, fantastic, I think this does indeed explain it. If one carb butterfly opens more then this explains the idle revs going up...
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eternally_troubled
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Re: Balancing carbs

Post by eternally_troubled on Wed 22 Apr 2015, 12:56 pm

Glad that all makes it more clear.

Now I think about it, I realise that is exactly why the manual suggests you reset the idle speed after balancing the carbs... doh!
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rob75
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Re: Balancing carbs

Post by rob75 on Fri 24 Apr 2015, 3:27 pm

Balancing the carbs is in fact ensuring that both butterflies are equally opened at idle and are equally opening when twisting the throttle.

So the reading should be equal at idle, but also checked at 1/8 opening and half/full throttle to be sure.

Please ensure your valve clearance is spot on, otherwise suction is off-set. Be sure there are no ignition issues. No vacuum leaks should be found either.

The last bit is to set idle mixture on both cilinders at such a point that smooth idle is obtained. At best this is done with a exhaust gas analyser, but listening will also help.

I've had my share on carbs with several Alfa Romeo engines with 2 double carb. So balancing and setting mixture per cilinder. It is a bit fiddling and getting the hang of it.

Succes,
Rob

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