My first attempt at carb balancing

[plc]

Hi All,

Well yesterday after checking valve clearances and spark plug gaps I decided to have a go at the carbs.

My bike was running a bit rich and I think it still is. I looked at the resources here and the Snowbum article and followed the procedure he outlines using my new Carbtune balancer. Tuned it up after the 20 minute ride to work.

Well all seemed good, on the ride home, it was a bit slow at idle when cold but i thought that will be OK when it warms up. Well when I was almost home the idle was rough and seemed out of balance. The same on the ride to work this morning, fine at first and then rough when hot. This morning I had another go and the bike idles but almost stalls when slowing down to a stop at an intersection or traffic lights. I usually need to blip the throttle to get it to idle then it idles somewhat happily.

Can't tell you exact revs (as tacho is low) it sounds around 1100 rpm. If i put the revs up higher with the throttle stops the bike loses revs slowly on gear changes. Should it do that?

I think I may have it rich. I didn't pull the plugs to adjust the idle mixture, butIi turned the screw out until the cylinder slowed and then turned it back in until it raised again. I am wondering if I should turn the mixture screws in a quarter turn and see how it goes. Somewhat paranoid about making it too lean and overheating the motor.

Thoughts?

Best to all,
Paul

[Barry]

If i put the revs up higher with the throttle stops the bike loses revs slowly on gear changes. Should it do that?

It certainly will do that if throttle stops are set too high and it's a sure sign that you need to lower the idle speed a little. You will often read that when the revs hang up at idle it is because of a sticking advance mechanism. While this can happen I think it is more often caused by incorrect idle settings. The aim is to achieve a smooth idle with the minimum opening of the butterfly. If for example the idle is set too rich so that the idle speed slows and is then compensated for by screwing in the throttle stops a touch more what happens is the very small holes just under the butterfly which are called transfer ports come in play and as they are not controlled by the mixture screw they flow more mixture which causes the revs to hang up.

So if this is what is happening with your carbs a 1/4 turn leaner on the mixture screws should not do any harm especially if that allows you to turn the throttle stop screws out a little. If you start from a basic setting of say 3/4 of a turn out on the mixture screws (the basic setting is quoted at 1/2 , 3/4, or 1 turn out depending on which publication you consult) I would turn them in until you get a lean stumble and then slowly out again until the stumble clears. That should ensure you do not have a rich mixture at least at idle.

[plc]

Thanks Barry,

I wound out the mixture screw 3/4 of a turn, then wound it out until it stumbled, then in slightly, so I now realise I was at the rich end of the scale. Last night I turned the mixture screws in 1/8th a turn and after 8 km found the bike idled better and wasn't stalling on rapid stops.  

It is slightly imbalanced now so I will investigate making it a little more lean and turning out the throttle stop screws.

So does the mixture screw control the amount of fuel coming from the idle jet? And does the amount the throttle stop opens the butterfly regulate the air?

Best to all,
Paul

[Barry]

Its not quite that simple Paul. The key thing to understand about the pilot circuit is while all the fuel flows through the pilot or slow jet only some of this is controlled by the mixture screw. The diagram also shows that the idle air jet allows air to bypass the butterfly to premix with the fuel flowing through the pilot jet. The idle mixture screw controls how much of this mixture enters the venturi down stream of the butterfly at idle to mix with the small amount of air passed by the butterfly. The transfer ports will begin to flow this mixture once the butterfly opens from the idle position, this is designed to smooth the transition to the needle jet.
The object in setting idle is to have very little if any mixture flowing through the transfer ports otherwise you have this potential for the revs to hang up. All of this explains why minimum possible butterfly opening at idle is important and also why if the carbs are overhauled that the butterflys should be accurately centred in the venturi.  

[Dizerens5]

I am wondering if there is any point in doing carb balancing by measuring temperature. Easily done now that some low-cost test meters (multimeters) come with a temperature-sensing lead. I tried the cylinder heads next to the plugs after a short warm-up - though my R65 seems to run and idle well there is about a 10 degree Centigrade difference between the cylinders. Does it matter and can it tell me anything useful?

[Barry]

I remember some time back this idea being suggested on another forum where it was dissmissed out of hand which I thought was unfair. I've often thought about using an infrared thermometer to compare exhaust pipe temperatures at idle.

In theory within the normal range of air fuel ratios the weaker cylinder runs hotter if and maybe its a big IF, all of the other things that influence temperature are equal like ignition timing and compression.  The thing to do is try it by altering the mixture on one side by a known amount and see if you can measure the difference.

If you are really interested some of the aircraft boys really know about this as they use exhaust gas temperature to monitor engine operation in flight. Here's a link which discusses cylinder head and exhaust gas temperature on Rotax engines.  http://www.800-airwolf.com/pdffiles/ARTICLES/part38.pdf and an another one where a very clued up guy tunes Bing Carbs using exhaust gas temperature.http://jackbhart.com/firefly/firefly121.html

[montmil]

Paul, the initial setting for your 1984 R65LS Idle Mixture Screw is 0.5 turns out from a gently closed position. A manometer is quite helpful to fine tune the screws. Usually, you will find your best results not more than one-eight of a turn off the factory stated initial setting... this with the understanding that the carbs are clean, clear and the carb floats are doing their thing. While tuning the idle mixture screws, throttle cables should be slack with no tension on the butterfly shaft.

Also, consider checking the condition of the rubber sleeves at the head/carb attach. Small cracks combined with hardened rubber can create all sorts of idle issues including a slow return to idle and random high/low operations.

Check Snowbum's site for correct carb tuning sequences and avoid saddeling your horse backwards. [smiley=wink.gif]

Monte

[plc]

Barry,

Thanks for the diagram and the explanation, there had been a number of discussions at work with other motorcycle riders about how the Bing's idle mixture screw worked. Thanks for clearing it up.

Also allows me to understand why I shouldn't let those butterflies open too much when setting the idle speed.

Best to all,
Paul

[Semper Gumby]

Hi Paul,

I use the Carb balancing procedure from the Bing Manual.  Seems to work best for me.  To get the idle mixture right I use a Colourtune plug from Gunnison to set the the Idle mixture on the rich side of the normal range (blue flame in the combustion chamber).  If I set the mixture on the lean side of the normal range the motor backfires (softly) when I close the throttle coming to a stop.  The Bing Carb Manual can be had from the Bing Agency in Kansas.  This will be the best $12-15 (?) you have ever spent.

Good luck.

[plc]

Hi Bill,

I have ordered the Bing manual, I hope it will get here this week. I will be keen to see what they suggest.

Paul

[Dizerens5]

Thanks Barry. If you put all your knowledge into a R65 engine manual (like the Rupert Ratio one for unit BSA singles) I'm sure you'd have plenty of buyers!

[plc]

Hi All,

Bing manual arrived today, their tuning method is interesting. Couldn't do it today, time was a little limited to be out on the road too much,

So I took your advice Barry. Wound in the idle mixture screws, then back out 0.5 turns (as per Bing manual). Then turned in until it hesitated then backed it out about a 1/4 of a turn until it idled smoothly. Then adjusted the burtterfly stops in a bit to slow the idle down.

On the ride home it was backfiring on the over run but idling nicely at stop (once warm). So I pulled over and turned each mixture screw 1/8th turn out. Then it was a little slow to lose revs, so I dropped the throttle stop screws slightly to test tomorrow morning.

Thanks for all the advice guys, it is easy to see how one can become obsessed with this.

Best to all,
Paul

[plc]

Hi Guys,

Well I found that the bike popped less, but still did on the way to work. So I turned the mixture screws ccw 1/10th turn today and I get an occasional pop on the over run now.

The Bing manual says that the bike will backfire through the carb when too lean, is that the popping I was hearing? I thought a the backfire was through the exhaust, but maybe I was wrong. I thought a carb backfire may blow the carb off, has anyone got a clear way to tell the two for the uninitiated? I should mention when the bike was lean the float bowls felt warm and my boots felt hotter than normal.

Best to all,
Paul

[Barry]

When an engine has a lean misfire the mixture doesn't burn instead it  flows out into the exhaust to be ignited on the hot exhaust gases from the next stroke. Hence the pop.  Well that's one reason. Popping in the exhaust can also be caused by air leaks.

[Semper Gumby]

Popping may also be the Air Injection system.  Is it still installed?

Many of us have removed the Pulse Air injection system.