Spark plug carb balance technique

[79beem]

Can anyone precisely describe the spark plug carb balance technique? I have a manometer but I'd like to learn the technique as my mech balanced the carbs this way and the bike ran better than when I used the meter.

[montmil]

Check the Snowbum's site for all the gory details.

[Barry]

You need a couple of spark plug extensions 3 or 4" long . Any longer than that and the HT leads might not be long enough to fit. I made them from spare spark plug screw tops and small bore steel tube. Devise a means of shorting out the extension to the head. I use long insulated screwdrivers and as long as you keep your hands on the plastic handle there is no problem.


I've only ever used this method to check idle balance and mixture. With the engine idling I do a vacuum balance first and then short out one side noting how the revs drop. Then short out the other side for comparison. What happens is the idle speed slows a bit more with each stroke. In some cases the engine might then stall. Mine drops from 1050 to 600RPM in 4 or 5 strokes and then seems to level out although I don't wait any longer to see if that happens.  If shorting out the right side slows the engine the most that means the left pot is not working quite as hard as the right.  I found only a small but noticeable difference.  Now this might not work for everyone because I run very lean idle mixture settings similar to the Bing manual method so I'm more interested in the checking the mixtures are balanced and assume the vacuum balance has set the throttle stops accurately.  It was more sensitive than I thought it would be and by making the smallest possible adjustment to the left carb mixture screw I got equal response to shorting both sides.

I haven't tried to balance the throttle cable pulls with the shorting out method because it would be impossible to do without fixing the throttle at the desired revs and I don't have the throttle locking screw to do that.

Overall I found shorting the plugs a useful supplement to vacuum balancing because it can address the most subjective adjustment which is the mixture setting.

It's very easy to do once you have devised the means of shorting. The only concern I have is it must stress the engine some running on one cylinder at such a low speed. Feels and sounds like lugging the engine. I guess 4 or 5 strokes at a time shouldn't do any harm though.

[79beem]

Thanks Monte/Barry.

Reading the Snowbum article jogged my memory a bit.

I seem to remember my mechs way was a tad different to Snowbums.

Firstly, he pulled the HT lead and shorted it by putting a spare spark plug into it and resting it on the fins.

Secondly,he adjusted the mixture by winding the mixture screw in until it all but died then out until it bogged, setting the screw approx half way between the two extremes at the sweet sounding spot.

This was done with the opposite side shorted.

The idle adjustments were as per Snowbums, with a fine adjustment with both pots running.

Thirdly the cable adjustments were different to Snowbums.

Shorting the right side first the rpm was brought to 1500 for the left pot.Then using a screwdriver he made a mark on the throttle grip in-line with the seam on the switchgear. Shorted the left side, turned the grip until the mark and seam lined up, adjusted the cable until the rpm was at 1500.

DONE!

She ran and sounded so sweet, the mech took less than 3 mins all up and he was casually chatting to me along the way.

I was amazed. [smiley=engel017.gif]

I did the points,plugs,valves,timing and start off settings prior to this.

He also stressed that the recommended mixture/idle screw settings were purely starting points and he too like Snowbum believed the shorting technique more accurate than the manometer.

 Any thoughts appreciated.

[Barry]

Firstly, he pulled the HT lead and shorted it by putting a spare spark plug into it and resting it on the fins.

I'm wondering how he did that with the engine running or if not did he start the engine on one pot. The thing is the HT lead must never be allowed to go open circuit with the engine running or you risk frying the coil or worse on an electronic ignition bikes. So it pays to ensure the shorting method is 100% secure.

Secondly,he adjusted the mixture by winding the mixture screw in until it all but died then out until it bogged, setting the screw approx half way between the two extremes at the sweet sounding spot. This was done with the opposite side shorted.

Taking the influence of one pot away is a good sensitive way to do the mixture. My only variance is I turn in till it starts to slow and back out only enough to restore the idle speed rather than back to the mid point. This gives me the leanest practical mixture.

Shorting the right side first the rpm was brought to 1500 for the left pot.Then using a screwdriver he made a mark on the throttle grip in-line with the seam on the switchgear. Shorted the left side, turned the grip until the mark and seam lined up, adjusted the cable until the rpm was at 1500.

Not heard of this exact method before. This where a throttle locking screw would be useful.

[Mike V]

I find the "old school" shorting method my preferred procedure for low circuit or idle balance.  I use a snipped old spoke from a wheel, crimp a spark plug tip on the end of the spoke. Thread the plug tip to the top of the spark plug in the cylinder and attach the plug wire to the threaded portion of the spoke.  I then attach an insulated jumper wire with aligator clips to the spoke and when I want to cancel one cylinder attach the opposite end of the connected jumper to the cylinder fin cancelling out that plug.

With a warmed up engine I screw the idle jet clockwise on the single firing cylinder until it just wants to quit, then counterclockwise to find it's highest idle point.  Repeat on the other side.  Does all that make sense?

After getting both sides to it's best idle point, I then balance the cables through the transition point and check at ±2500-3000 rpm via the regular method with manometer or Twinmax.  Done.  This has always worked for me.  Never allow the dead cylinder to run without grounding the plug lead.  Damage to the electrical system can occur.

I have many extra spokes and plug nipples if anyone is interested, happy to send them out with a PM.

[79beem]

 Engine off, attached spare plug, rested on fins, kicked her over.

[Barry]

Engine off, attached spare plug, rested on fins, kicked her over.

OK. So the engine was started on one cylinder.  Wouldn't have thought it was that easy. Perhaps much easier to start on one when the engine is hot than when it's cold which is our more usual experience of one cylinder running.

[79beem]

My mech said he rarely if ever used the manometer to balance cables or carbs. I recently rebalanced mine using his method with great results. Smooth through all revs, beautiful power delivery and sweet note . I have no problems starting on one pot , but this was after a long warm up run. No need for a throttle stop, but it couldn't hurt. Do make sure shorting plug has a good contact with pot  before starting engine.