'Dead Cylinder Method' for carb setting...?

[madjap]

Wanna make sure I'm gonna do this right. ONLY reference I saw to it in Tech section refers to 'earthing the lead'... So, do I need to pull my spark plug wire off and ground it to the motor??? I have the stock dual points on my bike, I'm running a dual output / single fire coil and BOTH my points are tied together at the coil. My bike WON'T run if I simply pull a plug wire off one side, it will die even at high RPM if that happens.

 

[gggGary]

Wrap some bare copper wire twice around a plug base and anything metal on the engine. yes, dual fire coils need both plugs grounded to work.

 

[madjap]

THANK you sir!

 

[retiredgentleman]

You can make up a dummy plug for testing use , such as dead cylinder. A piece of household wiring, solid copper 12 gauge can be used. Use a hose clamp to attach the wire to the plug base. Put an alligator clip on the other end of the wire.
Remove one spark plug lead, and connect it to the dummy plug, then clip the alligator clip to an engine cooling fin.

You should never allow any ignition coil to fire its high voltage, unless the spark has a path to ground. A dummy spark plug is a simple and easy way to do that. Without an easy path to ground, the high voltage will try to find a path (i.e. arc through the insulation), and that may cause the high voltage winding to be damaged.

Warm the engine up first. Turn up the idle speed screw, so that the engine will idle on one cylinder. If your carbs are working properly, the engine will run on one cylinder. In fact the engine should idle down to 700 or 800 rpm on one cylinder. The lower the rpm, the easier it will be to find the sweet spot when adjusting the mixture screw. You want to have the carb's butterfly closed as much as possible, so that the fuel mixture is mostly flowing through the mixture needle path. Use a fan to blow air over the engine.

 

[Bob Kelly III]

When I did my "Dead cylinder test" I went through this procedure..... its a 83' xs650 Heritage special.
.......
I started the bike up and let it warm up for about a minute warm enough to have the choke off and still idle....
I raised the idle to about 1500 RPM with the screw on the side of the carb linkage.
I turned the key off pulled the left spark plug lead wire and put my dummy plug in the spark plug lead and clipped the ground to the head
( the gas tank was already unbolted and propped up with a thick screw driver handle) so I could get to the idle air screws with my short screwdriver....
I started the engine back up and it ran fine on 1 cylinder I adjusted the idle down to about 800RPM or less and then turned the idle air screw for the fastest rpm I could get I turned down the idle again on the linkage and re did the process to make sure it was spot on.... it was.
I turned off the bike and then did the exact same thing to the other cylinder.... making sure I got the highest speed I could get by adjusting the idle air screw while keeping the RPM as low as possible..... its not necessary to go too slow just as long as it's not laboring at idle on one cylinder is a good speed.....
I then took off the dummy spark plug and put the lead back on the sparkplug in the cylinder and fired it up and set the idle on the linkage to around 800RPM.....
this procedure cured my intermittent Missing and dropping of the right cylinder and in general crappy running....
it idled perfectly and never missed a beat ! ..... I turned it off and the next day I synced the carbs by turning the screw between the carbs on the linkage ...it was a bear to reach but I got it as close as I could with my Uni-Syn carb syncer.
about a week later I made up 2M's design of a home made carburetor synchronizer with the reservoirs on top ... I started the bike and let it warm up and then hooked the carb syncer to the intake ports on the intake manifolds.... ( note I had to put the petcock lever to all the way back for a constant flow of fuel sense I disconnected the vacuum line to the petcock.) I started the bike with it all hooked up and the oil in the sight tube immediately started climbing up one side so I quickly turned the adjustment screw and it's climb slowed down.... turned it again and it started dropping before it hit the reservoirs ! ...... I let it idle a few seconds till the oil was about equal on both sides again and then barely touched the screw one last time to get it to stay put.....
the difference in how it ran was amazing I never felt that bike that smooth before !
i took it for a quick spin down the road and I was amazed at how well it was running it had power and smoothness like it was brand new !
.....
Hope that helps !
....
Bob........

 

[madjap]

TREMENDOUS instructions chentlemen! Now THATS what an old dum-dum like ME needs to see / read to understand! Feeling much more confident now...

 

[Skip]

My question with using this method is the fact that the cylinder which is unplugged is literally being washed down with fuel...I gotta believe this is a harmful senario..

 

[Downeaster]

I made up a grounding cable for checking spark on ANY small engine. Just a pair of large-ish alligator clips on about a 3 foot piece of heavy stranded automotive wire (for the flexibility) One is big enough to fit over the threads of the plug and the other clip can be grounded to any handy metal surface.

Way better than trying to hold a plug against a fin or whatever, not to mention getting knocked on yer arse if you grab it wrong.

 

[Skip]

I have a slick spark plug ground set up...that's not my issue....My question is regarding the unburned fuel washing the unplugged cylinder and the harm that can do

 

[5twins]

I don't doubt that could happen. I don't use the dead cylinder method because I've never found the need to. I can usually hear the differences my adjustments are making with both cylinders still running, at least on the '79 and older BS38s. The '80 and newer BS34s are so E.P.A. strangled, mix screw adjustments seem to have little effect. It seems you need to get them way far open or almost closed before something you can hear happens. I don't see killing one cylinder making any difference in that. You also run the risk of damaging the ignition or coil on the '80 and later set-up if the grounded plug loses that ground.

I never really considered what you're saying, about washing the cylinder walls, but it makes sense. Guess that's yet another reason I won't be using the dead cylinder method, lol.

 

[pamcopete]

On models that have a vacuum port on the carb holder, you can do the "dead cylinder" by simply removing the hose or cap from the vacuum port one carb at a time. The resulting lean mixture is too lean for that cylinder to fire it, so that cylinder just stops working. Much easier than grounding a plug because you can switch from one cylinder to the other without having to stop the engine. If you have vacuum operated pet cocks, just set the fuel selector to prime so disconnecting the hose will not cut off the fuel flow.
If you have dual exhaust, another method to sync the carbs for carbs with that center idle adjustment screw for the right carb, is to simply adjust that center idle adjustment until both exhausts sound the same..

 

[Jim]

My question is regarding the unburned fuel washing the unplugged cylinder and the harm that can do

I guess that with the small amount of fuel at idle and a hot cylinder and head, the fuel vaporizes as soon as it enters the cylinder. I can't really see it washing the walls.

 

[Skip]

That is certainly true....there would be bit of burn off from cylinder temperature....
I do like pamcopete idea of leaning out the dead cylinder via the vaccum leak...

 

[Bob Kelly III]

Skip....but your still having that unburnt fuel in that cylinder..... if that is your big worry ! ...
I've never had an issue using the dead cylinder method but on my 82' it was an absolute must to do, to get the fuel/air screws adjusted correctly
ear didn't work ( of course I'm hard of hearing now....but I can still hear it!) the adjustment screws on the BS34's like 5T said didn't seam to do anything.... but a long way out there was a change.... it wound up right about 4 turns out on those puppies ! but it purred like a kitten after that !
I Seriously doubt any damage whatsoever, can befall your motor by doing the dead cylinder test and WASHING DOWN the cylinders with gas...
because its a very small minute amount of fuel ! the only possible damage that could occur in my mind is the spark plug loosing it's ground and hurting the coil because of it......a spring clamp with a spark plug clamped to it with a hose clamp is what I use.... and It's never fallen off !
..... needless to say if the airscrews are that far out somethings wrong right ? ....well there is...its E.P.A. Crap.... ! to get it correct you'ed have to re-jet the carburetors completely.... but I didn't do that....
and at last check the bike started quickly and purrs like a kitten
..... I caution about feeling the exhaust or such methods to adjust the carburetor.... yes you can listen to the engine and hear one cylinder that is heavier than the other... but which one ? feeling the exhaust should tell you but it doesn't all the times on all bikes ! the XS650 has a cross over pipe in the stock exhaust....( I cut it off and plugged that) ... so I can take off one pipe if I want to ! that's very handy !
as you get to be a Pro like 5T and others here you will learn the sounds of each cylinder as its idling.... yes you can tune by ear ! but for those that have not acquired that expertise yet its best to use a method even a dummy can use..... such is the Dead cylinder method !
if you're having trouble hearing the change when you adjust the fuel /air mixture screw for engine idle, then do the dead cylinder method because it is allot easier to hear !!!!!
my 2 coppers !
Bob.....

 

[Skip]

Well put Bob....thank you