Topic: Seemingly No Change After Pulling Spark Plug Boot on Running Engine

[psykod]

I have a smooth running engine that idles well at 1000 rpm. If I pull the plug boot on cylinders 1,2, or 4, the engine dies immediately.  However, if I pull the plug on #3, nothing seems to change. 

I'm just wondering if this is normal or a further indication of a problem with #3 (see other post).

The exhaust is hot (sizzle test) and the plug is a good tan color with no wetness.

[camelman]

I wouldn't use the sizzle test as your only measurement.  The surrounding cylinders will pump enough heat through the cylinders/head and exhaust clamps to heat up that exhaust pipe.  Plus, any air passing through the cylinder will be heated up.

I bet you have an issue with that cylinder firing.  An engine can run smoothly enough on three cylinders, but will definitely lack power.

Camelman

[scrapvalue]

Are you doing this test by just starting the bike and letting it idle or are you reving it up before you pull the plug.
Sounds like you may have a pluged pilot jet in #3
Rev the bike up with all plug wires on and then rev it up with #3 off. Does it run differently?

[Hush]

Your SOHC4 will run quite happily on 3 cylinders and unless you new your bike really well you would have a hard job picking it, lack of power at speed would be a good indication.
As Scrapvalue said, sounds like you have a carb problem, the plug wont be wet if that cylinder is not getting gas, nor will it's colour change as there is nothing to coat it.
Drop the bowl on #3, my bet is it will be either dry or down below float level, could be something simple like a jammed float, I had this happen on my 650.
Also you could start with the old clear tube to the carb bowl drain trick and see exactly how much gas that carb is getting before you tear it apart.

Also check the plug lead, it sometimes helps to remove the plug boot and chop off 1/4 inch of the coil lead then thread the plug cap back on.

[Bodi]

It will run on three cylinders OK, but if it runs on 3 with one cap off you've got a cable problem - now you do at least. The spark has to jump to ground on both ends of the coil, through the 2 cables... usually on two spark plugs. The coil will try and deliver enough voltage to make a spark and it can make a LOT of voltage. If there's no plug attached to one end and the other end's plug is firing then the disconnected end of the coil IS sparking somewhere! Usually though the cable insulation to the frame or tank bottom, sometimes inside the coil housing. Both are bad ideas, the spark will create a burnt insulation carbon track which will lower the voltage needed to spark there... maybe lower than the voltage needed to spark the plug at some RPM and throttle level.
Basically you should NEVER let the coils fire with a cable - or both - disconnected.
If you have the original cables they're due for replacement by now anyway.

[Hush]

But Bodi, if his plug is dry on #3 then he is not getting gas to that plug anyway....carb problem.

[psykod]

Rev the bike up with all plug wires on and then rev it up with #3 off. Does it run differently?

Thanks,  I will give this a try.

If I turn the bike over without starting it and check the plug, it is wet with gas. So I'm sure I'm getting gas.  The bike runs smoothly as fast as I want to take it without stuttering.  I can easily take it to over 100 (indicated) without a hiccup and it rolls on power smoothly from idle right up to redline.

[fmctm1sw]

It will run on three cylinders OK, but if it runs on 3 with one cap off you've got a cable problem - now you do at least. The spark has to jump to ground on both ends of the coil, through the 2 cables... usually on two spark plugs. The coil will try and deliver enough voltage to make a spark and it can make a LOT of voltage. If there's no plug attached to one end and the other end's plug is firing then the disconnected end of the coil IS sparking somewhere! Usually though the cable insulation to the frame or tank bottom, sometimes inside the coil housing. Both are bad ideas, the spark will create a burnt insulation carbon track which will lower the voltage needed to spark there... maybe lower than the voltage needed to spark the plug at some RPM and throttle level.
Basically you should NEVER let the coils fire with a cable - or both - disconnected.
If you have the original cables they're due for replacement by now anyway.

Reading this reminds me of my coil mod woes: http://forums.sohc4.net/index.php?action=dlattach;topic=63141.0;attach=98495;image

[Hush]

OK so it's getting gas...good now revert to--->

Also check the plug lead, it sometimes helps to remove the plug boot and chop off 1/4 inch of the coil lead then thread the plug cap back on.

[psykod]

Rev the bike up with all plug wires on and then rev it up with #3 off. Does it run differently?

I checked and yes, the bike does run differently (a little more roughly) when the plug #3 is pulled and the engine is revved.  Also, there is a definite change when I run the engine up and hold at over 2000 rpm and pull the plug...but not as much as I would have thought.  It's like the engine just drops a couple of hundred rpms but still runs relatively well.