Topic: #4 cylinder won't fire -- tried all the standard checks

[moham]

spark plug boots.

replace em.

[db22]

#1 & #2 are firing again, but poorly.  Last night I picked up a cheap inductive timing light and found that it lit up when attached to #1 & #2 plug wires, but was dark on #3 & #4.  Removed #3 plug and found it wet with gas --  the carbs are OK but perhaps both coils have partially failed. . .

I have started trolling the For Sale page here for a set of second-hand coils. 

[steam-powered man]

ah, the vulture swoops!!!!!!!!!!!!

have used coils for sale, $22 plus ship (not much more that $8 am sure, depending) from 05735.  paypal is ok,
am just headed out the door, will check back here this afternoon.
bobp

[db22]

PM sent.

[Spanner 1]

Low battery voltage could cause what's happening..... are your points really shiny? Is the black/ whte wire 3-way connector at the coils clean and tight? ( sorry, it's under the tank ) What voltage do you have at each point when the points are held open ?  Stuff to check before blaming the coils  ;

[db22]

Spanner: Checked all the leads to/from the coils, cleaned the points with plain paper (took some soot away).  Everything looks as it should be.  If the weather here warms up a bit over the weekend, will try to get a voltage reading from the points. 

So -- Have begun to look in earnest for a good used coil set, and ordered new plug wires, caps and NGK splicers from Z1.  Can't hurt -- the ones in there now are 36 years old.  Now it's just "hurry up and wait".  And, I have my '68 Trail 90 to buzz around on. 

I remember "The Committments".  Good movie.  You a musician?

[Duanob]

#1 & #2 are firing again, but poorly.  Last night I picked up a cheap inductive timing light and found that it lit up when attached to #1 & #2 plug wires, but was dark on #3 & #4.  Removed #3 plug and found it wet with gas --  the carbs are OK but perhaps both coils have partially failed. . .

I have started trolling the For Sale page here for a set of second-hand coils.

That doesn't make sense. Unless all your plug wires are hooked up really wrong. Your 1 - 4 are the same coil and your 2 - 3 are the same coil. I would do some more research before spending the money on other coils.

[Duke McDukiedook]

Before you go blowing money on a set of coils test them with a voltmeter with an ohmeter according to the manual.
See if they are within specs.

I would be more willing to spend money on new condensors and points before I bought coils, coils just don't go bad that often while the points system parts takes a lot of abuse.

[db22]

Checked plug wires -- they are properly routed to their correct cylinders.  Okay, I'm going to think out loud here (stand back!!)  Juice flows from the battery to the points, where it is metered to the coils as needed (points open and close according to timing).  The coils increase the power and shoot it to the plugs, where jumping the plug gap causes a spark, igniting the fuel.  One set of points switches power to #1 & #4, and the other to #2 & #3.  One coil gooses #1 & #4, and the other #2 & #3.  Right now, #1 & #2 are firing, but the others are not.  This indicates that each point/coil circuit is functioning, but only half-way, right?  I checked the plug wires at the coils, and found no play or frayed insulation.  I trimmed the plug wires back .25" and re-installed the caps.  Same end result -- one and two hot, three and four cold.  #3 plug is wet with gas. 

So -- when I have some time this weekend, I'll try to measure voltage and resistance as advised in previous posts.  But I did spring $35 for a set of used coils, and ordered new plug wires, caps and splicers.  The weather is finally improving here -- I am getting antsy to ride!

[Duanob]

When you say hot and cold are you taking the plugs out and physically seeing if you have spark? Or are you just going by feeling the exhaust? #3 and 4 could have seperate things going on of you're getting fuel to one and not the other. You need to find out for sure if it's a spark problem or a fuel problem in each cylinder. 1 and 2 are firing off and getting hot so all is probably fine for now. 3 is not getting hot but getting gas could be a spark problem. 4 is dry? is it getting spark or gas? Could be a plugged pilot jet which is super common since the orifice is barely big enough for a C-hair. Does not take much at all to plug them.

You are correct in the flow of electricity from the plugs back but you need to test each component. Go down to harbor freight and get a cheap multimeter to measure voltage, ohms, etc. Then start the long process of elimination.

[TwoTired]

Juice flows from the battery to the points, where it is metered to the coils as needed (points open and close according to timing). 

Very close.  The points complete a the coil circuit which allows the coil to store energy.  When the points open the stored energy field collapses, which creates the high voltage need to create and arc at the spark plugs.

Right now, #1 & #2 are firing, but the others are not.  This indicates that each point/coil circuit is functioning, but only half-way, right?

I don't think so.  The points and coil are doing the job.  But something is preventing the arc at half of its output.  This makes the problem area narrowed to the wire, the plug cap, or the spark plug itself.  Further, since one side of the coil has produced an arc and current flow, the coil itself has proved itself good.  The non-productive side is shorted to the engine casework somewhere.

OR, the spark plug itself is shorted.  If the white porcelain insulator in the tip of the plug is coated with a conductive material (usually black), it is effectively shorted and the arc energy is diverted away from the gap.

Have you swapped spark plugs with working cylinders?  Have you put in new ones?  If the spark plugs are indeed fouled, you next have to figure out why they have fouled and correct it, or it will just happen again.

Cheers,

[db22]

Quoting Two Tired: "Have you swapped spark plugs with working cylinders?  Have you put in new ones?"

Oh brother.  I have committed a typical beginner's error: looking for a complex problem when the trouble was as simple as defective spark plugs.  At the start of this mess, I installed four new NGK D7EA plugs, and assumed they were all functioning, which led me (and all of you who tried to help) on a wild goose chase.  At Two Tired's suggestion, this morning I swapped the plugs, 1 and 2 with 3 and 4.  Right away, 3 and 4 fired up, while 1 and 2 remianed cold.  I installed the old D8EA plugs in the cold cylinders, and all four fired up.  You don't expect brand-new NGK plugs to be defective -- after all, they're Japanese, right?  Super-reliable!

So -- I learned a great deal about tracing faults in SOHC4 ignition systems.  I have become expert at removing and reinstallng SOHC4 carb racks, and hooking up choke and throttle cables.  And I realze that all the simple solutions must be pursued before blaming major components.  Now I will have a spare set of coils and new plug wires, and perhaps a little wisdom the next time something goes wrong. 

Thanks to all who responded!

[TwoTired]

I can make brand new spark plugs never work right by just coating the white porcelain with a conductive material

...like soot.

Cheers,

[Danno]

hey bud i hope you figured this out already but i had the same problem in the beginning with my 550 it was real frustrating to find that it was a tiny plastic insulator on the points allowing the points to short to ground but only when they bounced around from motor vibration you still get a spark on each cylinder but it is intermittent check the little insulator on the points where the wire attaches to the points it may be grounding out i used a tiny plastic pipe to insulate the screw from the frame of the points no more problem

[Danno]

hey bud i hope you figured this out already but i had the same problem in the beginning with my 550 it was real frustrating to find that it was a tiny plastic insulator on the points allowing the points to short to ground but only when they bounced around from motor vibration you still get a spark on each cylinder but it is intermittent check the little insulator on the points where the wire attaches to the points it may be grounding out i used a tiny plastic pipe to insulate the screw from the frame of the points no more problem