Topic: Coils 1 & 2 not firing - can't figure this out.

[murdrcycle]

Hello all - I am new to the site and looking for some help.

I have a 1978 CB750k with a very strange issue.  It is firing on cylinders 3 and 4, but won't fire on 1 and 2 (or vice versa).


I have coil #1 hooked up to the blue wire from the points and hooked up to spark plugs 1 and 4.
I have coil #2 hooked up to the yellow wire from the points and hooked up to spark plugs 2 and 3.
I have the black wire from both coils hooked up to battery.
So basically, each coil will only provide current to one wire.

I cant even figure out how this is physically possible.  Has anyone seen anything like this before?

[seanbarney41]

What have you done to check whether or not each spark plug is firing? Is it possible you have bad spark plug/s, bad spark plug cap/s, bad spark plug wire/s?

[scottly]

Hello all - I am new to the site and looking for some help.

I have a 1978 CB750k with a very strange issue.  It is firing on cylinders 3 and 4, but won't fire on 1 and 2 (or vice versa).

Hi, and welcome! What do you mean by "or vice versa"? Do you know how the cylinders are numbered?

[murdrcycle]

What have you done to check whether or not each spark plug is firing? Is it possible you have bad spark plug/s, bad spark plug cap/s, bad spark plug wire/s?

I ann using a timing light so the spark plugs are out of the equation.

[murdrcycle]

Hello all - I am new to the site and looking for some help.

I have a 1978 CB750k with a very strange issue.  It is firing on cylinders 3 and 4, but won't fire on 1 and 2 (or vice versa).

Hi, and welcome! What do you mean by "or vice versa"? Do you know how the cylinders are numbered?

Wires 1 and 4 are interchangeable as are wires 2 and 3.  Only one errors from each coil is working.

[scottly]

I ann using a timing light so the spark plugs are out of the equation.

Inductive timing lights are polarity sensitive; try reversing the pickup on the non firing coil leads.

[seanbarney41]

What have you done to check whether or not each spark plug is firing? Is it possible you have bad spark plug/s, bad spark plug cap/s, bad spark plug wire/s?

I ann using a timing light so the spark plugs are out of the equation.

I don't think that is an accurate way of testing for spark.  I can't exactly remember the technical reasons why it doesn't work.  Might be something to do with the ignition system being wasted spark and polarity, I dunno...I would remove each spark plug and ground it against the valve cover and crank the engine.  The spark should be visible and located between the ground strap and the electrode.  Also, test plug caps with multimeter.  5000 to 10,000 ohms is acceptable at this point.  Heck, I would just go ahead and get new spark plugs, it can save a lot of hassle and they are cheap and readily available.

More help here...
http://forums.sohc4.net/index.php?topic=5752.0

[HondaMan]

There are 3 things I've seen that can cause this symptoms.

1. If one of the sparkplugs on a given coils' 2 leads is fouled, it will drop the spark energy to less than 50%. This causes the other plug to not fire at all.

2. Likewise, if one of the sparkplug caps has resistance of more than 10% higher than the other cap on this same coil, only one side will fire.

3. A bad coil will also do this, but it's pretty rare for both coils to fail at once. It usually is #1 or 2 above.

The plug cap resistance of the post-1975 bikes is 10,000 ohms if the caps are OEM type. Unscrew them from the coil wires and measure the resistance between the brass screw on the wire side and the plug's socket inside the cap. On any one coil, the resistance between the caps must be equal within [less than] 1,000 ohms, and not over 11,000 in any case. Replace them in pairs. Today, we mostly find the 5000 ohm types in most bikes shops, which will work fine.

[murdrcycle]

Well here I am 6 months later and I am still experiencing this problem.  I am out of ideas here.  Does anyone else have any thoughts on the matter?

[Vinhead1957]

You need more detective work !   Pull the plugs. Are they wet with fuel? If not your problem could be fuel delivery. If they are wet change them. But before you install them put one in a plug cap and ground the base to the engine and turn the bike over to look for spark. A yellow spark is weak. A purpleish blue spark is good!  Be a detective and report your findings

[calj737]

Well here I am 6 months later and I am still experiencing this problem.  I am out of ideas here.  Does anyone else have any thoughts on the matter?

Did you follow any of the advice from your original post? If so, what have you discovered? Your information is incomplete and you have not listed any further steps taken to diagnose the problem.

[madmtnmotors]

Well here I am 6 months later and I am still experiencing this problem.  I am out of ideas here.  Does anyone else have any thoughts on the matter?

Did you follow any of the advice from your original post? If so, what have you discovered? Your information is incomplete and you have not listed any further steps taken to diagnose the problem.

+1

What steps have you taken?

[murdrcycle]

Changed the plugs (multiple times)

Took the boots off the coils, re-cut the wires, re-attached the boots

Changed the coils (I have now tried two sets of coils right out of the box)

Ran an entirely new wiring harness

Re-jetted the carbs (in case that was fouling out the plugs)

Changed the air filters (in case that was fouling out the plugs)

Changed the fuel lines (in case that was starving it for gas and fouling out the plugs)

Checked the timing

Replaced the points and condensers

Replaced the points plate

Checked the compression (125-135 on all cylinders dry, 140 on 3 and 160 on 1 wet)

Added an oil overflow bottle with a breather filter in case there was a venting problem

[madmtnmotors]

Did you replace, or at least check the resistance across the spark plug boots themselves?

if one of the sparkplug caps has resistance of more than 10% higher than the other cap on this same coil, only one side will fire.


The plug cap resistance of the post-1975 bikes is 10,000 ohms if the caps are OEM type. Unscrew them from the coil wires and measure the resistance between the brass screw on the wire side and the plug's socket inside the cap. On any one coil, the resistance between the caps must be equal within [less than] 1,000 ohms, and not over 11,000 in any case. Replace them in pairs. Today, we mostly find the 5000 ohm types in most bikes shops, which will work fine.

I just replaced a spark plug boot that would only show high resistance when hot. Checked out within 1K ohms of the other four when cold, but after warm up I started hearing a "ticking" noise that turned out to be the arc jumping to the head because the resistance through the boot had spiked to almost 20K when hot.

By the way, they are a P.I.T.A. to check when hot! 

[calj737]

Checked the timing

In your original post, you indicated that you were checking without plugs, and the issue of using an inductive light was discussed. Can you explain how you tested this time? The same manner? The same results?

[PeWe]

No spark on plug 1 and 2?
OR do these cylinders just not ignite? This can be lack of fuel to these carbs. I guess that you have a T-pipe between carb 1-2 where a fuel hose is connected. Hose is not blocked or have a kink?

[flybox1]

Did you replace, or at least check the resistance across the spark plug boots themselves?

+100
no mention of this being done.

[murdrcycle]

I got 1 & 2 to fire, but only for a short time.  After about 30 minutes they went out again.  I checked the plugs and 3 & 4 looked healthy, 1 & 2 were shot.  I used the timing light and a spark plug to check for spark, and 1 & 2 weren't receiving spark - the coils weren't pushing any electric out to 1 & 2.  I replaced the plugs on 1 & 2 and there was no difference.  I replace the plugs on 3 & 4 and 1 & 2 started sparking again.

Did you replace, or at least check the resistance across the spark plug boots themselves?

if one of the sparkplug caps has resistance of more than 10% higher than the other cap on this same coil, only one side will fire.


The plug cap resistance of the post-1975 bikes is 10,000 ohms if the caps are OEM type. Unscrew them from the coil wires and measure the resistance between the brass screw on the wire side and the plug's socket inside the cap. On any one coil, the resistance between the caps must be equal within [less than] 1,000 ohms, and not over 11,000 in any case. Replace them in pairs. Today, we mostly find the 5000 ohm types in most bikes shops, which will work fine.

I just replaced a spark plug boot that would only show high resistance when hot. Checked out within 1K ohms of the other four when cold, but after warm up I started hearing a "ticking" noise that turned out to be the arc jumping to the head because the resistance through the boot had spiked to almost 20K when hot.

By the way, they are a P.I.T.A. to check when hot! 

I have two new sets of coils.  Each one came with a new set of caps.  I also have the original caps for a total of 3 sets.  I have tested all of the caps and each one is good.  I have tried all 3 of the sets on  both sets of the coils and the results are identical.

Checked the timing

In your original post, you indicated that you were checking without plugs, and the issue of using an inductive light was discussed. Can you explain how you tested this time? The same manner? The same results?

I test with both a timing light and with a plug each time.  FYI - I can definitely see where a timing light could theoretically have an issue, but have never had an instance where the timing light wasn't accurate.

No spark on plug 1 and 2?
OR do these cylinders just not ignite? This can be lack of fuel to these carbs. I guess that you have a T-pipe between carb 1-2 where a fuel hose is connected. Hose is not blocked or have a kink?

I am running a dual-carb system from Cycle X.  I have replaced the fuel lines twice to rule that out.  The carbs are synched per instructions and I have had 2 other people check them to verify.

[madmtnmotors]

I have tested all of the caps and each one is good.

What tests were performed? Did you use an ohmmeter to measure the resistance across the spark plug boot? If so, what were the readings?

What is the status of the battery? New? Old? Fully charged?

What plugs are you using? D8EA? X24ES-U? Where did you buy them from? Some have reported counterfeit NGK D8EA plugs that fail miserably. For this reason alone I prefer the Denso X24ES-U.

I got 1 & 2 to fire, but only for a short time.  After about 30 minutes they went out again.  I checked the plugs and 3 & 4 looked healthy, 1 & 2 were shot.

"1 & 2 were shot"? What does this mean?

[murdrcycle]

I have tested all of the caps and each one is good.

What tests were performed? Did you use an ohmmeter to measure the resistance across the spark plug boot? If so, what were the readings?

What is the status of the battery? New? Old? Fully charged?

What plugs are you using? D8EA? X24ES-U? Where did you buy them from? Some have reported counterfeit NGK D8EA plugs that fail miserably. For this reason alone I prefer the Denso X24ES-U.

I got 1 & 2 to fire, but only for a short time.  After about 30 minutes they went out again.  I checked the plugs and 3 & 4 looked healthy, 1 & 2 were shot.

"1 & 2 were shot"? What does this mean?

I checked them with an Ohmmeter.  I don't recall what the readings were off the top of my head, but they were all good and all consistent with each other.

New and fully charged battery.

I have been using the D8EAs.  I purchased 2 sets from Autozone and 2 sets from NAPA.

Shot means that they were black with residue and wet with gas.

[madmtnmotors]

The only common denominator here is the dual carb setup, everything else has been replaced. Can the 1/2 carb be swapped with the 3/4 carb?

[murdrcycle]

The only common denominator here is the dual carb setup, everything else has been replaced. Can the 1/2 carb be swapped with the 3/4 carb?

I have switched them back and forth a few times.  I also changed to smaller jets in case the cars were providing too much fuel.

[madmtnmotors]

Then there's nothing left to replace. All I can suggest at this point is to try a set of Denso X24ES-U.

While you're waiting for those to arrive, remove the fuel tank and take close up pictures of your coils with the spark plug wires attached to the plugs. Be sure to get the coil wiring in the picture as well. Pull the points cover and get a couple close ups of that too.


One more question... The D8EA plugs typically come with screw on terminals for the posts on top of the spark plugs. The stock spark plug boots do not require, nor will they attach correctly, if these screw on terminals are used on the D8EA posts. Did you use the screw on post terminals on the D8EA plugs, or are the threaded posts exposed on the top of the plugs?

[madmtnmotors]

Another six month hiatus?

[calj737]

If they're black and wet (plugs are shot) isn't it more likely a case of fuel than lack of spark? Quite possibly his carbs are dumping so much fuel in, they're fouling not due to weak spark, but due to a too rich condition. If they didn't fire, how would there be carbon fouling? They'd be bright, shiny, and wet.