Author Topic: Inconsistent ignition/spark?  (Read 1204 times)

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Offline Davez134

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Inconsistent ignition/spark?
« on: September 01, 2013, 12:48:08 PM »
I have been trying to search, troubleshoot and figure this out for quite a while now with no success. I have a 1972 Cb750, just rebuilt top end, 836cc, 4-1 exhaust, k&n filters,pamco ignition with new Honda coils, NGK d8EA plugs and new NGK 5k ohm caps. Bike runs good and I am being really picky here, but I want it to run the best it can. The problem I am having is with #2-3 cylinders. Every tune up I have done per manual, everything is perfect except when I set timing with inductive light, 1-4 marks line up perfectly with light flashing consistently. When I use the light for cylinders 2-3, it flashes erratically, seemingly missing ignition and timing mark is to the left of the f mark. I know it's not just equipment/tool error as I can hear it in the idle and 2-3 plugs are darker than 1-4, #3 more so (not fouled just dark dark brown) and won't rev above like 6500-6700 rpm. It did this with factory Honda points plate too. I tried my best to explain the best I could.  I'm open to trying anything at this point.

Offline Davez134

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Re: Inconsistent ignition/spark?
« Reply #1 on: September 01, 2013, 04:02:10 PM »
Forgot to add, I have the idle set at about 1100 rpm.

Offline K3Owner

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Re: Inconsistent ignition/spark?
« Reply #2 on: September 01, 2013, 04:31:47 PM »
You know, I had this same thing happen on my 78 550. #4 flashed perfectly but #3 was erratic. The bike ran well but I couldn't get a reading stable enough to verify the timing on 2/3. I found that switching to #2 gave a better signal. Perhaps the frame plays a role - the inductive pickup is very close to that grounding point on 2/3 and today's inductive tools aren't the best of quality - at least mine is not. The old days of high quality lights - Sears Best - are things of the past. Move that pickup around and make sure the contact is not dirty. You can clean them with pencil erasers.
1978 CB550K4 - yeah, I'm not a K3 Owner - my bad

Offline Davez134

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Re: Inconsistent ignition/spark?
« Reply #3 on: September 01, 2013, 09:09:47 PM »
You know, I had this same thing happen on my 78 550. #4 flashed perfectly but #3 was erratic. The bike ran well but I couldn't get a reading stable enough to verify the timing on 2/3. I found that switching to #2 gave a better signal. Perhaps the frame plays a role - the inductive pickup is very close to that grounding point on 2/3 and today's inductive tools aren't the best of quality - at least mine is not. The old days of high quality lights - Sears Best - are things of the past. Move that pickup around and make sure the contact is not dirty. You can clean them with pencil erasers.

Thanks for the reply! I have tried switching between 2 and 3 with no difference, I will make sure contact is clean and give it another try tomorrow. I was just thinking if it was tool/equipment error I wouldn't hear a "stumble" in engine idle as light flickers.

Offline HondaMan

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Re: Inconsistent ignition/spark?
« Reply #4 on: September 01, 2013, 10:16:45 PM »
If the spark advancer shaft is bent more than .010", the 2-3 spark will 'jitter'. I covered this well in my book? You will have to remove the advancer, install a dial indicator nearby, and retrue the shaft. Use GENTLE taps with a hammer to do this: it should be within .003".

After that, NEVER turn the engine backward with the big nut there, with sparkplugs in the engine. This is what causes the bent shaft.  ;)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Davez134

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Re: Inconsistent ignition/spark?
« Reply #5 on: September 02, 2013, 07:26:47 AM »
Thanks! That makes sense, explains everything thats going on. Now off to buy that book!

Offline Davez134

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Re: Inconsistent ignition/spark?
« Reply #6 on: September 03, 2013, 02:10:30 PM »
If the spark advancer shaft is bent more than .010", the 2-3 spark will 'jitter'. I covered this well in my book? You will have to remove the advancer, install a dial indicator nearby, and retrue the shaft. Use GENTLE taps with a hammer to do this: it should be within .003".

After that, NEVER turn the engine backward with the big nut there, with sparkplugs in the engine. This is what causes the bent shaft.  ;)

It was out by .007". Got it to where the gauge needle moves less than .001". What a difference!

Offline HondaMan

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Re: Inconsistent ignition/spark?
« Reply #7 on: September 03, 2013, 08:25:12 PM »
If the spark advancer shaft is bent more than .010", the 2-3 spark will 'jitter'. I covered this well in my book? You will have to remove the advancer, install a dial indicator nearby, and retrue the shaft. Use GENTLE taps with a hammer to do this: it should be within .003".

After that, NEVER turn the engine backward with the big nut there, with sparkplugs in the engine. This is what causes the bent shaft.  ;)

It was out by .007". Got it to where the gauge needle moves less than .001". What a difference!

That sounds about right: it starts the jitter at about .005" out.

Search Google for "lulu coupon" before you buy the book: there are some big discounts out there if you use it at their website. I use them, myself! :)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com