Topic: Inconsistent ignition/spark?

[Davez134]

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.

[Davez134]

Forgot to add, I have the idle set at about 1100 rpm.

[K3Owner]

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.

[Davez134]

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.

[HondaMan]

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. 

[Davez134]

Thanks! That makes sense, explains everything thats going on. Now off to buy that book!

[Davez134]

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!

[HondaMan]

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!