Topic: '69 CB750: weird spark thing on 2 & 3

[chaintong]

Hi all,

I have finally gotten my 69 cb750 back on the road, but it has this strange problem:  a really weird ignition thing that’s beyond me…

The bike always starts on the outer two cylinders only. It will bring number 2 cylinders in after about 5 minutes warm up, and number 3 only after a good run - but not always.

When 2 & 3 aren’t running, if I pull the plug and hold it against the block, then is see there is no spark. The weird thing is if I lift the plug slightly away from the block I can watch it spark, across the plug gap and then from the plug to the block.

In fact I can hold the plug almost ¼” off the block and it still sparks fine. If I then put the plug back against the block, no spark again!

There is nothing fancy about the ignition system: it’s complete stock. I have followed the Honda book for setting up the points, advance etc.

Some pointers about what to try next would be really helpful.

Thank you

[nteek754]

Hey there  chain, maybe you already did this but new plugs? and then maybe snip just a bit off the coil wire to get rid of any freyed wires. and it could be just that the wires to shose cylinders are leaking  or come on and off as you move them around. also start the bike up in the dark and look for spark jumping around. good luck hope this helps Craig in Maine

[736cc]

Old coils, coil wires and spark plug caps cause all sorts of intermttent problems. Try strating and runnung your bike in a darkened garage and watch for sparks around the wires. Any cracks in rubber will allow this.

[OldSchool_IsCool]

Sounds like your spark is actually pretty strong if it can jump a quarter inch gap.  Just so we understand, you can get the quarter inch jump during the time that #2 & #3 aren't fireing, right?

Have you looked into other possibilities?  carbs, compression, valve adjustment, timing of the 2.3 points, etc?

hmmmm '69 750.  What are the first 6 or 7 digits of the engine serial number?  Sandcast, perhaps?

By the Way, welcome to the forum, Chaintong!!

[chaintong]

sandcast - no, I don't think there are any left in the UK-  it is one of the early UK ‘die cast’ bikes.

The coils (and leads) are new I have replaced plugs, (I've tried two different sets), replaced the plug caps, etc – there is no part of the ignition system (except the wiring) that is not new.

I have followed the manual religiously, I have timed the bike (statically), adjusted the tappets, carbs are overhauled (new jets etc), Not sure about the compression, but assumed to be good because when eventually it does run on four (when really hot)  then it goes nicely to 80, and there more from there but it get a bit of a wobble on, and since it has no tax, plates, insurance,  It’s talking all my will power not to take it out more, any way I’m going OT,   the carbs are balanced, and I think the timing of 2.3 points are right (I might have another go at these because this is where I think the problem lies)

Old school… you understand right …  I get about 1/4 inch when the #3 is not firing, in other words I lift the plug off the cooling fin, and it start firing, I put the plug back on the fin it stops.

[bryanj]

Check both points gap AND timing repeatedly as altering one changes the other

[KB02]

The coils (and leads) are new I have replaced plugs, (I've tried two different sets), replaced the plug caps, etc – there is no part of the ignition system (except the wiring) that is not new.

Is the points plate new or the stocker? It could be that the conde... comp... um... the little round things that sit on the plate... could be they are going bad and not functioning properly. (Those parts do have a name... I just can't think of it right now). You might check with your local Honda dealer to see if they have a new one available. (Or a Dyna...)

[chaintong]

ok.... here it is,

I measured the resitance accross the plugs, and it turns out the insulation has broken down, Ie. there is no resitence between the electrode  and the plug body.

the really weird thing is that its on 3 of the plugs I bought out of 8! - these are new NGKs. once I had replaced the plugs there wasn't a problem.

I have never known known plugs to go bad.

[Haggis]

Hi Chain,i've had quite a few problems with NGK plugs in the last few years,all be it on Zed motors and now use Denso plugs.Not too sure if its anything to do with the petrol we get here  now  In the past NGK was the only one to go for

[TwoTired]

I measured the resitance accross the plugs, and it turns out the insulation has broken down, Ie. there is no resitence between the electrode  and the plug body.

Huh?

The center electrode and the plug body should have infinite (very, very high) resistance between them.  If you have resistor plugs, then the internal resistor is in series between the top cap and the firing center electrode in the chamber.

If you HAVE resistance between the center electrode on the plug body, then it is likely that the plug is carbon fouled and shunting the spark voltage past the electrode tips, causing no ignition, and the center electrode insulator may have a black or certainly non-white coating where it fits into the chamber.

Cheers,

[chaintong]

Two tired...

You right!! At last the problem has been solved, I got the wire brush on my dremmel out and cleaned up the plugs, and sure enough the resistance went up to infinite!!!

I guess carbon is a really good conductor,  I guess I also need to look at dropping my needles a notch!