Topic: No spark

[Zax]

Hi. I have a 1978 CB750F that I picked up last year out of a shed. I have taken the year to get it road worthy (much thanks to this forum) for an out of province inspection which I have booked for tomorrow. Tonight I started it up to check a couple exhaust leak repairs completed yesterday. Shut it off after a couple minutes. Went back out a few hours later and it would not start. Pulled a plug and grounded it no spark. Pulled a plug from other coil no spark. Have voltage on black/white strip wire at coils. Have continuity from blue and yellow wires at points to coils. Checked pints gap and it is within spec. Pulled one condenser and tried spark test and nothing. I have no spark at points either (not sure if I should) when turning crank. But if I bridge point when open with feeler I get a spark there and at plug. This only works on one point.

Should I pick up a couple new condensers? What am I missing? A little desperate to get this sorted so I can finally ride it.

Thanks in advance.

Zach

[dave500]

had a look at any fuses?

[Zax]

Yes fuses are good. One thing I notched is voltage at coils drops to 9v while engine so   turning over. I wonder if there is not enough voltage to coils when starter is going.

[calj737]

What is the condition of the battery? Voltage before you turn on the key and ignition, and with the key and ignition on?

[Zax]

Voltage is 11.8 key off and 11.4 on. Holds pretty steady.

[Vinhead1957]

Check voltage to kill switch !

[Zax]

I have voltage after kill switch (at coils via black wire) and have continuity across switch.

[Spanner 1]

Looks like points are not actually grounding when closed.... a film of corrosion on the faces combined with less than optimum battery voltage when trying to start the bike will give results as described. Time to polish the contact faces of the points, charge battery and try again...... bike stored in damp weather conditions would not help this problem too !

Also, check your main ground cable connection at the motor mount bolt... must contact shiny metal of frame , not paint surface .

[Zax]

Thanks. A points file, is it a special type of file or can I use 1500 sand paper?

[Spanner 1]

A points file is always best....... sandpaper will leave some grit on the point faces = causes pitting. But if that's all you have go ahead. Hope that's the fix ...

[calj737]

To expand on the battery: 11.8v is an undercharged battery and a possible indication of a bad battery. Charge it fully to 13.2v, let it rest for 2 hours off the charger, then take a reading. If it doesn't hold at least 12.8v, replace it. Points issue as described could be your problem, but a bad battery will cause all kinds of other issues.

[Zax]

Battery was toast. Thanks for the suggestion. Popped in a new one and I am now half way there. For some reason I still have no power to one of the coils. The one fed by the yellow wire. Swapped the blue and yellow and still only get spark at plug from whichever coil the blue wire is feeding. Used some 1500 grit sand paper on point to no avail. I do see a spark once in a while while ranking at the pints attached to the yellow wire but never at the blue. I also swapped condensers but that was not it either.

[Don R]

 After sanding the points, polish them with an old fashioned business card by sliding it between the points. won't fix the problem but the points will last longer/work better.
 Have you put your test light on at the points and turned the motor to see if they are operating properly?

[calj737]

Battery was toast. Thanks for the suggestion. Popped in a new one and I am now half way there. For some reason I still have no power to one of the coils. The one fed by the yellow wire. Swapped the blue and yellow and still only get spark at plug from whichever coil the blue wire is feeding. Used some 1500 grit sand paper on point to no avail. I do see a spark once in a while while ranking at the pints attached to the yellow wire but never at the blue. I also swapped condensers but that was not it either.

Check the yellow wire for continuity first. If you're not seeing power on it at the coil, the problem lies somewhere along its path. It should derive the power feed from the ignition plate. Check the plate, and inspect closely the yellow wire connection, make sure its not grounding out. Perhaps a really clear, close-up picture of your plate would help?

[Zax]

Thanks for all the help. After replacing the battery I made up a wire to go from plate direct to coil and got spark on all plugs. Removed that and reconnected original yellow wire and it ran like a champ. No idea in the end what was going on but I do think the battery was the main culprit and diggin around in the electrical until the wee hours of there morning contributed to the issues.

Got it to the shop for its out of province inspection and things went back to bad. Got a call saying it failed because of, missing passenger pegs, one missing bolt on oil tank, one missing bolt on chain guard, rear brake fluid reservoir empty (pulled cap at home and it was full), needs new fork seals, seat lock and mount broken and the killer "bike will not shut off once running with out disconnecting the battery". I reminded him that I started and stopped it 3 times between unloading it and riding it into their service bay. It gets worse....picking it up later that day I dropped it on its left side, broke the rear signal light and dented the engine case (no crack thankfully). I cannot get it to repeat staying on with kill switch and key off and am starting on the he rest of hue he outstanding issues.

Sourcing the parts needed (including a new right side engine case) and hopefully next round will Finnish with a bike I can finally ride.


Thanks again for the support.


Zach