Honda-SOHC

SOHC.co.uk Forums => CB750 => Topic started by: Saesneg Shaun on May 04, 2022, 12:49:56 PM

Title: No Spark
Post by: Saesneg Shaun on May 04, 2022, 12:49:56 PM
Hmmm, my k6 is not running.
It blew a main fuse and broke down on a ride last week.
The original fuse box was a corroded mess and just fell apart which I (wrongly) assumed was why the fuse blew.
New genuine fuse box from DSS fitted but still no joy.
I have no spark at the plugs. I've got 12.6V at the fuses but only 10.8 at the coils?

Anything obvious i'm missing before I start pulling the loom apart?
Title: Re: No Spark
Post by: K2-K6 on May 04, 2022, 01:07:56 PM
Check inside the kill switch handlebar installation to see if it's got a problem there.

Also the electrical switch on the back end of ignition barrel can be vulnerable to years/wearing/corrosion too. The switching mechanism is brass pads and copper bridges spring loaded and swung around to make contact with this pins, you can carefully prize the plastic cover open to look inside if that's the area you pinpoint (do it over a tray or similar as the bits can piiiiing) then clean and reassemble with silicone grease or vaseline if you've not got any ;D

Obvious other elements are to just check all connection for obvious signs of corrosion.
Title: Re: No Spark
Post by: Saesneg Shaun on May 04, 2022, 02:56:33 PM
Thanks K2-K6

Just dismantled the ignition switch and kill switch but no change.
They were both clean. I now have 11.8v at the coils so its improved a bit but still no spark at all?
Title: Re: No Spark
Post by: Oddjob on May 04, 2022, 03:01:16 PM
Also check the connections on the block connector the fuse box connects with. It's possible they were the fault in the first place and it was them failing and connecting with something it shouldn't that caused the fuse to blow.
Title: Re: No Spark
Post by: K2-K6 on May 04, 2022, 03:18:40 PM
Thanks K2-K6

Just dismantled the ignition switch and kill switch but no change.
They were both clean. I now have 11.8v at the coils so its improved a bit but still no spark at all?

Should be ok to spark at 11.8v without any real problems.  If that's a stable v there, then it suggests that the points or something related to them is an issue. 

Points are switching the earth side of the coil on and off, but if they stay earthed all the time (removing earth causes the spark) you'll get no sparking.

Basics, sure you've realistic points gap ? All little washers where the wires bolt to the points themselves look ok ? Any sparks at the points when you crank it ?
Title: Re: No Spark
Post by: Saesneg Shaun on May 04, 2022, 07:01:36 PM
I checked the static timing and points. It all seems fine but I slackened them off and set them again and it now runs on 1 & 4.
Curious as i'd not touched the points since i've owned the bike but it runs, sort of!
2 & 3 still not sparking though. Its quite hard to set them as the centre cam that the points run off is really slack and can be rattled about on the shaft. I'm guessing it isn't supposed to be like that
Title: Re: No Spark
Post by: Saesneg Shaun on May 04, 2022, 07:08:12 PM
Also check the connections on the block connector the fuse box connects with. It's possible they were the fault in the first place and it was them failing and connecting with something it shouldn't that caused the fuse to blow.

Could be. I did find a bodged wire soldered between the back of the 15A fuse holder and the brown/white wire from the ignition switch, gawd knows why as the original wire seems fine!
Title: Re: No Spark
Post by: K2-K6 on May 04, 2022, 07:11:24 PM
Normally it wouldn't be rattling around, likely part of the problem for you.

Not hard to take apart, undo 10mm nut in centre, lift off the large "crank" nut, undo the securing screws that hold the whole points plate on and you can swing that out the way.

Then the cam (advance retard mech) can just be pulled off to look at it. It's got a location pin on rear so you can put it back in same place. Low risk to check it out and see if anything is out of place.
Title: Re: No Spark
Post by: Oggers on May 05, 2022, 02:51:18 PM
A slack cam is no good at all! ATU weights should move out freely, springs free of any crud. Oil the pivot points perhaps. Also check out all screws holding points for 2 and 3 are tight, and re-check the gap. Clean the points. Check out all electrical connections to 2 and 3 points and check out the condensor -  with ignition on and if it sparks when you open the points gap, then a possible condensor problem. Have you a spark across the points when it turns over? 
Title: Re: No Spark
Post by: Saesneg Shaun on May 05, 2022, 03:29:38 PM
A slack cam is no good at all! ATU weights should move out freely, springs free of any crud. Oil the pivot points perhaps. Also check out all screws holding points for 2 and 3 are tight, and re-check the gap. Clean the points. Check out all electrical connections to 2 and 3 points and check out the condensor -  with ignition on and if it sparks when you open the points gap, then a possible condensor problem. Have you a spark across the points when it turns over?

I can get it to fire but only on 1-4. I have checked the (new) spark plugs and 2-3 ins't sparking.
With 1-4 F mark lined up and the points gap set to 0.4mm the points don't break the circuit within the timing slots on the base plate.
Also I seem to have a problem where the points arm touches the cam as it doesn't look like the little contact arms touch the cam at its lowest point?

The cam nut wasn't tight but wiggle on the cam was just end float so that seems ok.
I don't get this, she's always been a sod to start but was running ok until the main fuse blew?
Title: Re: No Spark
Post by: K2-K6 on May 05, 2022, 03:44:04 PM
Quick reality check, swap the two wires over coming from coils to points and see if the 2/3 coil fires then. It won't run at all but will show (with confirmed switching from the 1/4 points, if the 2/3 coil will make a spark) and which part of the system is working.

It sounds like the small rod that secures the points cam may be bent. As my routine above, strip the points plate and advance mech, then spin the motor to see if the rod is bent (if so it can give all sorts of errors in points gap) and at least give a baseline to work from.
Title: Re: No Spark
Post by: Saesneg Shaun on May 05, 2022, 04:11:21 PM
Quick reality check, swap the two wires over coming from coils to points and see if the 2/3 coil fires then. It won't run at all but will show (with confirmed switching from the 1/4 points, if the 2/3 coil will make a spark) and which part of the system is working.

It sounds like the small rod that secures the points cam may be bent. As my routine above, strip the points plate and advance mech, then spin the motor to see if the rod is bent (if so it can give all sorts of errors in points gap) and at least give a baseline to work from.

Haven't swap the leads yet but i'm not so sure about the points cam now. The centre shaft runs true ish but theres lots of play in the whole assembly?
Couple of vids attatched

https://youtube.com/shorts/uI7pr4SpOKE?feature=share

https://youtube.com/shorts/wGs-SIJDHKI?feature=share
Title: Re: No Spark
Post by: K2-K6 on May 05, 2022, 04:29:01 PM
Definitely something wrong with securing the advance mech there.

Agree the rod looks reasonably ok, but mech shouldn't be moving like that and with any real slack at all.

You'll get no real accuracy in timing, gap and dwell to set it up correctly. Should have a washer under the six mm nut to secure the bigger nut, is there anything there ?
Title: Re: No Spark
Post by: Saesneg Shaun on May 05, 2022, 04:44:01 PM
Definitely something wrong with securing the advance mech there.

Agree the rod looks reasonably ok, but mech shouldn't be moving like that and with any real slack at all.

You'll get no real accuracy in timing, gap and dwell to set it up correctly. Should have a washer under the six mm nut to secure the bigger nut, is there anything there ?

Yes there is. The play is there with everything nipped up but not stupidly tight. When I stripped it down the 10mm nut was barely hand tight.
Title: Re: No Spark
Post by: K2-K6 on May 05, 2022, 05:21:48 PM
Wondering if the rod is backing out of the crank which will release the advance mech ?

If that's happening, then the securing nut may be reaching thread end before tightening mech properly.
Title: Re: No Spark
Post by: Saesneg Shaun on May 05, 2022, 07:18:19 PM
Wondering if the rod is backing out of the crank which will release the advance mech ?

If that's happening, then the securing nut may be reaching thread end before tightening mech properly.

The rod seems to be in rock solid. What is very worn is the large nut with the slot in the middle. Years of cranking the engine over on it has turned the centre oval. I don't know if you can just buy the nut?

Actually just looking at the cost of the advance unit/points/condensors on DSS I might as well do an electronic conversion!

Any recommendations? DSS unit or Boyer seem reasonable choices.
Title: Re: No Spark
Post by: K2-K6 on May 05, 2022, 08:27:31 PM
Likely you'd get someone with a nut on here to help you out if you put request out for one.

Electronic, many use the advance mech to then run various levels of electronic switching.

Boyer is fully electronic, triggering, advance the lot and has very good reputation for performance and reliability also. Probably the most proficient if you're changing.

Unsure if Boyer needs that nut too, or has alternative. 
Title: Re: No Spark
Post by: Oddjob on May 05, 2022, 09:55:29 PM
Sure I've got one somewhere, just need to find it.

CMS have them at 18 euros or so. DS don't stock them.
Title: Re: No Spark
Post by: Saesneg Shaun on May 06, 2022, 07:10:03 PM
Thanks for the help guys.

I'm totally fed up with messing about with the points so have ordered a Boyer Electronic Ignition kit.
I went for that one as it doesn't use the stock advance unit compared to the DSS unit and they have been around for donkies years.

Title: Re: No Spark
Post by: Saesneg Shaun on May 10, 2022, 08:18:08 PM
Just to update this I fitted the boyer electronic ignition today and she's running on all four again.

Its looks like my problem was the large nut had been used to crank the engine over and had gone oval in the middle. Even with the 10mm nut done up the big nut was still moving around. Fortunately the Boyer kit does away with the large nut and the points cam.

I have a bit of popping through the exhaust but i'm waiting for a strobe to be delivered then I can time it properly.
Title: Re: No Spark
Post by: K2-K6 on May 10, 2022, 09:36:55 PM
Good you've got to the bottom of it.

Those are timed at full advance as you can't set them like a mechanical system that advances the trigger point. They effectively run at full advance with a rpm register internal that allows them to retard only from set point for start and low rpm.

Seems a bit odd if used to conventional mechanical system, but logical once you think it through. 

Should be easier to start now  :)
Title: Re: No Spark
Post by: PHILIP2908 on May 11, 2022, 08:12:17 PM
I've got the same problem; the spindle and nut scribes an oval as the crank is spinning. You can set the timing OK but it plays havoc with the points gap.
As you say its caused by monkeys turning over the engine with the plugs still in: the compression proves too much at tdc and the spindle bends.
I had a look at a replacement but none seem to be available. From the parts diagram it looks as it just screws into the end of the crank.
I also thought about removing it and getting it straightened by an engineering shop but then had nightmares about it snapping off on removal which would probably necessitate a full strip down to get at the crank.
Consequently I will probably go for the Boyer system too. Can you please give some feedback on ease of fitment and what effect it has on the running?
Best regards
Phil
Title: Re: No Spark
Post by: Bryanj on May 11, 2022, 10:01:06 PM
The 750 spindle is available as a spare and has an O ring on it where it screws in
Title: Re: No Spark
Post by: Lobo on May 12, 2022, 01:25:55 AM
I removed my spindle recently - not easy. Heat, molegrips, rattle gun (on low) all to no avail. What worked in the end was a roller cam stud extractor, obviously down near the base. But still nail biting…
Title: Re: No Spark
Post by: Saesneg Shaun on May 20, 2022, 04:06:24 PM
Well dang, its still not sorted.
Got the ignition timed with a strobe lamp to full advance at 4500rpm.
Also did a carb balance whilst I had the bike partly disasembled (I know, I know, should have tackled one issue at a time)

Road tested today and she won't accelerate over 4000rpm in any gear. Tried with the fuel tap on and in res and with the cap open but with the same result.
Starts and ticks over fine.

Any ideas?

*edit, just pulled the plugs, No's 1,3&4 very black and sooty, No 2 tan/clean?
Title: Re: No Spark
Post by: Saesneg Shaun on May 24, 2022, 05:16:52 PM
Found the cause.
I just popped the float bowls off to find 1,3 & 4 main jets in the bottom of them  :-[
I didn't tighten them up and they rattled loose. No wonder the plugs were were black lol.
Title: Re: No Spark
Post by: K2-K6 on May 24, 2022, 06:44:01 PM
 ;D  ;D  ;D oops!

At least you found them quickly.
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