Engine cutting out / dies on throttle

Ok thanks! I’ll do some more testing and check the stator. Would the charging system be what’s fouling the plugs though?
Low voltage, however it is caused, can cause that. Best to sort the charging out first, in my opinion.

Check resistance across the rotor copper tracks and if OK, check the brushes. New carbon brushes, just a few $.

Check voltage drop across the ignition switch.
 
Ok thanks! I’ll do some more testing and check the stator. Would the charging system be what’s fouling the plugs though?

Yes Sir .. For once in my life I know what I am talking about I had poor charging for many ..many... Many years .At the same time never a problem with the carburetor
It is not always certain what is making the plugs black it can be to rich mixture which is rare and weak spark or other
I believe it is that the weak spark is having a lower temperature and is not able to fully burn off the soot

Check the rotor and brushes first .then regulator
 
If the 10.4 volts are measured at the brushes, when running, the problem must be before that. Other than the ignition switch, there is the fuse and the regulator relay. Maybe a bad relay?
 
If you ever think you have a fuel flow issue, the thing that happens is the bowls don't fill up where they should which a low bowl causes a lean mixture. While running if its bogging at a constant throttle, activate the choke lever which really puts more fuel in. It can be a good indication its fuel related.

I switched the stock relay coil activated regulator to the VR115 and could not be happier. My stock regulator was so far out of adjustment I believe it was the cause of my coil burning and also the rectifier popping a diode.

BTW, on a fully charged battery and a burnt rotor not even charging the battery, When I first got my bike running, I was able to idle and throttle up in neutral just fine. I think I had it going minutes before I stopped it, hooked up the meter to check the battery was charging then detecting the failed rotor condition.
In the end, I replaced rotor, rectifier that was blown, Regulator that was out of adjustment and cleaned up the runstop & Ignition switches, the connectors in the headlight which were dropping voltage going to the regulator & ignition.
 
Ok good advice, I'll keep going through everything.

The brushes are about 4mm.

At 200 ohms on the multimeter, the rotor measures 06.3 between the slip rings and nothing between a slip ring and a ground.
 
Ok good advice, I'll keep going through everything.

The brushes are about 4mm.

At 200 ohms on the multimeter, the rotor measures 06.3 between the slip rings and nothing between a slip ring and a ground.
Rotor looks good then, 6.3 ohms. Remove the stator, clean the copper tracks and fit new brushes. Should be a big improvement in charging.
 
Ok great, I'll get those brushes on the way. I'll check the ignition switch too.

I think I'm also going to order a new regulator/rectifier while I'm at it.

Thanks for the help!! I'll see how it all goes.
 
With the battery fully charged and lights off, your bike should be able to start and run fine with no charging system at all at least 30 mn.
If it is not able to do so, you have a problem not related with the charging system.
 
Hello, back again.

I was waiting on some parts and had other bike issues so only just got back around to testing this again.

I took the bike out this morning and it was fine for the first 20 mins or so, just some popping from the left on decel. I tested the charging system with the multimeter after that, I have recently replaced the brushes and cleaned the stator + rotor. It is 11.5V on the brushes, 1V higher than before, but still only getting to 13.5V on the battery under throttle.

I took it out again after the test and made it 5 mins before it died on idle at lights, started it up all good, sounded like the engine was missing every once in a while.

Another 5 mins after that, the engine just died while riding like before, like hitting the kill switch. Started straight back up. I changed the plugs, they looked ok but it happened again straight after changing.

It seems maybe heat related? I’m currently just waiting for the bike to cool down and try getting home, 15 mins or so away…

Any help would be appreciated!
 
Made it back with no issues, about a 15-minute ride. For reference here are the spark plugs after the engine died.

I have a spare all-in-one regulator-rectifier which I might try. I'll have a look for threads on that as I'm not sure where to mount it on the '75.

Also, the reason I think it's electric and not fuel-related is that I changed carb set, and cleaned them out well, same issue.
Otherwise, I've replaced the: brushes, coils, condensers, spark plug caps, spark plugs, cleaned and re-gapped the points, and did the timing and valves.
 

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I am not convinced your charging system is working properly. If the battery starts from fully charged then it'll be sitting at around 12.7 volts before you start. If there's 11.5 volts at the brushes then you are losing 10% of the battery voltage somewhere. I suspect the ignition switch. But it could be a corroded connection or similar in the wiring. Once you correct that, you might have a better chance of charging properly nearer 14.5 volts. I think I would try to sort this out before putting a new regulator/rectifier on the bike.
 
Ok, I replaced the regulator/rectifier, no difference.

I was losing 1V at the ignition switch, so I replaced that and now get the same voltage (about 13.1) at both ends.

I'm now getting 14.1 at the battery when I'm revving, which is on the right track, but it's still not sounding right. I'll go through and clean the connections again. I'm still getting 11.1 at the brushes, while 13.1 at the battery.

I also want to ensure it's not a fuel issue, or the gas cap vent trick (I'll try that again next time). The right side is stronger than the left, and I'm getting popping on the left. The left also seems to run stronger when the enricher is on. I'll do some more testing, although the bike was cutting out before I changed carbs + boots and started getting this popping.

I'm getting a bit lost because the symptoms seem to change. The first time it happened I replaced the spark plugs and it fixed it for that 3-hour ride. Coming back from the same way a week later, about halfway through, when I started getting to traffic lights, more stop-start it started cutting out while riding but didn't fully die. I replaced the spark plugs again but it seemed to keep happening, then when I got near home (about 3 hrs in) it kept dying when I was riding. It's not a 'stalling' like it's fuel-related but like someone hit the killswitch. So that brings me to yesterday, when after going through the charging system it happened again about 20 mins in where it died at lights, then when I pulled onto a side road it died while I was riding. I changed the spark plugs but no difference. I waited about 20-30 mins and rode it home with no issue.

Thinking about it, It seems to do better at higher revs, it's only been when stopped or slowing down when it's fully died (I think). Although it's definitely stuttered (but not fully died) at high stable rpm.

Should I just take it out and see how it goes? Is 14.1V enough?
 
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You probably have several issues and it is easy to get lost if you are not systematic. I suggest you forget the charging system for the moment. Disconnect it, charge completely the battery and test run your bike. Charge the battery before each test run.Once you get ignition and carbs perfectly sorted, then you come back to the charging .
 
The bike dying after a period of running OK is explained by the voltage on the bike dropping as the battery is steadily discharging.

As the voltage on the bike drops, the spark from the coils gets weaker and weaker. The high voltage to generate a spark from the ignition coils is caused by the voltage in the primary windings collapsing and inducing the high voltage in the secondary windings. As the voltage on the bike drops, the voltage induced in the secondary windings correspondingly drops too. And the bike doesn't run because the spark becomes too weak or completely absent.

I do not think this bike will run reliably until the charging system is working properly.

To prove this, charge the battery again fully. Note the voltage just before starting the bike. (13.1 volts was stated before). Run the bike until it starts to run rough. Then measure the voltage again at the battery. I expect it will be discharged.

Hope that helps.
 
The bike dying after a period of running OK is explained by the voltage on the bike dropping as the battery is steadily discharging.

As the voltage on the bike drops, the spark from the coils gets weaker and weaker. The high voltage to generate a spark from the ignition coils is caused by the voltage in the primary windings collapsing and inducing the high voltage in the secondary windings. As the voltage on the bike drops, the voltage induced in the secondary windings correspondingly drops too. And the bike doesn't run because the spark becomes too weak or completely absent.

I do not think this bike will run reliably until the charging system is working properly.
Yes, this makes sense. Wouldn't my battery die though at some point?

I'm not charging it except when riding the bike, and it seems to be ok when I let it sit for a bit. Thinking about it, I did charge it up before the longer ride I did and that's when it was running fine for the longest (except for before this started popping up)...

Or is it that it's getting enough charge for the battery but not for the coils? Sorry, still learning!!
 
Yes, this makes sense. Wouldn't my battery die though at some point?

I'm not charging it except when riding the bike, and it seems to be ok when I let it sit for a bit. Thinking about it, I did charge it up before the longer ride I did and that's when it was running fine for the longest (except for before this started popping up)...

Or is it that it's getting enough charge for the battery but not for the coils? Sorry, still learning!!
Please read carefully my suggested test again. - Start with a full battery, measure volts before starting. Run the bike until it starts to run rough, measure volts again. The battery, I expect to be discharged. Hope that helps. (It's probably helpful to set up a voltmeter that you can read while riding too. I think you'll see bike voltage steadily falling).

I think the charging system is not outputting enough power to stop the battery steadily running down.
 
Please read carefully my suggested test again. - Start with a full battery, measure volts before starting. Run the bike until it starts to run rough, measure volts again. The battery, I expect to be discharged. Hope that helps. (It's probably helpful to set up a voltmeter that you can read while riding too. I think you'll see bike voltage steadily falling).

I think the charging system is not outputting enough power to stop the battery steadily running down.
Ok I see, I'll give that a go!

So just to clarify, so the battery is slowly discharging which = a weaker and weaker spark. Changing the spark plugs helped this at first because the other ones were sooty (probably from a weak spark?), so improved the spark. But when I changed them after running it for a while it didn't help as the plugs were ok and the discharged battery was still causing a weak spark.

I don't understand why though after it running and dying, then letting it sit made it run ok for a while. I thought the battery would not have changed voltage thus = same weak spark. Although, after it died and letting it sit, I gave it more throttle and was keeping the idle higher when stopped, so was it just the high rpm that kept it alive? edit: yes I think it was
 
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Ok I see, I'll give that a go!

So just to clarify, so the battery is slowly discharging which = a weaker and weaker spark. Changing the spark plugs helped this at first because the other ones were sooty (probably from a weak spark?), so improved the spark. But when I changed them after running it for a while it didn't help as the plugs were ok and the discharged battery was still causing a weak spark.

I don't understand why though after it running and dying, then letting it sit made it run ok for a while. I thought the battery would not have changed voltage thus = same weak spark. Although, after it died and letting it sit, I gave it more throttle and was keeping the idle higher when stopped, so was it just the high rpm that kept it alive? edit: yes I think it was
There's only one way I can see for you to find out if I'm right or not. But one way or another, for sure you need to get the battery charging better. One thing I would do is revisit the voltage you are seeing at the alternator rotor brushes. It should be close to battery voltage. I think there's more work to do on that yet until it's 100%.
 
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