1972 R75/5 SWB Toaster/Coaster Mystery

[weberkane]

After searching the internet for ideas on why my R75/5 starts easily and runs smoothly then mysteriously stops running 1-2 miles from home, I thought it's time to post the question.
Plug wires, points and condenser are new (500 miles). Installed new Bosch coils from Motorrad Elektrik this week along with a new condenser (in case my recently replaced condenser was faulty). Got the tank back on and connected. The bike started right up and ran for about 5 minutes/1 mile, then quit. I was cautiously optimistic and started my test ride uphill. When it quit for the 5th time, at least I could coast the toaster home. After sitting a few hours, the bike starts again and runs as expected - till it doesn't. No coughing, sputtering or rough running before it stops; it just cuts out. First occurrence, I removed plug and did not see spark, leading me to think it's ignition-related. Plugs appear clean and in good condition. Just to rule out fuel, I checked float bowls and fuel filters - all good.
After it dies, the ignition key and idiot lights continue to function along with the starter. Battery is kept on trickle charger and cranks with enthusiasm...
I'm running out of ideas and would appreciate any help.
thanks,
Kevin

[San Arthur]

Hi Kevin, I always check the obvious and easy first. What about debris inside the tank and jets, or water?
Regardless how many miles the points, spark plugs, wires and boots (5k OHms) are they may have been shelve stored for years. Can you switch one electric component at the time with old/new units?
Does it start easy?

[schrader7032]

Welcome to the forum! Could you be getting a vacuum in the tank so gas doesn't flow? You said it stops abruptly...fuel flow problems tend to be a sputtering stop. You can do some fuel flow tests in the driveway. Petcocks closed, drop one float bowl, have something to catch the fuel (a graduated cylinder is best), then turn a petcock on. Should be around 350cc per minute...break that down for a 15 second run. Repeat on other petcock and also repeat at the other carb.

Or for the next ride attempt, crack open the gas cap in a safe manner and go for a ride. Does it quit again? Also, when it does quit, immediately (and safely) kill the ignition and turn off the petcocks. What's the fuel level in each bowl?

[srankin]

Hello, I thought I posted a reply but I don't see it

If the problem is no sputtering just a sudden swift cut out of the engine, I would suspect a loose, or bad connection in the ignition system.

First thing to do is recheck your work, the new coils, and condenser, make sure the connections are good for them.

If after that, you don't find anything, I am afraid you will have to bread out the schematic and go deeper.

Now Kurt and San, both have top notch advice for sputtering cut out. I would add if you didn't have the problem before, I kind of doubt it is a tank venting problem. I have that venting problem with my RS, when it acts up, it acts like it is running out of gas, not a sudden quit. I am shaking my head with it because the vent in the cap is clear, no spider died in it, yet, it won't vent.

Anyway, good luck, Cheers. St.

[weberkane]

Thanks for all the suggestions. I'll run through the fuel checks first as they seem easier to see. A loose or bad electrical connection seems more challenging to track down as the bike starts easily but dies after it's run for a bit. Wondering if old wiring or connections would be more prone to failing when warmed up. The wire that connects the coils in series appears original. I've not heard of wires working when cold and failing after running; then again, there's alot I don't know about machines that run perfectly one day and not the next.

[srankin]

Yep, electrical problems can indeed be a pain to figure out. I feel for you.

Did you have any luck rechecking your work? Are you sure your points are set and working as they should?

Did you replace the coils and do a tune up because the bike was failing? I want to be sure I am reading your original post properly. Was the bike quitting 1to2 miles from home at first then you changed the coils and tuned it up? If this is the case, it changes things to look for.

One question, how old is your battery, a dead short in it "might" cause the problem.

Like I say, I feel for you, I spent two years chasing down an intermittent cut out problem which started off minor and after two years was stranding me on the side of the road. I never found a smoking gun but a combination of cleaning every contact and ground on the bike seems to have solved the problem. I really hope and pray you don't have my luck.

One thing I did in my trouble shooting was to make a light bulb and a couple of wires tying it into sections of the bike's wiring harness, starting at the switch. The theory being, if the added test light went out when the bike quit, it was certain that part of the circuit was the one in question causing the problem. If the light stayed on, another section could be targeted.
Does this make sense? Sorry I can't explain it better or show you how to do it.

Sorry I cannot be much more help. St.

[Frank471999]

G'day mate,
There are two other possibilities you should check. Believe it or not, wear in the ignition key can actually cause it to switch itself off---this did happen to me on my R26. A new key and tumbler works wonders. An easier but more time consuming check is to physically separate and re-join each spade connection. A 50yo bike can easily have oxidisation build up on the electrical connections.

Cheers

[San Arthur]

srankin and weberkane both have great obvious suggestions.
Always check the easy stuff, like a battery poll, key not making good contact or vibrating away from the connection plate?

I remember during a long trip the generator frame that the points plate screws to cracked. The points plate felt lose all the time. The last condenser we installed also fell apart. The crack allowed the whole assembly to vibrate badly and shooked the condenser apart after 30-40 miles.
We removed the whole generator and weld the crack then tap the threads to 4mm and install new screws.
At the time it drove me crazy trying to find the cause.

[srankin]

How is the search going?

Have you been lucky and found your problem yet?

Just curious. St.

[weberkane]

Good news bad news. The problem is no longer intermittent - the bike doesn’t start. Fuel is flowing from the carbs. No spark Visible at the plugs. Tested the new coils with voltmeter and have 12.28 volts at the coils with ignition switch on. That portion of the ignition system appears to be working. Testing the wiring and connections between the coil, condenser (just replaced) and points (recently replaced). Will review a few you tube videos to make sure I’m using my multimeter correctly. Thanks again for the help.