Backfire then stall, then fine

Hi, chasing a few issues on my '72 R75/5 (I have also the annoying self-starting issue - ready to throw this bike away).

Here's a new one: I just did a huge ride around NE - no issues at all, now if I ride about 4 mins - all is fine, bike idles and runs well - its slows down like the gas is off (was not off) - then backfires (I believe on the right side) then dies...completely. I roll to a stop - two seconds later it will now start right up and idle fine and drive fine - if not a tiny bit off, or sluggish...hard to explain. I can ride it around for 20 mins and nothing, won't happen again.

This has happened twice now. Possibly related - I am seeing a lot of carbon on the intake tube to the right carb...

I checked for water in the bowls, and debris, but seems all okay. The plugs are perfect...

I was thinking to check valves, would a too tight valve (exhaust or intake) cause this behavior?

any thoughts?

thank you for your time

Chuck

In my experience and reading, valves tend to affect the idle more than running down the road. Still, worth it to check them. Consider cracking the gas cap...see if it's building up a vacuum sometimes. Also maybe a condensor?

Thanks Kurt, as always good thinking. I just checked my valves and the exhaust valve on the side that seems to be backfiring was very tight. The rest were okay. I will look up how to test a condensor, and I can open the gas cap if it's feels like its happening. I did note that all the valves were moving and didn't seem to be stuck...

Any thoughts on the carbon on the intake tube on the right? It's excessive. Unless that's somewhat normal.

thank you, Chuck

Carbon? Could it just be oil? The crankcase ventilation dumps oil mist into the right carb intake tube.

Condenser

There is not real way to check a condenser, they are cheap, put a new one in. I had a friend who's condenser went bad in his bike behaved the same way.

I have not seen valves causing your problem, usually when valves tighten up, the bike won't idle especially when hot. But to cause a back fire and loss of power, no, unless they are VERY messed up.

Crud in the gas, good thing you looked for water in the floats, It is possible to be the problem and you have cleared this off the list by checking it. Water in the gas, will cause the bike to Pop and sputter even die, however until the water is out of the carb, it won't just cool off and restart after time if you get my drift. Something that takes a specific amount of time too cool off or sit, most likely is ignition related, hence the condenser. Water, or gunk doesn't care about time, it just screws things up.

Finally, I hate to be a downer, but if you are having the issue with your starter, it may be a slim chance, slim but perhaps the two problems are related? I don't think so but I mention it, you never know.

So, after my ramble, change the condenser, check your points and such. Good luck. St.

All good thinking. I have a spare condensor - I'll pop it on. At least this forced me to check my valves and adjust! I totally understand about water or crud, that would not 'clear' so quickly...

And the deposit on the intake tube is sorta dusty, could be oil for sure with dirt stuck to it, as it's not really a liquid, more a dust, or greasy dust actually.

thanks guys,

Best Chuck

Dust

In regards to the dusty material you are talking to, is this between the air filter and the carbs? There should be no dust there? Check to see things are right and tight in your air intake system. St.

Another thought regarding acting up when hot: maybe a failed coil? Fine when cold or cool but as the coil heats up a break in the windings shows up so ceases to function. One sided? I've also found a broken coil mount cause a similar issue.

ST - yes, the carbon/oily dust is on the intake tube, between the air filter and the carb - I'll check the air intake to make sure all is tight and good.

And I wonder about the coil as this happens usually when the bike is NOT warmed up, then two seconds later it's fine...I can check the mounts though can't hurt.

thank you for input.

C

coils?

I replaced my condensor and checked timing as well, all good - started up and rev'd and all seemed okay - I need to do a test drive.

But, as part of my diagnostic test I will look my coils - in reading Duane Ausherman's articles I was surprised to learn that generally coils last 10-15 years and therefor should be replaced...I am pretty sure my 48 year old coils are original. He also states that regular 6v Bosch VW / Porsche coils (part no. 00016) at $50 a pop are good - I am not clear if Snowbum agrees. MAX and other shops have Bosch coils for our bikes but are over $100 - is there a consensus on this? Are the $50 VW coils good to use?

thanks Chuck

I wouldn't replace unless you had reasons to think they need to be. For aftermarket replacements, I'd consider working with Rick Jones at Motorrad Elektrik.

As a static measurement, what resistance do you get when reading from one spark plug cap all the way to the other cap? Generally, the coils should be 5K ohms each and you probably have the 1K ohm spark plug caps. So your end-to-end reading might be in the 12-15K ohm range.

I would take a hard look at the coils before I replaced them, check for cracks and burn tracks.
I've got the originals in my 78 R 80 and 78 633 CSi, they work fine.

Not to be a nit picking bonehead but when you say Backfire, where is the explosion's coming out of?
Technically backfire is coming out of the Carb end, Afterfire is coming out the exhaust.
Nick

I will test the coils thanks for the tips and metrics.

Nick the 'explosion' seems to be coming from the (right) carb, not the exhaust. I appreciate nit picking.

thank you chuck

plug wire may be the culprit

Hi - tested the coild using the plug cap to plug cap and was getting nothing. I do have a good multi meter...I tried it 10 times...

I tested the wires and was getting inconsistent and NO reading. Did this several times. I have a good spare set and those tested good, I put them in and re-did the cap to cap with coils and I get 22ohm, a bit higher than your range Kurt.

So I focused on the wires I had on the bike and it looks like one is okay, but the RIGHT side (the side that was backfiring) was not giving me a reading...but what's odd is the bike runs good...

I am thinking the right side plug has a weak (or at times no spark) condition and there would then be unburned fuel - causing a backfire?...not sure why it would stall then start right away...but I feel like I'm getting closer.

I tested the coils as per Snowbums article and Duane A's site and mine seem ok (if not a bit higher than the Snowbum range).


c

Looks like progress

Seems you are making progress, and in the right direction.

Refer back to Snowbum, or perhaps Anton Largidare, or Brook Ream's web sites.

Also check your wiring harness to the starter relay and well as the other ignition related wires. This is not related to BMW but, I had a cousin who chased a misfire in his Dodge only to find one of the small wires to the ignition module had been pinched in the distributor cap during his last tune up.

At least, you don't need a $700+ computer to scan and trouble shoot. Also, the replacement parts will be half or a quarter of some of the ignition parts on the newer bikes. Oh yes, you can do it yourself and save the shop labor charges.

I forget if you said what part of NY you are in? St.

Thanks St. I’ll check the wiring while the tank is off. I’m in Westchester. Regards. chuck

Sorry

Sorry Chuck, I can't pop over and give you a helping hand. Just a bit too far for me now. I am up in Rochester. St.

Update

Hi - I believe I solved the backfire and engine stall. I replaced the condensor and also later discovered one of my plug wires was bad, the wire on the backfire side. I replaced with a good used set I had (both tested good) and did a longish fast ride and no issues at all.

I may still have the starter engaging issue - but TBD...still working on that one...

thanks for the assistance. c

Glad for you

I am happy for you to have found the solution to one of your problems. Thanks for letting us know what you found and the fix.

Good luck on the starter issue, I am afraid I can't help much there. Cheers, St.