71 R75/5 dies when warmed up

[cvhd94]

Hey there! I picked up a ‘71 R75/5 a couple weeks ago and have been working on getting it on the road. I have gotten to a point where I no longer know what to do next.

It ran when I bought it, but poorly and hadn’t been on the road since 2008.

Symptoms

Bike starts almost instantly without choke when “cold” runs well and consistent but with some white/black smoke. Once it comes up to temp (maybe 3-5 min of idling) the idle slows down and eventually it completely dies. After letting it cool down for 5-10 min it will start back up no problem. And then die again when warm. If I try to restart it immediately I can get it to kind of “run” on one cylinder with the throttle about halfway open or more.

Things I have done:


Ignition - New points, condenser, cleaned advance and installed new springs, new coils, spark plug cables, and spark plugs. Oil and filter change.

Fuel - Rebuilt both carbs (cleaned/blew out all passageways, all new o rings, new idle jet, float needle, float, diaphragm and needle, and new gaskets)

New fuel lines, filters and cleaned out petcocks. New throttle cable and choke cables

Air - New air filter, new rubber carb to cylinder head coupler.



Troubleshooting

Immediately after dying turned off fuel taps and check carb bowls for fuel, both have fuel, and float levels have been set to the proper level.

Immediately after dying pulled plugs and checked for spark. Both cylinders have good spark. Spark plugs are quite sooty and seem to be running rich.

Checked/ adjusted valves and re-torqued heads.

Set points gap and using timing light I see “s” is in the window at idle and the advanced mark is showing when revved to 3000 and above.

Validated chokes are truly off when intended and installed correctly.

Adjusted carb to base settings then adjusted for best running, which seems to be mixture screw almost completely in. Also moved carb needles from position 2 to position 1 (longest) in effort to lean out mixture and that did seem help to cut down on smoke and improve running slightly but it still dies.

Adding choke while it’s running immediately kills it.

Validated idle jets are 45s and both main jets are 140s.

Carbs are Bing 32MM 64/32 - 3/4

At this point I’m just going back though everything I did again, but am only increasing my frustration because with each thing I check I’m validating that it should be running fine.

Any help or insight would be very appreciated, this is my first BMW Airhead so there’s a lot I have still to learn. But I am familiar with VW airheads and basic mechanics in general.

Thanks!

 

[dennisdarrow]

Most every negative indicator or symptom surely does seem to me that you are getting to much fuel at the carbs....IE cold start no choke, sooty plugs, running on one cylinder when warm, and black smoke.

Proud of you for doing what most new owners can't or won't do. If you are familiar with VW valves then hopefully yours are set correctly. The only thing that really catches my eye is "FLOAT LEVEL".

i dont know what method you used to set them but for me, the method that works best is to measure the fuel in the bowl when i turn on the pecocks with empty bowls. Then i let the fuel flow for a few seconds, and turn the petcocks off. Next, I CAREFULLY remove the bowl and with a measuring device down into the cavity in the bottom of the bowl meaure how much fuel is in the bowl.....i like 22 to 25 mm.

See what that does for you. IT'S A START...........Dennis

 

[cvhd94]

I appreciate your reply Dennis! When I said I checked the float level I just ensured the fuel cut out when the float was parallel to the carb body, but I never actually measured the fuel. I’ll give it a shot tomorrow once I get the carbs back together (again - after tearing them apart again today just to make sure I didn’t make any silly mistakes last time)!

 

[pglaves]

I think Dennis is right - the carbs are for some reason too rich. Fuel level is a possibility. Jetting is a possibility.

 

[52184]

Symptoms

Bike starts almost instantly without choke when “cold” runs well and consistent but with some white/black smoke. Once it comes up to temp (maybe 3-5 min of idling) the idle slows down and eventually it completely dies. After letting it cool down for 5-10 min it will start back up no problem. And then die again when warm. If I try to restart it immediately I can get it to kind of “run” on one cylinder with the throttle about halfway open or more.

You might want to check the discs in the enrichener (“choke”) assemblies on the carbs, to verify that they haven’t been swapped or installed incorrectly. It doesn’t seem right that the bike will start cold without any added enrichment, and the symptoms you describe certainly point to an over-rich condition.

Best,
DeVern

 

[cvhd94]

You might want to check the discs in the enrichener (“choke”) assemblies on the carbs, to verify that they haven’t been swapped or installed incorrectly. It doesn’t seem right that the bike will start cold without any added enrichment, and the symptoms you describe certainly point to an over-rich condition.

Best,
DeVern

On my initial tear down of the carbs I did notice that the discs were actually reversed. The one stamped L was on the right carb and vice versa. I corrected this when I reassembled them, and the left carb now has the left disc and the right carb has the right disc. The when looking internally the teardrops on the discs are both on “top” and when assembled the divot on the lever points to the side of the choke body with the “bump out”. From all the resources I have found this is correct, but do wonder if I may be misinterpreting something because it’s the most reasonable explanation I can think of.

One thing I just thought of, when the enricher is “on” by pulling down the choke lever on the engine case the arms on the carbs should be pulled up, correct? That’s how it is set up on my bike now, but if that’s backwards somehow that would explain a lot…

 

[20774]

The choke lever at the air cleaner is off when the lever is parallel to the ground. Choke is on when the lever is moved and points to the ground.

Another thought I had is fuel tank isn't breathing and a vacuum is being created. Try cracking the fuel cap slightly and see if the problem persists.

And welcome to the MOA and the world of Airheads!

 

[cvhd94]

Thanks for the sanity check, and welcome Kurt!

I did try to run it with the fuel cap open previously, but unfortunately found the same results.

This morning while cold and with the carbs off I did a compression test and found the right cylinder is at about 140PSI and left is at 130 PSI. I’ll test again when the engine is warm after it dies once I get it back together later this morning and will post an update.

 

[69zeff65]

Choke / enricher cables

Check to make sure your cold start enricher (choke devices) are fully disengaging when you flop that lever over to off. The braided cables and some of the older solid cables for the enricher don't completely shut off the enricher. This is due to age and bent or distorted cables. I have found this on four of my /5 through /6 bikes. I replaced the enricher (choke) cables, cleaned and serviced the lever assemble and the issue was resolved.

 

[cvhd94]

Alrighty, reporting back from a day full of adventure. Think I had a bit of an ah-ha moment, when tearing the carbs apart again I realized that I had only ever removed the main jet and main jet carrier, not the seat or atomizer since they were stuck in the carb body and didn’t fall out, so I assumed they were meant to stay in there.
After watching some more videos online I noticed they can and should be taken out, so with some careful persuasion I got them to come out and they were quite badly corroded and varnished up. I was able to clean them up okay, but the corrosion has definitely altered the shape of the atomizers.

I put the carbs back together and measured and adjusted the float level to 22mm. And then fired up the bike, it ran better than it had yet and didn’t die when it got warm, at least not right away.

I adjusted the mixture and idle speed the best I could and then bopped around my neighborhood for about 15-20 min and it was doing well, so I decided to venture out to get some gas, but a few block from home it died and wouldn’t restart with the starter. After a short waddle of shame I found a decent hill and rode it down until I picked up some speed and then dumped the clutch while in 2nd and after a while of only one cylinder firing while gravity pulled me down hill they both picked up and I was back in business and was able to ride home.

I then messed with the mixture a bit more and bopped around the neighborhood a bit more until I had the confidence to venture out and get gas again. I got to the gas station - just - as it died while I pulled in, and I rolled up to the pump in “eco mode”.

It refused to start after filling up, and again I waddled off to find a hill and hoped to repeat the miracle which had happened earlier, and so it did! I limped home, keeping the revs high when stopped and made into my garage as it died again.

While it was as hot as it has been so far I did a compression test and found 140 PSI on the right, and 125-130 on the left. Right spark plug looked quite clean and left was slightly sooty but not nearly as bad as previously. It was definitely the left cylinder that refused to fire and seems to cause the bike to die while at idle, but when revs are up and cruising it feels great, and I feel like I could ride forever… well, until I reach a stop sign…

My next thought is to get a pair of complete new main jet assemblies and see if that helps further, and possibly a leaner main jet for the left carb.

Does that seem like a reasonable course of action? Are my compression numbers within acceptable levels?

 

[20774]

Proper compression can only be done with the carbs completely removed, or at least with the slides in the carburetor manually lifted to allow free flow of air. Otherwise you will be getting restricted air and less than accurate readings.

 

[melville]

Proper compression can only be done with the carbs completely removed, or at least with the slides in the carburetor manually lifted to allow free flow of air. Otherwise you will be getting restricted air and less than accurate readings.

All that said, 125ish or more is just great. I think you're on the right track!

 

[20774]

I don't recall reading if the carbs have gone through a proper synching process.

 

[dennisdarrow]

Something is seriously wrong with those carbs.......

By the way, above, when you mention new springs. Are you talking about new forks, shocks, or advance mechanism springs???.........Sometimes i get the feeling that something is wrong with the ignition. Like a loose wire or something that is making a BARELY contact connection and sometimes works.

Yes, you need to sync the carbs as Kurt suggests. DO THIS WARMED UP, but then you have the problem with it not starting again when it is warm. Glad to hear that you are making forward progress. ONE STEP UP THE MOUNTAIN AT A TIME FINALLY GETS YOU THERE..........God bless......Dennis

 

[cvhd94]

I don't recall reading if the carbs have gone through a proper synching process.

I haven’t been able to keep the bike running long enough to do a proper carb sync, but I think I am on the right track with replacing the needle jets and atomizer, I got some new ones on the way and got another set of a slightly leaner jets as well. I can’t find what the standard jetting for this carb is and am suspicious of what’s in there now, as on my first tear down I found mismatched idle jets.

Dennis, I was referring to the advance mechanism springs. And I’m still curious about an ignition issue too, but the plugs have spark when I test them after it dies, and the timing is pretty much dead on when tested with a light. I also cleaned all the connections going to the coils including grounds when I changed the coils out.

New jets come early next week, hopeful they will do the trick!

 

[pauls1150]

from the Bing manual -



Looks like you've got the right ones; can you verify the needle jet and jet needle?

 

[cvhd94]

from the Bing manual -

View attachment 92306

Looks like you've got the right ones; can you verify the needle jet and jet needle?

Awesome, this is exactly what I have been looking for! Thank you! I do have 2.73 needle jets, but they and the needles and atomizer were quite badly corroded. I already replaced the needles, but not jets yet.

One other thing I ran across while doing research is spark plugs and wires, I got a new set of each, but am worried I may have the wrong kind now. I purchased new 5 Ohm wires with resistor, and NGK BPR6EIX which also have resistors. I am just running stock points so it’s my understanding now that I do not require resistors on either my plugs or plug wires. And having them on both could be giving me a weak spark. Would changing to plugs to BP6ES be suitable, or should I toss the wires in favor of non resistor models as well? Electrical stuff is definitely not my strong suit so please excuse my ignorance!

 

[20774]

Yeah, that's too much resistance. Ideally, you want low resistance wires, zero resistance plugs, and 1K or 5K ohm spark plug caps. I think the early /5s were 1K ohm but BMW when to 5K ohm caps for the /6-on bikes.

 

[stevenrankin]

Tune up and troubleshooting

You have quite a list of work done. recheck your points gap and timing.

Did you remember to set the valves one side and then rotate to the other?

All I can add is recheck your work. Good luck, St.

 

[pauls1150]

The needle and needle jet affect the mix from about 1/4 to 3/4 throttle... If either was corroded, it's easily possible that the other is also, or maybe worn out of round.
Do your needles have multiple slots for the clip? If so, raising the needle by lowering the clip permits more fuel flow, and vice-versa. (And make sure both sides are set the same, of course.)