1971 BMW R75/5: Power loss, sputtering, and dying when warm

[krisnys]

Hello, almighty forum.

I'm having problems with my beloved airhead. Here are the symptoms:

1. It starts right up from cold, no need for choke (always been like this)
2. Runs fine, except for sputtering somewhere between 2000 and 2500 RPM
3. After having been properly warmed up it starts to sputter, and the idle drops so that i have to constantly keep throttling it to keep it from dying (which i sometimes can't)
4. The exhaust pops when i try to start it again (which sometimes works)
5. The spark plugs have been a bit all over the place seeing as i have tried different carb settings, but generally they seem to indicate an engine running to rich
6. After it has cooled down again, it starts right up again and seemingly runs fine until the problems recur

These are the things i have tried:

1. Cleaned carbs and reassembled them correctly
2. Installed new fuel filters and checked fuel flow
3. Adjusted the carbs (no vacuum ports, so only by ear and feel, starting from baseline (which i was told was 0.75 out on the mix screw..?) I have tried several settings, both richer and leaner...)
4. Adjust valves and rocker arm end play
5. Set timing (A little difficult. Could be because the cam chain is worn?)
6. Swapped coils and spark plug wires (from another R75/5 awaiting restoration)

Newbie questions for the almighty forum:
- There seems to be a problem with the valves... After i have set them correctly (intake = 0.10 mm, exhaust 0.20 mm), they seem to not want to stay there. What does this indicate, and could it be the source of the problems?
- Could it be the condenser or the points?
- Could it be a carb problem?

I bought the bike in May, so i don't know a lot about it other than what i have been told by previous owners. It had an engine overhaul late 2011, where the valve seats were changed so it can run on unleaded fuel. The rocker arms were also supposedly changed.

 

[dennisdarrow]

Just an old fella here; but I have been setting my valves at .006 and .008 inch for a few years now. the .010 and .020 is new to me. Maybe the seat change required that difference. Condensors are cheap and if you did all that other stuff I would for sure do that easy fix. Had an old 53 Pontiac that did that, a 66 Plymouth, and an R50 that all did that same cutting out business.........Try it. Those valves need to be set correctly......God bless.......Dennis

 

[20774]

Your valve settings are generally OK. 0.10mm is 0.004" and 0.20mm is 0.008". You could go to 0.15mm which is 0.006". But the intake side stays a bit cooler with the inrush of fuel-air, so 0.004" is OK, but doesn't hurt to open it up to 0.006".

You should verify that the head bolt torque is correct...shoot for 25 ft-lbs, loosen one about 1/8th turn, then torque to 25. Do it in a criss-cross pattern, ending with the bolts at 12 and 6 o'clock.

I suspect you recheck your valve clearances after you've set them for a given side. Valves are set for the side that is at top dead center on the compression stroke. Rotate the engine 360 and do the other side. Then rotate the engine another 360 and recheck the first side that was done...finally rotate the engine another 360 and check the last side that was done.

It's a bit of a "dance" to tighten the adjust while at the same time using another wrench to keep the pushrod from turning.

 

[DonTom]

Hello, almighty forum.

I'm having problems with my beloved airhead. Here are the symptoms:

1. It starts right up from cold, no need for choke (always been like this)
2. Runs fine, except for sputtering somewhere between 2000 and 2500 RPM
3. After having been properly warmed up it starts to sputter, and the idle drops so that i have to constantly keep throttling it to keep it from dying (which i sometimes can't)
4. The exhaust pops when i try to start it again (which sometimes works)
5. The spark plugs have been a bit all over the place seeing as i have tried different carb settings, but generally they seem to indicate an engine running to rich
6. After it has cooled down again, it starts right up again and seemingly runs fine until the problems recur

You didn't say which carbs you had. But no need because I can tell by all your symptoms.

You have the junk Bings that came new with the 1971 R75/5. Perhaps the junkiest carb ever made. What you're describing is perfectly normal for those junky carbs. These junk carbs have a black "Bing" plate with silver "Bing" letters. There is NO fix for these other than the garbage can. I put Mikuni carbs on my R75. They will fix every problem you mentioned, but you will then need to use the choke on these carbs and the main jets will have to be changed at an elevation change of a few thousand feet, but they are very easy to change. They can be changed at a fuel stop, takes about a minute each side.

By far, the most common serious problem is when the 1971 Bing carbs get warm, they will not idle. Either at least 2,500 RPMs or the engine dies. But they are great when it is cold, and the choke is never necessary. However, not all of these old junky carbs have the exact same symptoms. But one thing they all have in common are the countless problems of one type or another.

There are other options such as newer Bings, which will fit better with your stock air filter and such.

But keeping those 1971 Bing carbs on the bike is NOT a reasonable option.

-Don- Reno, NV

 

[69zeff65]

Check your choke cables also

I would check the choke cables for wear, to much flex or the sheath coming loose from its attaching point on the carb and lever. If your choke cables are the braided type I would replace them with solid ones along with new attaching hardware. I have a late '70 with the clunky carbs, had similar problems and for the most part fixed it with a complete rebuild and new choke cables with all new attachment nuts on carbs and choke lever. The idle jumps around a little sometimes and on rare occasion it will stall at hard deceleration stop or panic stop but it fires right up and runs smooth after that. I may put in some slide return springs just to see if that does anything.

 

[krisnys]

You didn't say which carbs you had. But no need because I can tell by all your symptoms.

You have the junk Bings that came new with the 1971 R75/5. Perhaps the junkiest carb ever made. What you're describing is perfectly normal for those junky carbs. These junk carbs have a black "Bing" plate with silver "Bing" letters. There is NO fix for these other than the garbage can. I put Mikuni carbs on my R75. They will fix every problem you mentioned, but you will then need to use the choke on these carbs and the main jets will have to be changed at an elevation change of a few thousand feet, but they are very easy to change. They can be changed at a fuel stop, takes about a minute each side.

By far, the most common serious problem is when the 1971 Bing carbs get warm, they will not idle. Either at least 2,500 RPMs or the engine dies. But they are great when it is cold, and the choke is never necessary. However, not all of these old junky carbs have the exact same symptoms. But one thing they all have in common are the countless problems of one type or another.

There are other options such as newer Bings, which will fit better with your stock air filter and such.

But keeping those 1971 Bing carbs on the bike is NOT a reasonable option.

-Don- Reno, NV

That's interesting. I didn't know these carbs are problematic. If i recall correctly, the numbers on the side of the carb read 64/32/9 (and /10 on the other side). Do you know what carbs would be a good replacement? Or is there any way to modify the carbs so they'll work better?

 

[20774]

Snowbum indicates that the early carbs 1/2/3/4 were the ones with the problems.

http://bmwmotorcycletech.info/earlybingR75CV.htm

Read that page and get tips on ways to help the early carbs. He mentions the 9/10 carbs on the page, but I don't think they are as problematic as the earlier ones.

 

[krisnys]

Snowbum indicates that the early carbs 1/2/3/4 were the ones with the problems.

http://bmwmotorcycletech.info/earlybingR75CV.htm

Read that page and get tips on ways to help the early carbs. He mentions the 9/10 carbs on the page, but I don't think they are as problematic as the earlier ones.

Yes, i have read Snowbums information. That's why i didn't think my carbs were the problematic ones, but they could be...

 

[DonTom]

I would check the choke cables for wear,

When I bought my 1971 BMW R75/5 new in May 1971, I still remember the owner of the shop (Menlo Cycles in Menlo Park, CA) telling me to NEVER use the choke on these bikes. At least not in the SF Bay area where it never gets cold enough for the choke use.

With my Mikuni carbs, even on a very hot day, I will have to use the choke to start the engine for the first engine start of the day.

-Don- Auburn, CA

 

[DonTom]

Yes, i have read Snowbums information. That's why i didn't think my carbs were the problematic ones, but they could be...

What I recall is the black background plates on the carbs, with the silver "BING" letters are the junk carbs. The opposite, silver plates with black "BING" letters are decent carbs. Which are yours?

-Don- Auburn, CA