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. 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. I have also found that I can get it to run again when hot if I bump start it on a hill, but it will die again when it's time to idle. When not idling and just riding down the road it feels great and like it could go forever no problem, until it's time to idle.
The more it warms up it seems the engine has more trouble transitioning from open throttle to idle, if I can blip the throttle just right and make it land on idle then it will keep running but if I let it down normally or try to open the throttle from idle it just feels like mush unless I blip the throttle and get the revs up. And that’s also where it dies, in the “mush” zone. While cold and just started there is practically no "mush".
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 jets and atomizer, float needle, float, diaphragm and needle, and new gaskets). Checked float levels for proper fuel level (measure 22mm from the lowest part of the bowl)
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.
Opened fuel cap while running in case vacuum was building up.
Compression test right after it dies 140 PSI on the right, and 125-130 on the left (albeit with carbs still on, but throttle wide open while cranking)
Validated idle jets are 45s and both main jets are 140s.
Carbs are Bing 32MM 64/32 - 3/4
Running BP6ES plugs, and while troubleshooting found I'm running the wrong plug wires/caps with resistors and have some non-resistor ones coming.
At this point I have gone 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!
• Replace ALL the fuel with fresh (bought this week) gasoline.
• Adjust the valves. Airhead valves tend to tighten up.
• Set both carb pilot screws to 1-1/2 turns from bottomed-out.
• No air-cooled engine needs to idle for "3-5 minutes". Go ride the bike if you want it warmed. The object is to warm everything evenly, not just the cyl heads.
• When fully warmed (oil and all), then set both Idle Speed screws to just under 1000 RPM, as read on the tach.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Sorry trying to figure this site out on my phone wasn't the best idea.
I replaced all fuel with fresh when I got it, and took it on it's first trip to the gas station and filled the tank last week. That's when I found out that it can be restarted when hot - if there's a good sized hill to bump start it on.
I have adjusted the valves, and rechecked them just in case.
Both carb pilot screws are set to about 1 turn out from bottom out where it seems to run best.
I don't mean that I'm letting it warm up before I ride it, more that it dies whenever it's warm. Does matter if I'm riding around my neighborhood or letting it idle in the garage, once warm it will die when trying to idle. I can sometimes fuss with the throttle just right to get it to settle back to an idle, but there's a spot in the first 1/4 of the throttle where the engine bogs badly and feels like it's running on molasses. That's usually when it dies, it's almost like being in that throttle setting triggers the engine into a condition that it can't recover from. This is why I haven't been able to properly try and balance the carbs - I can't get the engine truly up to temp, and whenever it's warm and dies it can't be restarted with the starter. The battery is new and the starter cranks well, but engine seems to only fire on one cylinder, only when at half throttle or more, but never picks up beyond that - unless I let it cool down.
I have made some progress today, I think?
I thought the idle circuit may still be clogged so I cleaned them out again, and also tightened my clamps on the carb to cylinder head seals.
I fired it up with a bit of choke, and it ran well when cold as usual. It seemed to run better when warm and didn’t die on its own. When going from blipping the throttle to letting it idle again the idle speed came to a crawl and then slowly built up again. But it didn’t die.
I then turned it off to see if it would start again. Nope. Engine spins over and get some galloping from one cylinder with half throttle as usual but does not start.
On a whim I decided to take the intake tubes off that run from the carb to the air box to see if maybe the diaphragms were getting hung up. I tried to start it and I’ll be darned it started. I didn’t run well at all, but with a quick adjustment to the carbs it was purring. I let it run, turned it off and it fired right back up. Throttle response was great, and if felt just like a normal healthy bike.
The only thing I can assume now is that the intake air duct or somewhere between it and the air filter is blocked? It had a brand new filter and when I looked inside the housing when replacing it was clean.
Anyone have any other ideas?
I plan to take the tank off, and open the air box again to have a better look around.
Hmm. Well took off the tank and checked the air filter and air box and path to the intake duct and it’s all clear as day. So not sure where that leaves me now.
Remove the Starter Cover from the top of the engine and look for a nest that is blocking the air path. You'll need to remove the tank, but after that it's just 2 screws. Quick and easy.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Well, I double checked under the starter cover and it’s all clear. I don’t get what would be restricting the airflow, or if it is even really restricted, but then I don’t know why the bike runs so much better with the air tubes removed.
Your first sentence, “starts almost instantly with no choke when cold” … that shouldn’t happen. I know we are all suggesting things you have probably already covered, but here it goes; I’d go back and double check your enrichment circuits. Make sure the right and left disks are in the correct carbs, and oriented correctly. It sounds like the choke is on when it should be off, perhaps.
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