I have a 1980 BMW R100T motorcycle.
I was cruising down a 2-lane road at about 50-55mph yesterday, 40 miles into a day trip.
All of a sudden I felt something hit my left leg down by the ankle. Thought it might be a nut, washer, or a piece of rubber off my front tire. Perhaps pure coincidence, maybe something off the road flew up and hit me.
Then all I hear is a loud noise, like a massive exhaust leak, or a blown head gasket. I pulled over and check the bike. Everything looks normal on the engine, All the bolts are on the exhaust fittings. The large finned exhaust nuts appear to be tight as well. I walked back down the road to see if I could find what hit my pants leg, couldn't find anything.
I was able to limp the bike home, another 15 miles. It seemed to run ok, just loud.
After I got home and the engine cooled off, I checked all the exhaust pipe's nuts and bolts with wrenches, all good. None-the-less, I loosen them all and retightened them. Encluding the large finned exhaust nuts. Duh, still have the noise.
I checked the plastic tubes on both sides of the carburetors they're still snug.
Pulled the front engine cover off and checked to see if the seals behind the generator and my point-n-a-can are still intact. No oil leaks.
I then did a compression test, 130psi(left) the noisy side, 140psi (right). Not bad, within the 10% margin.
So I assume I don't have a blown head gasket or a stuck valve.
It was getting late by that time, so I didn't do a vacuum test(tomorrow). Too noisy to do any more testing.
The noise appears to be coming from the front left side of the engine, but I can't tell where.
My question/s, does anyone have any idea what this could be? And what else can I check?
Blue-1
It sounds like you are a victim of the Revenge of the Pulse Air System.
1980 was the first year BMW installed an air injection system on the USA bound models. Snowbum has a very thorough article on this system, including its removal, https://bmwmotorcycletech.info/pulseair.htm. In your case, it sounds like a previous owner removed the system and installed plugs to block off the holes in the heads where the pulse air system injected air. One of those plugs fell out and hit your left leg, and the hole in the head, right at the exhaust valve, began to roar.
You can see these holes/plugs in the head, but you have to look just below the finned exhaust nut. Snowbum's article has all the information you need to source a new plug and install it.
Andy
ABC# 15601
My thoughts exactly. Except I was sure it was called the Pulse Injection System, which would only mean you've been PISed on.
👿
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Andy, you hit the nail on the head!!!
After reading your reply I literally ran out into my garage, Sure-enough, the plug is missing!!! It's kinda hidden underneath the exhaust pipe, so I didn't notice it missing!
I just asked a question about these plugs in the "Wrenching" column, titled, "What's this port on my cylinder head?". I even attached a photo, the opposite side(right cylinder). The missing plug is from my (left cylinder).
I've got to run to an appointment but will read Snowbum's article when I get home.
In the meantime, is there a place I can buy a new replacement plug?
This is so cool, an easy fix to something I thought was a disaster!
Thank-you, thank-you, thank-you!!! This forum has proven to be a life-saver!
These plugs are not a common size, 16 mm by 1.5 mm pitch straight thread (not tapered pipe thread). One should be available from a hydraulics supply. It will probably come with an O ring, but you will need to use a crush washer, and maybe some thread sealant. And check the right side for tight.
Andy
ABC# 15601
Thanks, Scot,
Yes, I read what Snowbum said. I have about (5) engine oil drain plugs. New, used, some magnetic, some not. but they're all too big to fit in my Pulsed Air, hole.
Correction: My mistake, after re-reading Scot's comment, Scot said a drain plug from a Oilhead, all my plugs are from a Airhead. What was I thinking?
However, after contacting Ted Porter's Beemer Shop, the premier Airhead, head rebuilders, they have the right size stainless steel plugs in stock. I ordered two each with crush washers.
You're right, it was easy with everyone's help!
Thanks so much for your reply,
Mike...
FYI: Photo below of the Pulse Air hole hidden under the exhaust nut for those unfamiliar.
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