Backfire through carb

The Plunger · Post by **The Plunger** » Thu Dec 14, 2017 11:03 am

Well, the diagnosis seems correct. Vech responded to the video with this:

"Sounds like the rear main seal. The European seals sometimes do that. If you want to find out for sure if it is the seal, while it is running, making that noise, unscrew the dipstick. If it suddenly goes dead quiet when you lift up the dipstick out of the hole just a little, then it is for sure the rear main seal.
You see, when the pistons move down the bore, pressure is created in the crankcase. When the pistons hit bottom dead center, the window in the timed breather lines up with the crankcase vent, and pressure is released. As the pistons move up, a vacuum is created, and it will suck air in through the rear main seal. That is what they do. Now, depending on the seal design, sometimes it will cause the lip to vibrate as the air comes in, and it makes a squealing noise. That seems to go away at high speed but is there at idle when hot. It is harmless but irritating."

Also, "The noise may stop, but if it does not, and it drives you nuts, you need to change out the rear main seal.
This one won't make noise, we have it in stock: 11 11 0 001 120--$29.95--Viton rear main seal R26 R27 R51/3-R69S"

And, sure enough, the howl went away when loosening the dipstick. Yay!!

wa1nca · Post by **wa1nca** » Thu Dec 14, 2017 1:00 pm

Very good news
Glad you didn't have to tear the engine apart

I was aware the seal can do this but never understood why
Thanks for posting Vech's reply

Tommy

stagewex · Post by **stagewex** » Fri Dec 15, 2017 8:47 am

"And, sure enough, the howl went away when loosening the dipstick. Yay!!"

incredible diagnosis.

312Icarus · Post by **312Icarus** » Fri Dec 15, 2017 12:01 pm

Chalk up another one for Vech!!!

Icarus

808Airhead · Post by **808Airhead** » Fri Dec 15, 2017 4:34 pm

Vech rocks!!!!

wa1nca · Post by **wa1nca** » Fri Dec 15, 2017 5:04 pm

Yes indeed
One day I needed some advise
I didnt want to call without ordering some part
So I ordered a speedometer cable that I didnt need and then felt better to ask for some advise
I still have that spare cable but felt it was well worth the advise he gave me

Tommy

wa1nca · Post by **wa1nca** » Tue Dec 19, 2017 9:08 am

Oh, I found it, pages 191-192. Are you freaking kidding me?! So, in the meantime I'm supposed to ride it until the noise goes away or until the motor seizes up because it WAS a main bearing after all? Thanks, Tommy, we'll see...
BTW, the charging issue looks to be a bad rotor as it's showing continuity between the armature and commutator. Time for a puller and new rotor! This IS fun.

Did you get your charging system working

Tommy

The Plunger · Post by **The Plunger** » Tue Dec 19, 2017 7:17 pm

Tommy,
I wish! I’ve been awaiting a rotor puller tool I ordered over a week ago from Cycle Works....patience is NOT a virtue when you’re in a hurry. As soon as I get it off I can try another rotor.

miller6997 · Post by **miller6997** » Tue Dec 19, 2017 8:33 pm

In my experience, the howling noise never goes away entirely, and I have long since stopped fretting about it. If yours seizes, we'll grieve with you.

Vintage BMW Motorcycle Owners