Vintage BMW Motorcycle Owners

I've discovered that my left side Dellorto has a vacuum leak where it connects to the intake manifold. Specifically, when I spray carburetor cleaner where the carb connects to the manifold, the engine rpm falls drastically.

The carb has an insulator bushing that should seal on the manifold and mine are both brand new, but wondering if I could apply some sort of sealant on the manifold that would form an airtight seal? What would be recommended?

Thanks,

RickR90s

I'm familiar with the Bings...they have that piece of rubber hose that connect the output side of the carb to the flange screwed into the head. Is the Dellorto substantially different than that?

If you wanted to try a sealant, why not try same Hylomar or Permatex Ultra Gray or some of the other sealants used on the cylinder base. It's designed to work with heat and with a thin even coat, it should be able to form a seal.

One of the BMW books talk of taking a sheet of glass. Some wet & dry sand paper or..... And holding the carb flange on the sand paper on the ( flat) glass. Use a circular motion? The aluminum can distort from the heat cycles of the engine.

Hi Kurt,

yes, the Dellorto's are different in that there is no rubber connector between the carb and manifold. The manifold's are unique to the R90s also, due to the larger Dellorto's.

See the image, where #4 (insulator) is the part inside the carb that forms the seal around the manifold.

Thanks! I still like my Hylomar suggestion!!

I talked to Randy Long (BMW Machinist) and he suggested Permatex Gasket Maker. It's sold at NAPA and is used on intake manifolds and i fuel proof.

That still didn't work so now I'm thinking of using a very thin o-ring to slip over the manifold that will allow the carb to slide over that.

Better yet might be to machine a recess into the intake manifold to "hold" the o-ring.

I'm glad I found the air leak as it explains the high idle I've been fighting for a few years, but at the same time, this sure is frustrating.

RickR90s

There must be something seriously wrong if you can't seal the carb with gasket sealant. Look for cracks or pinholes in the carb, manifold and head. It should seal easily while it is dry.

Maybe the manifold is oval from being tightened with a pipe wrench or similar?

Mike

I used a Loctite sealant that worked.

I learned the following:

apply a thin bead (carb detached) and let it skin for a few minutes
assemble carburetor over the manifold WITHOUT tightening the clamp
wait 24 hours
Tighten clamp
Start motor and check seal with carb cleaner
Verify that rpm doesn't change


I rode her to work today (120 mile roundtrip) and no issues whatsoever. Even after sustained 80mph on the freeway, the bike idles at a nice, even 1000 rpm.

Note to self: Ride this bike more!!!