Thread: Brake venturi air leak

Hi there, this is my very first attempt at writing something here, and I'll try to sound sensible, (a folorn hope...)

I've had Tallulah, a S3 Diesel wagon for couple of years. She has brake assist with vacuum provided from behind a butterfly venturi on the inlet manifold.

I was having a gentle fiddle, and I noticed there was a lot of free play between the brass butterfly shaft and the aluminium venturi housing. This worried me, because unfiltered air will be continually sucked into the engine through gaps between the shaft and the housing.

I looked at getting it bushed, but the metal is sufficiently worn that it would still be difficult to get a good seal. Besides, I didn't know where to go, it would probably cost a bit, and I wanted to fix it for myself.

This has been a pragmatic bodge job, and if you have any good or bad ideas to make it better, or to stop it getting worse, please share your opinion, this has been a novel experience for me!

The brass shaft had worn by a bit under a mm on the same side on both ends: I guess the force on the butterfly from the airflow makes it push the the same way; the holes in the aluminium were also very slightly oval.

I wanted to stop the metal wearing any more: I did this by wrapping plumber's PTFE tape around the shaft until it was a gently tight fit when I put the shaft back into the housing, it had about 12 wraps a side. I then trimmed the tape ends, especially on the inside of the housing. One end of the shaft just stuck out slightly from the housing, and the other end and has a pivot arm attached. I found that a 1/4" grommet is slightly tight over the shaft. I cut one in half and slid it up the shaft to the pivot arm. There is about 3mm of unworn shaft sticking out of the housing, and I hope the grommet has made an airtight seal on the shaft. I used that blue "liquid gasket in a toothpaste tube" to seal the grommet-half onto the aluminium housing, and it isn't rotating (at the moment) on the housing.

The other end was easier: I used the same blue liquid gasket to glue a large washer onto the housing over the end of the shaft, and then glued a 5 cent piece over the hole in the washer to make an airtight seal.

On reassembly and test, I 've found that the brakes really work, all of a sudden! I guess the vacuum was largely compromised by those leaks, and I wonder what nasty stuff got into the engine until now.

It'll be interesting to see how this this home made repair will turn out to be.


Here is the washer and coin nicely glued on




And here is the grommet glued in place. The PTFE tape got a haircut with a trimming knife before it all got put back together again.


I'll keep an eye on it, and fingers crossed.

Cheers,

Mike.

I agree with John, I did quite a few test wraps before I got what I hope to be the right amount of tape in the right place. Tape tension wasn't a problem , and I counted the number of wraps on one end, ad then tested each end by inserting the shaft and simply felt for free play versus freedom of movement. When it started to get tight, the tape slipped up the shaft a bit, which is why you can see it in the bore, a sharp knife and tweezers were used to remove most of the excess.

I ended up with 12 wraps on one side, and 16 on the other. I used white tape, and I know you can get different colours for different types, thicknesses maybe??

I'm a bit worried about what will happen if any tape breaks loose. It is soft, so it shouldn't damage any downstream metal, I've never put this tape into a gas flame to see what really happens to it when it gets hot, I'll do that tonight!

A carby has more sticky-out bits downstream of the butterfly. What would happen if a bit of tape let go and covered a nipple or an orifice, or one of those annular shaped airflow management bits? There must be quite a breeze down the throat. I would treat this as more of an interesting experiment than a solution, and I will be very interested in your results!

Mike