VACUUM LEAK ON A CARB'D 308

I think I may have a vacuum leak on my Euro 76 308, and I'm preparing to track that down. Is it likely to be between the cylinder head and the intake manifold thanks, Steve.

 

It's hard to establish a steady idle, no matter what you do. That's why I think there's a vacuum leak, and we will use a smoke machine to diagnose that. It could be in the brake servo, but I'm very suspicious of the throttle shafts on the DCNFs as they tend to "oval out" and the shaft starts to wobble then the butterfly does not seal the venturi well. Steve.

 

I do not experience any backfiring with deceleration. Sometimes, when you have a failing brake booster, brake fluid can accumulate within the booster itself. You'll notice a loss of brake fluid in your reservoir, as well as being able to see it on a dipstick placed within the brake booster, which should be dry all the time. Thanks, Steve.

 

Thanx again, Brian. steve

 

@ProvaMo Thank you so much for your interest and insights. I probably will not get to the project until tomorrow. However, over the weekend we removed the rear deck lid, which was pretty easy with instructions from Brian and the fact that it's a Vetroresina with pull straps. More later.
thanks again, Steve.

 

The denouement! The professional mechanic who is helping me since I can no longer bend very well wanted to try out his new smoke machine. There were no leaks at the carburetors or intake manifold, nor at the brake booster. However, when we stepped on the brakes, bingo: smoke came pouring out under the dash and in the frunk. The vacuum leak appears to have been coming from the entry of the master cylinder rod into the booster itself. I guess the rubber seal there just gave up the ghost. Now, in retrospect it's easy to see why my idol was so erratic.

Unless my guy made more work than necessary, it was a huge job. Part of the mounting system is under the dash, requiring removal of the steering wheel, pedal box, some control cables, and return springs. The master cylinder comes out with the booster. It's on its way to be rebuilt. Thanks guys for all the help. It was way more complicated than I would've ever believed and way out of my pay grade. Here are a couple of pictures. s
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@Dockboy Thanks again for your advice. I never experienced any braking problems whatsoever, and it never occurred to me to try to correlate
braking with changes in vacuum, and thus idle. Good luck with your project. Who are you using to rebuild your booster? Might consider having the master cylinder gone through at the same time, as well. Hope you find it easier to remove than we did. steve

 

@Dockboy They were also on a list recommended to me, including Karp's. However, Power Brake Exchange which used to be in San Jose, now in Carson City Nevada, had done the power brake booster in my 330 GTC and it's still going after about 12 years of use. Interestingly, the symptoms I experienced with that failure were vastly different than those we're discussing here. They were very nice on the phone and told me that the booster turn around was one day, which must mean it's not going to be refinished to factory standards, but that's OK because you can't see it. However, I also sent them my master cylinder, so I don't know how long the turnaround will be, but we're about to find out. Good luck! Steve.

 

@Dockboy was removing yours as difficult as I experienced? s

 

I won't tell him.... he's a big guy probably weighs 225 and 6' 3 or 4, so that may have been part of it. At any rate, I cleaned up the reservoir today. With all of the jostling, the floats fell off so I had to fish them out. Now it's all nice and pretty. Fortunately, the reservoir is still in very good condition I'm very lucky, as the car that I bought almost exactly 3 years ago was a really good one. Steve.

 

@Dockboy Since I wasn't down there doing the dirty work, I don't know if the modifications I made to the pedals would have made my removal more difficult than yours. Here are some pictures of what my pedals look like, modified to allow me to "heel and toe". s
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You are correct for sure. Oh well it's water under the bridge. Thanks again. Steve.

 

Just a last follow up.....
On the erratic idle and inability to set a constant idle on my 308. I think the last time we looked in our hymnals, I felt that the problem was a vacuum leak arising in the power brake booster. This was only evident stepping on the brakes, the diagnosis being made with a smoke machine. Bingo, got the power brake booster repaired and thought I'd have the master cylinder checked out as well. Both items were returned and installed, and it looked golden.

However, after getting the carburetors synched nicely, I decided to take a peek at the master cylinder and reservoir, which are in a hard to access area. Unfortunately, or maybe fortunately, there was a leak of brake fluid that we discovered only when the brakes were being applied. It turns out, that the master cylinder, which was probably OK when I sent it in, was not properly repaired …. the large bolt on the end was loose, allowing brake fluid to squeeze out with every press of the pedal. Obviously, the “frunk” of the 308 is a complete disaster with brake fluid everywhere eating all the finish off of the inside of the “frunk”.

I believe that problem has been resolved by tightening that bolt that should've been properly tightened on the rebuild. I think we're good to go and maybe one last test of it tomorrow before we zip it up. The carburetors are nicely synched. One of the idle mix screws has been repaired, as it was really nonfunctioning, being unable to completely seat.

Thanks for your support.
steve