Resto File #3: Vacuum Leak Troubleshooting.

Admittedly this is somewhere in between troubleshooting and restoration but I figured I would share the process with you and maybe give a few pointers as well. For a while I’ve had an annoying whistle from a vacuum leak so I finally put forth the time to track it down. I’ve noticed that usually these old Keihins (and Webers as well) can get a little loose in their mounts, or the screws that hold the body of the carb together end up loose. So, your first step in finding the problem would be to simply jiggle the carb while the car is idling and see if you hear any change. In my case this was the problem, and so i’m just going to show you what I did to fix it. At some point I plan on doing a very comprehensive guide on the 3Bbl Keihin; As someone who has had years of dealing with the factory CVCC carb, I will be doing something a lot more in depth when I get a chance. I’ll try and help those who might be going through the same frustration. I will also attempt to reveal the magic of those “smog boxes” and how to make sense of the rat’s nest therein. In the meantime though: Vacuum leaks usually manifest themselves as a rough running engine: Unsteady or high idle, loss of power, poor fuel mileage, and hesitation during acceleration. Frequently you will be able to hear them as a whine or a whistle, but not always. If you are aware of a vacuum leak but unsure of it’s location an easy method is to use a spray can of carb cleaner and use little spritzes of it throughout your engine bay while the car is idling. A vacuum leak will suck in the cleaner and you will hear your idle speed increase for a moment. Once you have the problem localized to one area, simply inspect any and all lines or piping which are connected to the intake tract, or pull vacuum off the manifold. If you have an old Weber installed this is fairly easy, as chances are you only have the brake booster line, the advance line for the distributor and possibly a line to the egr valve. If you still have the factory carb and smog equipment on this may prove to be a bit of a nightmare but sadly there’s really no shortcut. If you have a Fuel injected model the process is the same, but it may be a little trickier to reach and inspect all possible leak spots.

In a worse case scenario there IS another technique you can use for spotting the leak that’s tricky to describe: If you have access to a fog machine (I know most of us do, right folks?!  anyone? ….no?) then this is an option..otherwise a big nasty cigar will do the trick as well. Ever seen smoke at night? When you shine a flashlight through it it becomes quite visible, hence the trick: If you can find a way to blow smoke into the engine bay while the car is running (at night) then you can use a flashlight at an oblique angle to illuminate the smoke. In theory you will see where it’s being sucked into the vacuum leak. That’s a bit of a last resort, as most of you will find the leak a lot more easily.

Anyhow, I was lucky in my case, and my problem turned out to be the mounting itself, I had a leak between some of the adapter plates for my Weber.  I had initially just tried tightening it down more but to no avail. You want to be careful with the main mount anyhow. If you tighten it down too much the studs can actually turn with the nut, and instead of tightening things down, the stud will go too low and end up pushing the bottom mounting plate away from the intake manifold, effectively creating a bigger leak and possibly damaging things in the process.

Just make sure that if you’re going to tighten those nuts down that the stud isn’t turning at the same time you turn the nut. I decided I would just take the whole thing off and “re-do” the gaskets while I was at it. To be honest didn’t use the right sealer when I put the factory gaskets on (I was told high temp grease would do the trick) and though it lasted for a while it obviously wasn’t a permanent solution. So, first I cut my new gaskets out, an exacto knife and scissors did the trick. I had the foresight to photocopy the original gaskets before using  them, so I used the copy as a template.

I used Karropak, a  multipurpose gasket material made by Felpro that’s available fairly cheaply at just about any autoparts store.

After disconnecting and plugging the fuel line,  I disconnected the vacuum advance hose, and was able to move the carb out of the way without disconnecting the throttle cable.

After the carb was off I continued to separate the stack of mounting plates.

As I looked at each layer of gaskets I think I found where the leak was. Look closely and you’ll see there’s a gap in the line showing where the seal would normally be made on this gasket.   It appears that despite tightening them down as much as I dared and despite the fact that I used some Loctite, the allen bolts on the middle plate came loose after a while, creating my problem. So, with everything apart it was time for re-installation.

I used some proper gasket sealer this time and although it was probably too old (It had the consistency of tar) it still worked. I’m going to be taking this carb off in a month or so anyhow so I’m not worried.

Anyhow I put everything back together, being sure to tighten things a little more firmly this time and Voila! No more whistle!

It runs a lot more smoothly now. The next step will be to re-set the idle mixture and speed, since they were tuned with the leak. That should be simple enough, but that will be covered another day. The journey continues…

Ethan – Editor

One thought on “Resto File #3: Vacuum Leak Troubleshooting.

  • December 9, 2013 at 12:55 pm

    Another handy way to find a vacuum leak…with the car idling, spray different areas of the carb/vacuum lines with carb cleaner. When you hear the engine speed up (from the extra fuel provided by the carb cleaner being sucked into the carb through the cracked vacuum line/whatever) you know where to look for the leak.


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