Diagnosing vacuum leaks

Carb conversion solving vacuum leaks

I’ve had this vacuum leak issue since installing my carburetors and I would attribute this to user error, aka my dumbass, than the parts themselves.

When I first set out to do my carb conversion I had no idea what I was doing and was vaguely following instructions from a myriad of youtube videos and forum posts that vaguely described what I was supposed to do. I don’t blame these sources either because my lack of experience on the fundamentals of engine and carb design really set me back.

My goal in this video is to describe the fundamentals behind a practical install of bike carburetors on a car. I wanted to simplify it enough that you could go in with a better understanding than I did when I first attempted this.

I’ve made a video on the principle of converting to bike carbs, but this one is a bit more practical. More than just hook this up, do this and that, and bango you’ve got a conversion, but with my experience and the errors I’ve run into, I think this will probably help the 6 people that are bored enough to do this… probably.

Let me reiterate, carb conversions are not for power. They’re aren’t for drivability. It’s just another option you can go towards reviving an old car you like. In retrospect, I probably should have gone for the full CE2 ABA swap, but I don’t think I would have known how to do that without going through this process of understanding the exchange between the fuel, intake, combustion, and electrical systems.

Diagnosing the vacuum leak

Let’s start with the current condition of the car, it has a vacuum leak.

A common culprit in any “rough running” condition is the result of a vacuum leak. Big or small, it will give you a bad day. There’s a few ways to go about this:

  • Propane*
  • Smoke testing*
  • Spraying carb cleaner
  • Using a cigar

Use any feasible method for you. For quick checks, use propane. For thorough checks, I recommend the smoke test. Avoid the carb cleaner test since things may catch on fire if you are not careful. The cigar method, if you’re into it I guess lol.

Go over every possible place that may leak air. Starting from your engine intake ports and work backward.

  1. Intake manifold flange
  2. Intake manifold tubing
  3. Hoses leading to intake after fueling
  4. Hoses before fuelilng
  5. Vacuum advance
  6. Vacuum sensors
  7. Brake booster
  8. And so on.

Once you’ve find your vacuum leak, we’ll go about the way on how to fix it.

If it’s a gasket you can replace, replace the gasket. If it’s a cut hose or loose hose, replace the hose and/or clamp that secures it. And if a connection is simply loose, tighten it up. My vacuum leak is coming from intake manifold flange. I have two gaskets there and I’m unsure which one it’s coming from since they’re so close together. This was not the biggest leak, but it shows how easy it is to find a leak.