The Stovebolt Site Forums The Shop Area Engine Shop Strange new sound

Picked up a hissing sound and truly horrible gas mileage, and of course, couldnt come at a worse time! My '63 Grumman Kurbmaster RV just finished week 2 of 5 consecutive weekends of fun.

So, I need to come up with a plan of attack so she's ready to roll next weekend!

I have traced the sound to the area generally on the left side of the intake/carburetor, leaning towards the intake as being the cause.

It is my 1st Chevy 292, and I havent been into the manifolds since Feb of last year, so I am fairly clueless.

After Google searching images, it looks like there may be a vacuum port {unused in my configuration} that may be the culprit. There does seem to be unused coolant lines {intake heat?}, but they dont fail the carb cleaner test. Something on the left side [out of sight for now] does fail...

Any other suggestions? Gonna pull off the dog house tomorrow, having any other ideas to check out would be sweet!

Sounds like a vacuum leak or it could be plugged up exhaust. Did someone stick a potato in your tail pipe? If its a vacuum leak, you should be able to to find it. Do you have vacuum wipers? A stethoscope can be helpful, or just a piece of rubber tubing to narrow it down. If you absolutely can't find it, put something like Marvel Mystery oil in a pump oil can and squirt it around the manifold. A leak will suck it in and the exhaust will smoke.

With the engine running, carefully spray a "little" starter fluid at the hissing sound, work your way along the intake manifold and carburator base. If RPMs change you've found the leak.

Use a propane torch with the gas turned on, but not lit. As you move the torch tip around the vacuum lines, brake booster, etc., the engine speed will pick up and/or the engine will run smoother when the gas gets sucked in by the leak. It's a lot less messy than liquids to find the vacuum leak. Acetyline works, too.
Jerry

Strange hissing sound ended up being some amateurishly sealed unused vacuum fittings, out of sight due to the dog house.

15 minutes and $3 later, a simple brass plug was installed, with the recently completed carb rebuild, she now turns a lazy 300rpm idle...

Time for another mileage check next weekend!

Another thing to watch on these if they have a remote air cleaner is the hose that goes to it. They can get soft and work ok at low speeds but will suck together at higher speeds and act like a choke. Took me most of a day to figure this one out.

Guess I'll have to do the mileage check by filling the tank and noting the gallons, since it went about 25 miles further {and all the way home} without emptying the tank!

If I can exceed 10mpg pushing that barn door around, I'll be tickled!