finding vac leaks using carb cleaner

[kuhlou]

Been putzing around quite some time locating vac leaks. After replacing hoses and plugging choke vac line and seeing some improvement, went for the carb cleaner test suggested in other posts. Sprayed the carb cleaner while idling, idle went up and smoothed out. So, something is certainly leaking vacuum. It seems to speed up the idle almost anywhere I spray it around the base, also on the carb body itself. I had the carb "rebuilt" last fall, though I'm getting quite skeptical about the rebuild.

Carb is an autolite 1100 non-scv on a 66 mustang 200 with non-loado dizzy, CA smog version. Plugged off the internal vac port also, as suggested in other posts. Dizzy vac hooked to manifold.

What items do I check on the carb to determine if there's a vac leak on the carb body?

Also, is it correct to assume that when the carb cleaner is sprayed, idle picks up and smooths out, that the leak is temporarily sealed by the carb cleaner liquid? As the liquid is sucked through the leak, the rough idle returns?

Finally, once vac leak issues are found, what is the correct procedure to set the idle. Set idle stop screw first, and then the mixture screw? I read about it in another post, and cannot find the post again.

Thanks for the help.

 

[ludwig]

Yes the carb cleaner plugs the leak and that is why it smoothes out briefly. The idea is to find out where the leak is located. So you spray systematically. You can do the same thing with oil (WD 3O or light motor oil as well. But oil leaves an oily residue.

Maybe you need a complete gasket kit for the intake. All new stuff. You can also get blank sheets of gasket material and cut your own to fit or beef up existing gaskets. Those older gaskets seem to dry up and split after 40+ years. Maybe even after only 20 years or more. Don't know why. Rubber does that too. Cracks you can't see will slowly spread, especially where they are stretched over some retainer bulge, like at the fuel inlet or the vac advance or where ever.

Once you have the vacuum leak prob sorted out, set the idle speed. THEN fool with the fuel mix. You can do this with a vacuum gauge. Hook the vac gauge at one of the manifold ports. With the idle at 700 in DRIVE, slowly close the mix until the engine just starts to stumble. Then back it out to where you get the highest vacuum reading [at least a minimum 18 lbs of vacuum]. Then back it off just a touch more but not enough to change the vacuum.

Finally, re-set the idle speed to 700 RPM. Should be good.

 

[JackFish]

Check for loose screws, especially those that hold the top of the carb to the body.

 

[kuhlou]

thanks, keep 'em coming!

 

[MustangSix]

Carb cleaner is messy and flammable. If you soak your carb and manifold, it could catch fire. Then the vacuum leak becomes harder to locate.

A better method is to use a small propane torch. Just turn on the gas (do NOT light it) and hold it in the vicinity of the suspected leak. The gas will get sucked into the leak and will cause a change in the way the engine is running. There's no fire hazard. Once you move the (UNLIT) torch, the gas dissipates very quickly.

 

[ludwig]

I always check for leaks in the natural gas lines of my house with a lighted match. Only wimps use soap and a brush.

Haven't had to shave my eyebrows any more since that one time...

 

[Eric Rose]

I almost always make my gaskets from the gasket material. I have even bought new gaskets, used them as templates, and then returned them to the store.

For carb gaskets I like to use a thick type of gasket paper called "carropak". You can also use thin cardboard (non-corrugated of course )... pretty much the same stuff. I once cut intake manifold gaskets for my old '79 Honda out of a shoebox. They worked great.

 

[Harte3]

Starting fluid works good...evaporates fast and no oily residue.

Cereal boxes make pretty good gaskets too.

 

[ludwig]

I'd start with that heavy Victoria's Secret gift bag. The paper is real durable and it has a shiny side you can use against aluminum or magnesium for a non-stick surface.

 

[kuhlou]

verrrry productive suggestions!

 

[kuhlou]

seems to good to be true, so probably is. Unfortunately, just used my last Victoria Secret bag, so got some gasket material to make some gaskets and get my vacuum leaks sealed. Per earlier post, I had detected some leaks, I thought, using the carb cleaner method at the carb base. Also got a tube of the blue hylomar from harbor freight.

So, fire up the stang, just for fun. choke is not functioning, so let 'er warm up, and it idles smooth, pulling 18 plus vac! What the heck! So I spray more carb cleaner at the base, no change in vac or idle. Any guesses what just happened here? Is it possible the carb cleaner left enough of an oily deposit on the old gaskets to allow them to seal? I'll chalk this up under the too good to be true dept. But, we'll take the stang out tomorrow and see how long our luck holds up. Strange, very strange.

I'll guess that after a nice half hour ride, the old vac leaks will probably reappear. Any thoughts on this?