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Happy New Year all,
79 bronco 351m auto, Cheyenne Wy, elev 6k-62oo. I noticed a week ago or so, I have a miss and thought I would try to find it. While I'm at it, I might as well try to set the carb for this elevation instead of the Michigan elevation of under 700'. It runs VERY rich.
I want to check for any vacuum leaks also. Is this the right spray to use?
I'm not sure because even when I spray directly into the air snorkel the RPMs do not increase.
So I'm trying to solve a few different issues today if I can. Thanks in advance.
Post a pic of a spark plug. The brake cleaner and similar chemicals are used on the outside of the carb or intake manifold gaskets &c to check for leaks. Certain types of brake cleaner are extremely hazardous when burned. I don't know what kind. An unlit propane torch is probably safer, and more effective. Lean out the idle mixture so engine labors at idle to increase effectiveness of this technique.
For general engine tuning purposes and diagnostics though a mechanic's vacuum gauge would be VERY useful. For altitude carburetor adjustment, both power valve & jetting are optomized. Make sure ignition timing, fuel pump pressure, float height & everything else is straight before changing carburetor parts.
It's not as easy to just buy and use, but building a little smoker works great for finding vacuum leaks, it's basically just a jar with a lid, a soldering iron stuck down into the jar through the lid, with a cloth soaked in mineral oil inside the jar, being heated by the soldering iron. Then a tube comes out of the lid, that's where the smoke comes out. You put that tube into one of your vacuum connections on your intake manifold, and then watch where smoke comes out of the engine, it will show up wherever a vacuum leak is.
I built one for my truck when I wanted to ensure no leaks, works great once the iron warms up
I almost always used Chemtool, because that's what I had laying around and handy more often than brake cleaner. It always worked well.
You don't need to only listen for the idle going up. It can go down as well, and all you're really looking for is "change" in the idle quality or speed. If it changes at all when you spray a certain area, some of it got inside the intake.
The good thing about general carb cleaners is that they evaporate reasonably quickly if you're not over spraying it too much. The bad news is that it does not evaporate fast enough for recently painted surfaces!
So if your engine is original paint you probably have nothing to fear, again unless you use too much and it puddles. But if your engine was painted more recently, be cautious about where you spray, as it could ruin the nice paint job.
I bet WD40 would work too, and it's pretty benign. Heck, water in a spray bottle would work!
Make sure your PCV valve and tubing are clean and free of carbon buildup. Make sure it's in good shape too. When was the last tune-up performed? Might just need some TLC again.
As said, to lean it out for higher altitudes you'll need to change jets and any other metering devices. What kind of carburetor?
ON a side note, but not a very accurate way to do things, I used to keep a '70's Granada w/302 PCV valve in my tool box and when I went up into the Sierras would swap it for my normal one. Literally just creating a semi-controlled vacuum leak. Since it's fed into the common plenum area of the intake usually, this worked perfectly for me.
I used to be kind of into this stuff, so would literally change jets before a camping trip. Run it a bit lean down here, but better up high. Never perfect, and even young and dumb gearheads get tired of re-jetting for just a few days of fun. The PCV "leak" worked sweet and I used that for years.
Then I went to EFI and never looked back!
The carburetor float height is the single most important carburetor "setting" you can make, apart from choosing the correct carburetor in the first place. What the float height does, is govern the actual fuel height in the carburetor bowl. Notice the fuel height spec is measured in 32nds of an inch. The entire carburetor is calibrated to this, due to the installed height of the high speed air bleeds, the fuel well, idle circuit, and everything else. Too high or too low, even by a little bit, it changes things by quite a lot and it won't run right and can't be made to run right if it's too far off the beam. The float height is set during rebuilding, on the bench, but keep in mind this is a "ballpark" setting. The fuel height must be checked after installation on the engine. Raise or lower the float height as required after installation to achieve the correct fuel height.
Getting this right in turn is dependent on the fuel pump itself being able to supply sufficient pressure and flow at all times under all conditions, from idle to wide open throttle. So a methodical approach to checking each along the way will make tuning much, much easier with better result. To accurately determine the correct jet size for example, the fuel height in the bowl must first be correct. Everything is pretty straightforward with carburetors but it has to be done in the proper order.
A vacuum gauge is a better approach to engine tuning than spraying chemicals. If the average manifold vacuum at idle is not within spec, then it might be time to start searching specifically for vacuum leaks. First determine if there really is a problem. A mechanic's vacuum gauge will also measure fuel pump pressure output. This is the first step to accurate tuning. A lot of the import fuel pumps are way, way out of spec and pose a fire hazard in my opinion. This is something to check.
The vacuum gauge will also prove valuable for jetting and power valve selection. Run a hose long enough to observe the vacuum gauge in the cabin, to check the average manifold vacuum at steady cruise condition, steep hills, etc. Save the brake cleaner for a brake job.
