Norm's information indicates that it's impor-tant to follow the 20-degree rule when using
More Info on Alignment
Norm Peterson, Cherry Hill, NJ: In your home alignment how-to, it is stated that the 20-degree turn-in/turn-out angles are not critical and the 20-degree figure is popular for reasons of accuracy. Neither is particularly true.
In truth, the 20-degree turn angles are specific to the commonly available caster-camber gauges such as the one you used. The short explanation is that when you make your caster measurement with this tool, you're actually measuring the camber change over a given amount of steering. The camber range becomes caster as you steer the wheels either way from straight ahead at roughly 1 ÷ (2 x sin ([turn angle])), where [turn angle] is 20 degrees in this case. Accordingly, the caster scale on the caster-camber tool is calibrated to use a factor of 1.5 for your 40-degree total sweep. Being off by a degree or possibly two in your turn angles probably won't make enough difference to matter for most street alignments, but if for any reason you could only get a turn angle of 14.5 degrees, you should be using a factor of 2.00 instead.
At least some of the commercially available caster-camber gauges are beveled on the end at the 20-degree angles for visual reference. I'd take this a step further and fabricate a 20-degree-angle jig from steel strip, as it will help to hit the 20-degree angle more reliably.
For those interested, there is an SAE paper readily available from Hunter Engineering (the alignment equipment company) at hunter.com/pub/under car/2573T/index.htm that goes into this in a lot more detail.
Jeff Smith: Thanks to Norm for this update. We were not aware that the bubble-type camber/caster gauges integrated this 20-degree requirement. Now we do.
This is Scott's orange '64 Olds F-85. His reference to being plagiarized is in regards to
Question that Sucks
Scott Renzenbrink, Massillon, OH: I have a '64 F-85 with a warmed-over 455 cam and a vacuum-secondary Demon 750. I noticed the vacuum gauge tends to bounce a little at idle, bounce a bunch at tip-in, and become steady once I am under way. I checked for leaks and adjusted the carb and the results are the same. Then last week I slapped the Q-Jet back on the motor and the gauge reads steady as a rock. Do you have any ideas why this would be occurring with the Demon? The car seems to be running fine, but I need to know what I am looking at. Is it just me? When I look at page 85 of the Oct. '09 issue, I feel like I am being plagiarized.
Jeff Smith: In the past when we've run into the situation, Scott, where the engine idle vacuum bounces around, it generally indicates that the engine is running lean. To get a little more feedback about this, we talked to Sean at Sean Murphy Induction (SMI) in Huntington Beach, California. Sean says it sounds like the engine is still suffering from a mild vacuum leak even though you did not immediately find one. Since you changed the carburetor and discovered that the engine runs fine, this indicates that the leak is located in the carburetor itself.
Vacuum leaks are one of the most common forms of engine idle and off-idle driveability problems. This is because the air that is introduced into the engine is not metered, which means the proper amount of fuel is not mixed with this air, and the engine will run lean. Depending on where this unmetered air enters the engine, it can affect all the cylinders or possibly just the front half or back half of the engine. A too-lean air/fuel ratio at idle is generally characterized by a rolling or inconsistent idle speed. Often this is described as an idle that hunts and is not consistent. The most common place for this to occur is the seal between the intake manifold and the heads. Loose intake manifold bolts or just a porous seal between the intake and the heads will create this same ragged idle quality.
There are several situations that can cause any carburetor to display these kinds of poor idle quality symptoms. Some areas to look out for are a rubber manifold vacuum port plug that's cracked. We've seen ones that look fine on the outside but once removed are actually split quite badly. We've also seen base gaskets between the carb and the manifold that do not completely seal the carburetor but only leak after the engine is completely up to temperature. We've also seen metering blocks on Holley and Barry Grant carbs that have been subjected to the gorilla-finger tightening technique. This is where the bowl screws have been overtightened, which tends to warp the sealing surface of the carburetor main body. It can create vacuum leaks that are not only difficult to locate but are also inconsistent, meaning the engine will not always exhibit these poor idle quality symptoms.
Another situation we've run across is when a chunk of dirt or solid debris tends to lodge in one side of a carburetor's idle circuit. One time that comes to mind was way back when Scott Sullivan and I drove his Cheez Whiz '55 Chevy from Ohio all the way to California. Somewhere in Utah, the Lingenfelter-built 496ci big-block began running very poorly at idle and part-throttle. After much messing around with the carburetor, we realized the black chroming of the carburetor had apparently deposited or dislodged something into the idle circuit. If we removed the metering block and blew it out with compressed air, the engine would run fine-sometimes for days and other times only for hours. The fix is to drill out the idle-feed restrictor and remove the offending debris. This is a rare occurrence, but it does happen. Somewhere in the middle of these suggestions is probably where the problem lies. We'd suggest going through the BG carb with a good cleaning, new gaskets, and a shot of compressed air through all the various metering passages and see if a more stable idle returns. My guess is this will solve your idle stability problem and let you go back to enjoying your 455ci Olds.
Sean Murphy Induction; Huntington Beach, CA; 714/843-9169; smicarburetor.com