locating that agravating MISS on an older muscle car

[grumpyvette]

theres several easier ways to locate a missing cylinder on an older muscle car with a carb
heres several
keep in mind a (MISS) is usually related to a lack of compression,in the cylinder, a badly supplied fuel/air mix ratio, or a lack of complete ignition,badly timed ignition or detonation, but can be linked to low voltage, bad electrical connections, or mechanical issues

(1) pull each plug wire in succession off the distributor cap,then replace it, if you find one that once removed has little or no effect on the idle its probably your problem., while you have the wire removed measure resistance with a multi meter on ohms, if its over 500 ohms per foot its most likely defective or at least not ideal, compare it to the others, any wire significantly higher is defective.
watch the spark, if you stick a Phillips screw driver in the distributor or wire and hold its un insulated shaft near the other it should arc bright blue,and snap audibly if its faint red or yellow and the arc not audible theres usually a problem

(2) use an infrared thermometer,measure exhaust headers right where they connect to the cylinder head, the temps not as important as consistency between cylinders but 450f-600f at idles about normal (depends on dozens of factors like displacement, cam timing,ignition timing, f/a mix etc.) a defective cylinder will tend to run cooler, but remember the front cylinders tend to run a bit cooler on some cars

http://www.professionalequipment.com/ex ... ermometer/


(3)learn to read spark plugs, set the gaps at .045 on new plugs and don,t forget to use anti-seize grease on the plug threads when installed.

(4) learn to adjust valves correctly, and do a compression test, a great many problems blamed on carbs and ignition are traced to badly adjusted valves or mechanical issues like detonation damaged rings or broken valve springs, check for vacuum leaks

(5) verify your TDC and timing marks, setting your ignition timing without verifying thats a hopeless waste of time

(6)verify your fuel and oil pressure , and carburetor float levels, simple things like low fuel levels, stuck floats, bad needle valves and vacuum leaks or fuel pressure thats not consistently near 5-6 psi drives carburetor tuning insane

(7) if the cars got a computer PULL THE TROUBLE CODES

(8) do a compression test, all cylinders should be within 10% ( high to lowest reading) if not rings or a valve job are suspect

(9)If nothing else is obvious, measure cam lobe lift, cam lobes wear, at different rates but its not unusually to have a worn cam lobe on some engines

do a google search on (SLIPV618)its a bore scope I use and it easily slides thru a spark plug hole
http://store.dejavujewelry.com/tlw-slipv618.html

just be aware that oil in the runners doesn,t necessarily indicate a bad intake gasket because reversion pulse at low rpms can and does suck oil out of the cylinders into the intake runners even on engines with perfectly sealed intake gaskets if the ring seal or valve guides are warn

obviously valve float and improper lash clearance can cause problems but in some cases, lash caps can reduce wear

in most cases when you see valve tip damage like this its the result of valve float or a weak valve spring , in many cases youll need to swap to a higher spring load rate and new springs to prevent or reduce this damage

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[Randy_W]

If it's an older Pontiac, just do the compression test, it probably needs a valve job! :mrgreen: I'm just sort of joking, the pre '67 heads were usually in need of a valve job by 25-30k miles if you ran them hard. The '67 on heads were about on par with all the others of the day. Most performance engines won't have this problem, but a lot of near stock cars with GM early style coils would develop a load miss long before the coil failed and it can be hard to diagnose. On the '60's style coils if a car has a miss and nothing obviously wrong, the coil will usually fix it.