Hello. Trying to get a handle on the current state of my new car that seems a little tired and has a slight miss at idle. Did "dry" compression test today and the numbers seem good (low 155- high 165). 1970 455 with 6 4 heads. Is this pretty solid? The plugs looked good. When I bought the car the distributor had an recent HEI kit installed. I notice the distributor was clocked funny. Yup put my timing light on it and it was off one tooth. I set it correctly and put the base timing to 12 BTDC as a guess. The light I have is basic I also do not have a tachometer so I do not know what my total timing is. I sprayed around carb and intake looking for vacuum leak with no luck. Only other clue is exhaust smells like unburned gas. Suggestions on where to hunt next? Thanks
Well, first thing to remember is that these older cars are not like todays fuel injection/electronic laden cars which most are so familiar with. The smell of gasoline from the exhaust and a slight miss at idle can all be "normal."
First, you can't guess as to what is wrong. You need facts. A vacuum gauge is a great tool to diagnose engine problems. They can be bought at your local auto parts store. A "Dial-back" timing light is really needed to dial in not only your timing, but your timing curve - mechanical/vacuum advance.
Compression numbers look fine because there is little difference in the numbers.
Next up is the carb. Many things can cause the carb to run rich and give you that "gas smell." Choke incorrectly setup, sticking, not working. Idle mixture screws incorrectly adjusted. Carb leaking internally. Carb running rich due to larger primary jets or power piston/rods stuck. Float setting too high. Aftermarket cam.
Slight miss can be cause by any one of the above from the carb, to include a worn throttle shaft allowing air in. Needs a complete tune-up. Spark plugs shot or incorrectly gapped. Bad plug wires. Corrosion in the cap at the contacts - often happens with the cheap aluminum contacts. Bad/worn rotor button inside the cap. Bad/worn rotor. Bad HEI. Bad HEI kit. Bad HEI module. Bad/weak coil. Bad power wire to the HEI kit. Incorrect voltage to the HEI. Sticking weights inside the HEI. Bad/weak vacuum advance can on the distributor. Bad/weak vacuum signal to the vacuum can. Poor gas. Poor engine ground. Spark plug wires touching or grounding out. Timing incorrect. Rocker arm coming loose or adjusted too tight. Weak valve spring. Intake heat crossover blocked. PCV valve bad or sucking air. Aluminum intake being too cold and not thoroughly heating the carb to provide even fuel/air distribution. Fuel puddling on the intake floor. Exhaust damper valve ( if you have one on the exhaust manifold) stuck or not operating correctly. Exhaust pipe/muffler collapsed internally or restricted flow.
I think that should cover it and give you a few things to check.