CHECK LIST FOR POOR PERFORMANCE
Lifter Preload
If the engine has been rebuilt before there is a chance that the pedestals could have been machined to adjust lifter preload. If the preload was not checked on this cam it could be holding 1 or more valves of the seat.
Valve Springs
High performance cams should always be used with high performance valve springs. High performance cams use increased acceleration rates and therefore require heavier springs to handle the higher forces generated. Low spring pressures can contribute to the lifter pumping up, holding the valve off the seat and causing intermittent missing and/or loss of power.
Valve Timing
The timing of the valve opening and closing in relation to piston movement is critical to the performance and efficiency of the engine. Dyno testing has shown that 2 degrees movement ( equivalent to approximately 1/8 of a tooth on the timing gear) can change power output by up to 10 hp. The timing specification supplied with Crow Cams list a checking figure for "lobe lift at Top Dead Centre on the overlap" which is a simple method of ensuring valve timing is correct. This check is particularly important on Holden V8 engines as the original timing gears came in several different timings. NOTE. Make sure you are using an dial indicator calibrated in inches or convert readings to metric before checking. Hint. If the engine runs better with a big increase in ignition advance, there is a good chance the valve timing is retarded.
Ignition Timing
If the timing mark on the harmonic balancer has moved or the balancer is not the correct model the timing will be incorrect. The distributor advance mechanism may also malfunction causing severe power loss. Check the timing mark by removing the spark plug on number 1 cylinder and feeling the top of the piston with a plastic pen. The piston should be at TDC when the line on the balancer indicates. EFI engines should have the ignition timing set as per the manufacturers instructions.
Throttle Linkage Adjustment
This is so obvious no one ever checks it ! Several race teams have had very red faces after finding that the throttle was not able to function to the wide open position.
Fuel Blockage
Restriction in fuel supply. My own engine had problems with a needle and seat that would not supply enough fuel. I have also seen blocked air vents causing problems. Damaged fuel supply lines, blocked filters. LPG engines sometimes suffer a build up of wax or tar in the converter. Some LPG mixers restrict the inlet flow enough to cause a significant reduction in power output. Fuel injection engines should have the injectors cleaned out and fuel filter replaced regularly .
Exhaust System Blockage
This has become more of a problem with catalytic converters. But a loose baffle plate in a muffler can cause a major loss of power.
Transmission Problems
A faulty torque converter (failed stator) can cause a severe loss of power.
Vacuum Leaks
This is especially important on V8 engines particularly when an alloy manifold is used.
The fit of the manifold must be carefully checked to ensure a perfect seal. A small vacuum leak will cause poor idle, oil burning and loss of power.
TESTING
The old diagnostic checking technique of Mechanical, Electrical, Fuel still applies to performance and modified engines. Check mechanical components such as valve timing and compression, then move to electrical and finally fuel mixture.
This is only a basic list and of course there are new problems arising all the time. However careful methodical testing will almost always get to the bottom of most problems.
DETONATION
If there is any oil leaking into the cylinders through valve guides or manifold this can cause abnormal combustion. This is most often in the form of detonation.
FUEL INJECTED ENGINES
Check computer for fault codes. Make sure base idle is set correctly. Clean out idle air control valve. Clean out injectors. Check type and size. The wrong model injectors can cause low power
Check that catalytic converter is flowing correctly. A converter can cause poor performance if it is blocked or if it has had the internals removed.
Exhaust system should be identical to system kit was calibrated with. Variation in back pressure may cause incorrect mixture.
Check inlet manifold and aircleaner. This should be stock or factory performance type. Free flow filter inserts are acceptable.
Check fuel pressure at maximum power. The fuel pump may deliver correct pressure at idle but may not be able to sustain this pressure at maximum power.
Check lambda or air/fuel ratio readings on exhaust.
NOTE
Special calibrations are available for high compression engines using PULP and stroker engines.