Sometimes we or others create problems, and then try to solve them by adjusting other systems
Most of the problems are easy to identify, check and solve with the basic tools and your eyes.
Fuel system ---- I’m not directly talking about the carburetor at this time, all that feeds the carburetor and engine. If something is out of standards or not doing its part it can have an effect that is not easily solved by adjusting other systems.
Fuel ---- I prefer to tune with the fuel that the engine is designed to run on (regular, premium or higher octane). The primary factor that determines the grade of fuel to use for a particular engine is compression ratio. It’s simple to figure out, see the chart below from (Advanced Engine Technology by Heinz Heisler) to figure out what fuel you should be tuning with. Generally speaking a street engine is 7 -1 up to 11 – 1 compression ratio, de-tuning a engine to run on a lower octane than it was designed to run on is a waste of effort and you end up with a very poor performing engine that will not last for the long run. There is no benefit in running a higher octane fuel than the engine was designed for, you’re not going to create more performance. Yes every one can find them self with a bad tank of gas or old gas that has a low octane level, adding some octane booster or a higher octane fuel to compensate is a solution and will not hurt any thing, but it’s not the fuel to tune with.
Gas Cap / Venting ---- The fuel tank needs to be vented in some manor in order for the fuel to flow, if it’s not vented the fuel pump will be creating a vacuum in the fuel tank preventing the fuel from flowing. For tuning my primary concern is that the vent system is working and not allowing a vacuum to be created in the fuel tank. It’s simple to check, just take a drive for about 15 to 20 minutes, stop and immediately remove the gas cap and note if you hear any air being sucked into the gas tank. If you hear air being sucked into the gas tank the vent system is not working and needs to be fixed, it might be the gas cap or a blocked vent tube, what ever it is needs to be addressed. Leaving the gas cap off or a rag in place of a gas cap is not a solution (smile). Note, some modern day cars are designed to have a slight vacuum/pressure in the fuel tank and when you go to fill them up you might hear some air at the gas cap, this is not necessarily a bad thing for them but is for an older carbureted engine.
Fuel Pump ---- They can be wore out, high performance/racing one, electrical or mechanical one. The question is, are they delivering the right fuel pressure for the carburetor. The carburetor is very sensitive to float level/fuel level in the carburetor, and the float level is directly related to fuel pressure being supplied to it. It’s easy to check just put a tee fitting in the fuel line between the pump and carburetor and attach your vacuum/pressure gauge, start the engine and read the pressure, pressure should be 5 – 7 PSI MAX 8 PSI, anything above or below the limits need to be corrected. This could be replacing a low or high PSI pump to one that delivers the right PSI or possibly a pressure regulator to reduce the high PSI pump to bring the pressure down to acceptable limits.
Fuel Filters ---- Often not thought about until there fully clogged and the engine won’t run. Normally changed by number of miles used as normal maintenance, often neglected during or thought about during those times. If there’s any doubt as to when the last time it was changed, put a new one in there cheap. On older cars and trucks there’s no way to know how well they’ve been treated and there is another filter that might need changing, it’s more or a screen in the fuel tank on the pickup tube. There’s no rule as to when to change that screen but it can deteriorate and fall a part allowing larger particles/dirt into the fuel line. All the changing of the inline fuel filter is not going to overcome feeding it dirty fuel from the fuel tank. This screen is just something that you might want to think about if you’re running into fuel filter clogging.
Positive Crankcase Ventilation System (PCV Valve System) ---- A lot of problems can be generated by an inoperative/disabled or some modifications that might cause it to not operate as it was intended. I’m talking about the PCV system not just the PCV Valve by it self, the valve could be right and the rest of the system causing the problem. There’s very little you can test in your garage to make sure the system is operating right except a visual inspection that all the parts to the system are there, if any of them are missing or disabled you have a problem. Most of the problems I’ve seen center around aftermarket valve covers and air cleaners, not that there a bad item, It’s just that they where not installed right or provisions where not made to keep the PCV system operating. I have a minimum list of all the parts of the system that need to be there and checked that they are there for a system to operate. What I would consider as obvious such as a capped off PCV port on the carb or damaged or pinched hoses or clean filters are not included on the list.
(First) Where is the filtered air coming into the crankcase? Air has to come in somewhere for the system to work. You can find this on the opposite valve cover to the PCV Valve or oil fill tube on V8’s or a tube down into the crankcase on I6’s, the filter can be a standalone on the valve cover or be connected to the air cleaner with a hose or tube, a separate filter in the air cleaner or using the carb air cleaner as the filter.
(Second) Where are the baffles in the system? They have to be there to prevent sucking liquid oil thru the system. Normally found in the valve covers, they work to separate the oil from the gases passing thru the system. They can be as simple as a plate or more elaborate baffle preventing oil from splashing up into the PCV Valve or air filter. Easy to check that their there, simply look down the hole where the PCV Valve is and the filtered air is going in, you should not be able to see the rocker arms in the valve cover. If you see the rocker arms the baffle is not installed and needs to be or the valve cover needs to be replaced with one that has baffles or provisions for one.
(Third) Where is the PCV Valve? I like to look at the PCV Valve as a controlled vacuum leak, drawing air thru the crankcase to eliminate gas buildup in the crankcase. There’s no test to check the PCV Valve except a visual inspection that it’s clean and the spring inside is working, spring not broken or restricting the free movement of the valve inside. As to what PCV Valve to use very little information is available to you or me regarding the flow rate/CFM of one valve over another. So the best choice for a valve is to find a valve for your particular engine or engine type and size down at the parts store thru there long list of valves to chose from. Generally you can find the PCV Valve in the valve cover or in the hose from the valve cover to the carb PCV port or maybe in the intake manifold going into the oil valley under the intake manifold.
This is a very simplistic look at the PCV System, there might be some additional items that can be found but this list meets the basic items that need to be there or it won’t work properly. Carburetors from 1968 and on or having a PCV port on them require a properly working PCV System or there will be troubles in tuning the carb.
Not looking at the parts/systems listed thru out this post can cause you problems and when your making adjustments to the ignition or carburetor your not trying to compensate for the above items not doing there part.
The next post will be addressing distributors and their curves, a must in understanding to bring the most potential out of a particular engine.
Enjoy Your Build