Some of these are gonna seem pretty basic. But everybody from the beginner to the seasoned engine-tuning veteran has probably been guilty of misdiagnosing a poor-running engine at some point in time. Engines are pretty simple critters when you strip away all the outlying accoutrements. All you need is fuel, air, and spark and then it’s suck, bang, blow. Larger tires sap power and a poor-running engine drinks more fuel. So check off a couple of these items the next time you’re poking around under the hood of your Jeep. Some more pep and better mpgs just may be the reward for a little time and money spent.
The ignition system can range from a basic points-type mechanical operation working inside an engine-mounted distributor to a complete distributorless setup with the coils on individual plugs actuated by a mixture of camshaft and crankshaft position sensors and an electronic control unit (computer). Assuming all your parts are in good working order, here are some ways to ensure proper operation and maybe even eke out a little more performance.
Electronic Ignition Conversion: If you’ve so far resisted the urge to convert to an electronic ignition, be well versed in checking your point gap and dwell every so often. If you’re not, mechanics who know how to work a set of feeler gauges and a dwell meter are getting fewer and farther between. A tip is to find a motorcycle or air-cooled VW mechanic since many of these vehicles retained points-type ignition systems longer than others. If that’s a no-go, several companies from Petronix to Crane to Accel to others offer electronic ignition conversion systems that will easily drop into your points-type distributor to convert its workings to solid-state operation.
Cap and Rotor: Sounds stupid, but many ignition woes are caused by moisture or corrosion inside the distributor cap. Pop off the distributor cap every other oil change and take a peek, especially if you frequent mud or water crossings.
Wires: Bad plug wires can cause erratic idling, backfiring, misfiring, and all sorts of other issues you may not immediately attribute to the wires. You should have a multi-meter in your tool box. You can pick one up at just about anywhere they sell electronic computer or auto parts. Or, Harbor Freight sells very inexpensive multi-meters with a high degree of accuracy. Pull your plug wires every so often and check the resistance in Ohms. Generally, anything under 7,000 ohms-per-foot means the wire is still good. Excessive resistance means the wire is corroded or breaking down inside and should be replaced
Plug Gap: The specs for the distance between the two sparkplug electrodes varies from engine-to-engine, but in any case, as plug life increases, the gap grows. Generally, every 30,000 miles or so you want to pull at least a couple plugs and make sure the plug gaps are still within factory-spec. If you’re running a hotter ignition coil and lower-resistance aftermarket plug wires, you can generally increase the plug gap over stock for a hotter spark. However, if your ignition system is still stock, it’s generally best to keep your plugs close to the factory gap-spec as possible.
Mechanical Advance: Non-computer-controlled ignition systems rely on mechanical centrifugal advance within the distributor to increase ignition timing as engine speed increases. The trouble is the factory advance spec is generally tailored more towards resistance to knocking than performance. Also, corrosion and wear can limit or restrict the advance mechanism’s operation. Pop off the cap and rotor and (normally) the points or electronic ignition plate to expose the mechanical advance. You’ll see eccentrics held in place by springs. Lighter-weight springs are available from companies like MSD, ACCEL, and others to allow the ignition advance to come into play sooner in the rpm range. Just be sure to test drive cautiously, as pulling too much ignition timing too soon can cause knocking and pinging.
Advanced Timing: If you don’t want to crack open your distributor to mess with your mechanical advance, you can still gain a bit of performance by loosening your distributor hold down and advancing your initial timing up a bit. Normally a 3- or 4-degree bump won’t harm anything. However, if the engine cranks hard or labored you’ve put too much initial timing in and should pull out a couple degrees.
Adjustable CPS: If you have a computer-controlled ignition system, the computer uses data from the cam and crankshaft position sensors to determine where in the rotation it should fire the plugs. You can’t spin the distributor to advance the timing in these engines, but you can either modify your crankshaft position sensor (or buy an adjustable one) to trick the computer into thinking the engine is farther into its rotation so the ignition fires earlier. Most Jeep engines position the CPS sensor in the bellhousing. Slotting the mounting tabs will allow you to either advance or retard the initial timing and, therefore, the timing throughout the entire rpm curve.
Whether a carburetor or injection, your engine can only burn as much fuel as there is air to support it. So, don’t go running straight out to buy 24 lb/hr injectors or a 950 cfm carburetor. Rather, make sure your current stuff is up to snuff first before determining the need to add or pull fuel for performance.
Injectors: For a Jeep TBI system, you’re pretty much stuck with the factory injector(s) unless you want to go with aftermarket parts and custom. If you’ve added a cold-air intake, header, and free-flowing exhaust system your factory MPI injectors can generally deliver enough fuel to keep the engine happy, but if you have a stroker or supercharger you’ll need something in the 21 or 24 lb/hr range.
Fuel Pressure: One way of cheating on installing larger fuel injectors is to up the fuel pressure so that more fuel sprays out when the injector fires. On OBDI systems, you can replace the external adjustable fuel pressure regulator. Later OBDII systems regulate inside the fuel tank, so you’ll have to bypass the internal regulator and run a dedicated return line with an external adjustable regulator.
Carburetor Jets: Tuning a carburetor is a dying art. Most aftermarket carbs have rebuild kits available to increase or decrease the amount of fuel on the primary or secondary circuit. On many factory carburetors, you can increase fuel delivery by drilling out the jet seats with an appropriately sized drill bit. Just take care to verify the size of the orifice you’re starting with so you don’t go too big too fast. It’s a game of trial and error.
Carburetor Idle Air Adjustment: If your carbureted rig loads up or idles poorly you can sometimes adjust the idle air mixture screws. They’re generally found on the bottom front of the carb facing the radiator. With the engine warm and the distributor vacuum advance disconnected, hook a vacuum gauge to manifold vacuum and turn the screws almost all the way in. With the engine idling, back off the mixture screws a quarter- or half-turn at a time until the vacuum gauge obtains its highest reading.
Tunnel Ram: Throttle body spacers are hit-or-miss, but adding a phenolic carb spacer between your carburetor and intake manifold (1-2 inches thick) generally increase performance at all rpms. A four-hole spacer will keep low-speed velocity on the intake charge, which is great for low-rpm torque. It’s been our experience, that an open spacer of about 1-inch will greatly help cylinder sharing and upper-rpm horsepower without having much, if any, effect on low-rpm torque. Open spacers more than 1-inch thick may or may not effect low-rpm engine performance.
Drained Dry: Mechanical fuel pumps generally don’t have much of a problem keeping up with the fuel demands of most carbureted systems because the carburetor by design maintains a reservoir of additional fuel in the bowl. However, if your factory mechanical pump has either low pressure or moves a low volume of fuel, you can experience fuel starvation during extended full-throttle use. An aftermarket fuel pump that can deliver an ideal 6 psi and at least 95 gallons-per-hour should be able to meet the fuel demands of most healthy small-block V-8 engines.
MAP: EFI engines use a manifold absolute pressure (MAP) sensor to determine engine load and fuel requirements. Less manifold vacuum means more fuel. You can usually build or buy an adjustable MAP sensor to trick the ECU into thinking there’s less manifold vacuum than there actually is so the computer will call for more fuel. This is normally only advisable if other go-fast goodies like a cold-air intake or high-flow exhaust have been added.
Gunkless: A good ol’ can of carburetor or throttle body aerosol spray can go a long way in regaining lost performance. In your throttle body, pay specific attention to the little idle air control window and valve, which can get gunked up over time and lead to poor idling and off-idle stumble. Same goes for the carburetor, with gunked up squirters and venturis leading to poor air/fuel atomization.
Off-Idle Stumble: If you stab the gas in your carb-equipped Jeep from a standstill and the engine momentarily falls on its face, many assume the engine is getting glugged with an excessive amount of fuel. In reality, it’s often a sign of an accelerator pump issue. If the linkage isn’t adjusted correctly or the rubber plunger inside has deteriorated, that immediate shot of fuel to get the engine up on its hind legs (so to speak) isn’t delivered. The result is a stumble until the primary and secondary systems can deliver the required volume to let the engine accelerate cleanly. Inspect the linkage actuation or buy a rebuild kit for your carb.
As with anything, the devil is often in the details. Underhood, even a seemingly innocent item can lead to poor performance. Here are just a couple of things to watch for.
Vacuum Leaks: Whether at the carburetor or throttle-body base, vacuum lines, intake gaskets, or even HVAC system, anywhere engine vacuum is plumbed can be a potential source of leaks. The easiest way to track down an engine vacuum leak is with a can of carburetor or throttle body aerosol spray. With the engine running, spray the suspected areas while listening to the engine. If the rpms increase, you’ve found a vacuum leak. Fix as necessary.
EGR Valve: If your vehicle had a rough idle and stumbles, the EGR valve could be sticking open. Once up to operating temperature, you can often rev the engine with the transmission in Neutral while watching the EGR plunger. If it’s not moving, it’s bad. You can sometimes also manipulate the plunger rod with a pair of needle-nosed pliers. If you pull up on it and close the valve and the idle smooths out, it’s time to replace the EGR.
Rattler: If your engine accelerates sluggishly or won’t rev beyond 2,500-3,000 rpm, there’s a chance you have a clogged catalytic converter. Sometimes the bricks inside get clogged with material or break up and get lodged sideways, blocking the flow of exhaust. If you hear sporadic rattling from underneath the vehicle at idle or lower rpms, chances are good your cat has bit the dust.
Filters: Naturally air and fuel filters are easy to replace, but often frequently overlooked. If you have a high-pressure EFI system the fuel filter may not be so easy to access or check, but on carbureted engines it’s often right under the engine bay. Old rusty tanks can plug a filter quickly even if it looks clean, so it’s best to keep a replacement in the glove box. If you’ve checked your pump and filter and still can’t get fuel into your carburetor, pull the fuel line off the carb body and inspect the sintered bronze filter. They clog frequently even with an aftermarket filter downstream. Replacements can be had at most auto parts stores, or you can omit the sintered filter altogether if you run good-quality filters elsewhere downstream.
Oil: If your oil is dirty you’ll not only increase your engine wear, but you could shorten the life of your catalytic converter and fail a smog sniffer test.
PCV: If your PCV valve gets gummed up or stuck closed crankcase pressure can build to the point of blowing out gaskets or forcing pressurized crankcase oil past the rings into the cylinders. Potential fouling of the plugs, increased sludge buildup, and large oil leaks are the reward for not spending the seven dollars, ya cheap bastidge.