Troubleshooting
This could have been the largest section on these pages, because the scope of the subject is endless. The possible troubles you can have are many, and if as it sometimes happens you have two things wrong at one time, the possibilities are truly endless. Most Haynes Manuals and similar have good basic troubleshooting sections, and you should start there. In this section we won’t deal with whether you’ve run out of gas or if the battery is flat. We’ll deal with troubleshooting tips that we found helped us (the hard way) and that could not be found in some manuals. They all assume you’re somewhat experienced dealing with your engine, and we’ll also assume your engine is in good shape and tune, see section Tuning
Getting it started
With fuel and battery power, your engine turns quickly but doesn’t start. This will be because of lack of fuel or spark. To determine which, pull the plugs, see section Reading spark plugs. If they indicate lean condition it’s a fuel issue, if they indicate rich condition it’s a spark issue.
Fuel issue:
Is the choke working properly? Tune it
Is it drowned in too much fuel? If so press down the pedal and keep it there. The initial pressing down of the pedal will give an accelerator shot of fuel, so it’ll make things worse, but keeping the pedal down allows the engine to suck air instead of fuel, fixing the too rich condition. When the engine starts, quickly release the pedal. In extreme cases (never experienced it) the spark plugs could be so badly contaminated that they wont fire, then you have to clean them or buy new ones.
The trick of keeping the pedal down also works when a too hot engine won’t start, won’t turn fast enough. We once had to give that tip to a competitor in a dragrace final, just to get him to be able to compete, and then we beat him of course…
Ignition issue:
Pull a spark plug wire, put a spark plug in it and hold its base on the engine, then try starting. You should see a spark firing. If yes it’s a problem in on of the other cylinders, if no, test the wire from coil to distributor in the same way. If no spark or very erratic spark, it could be the points, it could be a badly adjusted dwell angle. If that has happened and it seems to happen after a few hundred miles after tuning it properly, it’s because the part of the points that is touching the central axle is being worn quickly. This can happen if the axle is rusty or otherwise abrasive. You need new points a new central axle/distributor. If you need a new distributor, buy one with electronic ignition module, that’ll be the end of this problem.
If not spark is there, the coil could be faulty, or the wire that comes from the ignition switch. Try with a test light, is there current to the side terminal of the coil when the ignition is on? The wire could be broken. If it’s a points ignition there’ll be a resistor on the ignition wire, that can also be burned.
Low quality idle
So you got it started, but it doesn’t idle well, or it may even die if you don’t give it some extra gas. Again assuming the ignition and carb is not out of tune, here are some possibilities. First, a general comment. Ignition problems are usually very on/off, either it’s firing almost every time, or not at all. Carb and fuel mixture issues are more variable.
With the engine running, look at the vacuum gauge, see section Vacuum gauge, the wonder tool. If you don’t have a vacuum gauge, get one, but in an emergency use the tachometer. Then pull one wire at a time out of the distributor and notice what happens. If the vacuum drops, that cylinder was contributing. If the vacuum did not drop, that cylinder is having a problem. Put back the wire and go to the next. Note which cylinders have issues. And note that the faulty cylinders may contribute a little, so there will be a drop in vacuum but small. If it’s only one cylinder, check again for spark. If it’s there, check for faulty valve adjustment (the vacuum gauge would have shown this already, though not which cylinder). Check also for vacuum leak around that cylinder, and if it’s 2 cylinders next to each other, it’s probably a vacuum leakage near them or a blown head gasket between them. The latter can be determined by doing a compression test. If a few cylinders show low contribution (low or no drop in vacuum when wire is pulled), try checking out the pattern. If on a V8 it’s the 4 cylinders at the corners, it could be a fuel distribution issue, too lean on the corner cylinders. Did you buy that single plane manifold and too big carburetor, that could be the reason? If it’s 4 cylinders and you have a dual plane manifold on a V8, check if they are all fed by the same half of the manifold. We had that happen too us, the reason was simply dirt in one jet on the carb, so one half of the carb and thereby one half of the dual plane manifold was way too lean. If all cylinders are equal, it sounds like a general carb problem or a vacuum leak on the carb base.
You can find some vacuum leaks by spaying at the base of the carb or on the manifold to cylinder head gaskets. You can use WD40 or start gas or whatever. Some sprays will increase rpm, some decrease rpm, find out which it is by spraying down the throttle bores, and then look for the same effect as you try spraying around the carburetor/intake manifold, vacuum hoses.
Problems when cruising
If you can start and idle, then it’s almost never an ignition issue. The exception is the points issue we’ve discussed above. When cruising or at steady speeds on the freeway, one of two things can happen.
Either the car surges, goes up and down ever so lightly in speed. This is a lean condition that you need to fix, it could kill your engine. If the carb is properly tuned it shouldn’t happen, but if it comes suddenly, then it could be dirt in the jets, or a vacuum leakage. Most vacuum leakages that happen suddenly are old worn vacuum hoses that open up to the air around them. On some 70’s engines there are many of them.
The other thing that can happen is that the engine suddenly drops on rpm and can even stall. And if you start it after a few minutes it’ll run fine for a while, then it happens again. This is due to clogged fuel lines/filter. Most often it’s a filter that needs replacement. But it can also be a fuel line. Which brings us to the most embarrassing mistake we ever made on an engine. It was a boat and after the winter we had the problem that when going at full speed, after a few seconds one of the engines lost power, it dropped to 3000 rpm and stayed there. If we let off the rpms for a minute or so it could do top speed again but only for a few seconds. What was wrong? Well, after a long time spent trying many things, we found out that when we had changed the fuel filters in the winter, we had put on the double clamps nice and tight (everything on a boat has to be clamped with 2 clamps (stainless) for safety). However, we didn’t notice that one of the fittings on the fuel filter was shorter than the other three, so when we tightened the second clamp on that fitting, we in fact only decreased the diameter of the fuel hose, so like a clogged arterie in the body, fuel could only flow through very slowly. We put on another fitting that was longer, and the problem was gone.
Problems at Wide Open Throttle (WOT)
At WOT the engine is under most stress, everything has to work perfectly and there’s no room for error. The ignition has to be able to provide a steady and very fast spark, and those old points could be floating if you’re above 6000 rpm, you need to upgrade your ignition.
Most problems though come from the amount of fuel needed. There has to be enough fuel flowing, on a carburetted engine fuel pressure must not drop below 3-4 psi at sustained full throttle driving. It’s a good thing to measure, but keep the fuel pressure gauge outside the passenger compartment at all times. If the pressure drops to below 3 psi you could argue that there will still be fuel, but with that low pressure the carb can’t accurately mix the air and fuel, the carb goes lean, and that’s the last thing you want at WOT. If you’re lucky you lose power, if you’re unlucky you lose an engine.
Taking the gas
…is never a problem. We’ve yet to see an engine stumble because it got too much gas/fuel when the pedal is pushed down quickly. It’s the opposite that happens, the engine suddenly needs a lot more fuel not to stumble and it only gets some of it.
On a double pumper carburetor, this can only be an accelerator pump issue. On a vacuum secondaries carb, it can also be the secondaries coming in too quickly. The effect felt is very similar. Fortunately there’s an easy way to determine which it is. Try tying down the secondary arm with a strip or tape so it won’t be able to move. If this solves the issue, the that’s where the problem is and you have to tune as described in section Tuning a carburetor, step 1.
Otherwise it’s the accelerator pumps. Try firing them with a stopped engine while looking down the carb (wear glasses). If there’s a pump shot you need to tune the accelerator as described in section Tuning a carburetor, step 1. If there’s no shot, is the linkage broken or maladjusted? Or it’ll be the diaphragm in the accelerator pump housing being broken.