Diagnosing Engine Trouble

by David Schwarze

For the benefit of those who may someday be stuck out in the boonies with an inoperative or semi-operative bus, I suggest the following. Comments / corrections welcomed, of course. All of this is in reference to 72-74 bus engines with dual Solex carbs, but most of it is applicable to other busses as well.


Your engine needs three basic things to run - compression, fuel, and spark. Your engine needs three basic things to run - compression, fuel, and spark.

I repeated that line because it is fundamental. An elderly man told me that when I was in my early teens, then proceeded to diagnose and repair the lawnmower-engine-powered minibike I was riding (well, pushing). If you are having engine trouble, it will likely be lack of compression, lack of fuel, or lack of spark. Confronted with a problem such as what happened on the baja run with Brian's bus, I would have attacked these areas one at a time in the following order:


Check compression first. A compression tester is something that I always bring on long trips. Pull all four spark plugs *first*, then remove the coil wire and check the cylinders one at a time. Hold in the clutch so your starter won't have to spin the clutch disc and transaxle input shaft. All four cylinder readings should be over 100 psi and within 15% of each other.

(Aside: If you have a good ear you can do a pseudo-quickie compression check by simply disconnecting the coil wire, cranking the engine, and listening to the pulses. If you hear "da da DA da da da DA da da da DA da" you probably have a weak cylinder. If the pulses all sound the same compression is probably okay. A real compression test will tell you a lot more. I've also noticed this doesn't work very well on type I engines for some reason.)

The results of the compression test will tell you whether to proceed or think about a tow truck. If compression is low on one or more cylinders, there are a couple of things you can do. First is check the valve lash. Tight valves will lower or eliminate compression. If the valves are okay, you might look for loose cylinder head bolts, but that's about it. Failing those two things, you have serious internal engine trouble. It's good to know sooner rather than later. If you're a glutton for punishment you can get some more information by pulling the valve covers and looking for motionless valves while the engine is being cranked (watch out for flying oil!).

If the engine passes the compression test, move to electrical next. I have found that electrical problems are much more common than fuel problems.


If the bus won't start, have someone get ready to crank it for you. Pull the center wire off of the distributor and hold it very close to where it plugs into the center of the cap. Preferably hold it with an insulated tool, or use a glove. Don't hold onto the body of the bus with your other hand. Don't have bare feet. Have your friend crank the engine with the key on and a decent sized blue spark should jump repeatedly from the end of the wire to the place where you plug it into the cap.

If you see the spark, you are doing pretty well and most likely don't have an electrical problem. The only other electrical thing that could keep your engine from starting in this case would be if the timing was way, way off.

If you don't see the spark, check that the coil is getting current at the + terminal. Use a multimeter if you have one. A multimeter is something that I always bring on long trips. If the coil has 12V and you aren't getting spark, you have either a bad coil, bad condensor, bad points (check that the point gap is reasonable: .015-.020"), or bad coil wire.

If the bus is running at all, do the following. While the engine is running, and using the same cautions listed above, pull one wire at a time from the distributor cap, then replace it (not the center wire though). Each time you pull a wire, the rpm of the motor should drop considerably, and should increase when you plug the wire back in a couple of seconds later. Do all four and note which (if any) are not changing the RPM of the engine. If all four cause the RPM to drop, you probably do not have an electrical problem. In fact, you probably don't have any problem at all, so let's assume at least one wire does not cause a drop in engine rpm.

*** This next section is important ***

If ONE wire out of four does not drop the RPMs, suspect the following:

a) that plug wire (switch two wires to test)
b) spark plug (pull it, look for cracks in the porcelain, obvious damage)
c) distributor cap (look for cracks, burnt contacts)

(yeah, we're switching to fuel right in the middle of this test!)

If TWO wires out of four do not drop the RPMs, and those two wires are both on one side of the engine (i.e. cylinders 1 and 2, or cylinders 3 and 4) suspect the following on the carb/intake from *that* side of the engine:

a) idle mixture screw in too far
b) idle fuel cutoff solenoid plugged/inoperative
c) vacuum leak between manifold and head

For a), simply try backing out the screw (turn counterclockwise). The screw is in the throttle plate just forward of center, and angled forward towards the outside of the bus. If you back it out and the engine picks up rpm and smooths out, great! Do the wire pull again to make sure and drive off happy.

For b), first pull the wire off the end and wait 5 seconds. If the rpms of the engine drop, b) is not the problem. Put the wire back on and go to c). If the rpms of the engine don't drop, check to see that the wire is getting 12v. Use a meter or circuit tester. A multimeter is something that I always bring on long trips! If the wire is live, reconnect it, reach up and very slowly loosen the solenoid 1/2 turn while the engine is running. If it is plugged, the engine will begin to pick up speed briefly when you have loosened it about 1/2 turn. It won't last long though - the engine will be stumbling again in a few seconds. Shut the motor off, unscrew the solenoid and look at the little hole in the very tip of it. Three times now I've found it plugged with debris. Blow it out or poke it out gently with wire. I usually find a piece of 14 or 18 gauge wire and use just one of the strands to clear the obstruction. Don't want to enlarge the hole.

Install the solenoid (then replace your fuel filter). In addition, the solenoid should make an audible click when you connect or disconned the wire. If it doesn't click, it probably needs to be replaced.

If b) failed, diagnose c) by running the engine then spraying carburator cleaner at the base of the manifold (where it bolts to the head). Be liberal and hit it from all sides if possible. BE CAREFUL and have a fire extinguisher handy, although I've done this for years without the slightest hint of danger. If the engine RPMs drop at all, you have a vacuum leak between the manifold and head. If the engine stalls, you have a pretty bad leak. Tighten the four nuts holding the manifold to the head and try to limp to someplace where you can replace the gaskets or have them replaced. These gaskets are metal, and don't seal very well unless the mating surfaces are flat and smooth. I had horrendous trouble with this until I removed the engine and polished the surface of the head with a dremel sanding disc. Also check the surfaces of the head and the manifold with a straight edge to make sure they're perfectly flat.

Backing up a minute, if the engine won't start at all, you only have to do one test to find out whether it's fuel related. First, remove the air cleaner, make sure the choke plates are open and crank the engine for 30 seconds without giving it any gas at all. This will clear any flooding in the carbs. If the engine is flooded it will crank for a while without catching at all, then very slowly start catching on one cylinder, then two, then three. Avoid the urge to give it any gas - just keep cranking until it stays running.

If it never sounded like it was catching, get a couple of ounces of fuel (helps to carry a spare gallon here) and pour half into the top of each carb. Try to start the engine again. If the engine catches briefly and then dies, the carbs are not getting fuel. Check fuel filter, fuel pump, fuel lines. Open the gas cap and rock the bus back and forth and see if you can hear gas sloshing around in the tank. Maybe you're out!


The engine has two separate fuel systems. Well, three actually, but bear with me. The one we've been testing up till now is the idling system. Assuming the bus idles smoothly and passes the plug wire pull test, the main fuel delivery system could still be malfunctioning. Symptoms would be a smooth idle (of course) but bad/inconsistant acceleration when you hold the throttle down. This is most probably caused by the main jet being plugged. Find out which carb it is by removing the throttle linkage from 1 carb at a time and trying to drive. When the good carb is hooked up, the bus will drive a little. With the bad carb hooked up, it will barely move. You can open up the bad carb by removing it and either taking the top off (five slot head screws) and/or reomving the 13mm plug in the side of the bowl. The latter is easier but doesn't give you as good of a view. Stick a screwdriver through the plug hole and unscrew the jet. There will probably be a bunch of crap sloshing around in the gasoline at the bottom of the bowl. Make sure the jet isn't plugged, then clean out the bowl and put the jet back in. The third system I mentioned is the accelerator pump mechanism, but you can live without that one.

Argh, this is getting long. I know I've left out a lot of cases, but these are all of the things that have happened to me over the years, so hopefully they will be useful to someone else.

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