Decomps for Dummies

The SIMPLE way to optimum decomp starting performance

For decades, the motorcycle industry has enjoyed the monetary benefits of making bikes APPEAR to be more complicated than the average shade tree mechanic can manage. This helps maintain a generous flow of traffic to the Service Department of your local dealer, where the technicians know all the shortcuts and tricks of the trade, to make life easy and the bosses happy.

Enter the dreaded Decompression adjustment procedure for the LC Intruder!! Ever read the manual on how to adjust these things? AARRGGHH!!! Visions of major oil spills, parts strewn all over the property, and a basket case engine that will never run again!! Quick! Load her up and haul her to the shop for this one!!! NOT!! The Factory Service Manual shows how to initially setup the Decomp system on a brand new factory assembly. It never goes beyond that point to explain how to actually tweak them for the individual bike, so once you know how to get TO the adjustment points, put the manual away.

Decomps have a very simple job of reducing compression inside the cylinder by slightly lifting the exhaust valves. When engines are so big that they require more power to turn over than the starter and battery are capable of, the use of decomp valves cams is preferable to beefing up the electrical components. If your starter and battery are good, but are having trouble spinning your engine over, there's too much compression fighting against them. While this condition is often found in the rear cylinder (because of the weak cable system of linking the decomps together), it is possible to have either or both of the cylinders causing the trouble, so adjustment  is a two-step procedure. First you make adjustments to the rear lobe to insure that both lobes move at precisely the same time, then you adjust the WHOLE SYSTEM by tightening or loosening the front lobe cable at the solenoid, until the starter doesn't have to work so hard.

Because the ideal setting for decomps is achieved by sound, and not something you can measure with a stick and take a picture of, I could type for days and it wouldn't make any difference. It isn't rocket science, just make sure they both move at the same time, then make sure they both open up enough for the engine to spin without a lot of work. It took me longer to type this than it does to actually fix your decomps.


Realistically, diagnosing the problem behind a bike that won't start, can be a frustrating experience, as there are many potential problems that can create the same symptoms. Here are a couple of guidelines to help determine the root of a "labored" starting problem, and by "labored", we mean that the starter motor kicks in and attempts to turn the engine over, but either can't get it over the "hump", or does so with a lot more effort than usual.

If your battery is over two years old, and you CONSISTENTLY have a labored start, then the first suspect will be the battery.

If she starts cold with no problems, but CONSISTENTLY has labored starting while hot, then the charging system should be checked. A grounding rectifier can actually discharge your battery while you ride. The battery can somewhat recover while sitting, so it may well start the bike while its cold, but then not be able to start it after only a few miles of riding.

If everything on the bike seems to be in perfect order, and you OCCASIONALLY have a labored start, or no-start, for no apparent reason, and with no perceived pattern, then very likely, the rear decomp cam is loose or lagging behind. A good test for this is to place the bike in second or third gear when it won't crank, and push it backwards a few inches while in gear, then try again. If it starts then, you definitely have a loose rear cam. Pushing the bike backwards while in gear moves the rear piston back off its compression stroke enough to give the system a running start at it when you push the button. This will only happen when the rear cylinder stops on its compression stroke. Therefore, it seems impossible to predict when it won't start. Might be cold, might be hot, might only happen once today, but maybe three times tomorrow...This occasional no-start is the prime indicator that the problem lies with the rear decomp cam. (If the front cam were out of adjustment, it would NEVER start properly, as both cams would then be off their specs.)


The rear cam is actuated via a cable from the front cam. This cable makes an "S" turn along its path and its metal sheath is lined with nylon, for a smooth action. Unfortunately, due to the pressure on it during actuation, it tends to cut its way into the inner nylon lining, leaving it with ever increasing slack. Once the slack occurs, the rear cam moves slightly behind the action of the front cam and does not open up quite as much. It only takes a tiny bit of slack in the cable to render the starting system incapable of turning over the rear cylinder.

Therefore, the REAR CAM CABLE is the weakest link in the entire decompression system, and it is more than likely the culprit behind your occasional no-starts.


The decomp cams are located right beside the spark plugs, so in order to make adjustments, you will need to "skin" the LC down for spark plug replacement (see "Skinning the LC")


Adjustment of the rear cam is made at the right of the front cylinder. On the right photo above, items 1 and 2 are the adjustment nuts used to tighten or loosen the cable to the rear cam.

To tighten the rear cam you simply loosen the lock nut on the front end of the cable bracket, then turn the adjuster on the back end of the bracket counter clockwise to back it away from the front cam, then secure the adjustment with the lock nut on the front.

With the front cam resting on its stop plate, tighten the rear cam adjustment until the rear cam lifts from its stop plate, then loosen it back up until it just touches the stop plate. Secure the front lock nut, and visually check to ensure that the rear cam moves at precisely the same time as the front one. Any slight delay in "lift off" between the two cams means the rear valve is not opening as much as the front valve. They should be perfectly in sync.

Now, pull both spark plug wires from the plugs, and hit the starter a few times to make sure all is well. If starting is still "labored" a very slight adjustment of the front cam may be in order, but this is rarely ever needed.

Important note: The stop plates for the cams are used only as a fixed reference for synchronizing the two cams. You may need to loosen your front cam to get it to rest on the stop temporarily, but final adjustment and distance from the stop plate will depend on the wear factor on your bike. On a used bike, there is no set amount of space or play, just whatever it takes to make the system function properly.


Items 3 and 4 in the right photo above are used to adjust the front cam. It is very unusual for the front cam to get out of adjustment, due to the very short and direct cable routing to it. If the solenoid is working, it will not lose retraction depth, that cable will not stretch, and there is no cable sheath to wear. High mileage engines might be the exception here, as the cam lobe can possibly eventually wear to some degree. If the cable adjustment lock nut has not come loose, and no major repair work (requiring removal of the solenoid) has been done, the front cam is likely still set properly.

If you feel that the front cam needs adjustment (which adjusts BOTH decomps at the same time), its easier to do trial and error adjustment than to actually set it to specs. Adjust the FRONT cable (that goes straight to the solenoid) by one half turn at a time, then hit the starter (plug wires off but plugs still in) and listen. If the engine spins too easy, as if there are no spark plugs in, the decomps are too tight. If it is labored or won't turn over at all, they are too loose! The trick is to find a happy medium so that the battery and starter aren't working so hard to spin the motor. (after a couple of revolutions, the decomps automatically disconnect to give the bike full compression) This procedure beats the heck out of pulling the generator cover and locating TDC, then trying to measure free play in a difficult area to access.


The Decompression system is nothing more than a pair of cheater cams that lift the exhaust valves slightly during starting, to ease the burden on the starter and battery. They are actuated by a single large electrical solenoid that retracts the cams when you push the starter button. The solenoid (upper right in the photo above) is cabled to the front cam lever, and it in turn is cabled to the rear cam lever. Pull the front cam and the back one pulls with it.

The exhaust valves are only opened a very slight amount, as too much retraction would leave insufficient compression for the engine to run with, so there is a very fine line between to much decomp and not enough. Too much retraction and the engine will spin over like somebody left the spark pugs out. Not enough retraction and you will think you have a dead battery!

The trick to working with decomp adjustment is to make sure that you really DON'T have a dead battery first!!

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