By Ron Schmidt
4 Cycle Engine Basics
Currently, all BMW motorcycle engines are 4 cycle engines. This is a brief explanation of what happens inside that block of aluminum every time you use it.
The term “4 cycle” (or sometimes also called “4 stroke”) describes the engine operations that make it function. The cycles will be called “intake”, “compression”,” power”, and “exhaust” in this seminar, though other terms are often used.
Like the chicken and the egg, it is debatable which of these cycles happens first, as at least two will be happening on our multi-cylinder engines when you hit the starter button. I like to start with the intake cycle because until you get something in the cylinder to burn, there will be no power and since power is what we all are really interested in, we’ll begin there. Also, since I’m the one giving the seminar, I get to do it my way!
I am going to over-simplify the engine cycles by telling what would be happening if the engine were to be operated at extremely low rpm. In the case of the real world, since the gasses we are moving around have considerable mass and are reluctant to be moved (an object at rest will remain at rest unless a force is put upon it—like when the alarm goes off each morning, we do not just jump out of bed at full speed!) the real operation of the valves and when the spark plug sparks are advanced from the piston locations what we will discuss here. If any of you wish to know how much, why and what happens by this advancement and has a few hours to discuss the very basics of this subject, let me know!)
Intake Cycle:
The piston isat Top Dead Center or TDC (would really be “side dead center” on the boxer engine!). The exhaust valves are closed, the intake valves are opened. As the piston goes down to the bottom of the cylinder it will allow a mixture of air and fuel (and maybe some dirt if you have a K&N air filter) to fill the cylinder.
Compression Cycle:
Intake valves now close, exhaust valves are still closed. As the piston moves back up to the top of the cylinder, the air and fuel mix is compressed. This compression raises the temperature of the mix, making it more combustible.
Power Cycle:
The piston is back at top dead center. All valves are closed. The sparkplug sparks, a big bang happens. The burning fuel air mix is expanded and pushes the piston down. Yahoo!! Power!!
Exhaust Cycle:
The exhaust valves are opened. The intake valves remain closed. As the piston goes back up to the top, what is left of the burned air fuel mixture is expelled through the exhaust valves and out through the exhaust system. As an interesting note, the exhaust sound that you hear is the release of these dead gasses—well after the power has been made—so never confuse a loud exhaust with power!
Valve Adjustment Procedure:
1) The engine needs to be cold. It is best to leave it set over night before performing the adjustment. At the very least, you should not feel any warmth from the engine when you touch it.
2) Park the bike on the center stand and secure it in some manner that will not allow it to tip over. You will be tugging on it some, so make sure it is secure!!
3) On some models the fairing panels will need to be removed to perform the valve adjustment. Remove panels if necessary.
4) Remove the sparkplug covers by pulling them from the rear, to the rear then pivot to the front.
5) Remove the sparkplug wires from the sparkplugs using the tool in your tool kit if you do not have the $50.00 BMW special tool.
6) STOP NOW AND READ THIS!! IF YOU MISS THIS STEP YOU CAN CAUSE YOURSELF LOTS OF GRIEF!! Look down into the hole that the sparkplug wire was just removed from and all around the outside of the valve cover. There will certainly be dirt there; lots of dirt if it is a properly used GS. This dirt is your worst enemy and must be removed BEFORE you take the sparkplug out or remove the valve cover!! The dirt can generally be blown away with compressed air (be careful to shield your eyes if you remove the dirt this way!). If you do not have an air compressor at home, you might be able to vacuum it, or you could even use a garden hose and flush it away. Whatever means you use, make sure the area around the spark plug and all around the outside of the valve cover is squeaky clean.
7) Remove the sparkplugs from both heads. There is a socket in your tool kit if you do not have the $85.00 special BMW tool.
8 ) Remove the rubber plug from the timing view hole. It is on the right hand side of the engine, just to the rear of the cylinder. This plug is about 20mm (3/4”) in diameter.
9) Put a drip catcher pan under the left valve cover. Loosen the 4 bolts that hold the left valve cover to the head until they no longer are threaded into the head. These bolts are quite long and you do not want to actually take them out of the valve cover. They are going to be “stuck” in the valve cover by their sealing grommets, so just let them stay there. The valve cover can then be pulled off. “Pull” can be where they almost fall off by themselves to having to VERY LIGHTLY persuade them with a soft mallet. If they are stuck and you do not have a soft mallet, you can use a regular hammer—just put your fingers between the cover and the hammer so you do not damage the valve cover or be tempted to hit it too hard. When you remove the cover a bit of oil will come out.
10) Put the transmission in high (5th or 6th) gear. Turn the rear wheel in its normal direction of travel and watch the intake valves. You want to see them completely open (pushed down all the way) then close (all the way out). When they are completely closed, look into the timing view hole and continue to turn the back wheel in its normal direction of travel until the “OT” marking tab on the flywheel is centered in the hole. The engine is now at Top Dead Center (TDC) with the left side on the compression cycle, which is where it needs to be to adjust the valves for the left side. Note that once the intake valves have closed, the engine will be near TDC; if you have to turn it very far you probably have missed the timing mark and will be on the wrong cycle when you see the OT mark come up in the window. When the engine is at TDC on the correct cycle, there should be clearance at both the intake and exhaust valves.
11) Check the vertical clearance of the rocker arms. This is done by measuring the clearance between the bottom of the rocker arm and the steel lower carrier that is held onto the head with three 8mm bolts (these are usually gold colored, use a #45 Torx tool, and have shallow heads) and one 10 mm nut (uses a 15mm socket). The clearance needs to be between 0.05mm and 0.40mm. You will find that the valve train will be quieter if the clearance is closer to 0.05 than 0.40mm. Personally, I set them as close to but not less than 0.05mm as possible. This is an extra step and is not necessary if within the specified 0.05mm and 0.40mm as far as BMW is concerned, but I feel it is just the proper thing to do. You will definitely notice a reduction in noise if the arms are set closer to the minimum spec.
12) If you are not going to reset the rocker arm vertical clearance, skip this paragraph. To set the rocker arm clearance you will need a torque wrench, an angle torque gauge, and a hard plastic or brass hammer. Loosen the three 8mm bolts and the 10mm nut that hold the steel lower carrier onto the cylinder head. Then make them all just a bit more than finger tight. GENTLY tap the steel lower carrier up until the clearance is close to but not less than 0.05mm. Torque the three 8mm bolts and the 10mm nut to 20NM (Newton meters). Recheck the clearance. If it is correct, install the angle torque gauge and angle torque the 10mm nut an additional 180 degrees. Recheck the clearance. If it has changed (it often will) you will need to repeat the whole procedure. Make very sure that when you are done there is at least 0.05mm clearance. If there is not, you can damage the camshaft and followers due to excessive drag on the rocker arm. You should be able to get the desired clearance between 0.05 and 0.10mm within a few tries.
13) I suggest that you adjust the intake valves first, but I have no idea why as it really does not matter—just the way I like to do it! Slide a 0.15mm feeler gauge between the top of the upper intake valve and the rocker arm. There should be a slight drag as you slide the gauge through. If there is no drag, the valve is loose, if there is a lot of drag, the valve is too tight. What is a slight drag compared to a lot of drag, you ask? A good rule of thumb is that the drag should be just enough to hold the feeler gauge in position when you let it go. If in doubt, a little loose is better than a little tight. I have found that good mechanics will vary about 0.002mm according to what they feel is the correct amount of drag and argue at length about who’s feel is right. In view of the fact that the clearance is temperature sensitive and will change as the engine warms up, don’t loose sleep over it. If you want to check your work, try a 0.17mm gauge, it should feel very snug, and a 0.13mm should feel very loose. To adjust the valve, loosen the locknut with a 10mm wrench, turn the threaded adjuster with a 3mm allen wrench. Turn it clockwise to decrease the gap, anti clockwise to increase the gap. Then torque the locknut to 8 NM and recheck the clearance. Correct if needed. Then perform the same procedure to the lower intake valve. Move to the exhaust valves, using the same procedure, but the using the 0.30mm gauge instead of the 0.15mm gauge.
14) Wipe all the oil off the valve cover and the outer gasket, and the head. I suggest you replace the inner valve cover gasket because they very often will not seal again. The part # for that gasket is 11 12 1 341 707. They are cheap. Oil leaks are aggravating. Put the valve cover back on, tighten the 4 bolts that hold it on finger tight in a cross x pattern (upper left, then lower right, then upper right, then lower left). Then, again in a cross x pattern, torque them to 8 NM.
15) Move to the right side of the motorcycle and repeat steps 9 through 14.
16) Reinstall or renew sparkplugs. Torque to 30 NM.
17) Reinstall spark plug wires. Note that a bit of dielectric grease on the plug wire where it slides over the sparkplugs will make them easier to remove next time!
18) Reinstall the sparkplug covers. To do so, slip the rubber nubs on the rear of the cover into the recesses on the valve cover, the push the clip at the front on the valve cover. If you do it the other way, the cover will likely fall off even though it felt secure when you installed it.
19) Put the rubber plug back in that was removed in step #8. be careful not to push it all the way into the hole and loose it—I have seen many mechanics do that. If you do loose it, don’t worry—you can’t retrieve it but it won’t hurt anything either.
20) Reinstall body work removed in step 3 if applicable.
21) Start engine, let it run for 63 seconds, turn it off and check for leaks at the valve covers. Be careful to note that the exhaust header will be hot enough to burn the poop out of you already! Correct leaks if they exist.
22) Go ride the bike a few hundred miles in some beautiful place to enjoy what a good job you just did!
