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Overheating, Excessive Idle Time, etc.
on your BMW Airhead Motorcycle
© Copyright, 2012, 2017, R. Fleischer
There is a relatively common misconception that even moderate idle time will cause overheating on our BMW air-cooled bikes, yet this is not so. There are of course, limits to even our well designed & well-finned BMW Airhead motorcycles. Truly excessive idle time can certainly overheat the top end & cause problems from warping. It is even possible to loosen valve seats, besides the usually spoken-about frozen or seized parts. The oil, if excessively hot for periods of time, will deteriorate.
Airheads have a large amount of cooling fins. There is a considerable total surface area. The cylinders are outwards & have good convection cooling. A few minutes of idle time at a stop light, even in the hottest weather, is not going to do any harm. This assumes you will ride off a bit after that ....& not be idling for several minutes at every block in a city, for miles. Being stuck on the freeway in a traffic jam for 10 minutes and longer in hot weather is not a good place to continue to idle the engine. It is especially not a good idea for a low RPM idle constantly, because as the oil thins from heating, the oil pressure in the system drops at low RPM, and there is less and less oil coming out the pressure relief valve located above the crankshaft sprocket. Thus, the timing chest parts are not getting enough oil for lubrication and cooling.
But, for brief stops, there may be thoughts of engine over-heating. Human beings know what outdoor temperatures are comfortable to them, & may unconsciously transfer that idea to our engines.
Most engine wear happens in the first few minutes of cold engine start-up, where engine clearances are decreased from cold shrinkage. Even in mild weather, initial start-up is where an increased amount of wear occurs.
Idle & low power levels produce vastly less engine heat than cruising down the road at highway speeds. The faster you go, the more heat is produced by the combustion process because you are using more power, burning more fuel.
The heat is gotten rid of primarily by the exhaust and the oncoming air, with its turbulence effects as it passes very irregularly around the fins. Some cooling comes from air flowing over the motor in other areas. Conductance, convection, and radiated heat .....all are happening. BMW has designed out Airhead engines to have more cooling ability than usually needed, for any power output level of the stock motor.
Airhead owners who have the BMW oil-cooler radiator (with thermostat version) occasionally think something is wrong with the thermostat if the oil-cooler radiator did not feel hot to their hand immediately after they stopped. The thermostat will sometimes open for the additional oil cooling, giving a feeling of warm or hot at the oil cooling radiator, when driving slowly in town, or moderate stop and go traffic. This is because the oil is hot enough to open the thermostat. The GS models don't have a thermostat, but a bypass hole, so they might feel warm to hot much of the time. But, it takes a fair amount of oil heating to cause the thermostat models to pass oil through the radiator. If you are cruising at speed, it is more likely that the thermostat version will have a hot radiator, because you are using more horsepower, creating more internal heat, which heats the oil more. This does not happen quickly, so it takes some time at highway cruising speeds.
I personally think that "ten minutes is OK for idle", as has been cited in BMW's literature, canbe excessive. I think 3 or 4 minutes is OK, but that nearing a reasonable limit, especially if there is not a mile or or so of riding between multiple stops. In that instance I'd pull off the road, turn off the engine, and let it cool down. I am speaking here of both heat effects on expanding metal and lubrication concerns. Petroleum-based compounded oils, if they get hot enough, will deteriorate much faster than part synthetic or full synthetic oils. The deterioration usually is not reflected in lubrication qualities, until extreme. By deterioration, here, I mean not only the base oil itself, but the additives (which may deteriorate and even evaporate at high temperatures).
While quite excessive idling time can be worrisome for overheating of various parts, especially the top end, I would also be concerned about inadequate lubrication to the timing chain, sprockets, etc. .....This is particularly so if your engine is worn moderately or more, and/or your normal idle is below 900 rpm, which is a BAD idea. Worn engine bearings allow more oil to flow out of the bearings to the crankcase, causing lowering of oil pressure at low rpm. Additionally, as oil heats up, it thins, oil pressure at idle rpm drops even more. While it is true that thinned oil passes through bearings & other places much faster, which can be good, as it carries away heat, the pressure reduction at idle leaves less oil for the overflow from the pressure regulator, which is how the timing chest area parts are lubricated.
Explaining a bit differently: The timing chain, chain guide, and sprockets, are all lubricated by the oil output of the high oil pressure release valve (plunger type) located just above the crankshaft sprocket in the timing chest. It is set for ~75 psi actuation by an internal spring. Oil viscosity reduces as oil temperature rises. Some oils have a poorer viscosity index than others so are worse at this thinning, & although thinner oils get through the system faster, and probably can carry heat better due to the velocity, there can be much lower idle rpm oil pressure, particularly on an higher mileage Airhead, where bearing clearances are increased due to wear. Thus, there may be very little or no oil at low idle RPM to the timing chain & sprockets, etc. It is the overflow from that pressure relief valve that provides all the oiling for the timing chain, chain guides, and timing sprockets. Enough experience & tests have shown that idling rpm should not be too low, certainly not lower than ~900 rpm, for just this reason (let alone others, such as carburetor functioning at low rpm as you transition to the off-idle mode), and, chain system jerkiness ...which adds extra wear.
I think the worst situation for overheating would be either quite long waits in stop & go driving in the city ....or; worse ....an excessively long low rpm stop right after quite a few miles at high speeds. Blipping the throttle now & then at a stop will help with oiling, some anyway. One of the worries is that excessive cylinder head heating could cause head warping. We do see R100 engines warped now and then ....usually when someone complains a regular valve cover gasket is not sealing well, or there are leaks at the head gasket, or a dual-plugging job is being done and the head is being checked; or, checks done during a top end job. Extremely overheated heads may have loosened the valve seat rings, I have seen them come out of the proper area, hang up the valve, and cause a lot of engine damage. This usually happens only with improperly done valve seat installations. Valve seats must be shrunk into place properly.
If you know you will be stopped in traffic for quite a few minutes, & perhaps there is intense stop & go with long stops, you should probably consider turning off the engine. The cooling is much more rapid than you may think.
Oil, particularly some petroleum-based oils, deteriorate quite fast, above ~250°F. Some might say the accelerated deterioration starts around 230°F. Most part-synthetics are better, and good quality part-synthetics, such as Golden Spectro 4, 20W50, are quite good. The hottest area for oil is not inside the main engine case, but around the valve stems in the heads. Thus, oil is heated quite high, briefly, as it passes those areas .....but the bulk of the oil is not going to reach such high temperatures. Dino oil that reaches towards 300°F is deteriorating rapidly. Because most of the engine is much cooler than around the exhaust valve stems and cylinder head in general, the bulk of the oil deteriorates slowly over many miles; and, oil changes bring fresh oil, with fresh oil additives.
Full synthetic oils, such as Spectro's, are good with hot engines with quite hot oils. However, the additives the oil manufacturer put into the oil will burn-off or otherwise be less usable; and, for most riders, I would suggest a semi-synthetic oil, such as Golden Spectro 4.
Oils do not need to be changed too often ....but do need to be changed. When the change should be done will depends on the conditions of use of the motorcycle. This has been talked about, and argued about, forever. Changing oil too often is also not the best thing, and few know anything about that ....the information, from SAE no less, & is in my oil articles: https://bmwmotorcycletech.info/Oil.htm.
If you wish to learn about how oil really works, how it really lubricates, read this:
There is a tendency, perhaps carried over from the /2 days, for people to set the idle rpm on the Airheads too low. In the /2 era, the engine oiling system was quite different, and, the flywheels were very heavy, helping to keep the engines quite smooth. In that era, it was sometimes considered a matter of pride ...or ego of some sort ...that the engines could be made to idle very slowly & smoothly with a very careful carburetor and other adjustments. This was actually foolish. Rather too many still carry over these ideas to the Airheads.
I have noted, well above, that excessively low idle rpm on Airheads may provide far less oiling to the timing chain/sprockets/guides. The chain & sprockets will jerk about more at very low rpm, which is harder yet on these items, as well as other areas of the engine, including the camshaft, and even various bearing areas. I have long recommended that the ideal rpm for idle is likely ~ 1025, or even slightly more. An added advantage is that the Bing CV carburetors work better at that idle RPM. Not only can idle itself be smoother, which is better for the timing cover innards, but, the transition period (that point you just barely open the throttle from idle) is much smoother, & the carburetors synchronize and adjust much better. Those with dual-plugged engines & Bing CV carbs have further reasons for having a 1025+ rpm idle, as the carburetor adjustments are even more critical (as the butterflies are closed more on dual-plugged bikes).
11-24-2010: Review article, make MINOR updates, strictly for clarity; add copyright notice at lower area, check meta-coding.
05-27-2011: Clean up some, nothing major.
09/27/2012: Add QR code, add language button, update google ad-sense code, minor other editing.
2013: Remove troublesome language button code.
09/22/2014: Clean up article.
02/19/2016: Update metacode; increase font size, justify left, clean up article; clarify a few items.
06/15/2016: Update metacodes again. Clean up article. Modest changes for additional clarity.
11/05/2017: Go through entire article, reducing fonts, colors, html. Fix typos and clarify statements.
© Copyright, 2012, 2017, R. Fleischer
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Last check/edit: Monday, July 22, 2019