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Starting Your Airhead Motorcycle in Cold Weather
© Copyright 2021, R. Fleischer|
It was in November of 1971 that BMW lastly (?) recommended opening the throttle some during cold starts. This can be a necessity depending on the bike, temperature (does not have to be extremely cold), & how the carburetors are adjusted...and the general condition of many things. This is still true for later Airhead models, but BMW dropped the recommendation of slightly opening the throttle during starting. Too bad, because that may be necessary or quite helpful, this is especially so with the Bing CV carburetors. Method is to open the throttle a little during the initial pushing of the start button. |
| WARNING! Be careful NOT to use excessive RPM when starting the engine, particularly on a quite cold day, this means in particular with engine temperatures below 40°F. Use of proper viscosity engine oil is also important. If you blip the throttle too much, or otherwise use excessive RPM, perhaps over 1500 or so at the very beginning of the starting process, you run the risk of damaging the oil filter; and even the oil cooler if you have such! |
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Cold weather starting is nearly always done, & properly so, by using 100% full choke (full Enrichener setting) and slightly manipulating the throttle during cranking. Have the clutch lever at the handlebars pulled-in during the cranking to reduce loading of the starter motor by the transmission with its cold thick oil. Helps the battery and starter motor too. Many manuals, including the factory Owners Manuals, will say to not touch the throttle. In my experience, that is wrong. I have found nearly all Airheads require some light throttle manipulation upon starting in cold weather, and often in mild weather. As soon as the engine is running, reduce the amount of choke as quickly as you can, yet if you need to, and you likely will, keep some choke on, until you have smooth running, including when in initial riding mode. Typically the choke lever is returned to ~half-way within half a minute. Even in the coldest weather, the choke lever should be returnable within a few minutes to/from the half-way position, usually to full off ...or, nearly so. For very cold weather, try to keep the rpm between 1200-1500 (during non-moving time) until some decent warmup is had. No matter how cold, never blip the throttle to high rpm when starting; wear will be high, in some situations you can break rings or collapse an oil filter with a quite cold engine. Generally, you can start an engine & take off modestly, using moderate rpm, after 30 seconds to 2 minutes of high idle rpm (1200-1500); if the temperature is down to as low as 30°F or so. If the temperature of the engine is below 30°F at startup, use similar rpm advice, but warm the engine a bit longer the colder it is. An engine at 0°F may seem to require 5-10 minutes of warm-up, but you can start the engine, let it run for a minute or two, then take off gently, using moderate throttle & quite moderate RPM, until the engine is well-warmed. Be cautious when first moving off, if the temperature is below 30°F. Not only is the engine affected, but so is the transmission and rear drive, even such as wheel bearings, etc. Taking off in quite cold weather can result in oil spewing out the top oil breather/inlet of the rear drive ....worse if speed is increased too soon before the rear drive warms up. If you consistently are starting in near freezing weather, you should change to a Winter grade of oil, & the grade to select is in a chart in your owners booklet, but I have the very latest BMW chart in my website, about half-way down this article: For Airheads with the choke lever mounted on the air-cleaner clam-shells (pre-1981), the lever is down for choke on, & the lever points to the rear for choke off. Some Clymer's manuals are wrong, having this backwards! For pure-slide carburetors, they usually have a button to depress (or?) to increase the richness of the mixture for starting. |
When the engine is bitterly cold (some will start the engine at very considerably below freezing) it is best to warm the engine before starting. It is better to have the entire motorcycle above freezing; in, say, a heated garage. Wear on the engine, transmission, rear drive ...and other parts ...is vastly increased during extreme cold-starting. But, warming the motorcycle is hardly always practical.
There have been times, in extremely cold weather (considerably below freezing), when I have warmed an engine slowly, overnight. Lots of ways of doing that. I have even thrown a big insulated blanket over the bike and used a 150 watt lamp on the floor under it. One must be exceedingly careful about such heating...fire hazard, gasoline fumes ....etc.
Most of you will not start nor drive in quite cold let alone quite extremely cold temperatures, but some will....there are even Rallies in such weather!
If you start any engine at ~40°F (4.4°C) or below ....be cautious ...Again I caution ....don't excessively rev the engine when cold, it is hard on the pistons, camshaft lobes, oil filter element & many other places. Most engine wear occurs within the first minutes during startup. This is true at any temperature, but very especially so when the engine and oil is cold. If the motorcycle has not been started in several months, use a kickstarter, if you have one, to rotate the engine a few times; then start the engine with the starter motor; and have the clutch lever pulled back. Be cautious about kickstarters ....early versions, especially 1974 models, were weak, and best to be used only for emergencies if the starter motor system failed. Under severe conditions, arctic, for instance......especially if the engine has not been started in months or years, it would be best to drain the oil and filter, install a fresh filter, and install heated oil, preferably into a reasonably heated engine. That heated oil should be installed not only into the crankcase, but the valve gear covers should be removed and some of the oil put down both pushrod tubes in both cylinders.
Aluminum expands & shrinks fast with temperature changes. Tolerances get taken up. Metal can rub against metal. When it is cold, be extra cautious!
For regular use in relatively cold temperatures I suggest using a premium quality motorcycle-rated part-synthetic or full-synthetic multi-grade oil of lower viscosity range and containing a goodly dose of ZDDP. The winter grade oil with ZDDP is not just for starting ease and some ultimate surfaces protection; but, for quicker lubrication, and the fact that premium oils usually stick better to the steel surfaces during storage. You need oily surfaces during even the first engine rotation during starting-up. Since the oil temperature is not likely going to be reaching into the upper range during riding under such conditions, use of thinner oils is quite appropriate. Be sure to see the BMW chart in the viscosity article.
Full synthetic oils (or, if not bitterly cold, compromise with a part-synthetic, such as Golden Spectro 4) are good choices for cold weather.
The latest BMW factory recommendations, in chart form, is located inside the following linked article: https://bmwmotorcycletech.info/viscosity.htm. I suggest you also consider the air temperature after you start the bike, and during and ending your trip. You may have to make a compromise in oil viscosity. Oil compounding has changed! ....that chart has changed! Be sure to read the comments!
Here are two things not usually mentioned anyplace: It is better to err on the side of a thinner oil, than a thicker one, if you are beginning the ride in quite cold temperatures, and continuing later in warmer temperatures. Oil flow, which is much easier with thinner oils, is more important, particularly with modern oils, when you start out on a quite cold day. I wouldn't worry about the bearings, etc., holding up with a thinner oil. You may want to read my oil essay, because many have the wrong idea about oil thickness, oil film strength, etc.:
https://bmwmotorcycletech.info/oilessay.htm
In some areas of the world multigrade oils are not easily available. You may have to make compromises.
For normal temperatures, from around 40°F to as hot as it gets anyplace in the world, I suggest you use a quality 20W50, such as a top-grade semi-synthetic like Golden Spectro 4; or Spectro's full synthetic. Short rides (under 20 miles in even mild weather) do not allow condensation products in the oil to be 'burned-off'; plus, there are more such condensation products, and they mix with sulfurs, etc., in petroleum oils, and the acids formed are not good for the engine parts. Full synthetics are better in very short ride situations or severe to extremes. Quality part-synthetics are reasonably good too, such as Golden Spectro 4.
Your Airhead will start more easily with the proper grade of oil, if the starter motor is in good condition, has a good battery... and the clutch lever at the handlebars is pulled-in during starting. The battery, cables, connections, starter and starter solenoid must be in good condition. You could take voltage drop measurements to be sure of proper system operation. Do a formal battery load test yearly or even every 6 months, to avoid bad battery surprises. Anything marginal, such as in the ignition or having valves with too little clearance, can cause poor starting in cold weather.
Helpful ideas for cold weather riding:
Spark plug gaps can be worn to .032" maximum. Stock setting is better, but it is best to not change spark plug gaps on used plugs; that weakens the grounding stub. Check the ignition coils and towers inside and outside & clean these things. Check for cracked ignition coils on the two-tower gray plastic case types. Clean and check the wires & spark plug caps. Proper ignition points gap if you have points. For POINTS models, I suggest 1000 ohm early style caps or 1000 ohm NGK spark plug caps. You can not use 1000 ohm caps on the 1981+ electronics models, they must have the 5000 ohm caps. Do not use resistor spark plugs on any BMW Airhead.
Valve & valve seat condition (& valve clearances especially), compression pressure... & ignition timing ...must be good. Tight valves, or a worn top end, particularly with low compression, will make starting difficult ...or even impossible.
The engine must spin over relatively fast to get a good start in cold weather. Kick starting may be especially difficult and should be avoided. Kickstarting is hard on the transmission parts associated with the kickstarting mechanism. The 1974 5 speed transmission is particularly weak in the kickstarting parts area.
The incoming mixture will tend to be less homogenized. A very fuel-rich mixture is needed in cold weather starting and the first portion of engine warmup; which is obtained on the CV carburetor models by using the 'Choke' lever, which is actually a lever to turn on the Enrichener. Be sure the Enrichener is properly assembled and adjusted. There have been a considerable number of Bing CV carburetors in which the enricheners are wrongly assembled, even some where the Bing factory mis-marked the enrichener shaft dot identification. Photos of correct and incorrect enrichener parts assembly are in my website in the Bing CV articles. Carburetor choke (enrichener) levers that do not reach full-on stops (or at least nearly) may not give full enrichment. If the enrichener circuits, etc., on the Bing CV carburetors are not assembled correctly, or the enrichener gasket is leaking, or the jet in the side of the bottom of the float bowl is clogged, or the dip-pipe cracked, starting may be very difficult.
If the gasoline is old, or summer-grade, starting can also be very difficult. Starting may be nearly impossible if you are using aviation fuel....it does not contain the highly volatile ingredients of road-going-vehicles fuel.
Spark plugs must be clean & dry at the electrode end. If spark plugs are carbone'd up, or otherwise fouled, they must be cleaned or replaced, & the reason for such carboning, if excessive, fixed.
If your Bing CV carburetor'd bike starts, & then seems to die after maybe 10 to 20 seconds, even if holding a small amount of throttle, inspect the carburetor bowl contents, & especially check the bowl gasket ...that it is intact and good around the enrichener well area .....yes, that gasket needs to be good in order for the enrichener to work properly. Be sure to inspect the enrichener's jet in bottom of the corner well of the bowl and its side-hole inlet at the bottom of the same bowl well. It is not unusual to find that jet clogged. Failure to have fuel properly flow from the bowl into that corner well area will cause this particular 10-20 second problem, & a clogged or partially clogged jet is commonly found. If the bike does not want to start at all check to see if the corner jet well has fuel in it. No water is allowable to be in the bowl. A rare occurrence is a split brass tube that dips into that corner bowl area, this can come from water freezing inside that very tiny pipe.
Other jets can plug-up with crud, causing starting & running problems. In some particularly neglected motorcycles, even the bottom of the tank & petcocks could have frozen water in them. Be sure the petcocks allow enough fuel to flow into the carburetor bowl. Be sure the fuel tank can vent...otherwise, as you try to use fuel, the tank will have a vacuum, and fuel flow will greatly lessen...or, cease.
If you have a GS, it probably has no oil thermostat at the canister area. The BMW factory recommended that in quite cold weather you cover the oil radiator. BMW sold covers for this. You can make one. Over-cooled oil that never gets even close to 200°F will not lubricate correctly, and will gather contaminants faster.
Dual-plugging, done correctly, will make starting far easier.
Watch out for water in the float bowls & fuel tank; water condenses in the fuel tank overnight; this is particularly so in areas where you see dew collecting; and very particularly so if the fuel level is not quite high. This water will rot out the tank bottom! Clean your tanks. I do it yearly. In some areas, you may have to do it more often. If you have aftermarket paper fuel filters, be sure they really will flow sufficient fuel. You can clean the tank and then treat the insides with a weak phosphoric acid mixture (screen door or other aluminum cleaner). It leaves a protective anti-rust gray coating.
Many street tires are very slippery when cold, so be cautious about your tires when riding off.
Both disc and ...especially drum brakes... in cold weather ...and sometimes mild weather ...can be grabby on first braking attempt, especially if damp or in high humidity overnight. Be cautious about the first use of brakes. The type of friction material can have quite an effect on initial grabbiness. See: https://bmwmotorcycletech.info/brakes.htm.
Keep in mind that the colder it is the more the engine is affected ...but so is nearly every other part of the motorcycle. Even something like a plastic tail-light lens is more brittle ....and if its screws had been even slightly over-tightened, a crack could occur.
Revisions:
02/03/2003: Add Clymers caution and how cold Wx starting is done.
04/04/2003: Edit for clarity, especially about 1000 and 5000 ohm caps.
09/28/2003: Check-over. Moderate clarifications.
02/19/2007: Minor editing, mostly re-arranging areas and adding emphasis to existing items.
08/25/2009: Minor.
06/16/2011: Clarity fixes.
09/23/2012: Add QR code; update google code; add language button; clean up article a bit, fix some typos, change oil grade recommendations a bit; clarify things in the article and eliminate some redundancies.
2013: Remove troublesome language button code.
09/27/2014: Update slightly.
02/05/2016: Update article. Narrow. Font increased. Metacodes.
05/24/2016: Final work on metacodes, layout, clarity of information...etc.
08/22/2017: Go through entire article. Reduce use of fonts, colors, bolds. Clean up and minor to modest edits for clarity.
04/24/2019: Add red outlined table, with excessive RPM warning.
05/05/2021: Clarifications, only.
© Copyright 2021, R. Fleischer
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Last check/edit: Wednesday, May 05, 2021