Service

SERVICE

Joe Minton

Warming wisely

My question concerns the warm-up of air-cooled engines. How long should they run before being ridden away, and how long should the choke stay on? Also, is it better for the engine if the idle speed is temporarily increased to cut down on the use of the choke? Like most riders, I want my bike to last as long as possible with as few problems as possible. Can you give me a general rule of thumb for maximum durability?

Christopher Lambert North Vancouver, British Columbia

Testing done by General Motors many years ago showed that an engine will last longer if it is warmed up quickly with a light driving load at moderate rpm. As soon as an engine has full oil pressure and can operate safely-meaning that it will pull cleanly with no danger of stalling-it should be driven. This applies to air-cooled and liquid-cooled engines alike.

To avoid an excess of unburned gasoline in the combustion chamber that can dilute the oil lubricating the piston rings and cause accelerated wear, don't use the choke any longer than necessary. And increased idle speed during engine warm-up is a good thing; it helps prevent stalling and the aforementioned oil-dilution that can result from the incomplete combustion that usually occurs in a cold engine at very low rpm.

Long warm-up periods became commonplace more than 20 years ago when the EPA began placing limits on automobile (and later; motorcycle) exhaust emissions. Initially, those emissions standards were met by simply leaning-out fuel-air mixtures; but that tactic resulted in huge performance compromises, including interminable warm-up periods and terrible throttle response. Many American automobiles of that era needed 10 minutes or more of running in neutral before they could be coaxed out of the driveway during cold weather. And I once tested a CV-carbureted, early-Eighties motorcycle that was so lean, its engine would stall if given full throttle at low rpm, even when fully warm.

Things are a lot better these days. Car and bike manufacturers have developed more sophisticated ignition systems, higher-tech fuel-management technology and more-efficient combustion-chamber designs that permit engines to meet ever-stiffening emissions standards while still providing excellent all-around performance and relatively short warm-up periods. But the method you should use to warm your motorcycle s engine remains unchanged: Ride it. Just don V be in such a hurry to do so that you can 7 depend on the engine to get you out of the way in an emergency.

Remaining biased

It’s time to replace the rear tire on my 1989 Yamaha V-Max. Would the bike perform better with a radial rear tire, or should 1 stay with my present Dunlop Qualifiers?

Jason M. Zoufal

Grosse Pointe Woods, Michigan

Motorcycles depend enormously on their tires for stability, so the two must be compatible. Because radial tires are designed to fit on wider rims than those on a V-Max, your bike doesn 7 provide the required compatibility.

Radial sidewalls are quite short compared to their bias-ply counterparts and must operate at the proper angle relative to the tread. If the rims pinch the tire beads too close together, the sidewalls will be angled improperly and won 7 be able to provide the support that is so critical to the stability of the tread and, consequently, the entire motorcycle. So important is this compatibility that some countries have made it illegal to use tires that have not been tested and approved for the particular motorcycle on which they are mounted.

The bottom line: Stick with bias-ply tires designed to work on your V-Max.

Hot-oil wrestling

I read with great interest the letter titled “Slippery Business” in July’s Service column, and in particular your statement, “BMW Boxer Twins of all sizes also maintain comparatively low oil temperatures.” This is contrary to my experience with my '78 RIOOS. Several years ago I installed a Luftmeister gauge that senses the oil temperature just above the oil-pump pickup screen, approximately in the middle of the sump. The gauge reads accurately at 192 degrees Fahrenheit, the boiling point of water at this altitude. Surprisingly, the indicated oil temperatures were much higher than I expected.

For example, today I went on a solo ride on local freeways, 60 degrees ambient temperature, speeds in the 60-to-70-mph range, and the gauge read 240 degrees F. On warmer days the gauge often indicates 275 degrees on spirited solo rides, and reaches 300 on two-up rides in the mountains.

Adjustments in jetting and ignition advance have had minor impacts on my bike’s oil temperature, and an oversize sump had no apparent effect whatsoever. My experiences with high oil temperatures are shared by other BMW owners in Denver.

Lars Hansen Littleton, Color?'

The 300-degree oil temperature you report is the highest I 've ever heard of in a BMW Twin street bike. Your sensor is located in a good place, so its readings should be representative of the oil temperature as the oil is fed into the main bearings. But aside from the fact that the thinner air at high elevations does not cool as effectively as the heavier air at lower altitudes, I have no explanation for your higher oil temperatures.

I can tell you, though, that your Beemer could put an oil cooler to very good use. A cooler rated at around 7000 B TUs per hour should do the job. BMW's factory cooler makes for a very’ neat installation, but it may not have enough heat-shedding capacity for your needs. In testing I did some time ago, Lockhart oil coolers proved to shed more heat for their size than any others I evaluated. Borg-Warner makes coolers that are not quite as compact as the Lockharts but are available with greater heatshedding capacities. The Borg-Warner coolers are available in a wide range of sizes through any auto parts store that handles B-Wproducts, and they’re rugged, attractive and compact enough to fit across the front downtubes of your Beemer s frame.

While the oil temperatures you report are high (in my experience) for a BMW Twin, they are low compared to those of numerous other motorcycles, particularly a few older bikes with engine designs that routinely allowed oil temperatures to get as high as 310 degrees F. Using high-quality synthetic oils in hot-running engines helps protect metal engine parts against accelerated wear at elevated temperatures, but they cannot protect temperature-sensitive parts like oil seals or rubber cam-chain control pieces. That ’s why I recommend that you do something to get your engine ’s oil temperatures down into the 180-220-degree range.

Shave your head

I have a 1983 Honda VT750 Shadow V-Twin that has blown a head gasket and burned a groove in the head where the gasket seats. I have been told that I can have the head milled .010-inch. But before I go ahead with the job, I’d like a second opinion.

Thor Hansen Yerington, Nevada

You should be able to mill .010.015-inch from the head without any significant consequences. Understand, of course, that milling the head will shorten the distance between that particular cylinder ’s chain-driven camshaft and the crankshaft that drives it, which will retard the cam timing and raise the compression ratio ever so slightly. But because those changes are small and will affect only one cylinder, neither should be noticeable as you ride the bike.

If you need to mill more than .015inch off the head, just add a second cylinder-base gasket. That will raise the entire cylinder and the head mounted atop it, thereby increasing combustion-chamber volume and cam timing back closer to their original specifications.