Service

SERVICE

We welcome your technical questions and comments, and will publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return any personal replies. Please limit your “Service” letters to technical subjects only, and keep them as brief as possible. Send them to: “service,” CYCLE WORLD, 1499 Monrovia Ave., Newport Beach, Calif. 92663.

OIL-SPITTING KAWASAKI

About six months ago I purchased a new 1980 Kawasaki KDX250. I love everything about it, especially the power, except one thing bothers me. It spits lots of black exhaust oil onto the back fender and all over me. I leaned it out one notch but that didn’t help. Should I go up one or two more notches on the jet needle or would this risk causing a seizure? I'm not worried about trying to get more horsepower, just whether it should spit exhaust oil like this.

Chris Iaconis Rural Valley, Pa.

Oil coming from a two-stroke engine’s exhaust is a fundamental characteristic of this engine type; the engine is lubricated by oil mixed with the gasoline the engine burns, and the unburned remnants of the oil are expelled out the exhaust. Because your engine is making good power, it’s unlikely that it’s running so rich that unburned gasoline is contributing to your oil spitting problem.

Kawasaki recommends a 20:1 fuel/oil ratio for the KDX, but our experience is that the KDX 250 will run well on considerably less oil than that. We’ve accumulated several thousand off-road miles on KDX 250s running them on Bel Ray MCI + oil mixed 2 ounces of oil to each gallon of gas, a ratio of 64 to one. This minimizes the amount of oil coming out the exhaust, and hasn't led to any other problems.

EASIER PISTON INSTALLATION

I just finished reading your article on modifying the 650 Seca, and would like to pass on the no sweat method of installing new pistons in four cylinder engines.

The first step is to install one piston clip in each piston, making sure it’s the clip closest to the cam chain in each. The inner throws of an inline Four crankshaft move together, so slide the two outside pistons all the way in and the inside ones in until the pin holes are just accessible beneath the cylinder sleeve. Rotate the crankshaft so that two inner rods are at the top of their stroke and line up the cylinder block over the studs. Slide it down so the pin holes line up with the small ends of the inner rods and prop the block in place. Slide the inner pins home, clip the two clips in and you're half done. Push the outside pistons down so their pin holes are visible. Remove the prop holding the cylinder block. Now carefully rotate the crankshaft and lower the cylinders so the inner pistons don’t slip out. When you’ve rotated the crank enough, the outer piston pin holes will line up with the outer rod small end holes. Prop the block in place again, slide the outer pins in place, install the clips, and there you have it. Pistons installed without pain or cursing.

George Fallar Mamaroneck, N.Y.

Your method is certainly superior to the one we used on the 650 Seca engine build. The only addition we’d make is to cover the crankcase openings around the rods with clean shop rags to prevent c piston clip from jumping down into the engine.

WATER VS. AIR-COOLING

In your January 1983 Cagiva 200 review you say “and as with any water cooled engine, the relatively constant engine temperature means the piston anc cylinder life will be greatly extended.”

Julius Mackerle’s book Air Coolea Automotive Engines explains that bore wear is very slight when cylinder wall temperatures are between 150° anc 390°. At temperatures above 390° qil breakdown occurs and rapid wear results. At temperatures below 150° rapid wear occurs because acids formed by combustion condense on the cool cylinder walls.

While properly designed engines should not have cylinder wall temperatures above 390°, all engines start below 150° and this is where most wear is incurred. Water-cooled engines take longer to warm up than air-cooled engines and as a result water-cooled engines typically show twice the cylinder wear of air-cooled engines.

That's one reason my new bike is going to be air-cooled.

David Westerfielü Dix Hills, N.Y.

We won't dispute Mr. Mackerle’s comments on the relationship between cylinder wall temperature and bore wear, but it strikes us that the conclusions you’ve drawn from those com -ments are more relevant to choosing between a Deutz air-cooled diesel industrial engine and a water-cooled Cummins than to choosing between ai$ and water-cooled motorcycle engines.

The limiting factor in engine life ip small two-strokes like the Cagiva isnkt cylinder bore wear, but instead the problems are piston cracking, piston skirt collapse, and loss of the ring seal from increased piston rocking. The watercooled two-strokes, because of their more constant cylinder wall temperature and freedom from hot spots, can rup considerably less clearance between the cylinder and piston than their air-cooled predecessors. This keeps their pistons from rattling about as much as in the air cooled engines, and allows the pistons to run cooler. In real world use, watercooled motocrossers have three to five times the top end life of the air-cooled versions they replaced, and make more power while living longer

Four-stroke street bike engines are a different story, but the conclusion à similar. Bore wear is not likely to dictate the first engine rebuild, at least not if our experience, but instead problems are more likely with valve sealing or cam chains. We won’t claim that water-cooling will aid cam chain life, but the lower temperatures water-cooled motorcycle engines run around the valve seats ana guides should aid valve life. The experts ence of thousands of Gold Wing owners indicates, if nothing else, that watercooled engines aren't shorter lived that: any of their air-cooled counterparts.

OIL COOLER?

I would like to know what you think of oil coolers. I own an 850cc Suzuki ai^a I'm thinking about putting an oil cooler on it. But my dealer tells me the bike doesn't need an oil cooler because the bike was built to handle the engine heat. I'm not so sure; everytime I take a lon| trip I have to add a quart of oil and the engine runs really hot. My dealer alsd told me that an oil cooler would probabîy leak. What's your opinion?

Duane Becker Freeport, 111.

We agree with your dealer that your bike was built to handle the amount oj heat it generates under normal conditions, but we'd disagree that an oil cooler has to leak. Motorcycle companies do considerable pre-production testing of their bikes, and if the testing indicates excessive oil temperatures; something is done to fix it. Usually that means more fins (often on the sump), bug sometimes an original equipment oil cooler is installed. The testing we’vg done on oil temperatures shows that tkg companies are usually right, but that there are adverse conditions that can raise oil temperatures to unacceptable levels. You’re most likely to need an oil cooler if you operate your motorcycle at high load or high rpm at low roa<y speeds; this adds heat to the engine when there isn’t much air rushing over it to cool it. We’re talking about unusual operating conditions; some examples would be pulling a heavy sidecar rig running up a long mountain road using the lower gears and high rpm, or towing a trailer up a long grade. If your riding doesn't fit this description, you probably don’t need an oil cooler.

In any case, an oil cooler would drop your engine temperature on trips, but probably wouldn’t drop your oil consumption. As far as the question oj leaks, we’ve seen many aftermarket ofl cooler installations that don’t leak, if it’s a real worry, some of the oil cooler companies are offering oil cooler kits that use aircraft style fittings in place of 'rubber hose and hose clamps.

BIGGER 650 KAWASAKI

I presently own a 1978 KZ650B, which Eve bored to 717cc using pistons purchased from Action Fours. I was .thinking of increasing the bore even more when I do some more work on the .engine. What 1 need is some information dn the biggest bore I can get with other sleeves and pistons. I know machining will be necessary for what I want.

W'alter A Martin Leghorn, Italy

The largest aftermarket pistons readily available for the KZ650 are 965mm, the same size you already have in your 717cc 650. However, Champion Kawasaki, 1980 Harbor Blvd., Costa Mesa, Calif. 92627, phone (714) 6422040, has built considerably larger versions for use in Superbike road races. Whitney Blakeslee, the man responsible for the KZ650 based Superbikes, tells us the most ridiculous version they built displaced 1021 cc. It used a 73mm bore (I I mm larger than the stock 62mm) and a 60mm stroke, and possessed the reliability that might be expected of such a grossly enlarged engine. More luck was had with an 855cc version built using oversize 71 mm KZ1000 pistons from the fKawasaki parts book and the stock stroke. Blakeslee pointed out that the piston crowns had to be extensively recontoured to match the KZ650 combustion chamber, the connecting rods’ small ends had to be enlarged to match the larger KZ1000 piston pins, and new cylinder liners had to be installed. The *855cc engine worked well in a race bike, but might take too much effort to duplicate for a street machine. Blakeslee suggested that a more reasonable street engine might be built using KZ750 or GPz750 pistons in the 650. One millimeter oversize pistons from the Kawasaki catalog would give a 76lcc displacement, but note that modifications to the piston crown may be required.

Any of these engines will take more care and effort to build than your original 3mm overbore, so be prepared to become friendly with your local machinist if you attempt further enlargements. A partial shortcut would be to substitute a KZ750 cylinder assembly for the 650 unit; the cylinder liners that come in the 750 block could be used without replacement for the 761 cc engine, and the additional aluminum around the liners would be beneficial for any big-bore engine.