Service

SERVICE

WLe 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 1brief as possible. Send them to: “Service, ” CYCLE WORLD, 1499 Monrovia Ave., Newport Beach, Calif. 92663.

HIGH COMPRESSION BLUES

I have a 1978 Honda CB750-F2 that I have extensively modified. About the only thing I haven’t done to my scoot is drop in a big bore kit. All the kits available now fun 10.5:1 compression and require premium fuel. Unfortunately, premium gas has virtually disappeared where I ride and octane-booster is both expensive and impractical for street riding. Are the speed shops aware of this problem and planning to do something about it? Could high-compression pistons be milled? Please advise. Like most riders, I’m always on the lookout for a way to get a speed buzz. Peter Del Nagro Snyder, N.Y.

There are ways of living with the high compression ratio big bore kits. If you’ve çdded a hotter cam to your bike, that can be one solution. Camshafts with more duration and overlap reduce peak cylinder pressure in exchange for flowing more gas 'through the engine at peak engine speeds.

Because of this, the high performance cams not only give you more peak power, but lessen the tendency for pre-ignition that the high compression ratios may bring. Most pistons do have enough metal in the crown for a good machinist to remove some for lower compression ratios. Two other good methods for reducing compression are the addition of thicker or additional cylinder base gaskets and opening up the combustion chambers of your head. When the chambers are cc’d it’s easy to have them ground out a little larger for lower compression. A combination of these techniques can probably result in a powerful motorcycle that can run on gasoline now available.

ARCO OIL

A Honda dealership mechanic told me that a new Honda motorcycle guarantee would be voided if the owner used graphite oil (specifically, we were talking about Arco Graphite). Is this true? If so, why? And, if true, would these reasons apply equally to automobile engines? David R. Voth Denver, Colo.

Not only does Honda recommend against using Arco Graphite in its motorcycles, but Arco recommends that motorcyclists not use that oil in motorcycles with wet clutches. This is not to say that Arco> Graphite might not work in your bike. The problem is one of testing. Arco hasn't tested that oil with all the different kinds of wet clutches available and doesn’t know if the graphite will leave deposits on the friction plates that could cause problems. The same goes for Honda. Honda has tested its engines with standard SE-rated engine oils, the kind Honda recommends you use, and knows these oils will work. But Honda hasn't been able to test all the available oils that are substantially different from normal engine oils. Arco says that on motorcycles with separate engine and transmission cases where there’s a dry clutch their oil will work fine. That’s why there isn’t any reason not to use Arco Graphite in a car.

THROTTLE TROUBLES

I own a 1979 Suzuki GS850 (shaft) with 16,000 reliable miles on it. Even after routine tuneups, from its first mile on the road, the engine has always refused to accept full throttle without a lag in acceleration. No missing or backfiring, just have to roll open the carburetors gently to increase speed. Eve read everything I get my hands on concerning rejetting but nothing relating to my specific problem. Black plugs indicate a rich mixture but I know the bike is jetted EPA lean. Our elevation here is 4292 ft. with about 10 to 20 percent humidity At this elevation should I rejet one step leaner or richer? Reginald Exum Clovis, N. Mex.

Here is a case where the EPA is unjustly blamed. Just because your motorcycle was made to comply with EPA emission rules, don’t assume that it’s jetted too lean. The emission standards in force for the 1979 models could be met with any four-stroke motorcycle jetted for the correct mixture. Now some bikes were jetted leaner than they had to be and production variations could cause a problem on some individual motorcycles, but if the plugs on your bike are black and sooty, rest assured the bike isn’t jetted too lean. The problem, and there could be more than one problem, begins with the altitude. At nearly a mile above sea level any engine runs much richer because of the lower atmospheric pressure. That’s why some motorcycle companies offer leaner-than-stock jetting for high altitude use. As for information on changing the jetting, refer to the article on carburetor jetting in the March, 1981 Cycle World. Another problem that could be affecting your bike is carburetor synchronization. The Suzuki Tours, because of the intake tract differences between the inside and outside cylinders aren’t synchronized properly by having all four balls of a vacuum synchronizer lined up. The out side cylinders should be set with the balls a half-ball distance higher than the finer cylinders.

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MILD MANNERED FRAME

I was thinking about buying a new Honda XR500 but in your March issue ypu mention that the frame material is mild steel not chrome-moly. Because I V/eigh 225 lb. and plan on riding the bike in the hills and on the dunes, do you think -the frame is strong enough for me? Dave Surmeyer * Albany, Ore.

^ Honda’s XR500R isn’t the only thing made out of mild steel. So is the Golden Çate Bridge. Because mild steel tubing doesn’t have the same strength as does ichrome-moly or other alloys, the frame tubes are beefier. That’s why the XR500R *ks+a 299 lb. motorcycle. It is certainly strong enough. What it isn’t, is lighweight. Piad Honda used chrome-moly steel tubing the bike might be 10 lb. lighter, but it would also be a couple of hundred dollars nyjre expensive. If you like the bike, don’t let the frame material bother you.

GETTING THE LEAD OUT

I ride a Triumph Bonneville and a Triumph Trident, both of which prefer premium gas. Regular gas causes detonation finder moderate load. My question is, since leaded premium fuel is becoming a lining of the past, can I use unleaded premium without harm or extra wear on my •engines? Gary Du Bois Sandy, Ore.

Unleaded premium gas is probably the best fuel you can find for your bike. There’s only one shortcoming of unleaded gas and that concern valve wear. “Tetraethyl lead used in leaded gas provides some lubricating qualities for the valves and valve seats. New cars and motorcycles designed to use unleaded gas come with hardened valves and valve seats ayid some companies have used an initial tankful of leaded gas in the vehicles to \adequately coat the valves and seats. Exactly how your Triumphs will hold up with unleaded gas is still a question. Unless you’re putting on really high miles on the bikes you’ll probably wear out other p¡prts of the engines before the valve seats. An occasional partial tank of leaded regudcyr gas will help keep the valves protected. If you want to run leaded regular gas all *the time you’ll need to reduce compression in the engines. The two easiest ways to do *?hat are either to use an additional base basket under the cylinders, or to enlarge che combustion chambers. A good machine shop can cc the combustion chambers, grinding away at some of the metal in the heads so that the combustion chambers hold more volume and are exactly the same size. And by using thicker base gaskets instead of thicker head gaskets you lessen the risk of blowing out the gasket, because the base gasket doesn’t contain such high pressure.

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CARB FIX

I am thinking about installing a Yoshimura 4-into-one pipe on my 1980 GS550. Also, if I install some slide throttle carbs from an earlier model GS750, would it help some of the terrible flat spots in the low rpm range? Will the carbs boost performance or choke the motor with too much fuel? Rob Heim Hutchinson, Kan.

Changing carburetors isn’t an instant fix for your problems. It’s going to take at least as much work to get the 26mm Mikunis from a 750 to work on your GS550 as it would take to get the 32mm Mikuni CV carbs on your bike working right. A good fix for the flat spots you describe would be to raise the needles in the carbs. Refer to the article on the Suzuki GS450 carb fix in the January, 1981 Cycle World. That describes a simple fix for lean running that should also work on your 550. If you install the 4-into-l pi you’ll probably need to rejet the carbs more than that, whatever carbs you use. The carbs that came on your bike are large enough to flow as much air as the 550 needs. If you figure out how much air the 550 can suck in by multiplying the displacement times half the maximum rpm you will find that the bike won’t need more than 97 cfm even if you have 100 percent volumetric efficiency. Because the carbs from the 750 are jetted for the larger displacement engine they’d need complete re jetting if you use them. Better stick with the stock carbs and get the jetting right in those.