Service
SERVICE
Cool Yamaha XT
I have a 1981 Yamaha XT250 that I ride in the winter. But when I start it when the outside temperature is around 10-15 degrees F., it doesn't idle very smoothly for the first 10 minute of riding, especially at altitudes over 8000 feet. A friend suspects the cold temperature of the intake manifold won't allow proper gasoline-droplet atomization. Is my friend right? What can I do to make my mountain bike more ridable?
John Best Bozeman, Montana
Your friend is right. The main prob leni is I/ia! gasoline is not properly vaporizing for the first /0 minutes while your motorcycle is warming up. Norma//v. the enrichening circuit (the "choke') compensates for this b suppli'ing excess gasoline, on the prin ciple that m,iore liquid gasoline will create more gasoline vapor. and hope-
fully enough to make for a burnable mixture. But in the extreme cold weather in which you are riding, this may not be enough. There are three things that would improve this situation. First. the enrichening circuit has its own jet that regulates how much added fuel is sup plied. and f the orifice in this jet
drilled slig/itli' larger. the fuel added during warm-up will be increased. You `II need a set of number drills, probabl~' 60 through 80, for this. Find the largest bit that wilIfli through the jet. then drill it with the next larger one.
Second, the cold, dense air you're riding in will actually lean-out the> entire range oJcarburetion, even when the engine is warm. The engine inai' respond well to a slight richening by raising the needle, adjusting the air screw to a richer setting. and perhaps even installing a one-size-larger ,iiain jet.
Third, turning up the idle-speed screw during the warm-up period should help.
Power versus displacement
Can you explain to me why a bigdisplacement motorcycle like the Suzuki GV 1400 only produces 99 bhp while Yamaha's new FZ750 produces 100 bhp? Also, this seems to hold true on dirt bikes, as most motocross 250s produce around 40 bhp, the same as most 500cc class enduros.
Martin D. Bledsoe Elk City, Oklahoma
Power iSn `I related strictly to disp/ace~nent, as you `ye noticed. In stead, it `s a function of engine speed, engine efficiency. type of engine cycle (twoor four-stroke), and displacement.
The amount of power an engine ,,zakes is directly related to the amount of heat generated by burning fuel. But burning fuel requires air, so power capabilityfinally resolves around air: How much of it can an engine pump through in a given time? The more air, the more power.
There are two basic ways to pump more air through an engine. First, an engine can be given a larger displace nent so ii moves more air with each cycle. Or the displacement can be left the same, and the engine can be spun faster. The same amount of air (more or less) shuttles through with each engine cycle, but now there are more cycles for any time period. Both met/i ods move more air, and result in more power.
Those principles can be related to the FZ and the G V. Assuming that the FZ moves 100 percent of its displace imient every two engine revolutions, at its 10,500-rpm power peak it will move 3940 liters of air a minute (0.750-liter displacement times 10,500 rpm divided by 2for four-stroke cycle,). Using the same assumption, the G VI 400 pumps 4420 liters at its 6500-rp#iz power peak. That analysis indicates the G V should make about 10 percent more power than the FZ,> much closer to the actual case than the 80 percent larger displacement of the G V might indicate, but still not quite correct. That 10 percent discrepancy exists because the FZ is either a more efficient air pump than the G V or because it's more efficient in turning heat into work.
As for differences between 250 inotocrossers and 500 enduros, two factors are at work here. Many of the 500 enduros are four-st rokes, and fourstrokes have a basic disadvantage when compared to two~strokes. They take Iwo engine revolutions to move roughly as much air as a two-stroke does in one. And comparing 250cc two-stroke inotocrossers with 500cc two-stroke enduros isn `tfair because of the very different state of tune of the two engine types. The inotocrossers rev higher and are designed to be very efficient over a narrow range of engine speeds. Because of that, they make relatively more power at those speeds, but may make less than an enduro tuned engine at lower speeds.
Hot-rod Suzuki
I am looking for technical advice concerning high-performance modifications for a 1981 GS75OEX Suzuki. My main interest, besides making the bike handle better, is to improve the power-to-weight ratio. I would love to purchase a new FZ750, which would take care of everything except the lump sum ($). So the alternative is to work with what I have, building up my GS750 at a rate I can afford. I'm consid ering aftermarket 4-into-i pipes, K&N filters, Accel wires and ig nition system or Dyna coils. An oil cooler is also a consideration.
I would appreciate your advice on choosing the appropriate hardware, and especially any suggestions as to improving my motorcycle's han dling. At this point I think it wise for my wallet to limit modifications to those involving the least difficulty from a mechanical and technical point of view and without getting into extensive internal engine machining. These modifications aren't for racing, just for keeping up with the Joneses.
Scott W. Petrie Philadelphia, Pennsylvania
A 4-intoI pipe would be a good start for your modifications; it may not boost power all that much (if at> all), but it `ii trill? pounds Jioin your Suzuki. We'd recommend against tile K&N filters unless they're available wit/i a rejetting kit, while individual air filters can significant/v improve power. you only achieve that benefit wit/i correct carburetion, and that will definitely be considerably different than stock. Ignition changes and an oil cooler probably fall into the category of modifications that may offer some benefits, but with small direct effect on peak performance.
The best way to markedly improve i'our motorcycle's handling would be to use any of tile excellent aftermarket performance tires on the market, and to bolt on a set of top-qua/it v rear shocks. Koni, Works Performance and Fox all have excellent shocks available for tile GS750.
More on Bimota spares
In the August. /985. Service col u,n,,. a reader complained that he was having problems obtaining a replace menifairing and seat for his fivei'ea r old Bimota KBJ from Bimota distributor Comopolitan Motors. We recommended that he tr~ a d~fferent Bimota distributor.
A spokesman for cosmopolitan has since informed us that he had telexed Bimota about the parts, but was in formed by the factory that they were no longer available, although almost all non-fiberglass parts for the KBI were still stocked.
AdditionaII~ we have recently had the opportunity to discuss the spares situation directlr wit/i Mr. Morn, the owner/manager of Bimota. He recoin iiiends that if an y Bimota owner has any difficulir obtaining ani' Bimota parts, he should contact the factory directl~ Bimota `S address is Via Giaccaglia. 38, 47037 Rim/ni, Italy. The compan r'S telex number is 551031 BIMOTA. And if you speak Italian, you can call t/iefactori' at 3954 1-83493.
