W ith the prices of new bikes and insur-ance rocketing, the step up from a250 can be an expensive one, yet you

can probably get just as much enjoyment froma super-tuned 250 as you can from most400s. Of course, the handling must bereasonable, but the real essentials for tuningare an engine with a robust bottom end and aclose-ratio gearbox.

One that neatly fulfills these requirementsis the Suzuki GT250, preferably in the latest Aform, but also as the previous GT250 M. Theengine is well-engineered, and is protected by

iSpeedTuneSeriesfthe cylinder immediately above the inlet port.Of course, the idea with the Schnurle loopscavenge system used on modern two-strokesis for the inlet gases to be driven up at theback of the cylinder and then downwards atthe front, so that the exhaust gases are pushedout without being mixed with inlet gases. Intheory, therefore, the use of the twisted portsshould improve combustion quite a lot com-pared with the previous arrangement.

To replace the 26 mm carburettors of the

some production racing, or want to get somuch power that the engine won't pull below5,000.

In standard form, the GT250 is claimed toproduce 32 bhp at 7,500 rpm, 2 bhp morethan the M, but when we road tested one, a 'figure of 26 bhp at about 8,400 rpm wasmeasured at the rear wheel, with 17.5 Ib ft

' torque at 7,500 rpm. That's pretty good for a250, and in practice it means that there ispower from about 4,500 to 8,500 rpm, with

HOW TO TUNE THE SUZUKI 250A AND M MODELSthe CCI lubrication system, while the power ispassed through a six-speed gearbox. With amaximum speed of just over 90 mph, and theability to cover the standing quarter from restin under 16 seconds, the current GT250 hasquite a brisk performance anyway, and pro-duction racing results indicate that there's notmuch wrong with the handling.

Before going into detail on how to improvethe performance it will help to differentiatebetween the M and A models, because theinternal differences are quite marked. Forget-ting the cosmetic differences, like the Ram Aircylinder shroud on the M, the main differencesare in the ports, the carburettors and inletstubs, the exhaust silencers, the crankshaftand bearings, and the gears. Whereas the M isbased on a three-bearing crankshaft, there arefour bearings on the A, while the lubricationsystem has been revised.

Interestingly enough, one of the mods Bar-ton Motors have been making'to the 250 forracing is to fit two centre bearings instead ofthe standard one, so the A is obviously a bet-ter bet for high performance and durability.However, the main differences between thetwo engines are in the ports with the M hav-ing two simple transfer ports and the A havingfour ports and revised timing to give a bitmore top end power. The layout of the extratwo ports on the A is unusual, in that theyaren't just alongside the other ports, but twistaround so that they come out at the back of

Shepherd racer with monocoque chassis, cantileversuspension and water-cooled engine taken out to350cc. It uses Yamaha YR5 pistons.

M, the A has a pair of Mikuni VM28SSs, andthese are spigot mounted to rubber adaptors.Initially, the compression ratio of the A modelwas 7.3:1, but following some problems withpiston burning, this has been reduced to 6.8:1by the expedient of fitting a thicker head gas-ket. The plugs have also been changed toNGK B9ES, and the plugs, points and timingare all pretty important if local overheatingproblems are to be avoided. The ignition tim-ing is 2.05 mm before TDC, compared with2.7 mm on the M. The other main change asfar as the engine is concerned is the exhaustand here the need to meet noise legislationhas resulted in a highly-restrictive system onthe A model that limits power.

At the same time, some efforts have beenmade to make the lower ratios closer, so thenew ratios with the old ones in brackets are:2.333, 1.32(1.50), 1.05(1.105), 0.905, 0.782and 0.708:1. Although this increases the stepbetween first and second, it does mean thatfor most use the rider has better matchedgears, yet he can still get away from lightsquickly. In fact, the gap between first andsecond is large, since a shift at 8,500 rpm infirst is made at 28 mph, and the revs dropright down to 4,800 when dropping intosecond. Changes between the other gears atthe same Tevs keep the speed over 6,500rpm. To overcome this problem, Eddie Crooks,the Barrow in Furness Suzuki specialist, sup-plies a special driven gear for first with 25instead of 28 teeth, to raise the ratio to2.083:1, which means that 9,000 rpm in firsttakes you to 33 mph, with the shift to seconddropping the revs to 5,800. The gear costs£24 and is worthwhile if you are going to do

the power building up sharply at about 6,000.So how do you go about coaxing some

more power from the M or A models? We'dbetter start with a few pitfalls, of which thefirst is that almost any change is likely to in-validate the warranty. It's best to wait until thesix months or 6,000 miles is up before youstart changing things. The most importantthing about tuning a two-stroke is to remem-ber that once you start looking for real power.

On the 250M the transfer ports should be openedout to match the gasket. The sharp inner edge

should be radiussed off smoothly.

The inlet port on the M model should be ground sothat it matches the carb flange exactly.

you are dealing with a complete system — ifyou alter the port timing, you'll need to alterall the ports, and the exhaust as well — not tomention the ignition timing. In other words,you need a dynamometer, and it is for thisreason that some of the expert tuners won'tpromise you a vast increase in power on aroad bike. -

GAS FLOW

At the same time, there is quite a lot ofscope for tuning, so long as attempts aY reallywild timing are avoided. To start ^with,although the 250 engine is nicely made, it ismass-produced, and as a result, the ports inthe Hners don't usually match those in the alu-minium barrel precisely, and the shapes areformed pretty roughly in some areas. So, thegeneral aim is to blend the ports in, and thenmake sure that the gas is directed where it ismeant to go. Terry Shepherd of Scarisbrick,Lanes, is one of the experts on tuning Suzukis,and he1 says that it is most important to makesure the gas is directed from the transfer portsto the back of the cylinder. He also points outthat a difference of 0.01 in between theheight of the transfer port on one side of thecylinder and that on the other can rob themotor of a lot of power. This is because ittends to spiral up the back of the cylinder,rather than push burnt gases out through theexhaust. He aims to give a good spread ofpower from about 5,000 to 9,000 rpm, andcan supply exchange barrels suitably modifiedfor £36. With his barrels alone, he reckonsthat a GT250 M can do 100 mph, withacceleration to match.

On the 250A the use of four transfer ports insteadof two gives a significant increase in power.

The side of the transfer ports on the A model can becut back and radiussed off.

If you are tackling the ports of a GT250 Myourself, start by placing the barrel-to-crankcase gasket over the bottom end ofthe barrel, and you will see that the transferport is quite a bit narrower than the gasket.So the first thing to do is to open out thetransfer ports to match the gasket, and tomatch the crankcase mouth. In this case youare matching the port and increasing the areawhich should then be blended into the portitself. The bottom edge of the liner that actsas the inner wall of the port is chamferedroughly in manufacture, and you should radiusthat off smoothly. It is worth rememberingwhen you do these things what the gas isdoing at each stage. When it enters the port itis being blown horizontally, and so must beturned round to go upwards as smoothly aspossible — hence the radiussed edge, and nota sharply inclined face. Best results areobtained when the cross-sectional area of thetransfer ports reduces slightly from the bot-tom to the top, so that the gas is speeded upwhere it enters the cylinder.

Having pumped the gas smoothly fromunder the piston into the mouth of the transferport the port itself should be polished and anyrough areas smoothed down so that the gasmaintains its speed as it approaches the topturn of the port. This is the critical area of theport which determines whether the bike willturn out to be a flyer or a lame duck. The topradius has to be smoothed round so that itblends into the port window without .slowingdown the gas speed. Any steps where the portmeets the window must be smoothed out. Indoing this you may (very slightly) alter theport timing. If this is the case the other portmust be altered by the same amount as it isbetter to have two evenly matched ports thatare both timed slightly wrong than to havetwo ports with the standard timing but flowcharacteristics that do not match.

To get it right it would be well worthwhilemaking up a small template after moding theworst port and then radiusing the other tomatch. If you want to go to extremes youcould blank off the port windows and measuretheir volume with a burette as a check on theaccuracy of your port matching.

PORT SHAPE

When it comes to the A motor the samebasic approach is adopted, that of cleaning upand matching the ports for identical shapeand timing. The additional transfer ports oftenneed quite a lot of cleaning up to obtain asmooth entry into the bore. If in this process,the port heights are altered slightly then themain transfers must be matched for timingbecause it is very important that all the portsopen at the same time.

With the inlet and exhaust ports, it is aquestion of blending the ports to suit the car-burettor or pipe. With the M motor, there isusually a slight step at the top or bottom ofthe port near the flange mounting for the car-burettor, and this should be ground away sothat the, gas can enter the port without anyrestriction. Then, the walls of the port can bepolished, but there is no point in enlarging theport, since it is the carburettor that deter-mines the amount of gas that can flow intothe cylinder. Although the surface finish of theexhaust ports is pretty good, if you run yourfinger along the bottom of the port, you willcome to a distinct sharp corner where theinclined part of the port meets the horizontaloutlet from the cylinder. Now this ledge tendsto throw the gases against the roof of the port,

which doesn't help at all, so the cornershould be ground off into a smooth curve.

But why don't we recommend increasingthe duration of the port openings? Althoughthe inlet port can to some, extent be con-sidered independently, the,transfer ports andthe exhaust ports, and the exhaust system areall closely related, and a change to onerequires a change to the other. In fact, most ofthe experts reckon that the standard Suzukisilencers are well matched to the timing of thestandard engine, and so any alterations in tim-ing will just be wasted unless some specialexhausts are used. If you fit racing expansionboxes, assuming the law doesn't hound youevery time you appear .on the road, you willonly get power at the top end, and the bikewill be pretty unpleasant to ride. If you shouldbe tempted to increase the opening period,remember that raising the exhaust and trans-fer ports by 5 mm means increasing theperiod by about 20 degrees, and that is quitea lot, bearing in mind that the timing of theGT250 is fairly sporting anyway.

Although the M motor silencers work quitewell — so long as they are kept very clean —those on the A do restrict the output. JohnWren, the mechanic at Eddie Crooks' whotuned their production racer reckons that thebest way out of this one is to remove thebaffle, and to cut it off just forward of the rearsupport, and then put it back again. Thisreleases a bit more power without increasingthe noise level too much. Of course, you canalso buy a pair of 'street legal' exhausts, andthese can help, so long as they are designedfor go and not show.

With these alterations, Eddie Crooks' pro-duction racer was pulling about 9,000 rpm intop in the Island, and that is about 100 mph.

If you really want some power, and youwant something that looks different, whatabout the Shepherd water-cooled 350? Thestandard bottom end is used for roadmachines, although a Barton four-bearingarrangement is used on the racing versions.On to this is grafted Terry's own design water-cooled barrels and heads, either 250 cc or350 cc, the larger engine having a bore of 64mm, and Yamaha YR5 pistons. The transferports are set slightly tangentially to the cylind-ers to force the gases to the back of thecylinder, and small, deep combustion cham-bers are used. Although the ports are intendedfor racing, the barrels are machined slightlydifferently for road use to give a wider powerband. To suit the ports in the barrels, themouths in the crankcase must be opened up alittle.

For road use, Shepherd recommends thestandard, silencers and carburettors, and sug-gests that the bike is geared up by two teethon one end and three teeth the other, sayfrom 43/14 to 40/16, which would give aspeed of around 121 mph at 9,000 rpm —and Shepherd puts the maximum speed atquite a bit above that! Of course, this is apretty expensive exercise, since the conver-sion costs about £350. Fortunately, even afterall these modifications, little needs to be doneto the chassis. For production racing, the frontend is lowered on to the forks by about -J- in,Girling Gas Shocks are usually fitted andthat's that. On a road bike, the use of GirlingGas Shocks is also a good idea, especiallynow that there is a range of two-rate springsavailable. The beauty of these springs is thatthey give a nice soft ride over little bumps,and then soak up the big bumps without bot-toming. They are also better suited to a bikethat is used for one-up and two-up riding.