Frequently Asked Questions about the Suzuki GS Twins

Thanks to Bruce Brown of the GS Twins list for putting together this first version of the GS Twin FAQ. It will be expanded as we get more feedback from people on the list.

Suzuki GS Twins FAQ v1 (03/17/98) compiled by Bruce Brown (91 GS500)

Preface:

I enjoy my GS500 and the GS Twins list so with Michael Moore's consent this is an attempt at a FAQ section for the GS Twins. Most of the information for this section is coming directly from the posts to the GS Twins list. This is just a version of the FAQ and with everyone's input it will get better and more complete over time.


Introduction:

I believe the GS line of twins began life in 1976 with the GS400, from which descended the GS425, GS450, and the latest model the GS500. These twins were essentially the middle half of the larger GS 4 cylinder line of models. They are all air cooled with two cams controlling a total of 2 valves per cylinder (some countries did receive engines with 4 valves per cylinder).

The newest GS model from Suzuki was introduced in 1989 and is currently on the market with minor changes from its introduction.

Caption from the Motorcycle Online database) A vertical-twin engine with a broad, mellow powerband keeps the GS500E slim, light and easy to handle. Wide, upright handlebars, a flat seat and lots of legroom make it comfortable. More experienced pilots will appreciate the engine's six-speed gearbox and high-rpm redline - not to mention the rigid box-section frame, single-shock rear suspension and hard-stopping front disc brake. All of which make this Suzuki a backroad fun machine.

Magazine articles:


Specifications

ENGINE

Note: most of the specifications for the 400-450 are pretty much the same. I've left cells blank where I'm unsure of exact numbers, but if you see a blank cell for a model, and the data to either side are the same, you can assume the info is probably the same for the blank cell. Note the change of stroke between the GS425 and 450, when they went from a roller bearing big end to plain bearing rods.

GS250 GS400 GS400E GS425 GS450 GS500
Type: Air-cooled, transverse, in-line, 4-stroke twin SAME SAME SAME SAME SAME
Valve arrangement: DOHC, 4 valves: rocker arm with adjusting screws DOHC, 2 valves: adjusting shims over buckets DOHC, 4 valves: rocker arm DOHC, 2 valves: adjusting shims over buckets SAME SAME
Displacement:
399cc 399cc 423cc 448cc 487cc
Bore x stroke:
65 x 60mm 65 x 60mm 67 x 60mm 71.0 x 56.6mm 74.0 x 56.6mm
Compression ratio:
9:1 10:1
9.0:1 9.0:1
Carburetion:

2,BS34SS Mikuni constant-velocity SAME

2,33mm Mikuni constant-velocity
Ignition: Battery-powered, electronically triggered SAME SAME SAME SAME SAME
Exhaust system: 2 into 2 SAME SAME SAME SAME 2 into 1
Air filtration:



Polyurethane foam element Paper element, disposable
Oil filtration: Paper element, disposable SAME SAME SAME SAME SAME
Bhp @ rpm:
30.61@8500 (Webco Dyno) 41.5 @ 9000 (claimed)

39.39 @ 9500
Torque @ rpm:
19.73@8000 (Webco Dyno) 3.61 kg-m at 7500 rpm

24.59 @ 7500
Transmission type: 6-speed, constant mesh, wet clutch SAME SAME SAME SAME SAME
Primary drive: helical gears



76/28, 2.71:1 SAME
Final drive:

# 530 chain, 2.812:1
DID50UB - 104 L, 45/16 sprockets, 2.81:1 #520 chain; 39/16 sprockets, 2.44:1
First gear:

32/13; 2.461:1
32/13; 2.461:1 SAME
Second gear:

32/18; 1.778:1
32/18; 1.778:1 SAME
Third gear:

29/21; 1.380:1
29/21; 1.380:1 SAME
Fourth gear:

27/24; 1.125:1
27/24; 1.125:1 SAME
Fifth gear:

25/26; 0.961:1
25/26; 0.961:1 SAME
Sixth gear:

23/27; 0.851:1
23/27; 0.851:1 SAME



GS400

From 'Cycle' magazine's 400cc four stroke comparison - Honda CB360T, Kawasaki KZ400D, Suzuki GS400, Yamaha XS400D(05/77):

They say the GS400 has a bore/stroke of 65x60mm, which contradicts some other numbers I've gotten. I'll resolve that when I can get numbers from a factory manual.

The Suzuki was picked as the best handler, second fastest, and overall choice.


GS500: (1989-present)

TRANSMISSION

Gear ratios (overall):

CHASSIS

Type:  		Double-downtube, full-cradle, steel perimeter frame, box section steel swing arm
Front suspension:  Center-axle KYB fork with 37mm tubes, producing 
4.7 in.(120mm) of wheel travel
Rear suspension:  (1) KYB shock absorber, adjustable for spring 	preload, producing 4.5 in.(115mm) of wheel travel
Wheelbase:  	55.5 in.(1410mm)
Rake/trail:  	25.5 degrees/3.7 in.(95mm)
Front brake:  	Single-action, dual-piston caliper, 310mm disc
Rear brake:  	Single-action, 250mm disc
Front wheel:  	Cast, 3.00 x 17
Rear wheel:  	Cast, 3.50 x 17
Front tire:  	110/70-17 Bridgestone Exedra G547
Rear tire:  	130/70-17 Bridgestone Exedra G550
Seat height:  	31.1 in.(790mm)
Ground clearance:  6.1 in.(155mm)
Fuel capacity (main/reserve):  3.6/0.9 gal.
Curb weight (full/empty tank):  416.5 lbs.(188.9kg)/388 lbs.(176kg)

ELECTRICAL

Power source: AC generator, 225 watts

Charge control: Solid-state voltage regulator

Headlight beams (high/low): 60/55 watts

Tail/stoplights:(2) 5/21 watts

Battery: 12V 12AH

Instruments

Includes: Speedometer, odometer, tripmeter, tachometer with 11,000-rpm redline, indicator lights for oil pressure, high beam, turn signals, neutral

Speedometer error: 30 mph indicated actual 28.2 mph, 60 mph indicated actual 56.9 mph

PERFORMANCE

Fuel consumption: 49 to 59 mpg, 53.9 mpg average

Average 200-yd. Acceleration from 50 mph, terminal speed: 6th gear, 69.2 mph; 5th gear, 72.3 mph

Corrected best quarter-mile acceleration: 13.60 sec. @ 93.17 mph

Measured top speed: 108 mph

Maximum (theoretical) speed in gears @ 11,000 rpm:


ENGINE, CLUTCH, AND TRANSMISSION

GS500 - For an oil and filter change the shop manual suggests a SAE 10W/40 (min of SE or SF grade) oil, capacity of 3.1 quarts (2.9 liters). The listers suggest a variety of weights and brands of oil, read the archives for more details.

If you plan on a motor rebuild looking for more power Michael Moore has had GS450 motor built for one of his road race projects that makes at least mid 60's on the dyno. Megacycle has cams/springs for the GS motor and Michael can recommend his friend Craig to do the head porting, big valves, twin plugs, etc. as well as lightened crankshaft rebalanced to eliminate the balance shaft. It is suggested that those who rebuild their motors without a balance shaft have the crank rebalanced so the engine vibrations will be tolerable.

There have been a few racers who have experimented with 4-valve heads using the inner two cylinders from GSXR motors. Again read the archives for more details.

FUEL AND EXHAUST SYSTEMS

General items to inspect and/or replace are fuel lines, hoses, carb gaskets, and also check the intake boot between the carbs and cylinder head.

One reoccurring item of annoyance on the GS500 is the fuel petcock. Many emails talk about a fuel starvation problem which was remedied by placing the fuel petcock in the reserve or prime position. Some folks have replaced the stock petcock with a dry brake quick disconnect. However both of these remedies leave you without a reserve.

Replacing the stock air filter with a Uni-filter or K&N filter is popular among the GS Twins listers. The K&N pod filters are considered to restrictive by the engine builders of this group for good performance. Also the K&N filter that fits in the stock air box should be lightly oiled not saturated with oil.

Vance & Hines and Yoshimura both make aftermarket exhaust systems for the GS500. One racer installed the Yoshimura exhaust and reported the pipe required some grinding on the oil pan fins and passenger peg bracket in order to fit. The pipe also has to be removed to replace the oil filter.

Remember the bike will need to be rejetted when a non-stock filter and/or exhaust system is installed. In the archives there are many posts describing different carb set-ups.

FRAME AND SUSPENSION

The front end is considered too soft even for riding on the street. Progressive Suspension and Race Tech both make aftermarket fork springs to fit the GS500. Many of the listers have installed straight rate springs (like the type from Race Tech) and report improved handling. Race Tech also sells a gadget called an Emulator which when installed in the forks allows the rider to tune rebound and compression damping independently. The shop manual calls for 10-weight fork oil although many of the posts mention using 15-20 weight oil.

Those who have replaced the rear shock of the GS500 usually find a used stock GSXR shock because they can be had for $50-$150. Again go to the archives and read the details about making this kind of modification.

BRAKES

Although the stock front brake on the GS500 is good additional feel and performance can be achieved by installing after market brake pads with a steel braided brake line. EBC green sport formula pads are a good choice and Russell makes a brake line kit. Also change the brake fluid at least every two years since the fluid will absorb water as time passes.

Racers report that after a season on the track the front rotor will warp. EBC makes a replacement rotor for about $190.

Read the archives for owners who have installed front ends from larger sport bikes and other modifications.

TIRES

For the older GS models the Avon Roadrunners or Super Venoms are recommended.

For the GS500 Metzeler, Bridgestone, and Dunlop make good sport rubber that will fit the stock rims. The racers who use Metzeler's recommend the ME-33 Comp-K 110/80-17 for the front and the ME-1 Comp-K 130/80-17 for rear. The stock rear wheel can accept the 140/80-17 size and some listers recommend this size. The Dunlop in the 591's series or the Bridgestone BT-39 Battlleax are also popular replacement tires.

BODYWORK, HANDLEBARS & CLIP-ONS

The 89 GS500 came stock with clip-ons that attached to top triple clamp. All models since then have a wide upright tubular handle bar that also attaches to the top triple clamp.

The cheapest and easiest way to lower the riding position is to replace the upright tubular bar with tubular clubman bar, cost about $20.

If you want to install a set of clip-ons you will need to modify the bike. One option is to raise the fork tops up in the triple clamps just enough so the clip-ons can mount to the portion of the fork tube that is now above the top triple clamp. Another option is to mount the clip-ons below the top triple clamp however this requires modification to the headlight mounts that also attach to the fork tubes. Some listers have installed clip-ons from the Honda 600 Hurricane (87-90) because they are available used and are the same size as the GS's fork tubes (37mm). Aftermarket clip-ons have also been installed on the GS and cost at least $100 a pair.

Full frame mounted fairing are available from Spec II and Airtech. The complete full front fairing with frame mounting kit will run about $400-500 unpainted if you install it yourself.


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