Nederlandse versie

Introduction

This guide covers the classic Benellis from Pesaro, from the small singles of the fifties to the six-cylinder Sei of 1974. These are four very different engine families; the guide gives the main data and general maintenance per family.

Models in brief

• Singles: the Leoncino 125 (from 1951, mostly two-stroke, also a four-stroke version) and the four-stroke 125 Sport Special (1968–1973) with horizontal cylinder.
• Twins: the 650 Tornado (1971–1976), a British-style overhead-valve parallel twin; the Tornado S got electric start in 1972. Later came the small 125/250 2C two-stroke twin.
• Fours: the 500 Quattro (from 1973) and 350 RS (1978–1980), plus the small 254 (1977–1984, 231 cc).
• Six: the 750 Sei (1974–1977), the first production motorcycle with six cylinders, and its successor the 900 Sei (until 1989).

Background: after the takeover by Alejandro de Tomaso in 1971 the fours and sixes were visibly derived from the Honda CB500 Four: the same bore and stroke (56 × 50.6 mm), cam chain in the middle, plain bearings in the con-rods and a Hy-Vo primary chain. So many of the Honda’s maintenance principles apply here too.

The text is our own summary of public sources. Few settings can be found online for these models; a workshop manual per model is essential.

Technical data

The 500 Quattro and 750 Sei share the bore and stroke of 56 × 50.6 mm; the Tornado is a completely different, short-stroke overhead-valve engine.

Other features

• Tornado: horizontally split crankcase, crankshaft on four roller bearings, camshaft via gear; five-speed box, drum brakes (front 230 mm with four leading shoes). Dry c. 210–220 kg.
• 500 Quattro: wet sump, 12 V 15 Ah battery, electric start, five-speed box, dry c. 200 kg.
• 750 Sei: alternator and starter motor behind the cylinders (narrow crankcase), wet sump with c. 3 litres of oil, two 280 mm discs front, drum rear; dry c. 220 kg. Each carburettor feeds two cylinders via a shared manifold.
• One source gives 29 mm Dell'Ortos for the Sei; most sources say 24 mm.

Layout per family

First establish which family your engine belongs to: that determines which method applies.

• Tornado (overhead valve): camshaft low in the engine driven by gear, pushrods to the head, crankshaft on roller bearings. The starter motor (from the S) sits behind the cylinders and drives the crankshaft via a chain.
• 500 Quattro and 750 Sei (SOHC): camshaft in the head, driven by a chain between the middle cylinders; plain bearings in the con-rods and a Hy-Vo primary chain. On the Sei the alternator and starter motor are behind the cylinders, driven via gears.
• 350 RS and 254: smaller fours of a different design (the 254 by Lino Tonti), high-revving; the 254 revs to c. 11,800 rpm.

Because the 500 and 750 are so similar to the Honda CB500, Honda methods for cam chain, valves and synchronising help if you have no Benelli manual. Do always check the values.

Periodic maintenance

All four-stroke Benellis in this guide have a wet sump; the 750 Sei holds about 3 litres of oil, the 500 Quattro about 2.5 litres.

Oil: use a 20W-50 or 10W-40 engine oil without friction modifiers; the wet clutch shares the oil with the engine.

Known weak points

• 750 Sei: sources report problems with gearbox and crankshaft. Listen for noises from the engine and watch the gear change.
• 650 Tornado: vibration; many parts are rubber-mounted and there are weights in the bar ends. Check rubbers and mountings.
• Fours: leaking inlet rubbers and dirty carburettors after standing.

Valve clearance

For the 750 Sei a parts database gives a value; for the other models we found no reliable online value. The Benelli workshop manual for the 500 Quattro and 750 Sei has separate chapters on valve clearance and ignition and is available as a scan on ManualsLib.

The 500 Quattro and 750 Sei have rockers with adjusting screws, just like the Honda CB500. Owners find that the valve clearance on the Sei drifts faster than on a Honda and that the cam chain needs tensioning more often.

Tornado (overhead valve, adjusting screws on the rockers)

1. Engine cold, plugs out, valve covers off.
2. Turn the engine until the left cylinder’s piston is at TDC compression; both valves of that cylinder then have clearance.
3. Measure between rocker and valve stem and adjust with adjusting screw and lock nut.
4. Turn the crankshaft one revolution further and do the same for the right cylinder.

500 Quattro and 750 Sei (SOHC, adjusting screws)

1. Work on the same principle as on the Honda CB500/CB750: each cylinder at TDC compression, with both rockers free.
2. Find the TDC marks on the rotor or points plate and follow the cylinder order from the manual; with six cylinders three or more crankshaft positions are needed.
3. Use a clean feeler gauge and hold the adjusting screw while tightening the lock nut.

Tight clearance is more dangerous than generous: a valve that does not close burns. A slightly ticking valve train is no reason for concern.

Ignition

The Tornado, Quattro and Sei have battery ignition with contact points and coils; always set the points gap of each set first and only then the timing.

Spark plug 750 Sei: NGK D8EA (or DR8EIX), electrode gap 0.5 mm. The Sei had contact points from the factory; experts strongly recommend electronic ignition. Modern replacement systems use about 32–39° advance at high revs.

General method

1. On each set adjust the points gap with the points fully open (typical for this period is 0.3–0.4 mm; check the manual).
2. Put the first cylinder on the ignition mark and adjust the matching points set so the points just open there (test lamp).
3. Repeat for the other sets with their own marks.
4. Check with a strobe that the advance works.

Many owners replace the points with electronic ignition; that avoids repeatedly adjusting several sets.

Carburettors and synchronising

The Tornado has two Dell'Orto VHB 29s, the 500 Quattro four VHB 22s and the 750 Sei three VHB 24s that each feed two cylinders. Synchronising is necessary on all multis.

Synchronising (Tornado, Quattro, Sei)

1. First set the valves and ignition correctly and warm up the engine; put a fan in front of the engine.
2. Set the slides equal mechanically: each slide must start to move at the same moment.
3. Connect vacuum gauges (one per carburettor; on the Sei per manifold).
4. At c. 2500–3000 rpm adjust the cable adjusters or link screws until all gauges read the same.
5. Return to idle, set the idle screws equal and the speed to c. 1000–1100 rpm.
6. Set each carburettor’s mixture screw for the smoothest running and check the gauges again.

On the Sei: each carburettor feeds two cylinders via a split manifold. A leak in one branch of that manifold disturbs two cylinders at once; check the rubbers carefully.

Idle (single): air screw 1.5 turns open as a starting point, set warm with the stop screw and the air screw for the smoothest running.

Clutch, brakes and fault finding

All models have a cable-operated wet multi-plate clutch and a final chain; the Sei and later Quattros have front disc brakes, the earliest Quattros drums.

• Clutch: 2–3 mm free play at the lever.
• Tornado: a very short first gear with a big jump to second; some clutch slip when pulling away is normal.
• Brakes: Tornado drums (front four leading shoes, adjust both sides equally); Sei two 280 mm Brembo discs at the front and a 200 mm drum at the rear.
• Electrics: 12 V; on the Tornado S and the Sei the starter motor and alternator are behind the cylinders.

Sources

Consulted on 18 October 2026; all text in this guide is our own work. For the Sei the valve clearance and spark plug came from a parts database; for the other models the workshop manual remains necessary.

• Wikipedia: Benelli Sei
• Wikipedia: Benelli 650 Tornado
• Wikipedia: Benelli 254
• Wikipedia: list of Benelli models
• Motorcyclespecs: Benelli 750 Sei
• Motorcyclespecs: Benelli 500 Quattro
• Motorcyclespecs: Benelli 650 Tornado
• Motorcyclespecs: Benelli 650 Tornado S2
• Le Repaire des Motards: Benelli 750 Sei data sheet
• Motoplanete: Benelli 750 Sei 1973
• Bikesales: Benelli 750 Sei
• Old Bike Australasia: Benelli 900 Sei
• Classic Mechanics: Benelli 650
• Autoevolution: Benelli 350 RS
• Bikez: Benelli 500 Quattro 1973

Additional databases and manuals:

• Louis: Benelli 750 Sei
• ManualsLib: workshop manual Benelli 500 Quattro and 750 Sei (scan)
• Classic Mechanics: buyer’s guide Benelli 750/900
• John Nutting: Benelli 500 Quattro
• Officine08: Sachse ignition 750 Sei

← All workshop guides NL