Fitting Instructions for the RB25 Cylinder Kit

Now that you are the owner of the most advanced engine kit ever designed for the small Lambretta, please read and follow the instructions listed below, so that the future will hold thousands of trouble free scootering miles. Due to the nature of the RB25 and it’s intended use, NO guarantee can be given against wrong assembly, incorrect petrol oil mixture (4% No Less), incorrect timing, general abuse in driving causing excessive wear and tear, or any other part in the engine that fails causing damage to the kit. The only warranty offered on this kit is against manufacturing fault, in which case the parts must be returned to us before assembly. By fitting the kit you are accepting our terms for its use and warranty, if you do not accept these terms please send the kit back immediately for a refund.

The Kit Consists of: - RB25 Cylinder, Piston Complete, Dual Reed Valve Assembly, Inlet Manifold, Inlet, Base, Head and Exhaust Gaskets, Six Inlet Manifold Bolts and Two Exhaust Studs.

Recommended Minimum Engine Spec.
Based on a brand new GP 200 Engine the following is the minimum engine specification required to reliably run the RB25. Cutting corners will result in greater expense and possibly life threatening failure. This is a high performance kit not for the foolish or faint hearted.
RB25 Kit, RB25 Head, RB AF 60mm Race Crank, Race grade clutch assembly 6 Plate or similar, Aluminium Top Chain Guide, possibly a Strong Selector Spring. This kit should only be run with an appropriate Expansion System and Suitable Open Carb, we recommend our RB Rally NK Expansion or our RB Road Race NK Expansion and the 34mm VHSB.

The engine should be thoroughly checked to ensure it has no other problems with repairs and replacement components fitted as required. The crankshaft is especially important. It must be a 60mm stroke crank with a top quality big end such as the IKO18 roller used in the AF RB 60mm Crank. You will also require the crankcase neck to be reamed out to 78.2mm to a depth of 11.5mm to accommodate the larger cylinder spigot. We charge £20 for this service.

Assembly of kit
  1. 1. Remove the engine bar and lower the engine to ensure ease of assembly and for checking frame clearance.
  2. Clean the crankcase base gasket surface completely. Take the new cylinder base gasket, lightly grease and fit to neck of crankcase.
  3. Fit piston complete, make sure the arrow on the crown of the piston points towards the exhaust port and that the circlips have positively located.
  4. Fit RB cylinder on to the engine, take care not to damage the piston or rings. Check to make sure the cylinder is sitting down flat on the base gasket. If there is any resistance find the cause, do not apply excessive force. Hold the cylinder in place and gently turn the flywheel and feel for any resistance or problem as you watch the piston travel through its stroke.
  5. Fit the head gasket and secure the RB25 cylinder head to a torque setting of 18 ft.lbs. If you are fitting a skimmed head please ensure that it has been opened out to the correct diameter. Check your squish clearance (smallest distance between piston crown and cylinder head at TDC) is not less than 1.5mm and not greater than 2mm. Alternate gasket thicknesses are available to help you set this.
  6. Check the bolts holding reed petal stops are tight. Pinch the dual reeds together, and fit the reed valve assembly in to the cylinder. Lightly grease the inlet gasket and loosely fit the inlet manifold to the cylinder. Fit the six inlet bolts with loctite, clamping the inlet manifold to the cylinder. Fit exhaust studs with loctite.
  7. Loosely offer up the cowling, mark around the cowling the inlet manifold position and the exhaust port position and cut the cowling to suit.
  8. Lift the engine back up and locate the engine bar. Check you have sufficient clearance between the inlet manifold and frame by removing and refitting the rear shock (to simulate full suspension travel). Make any modifications as required to the frame, remove battery tray if fitted, fit series III rear shock (300mm) if series II is fitted.
  9. Fit the exhaust stub and cowling. Proceed as normal for the complete engine assembly. You will require a piece of petrol pipe approx. 18’’ in length. Thread the pipe under the frame, across the head cowling to the carburettor. You may use cable ties to ensure the pipe stays in location and will not be trapped.

Plug and Oil mixture
Run the machine using a Champion N2 plug with 30 thou plug gap, or equivalent from another manufacturer, you may need the hotter N3 in winter. Use a petrol to oil mixture of 25 : 1 (4%, or two AF mixing jugs per 5 litres). Never drop below 4% mixture regardless of brand of oil or what “your mate said”!
Always use 99RON Unleaded if the head has been skimmed to a higher compression. Regular 95 RON unleaded should be okay with the standard RB25 Head.

Starting point for jetting Dellorto VHSB 34LD
Slide – standard
Needle K21 or K24 2nd clip from top
Needle jet (atomiser) DP267
Main Jet 198 Pilot Jet 45 to 55.
Float Valve 200 minimum Choke Jet 50
Mixture screw 2.5 to 3.5 turns out

N.B. Do not fit an inline fuel filter. This jetting is not suitable for running with an air filter.

Expansion System and Gearbox
The expansion system you use will affect the final drive ratio you will need, and the desired final drive ratio will dictate which front sprocket, and crownwheel you will need to make your gearbox work well. Ask the supplier of your expansion system what they recommend letting them know your weight, if you are using the bike two up and the area you will be riding the bike in, i.e. Hilly, Flat, Town, etc.

RB Rally NK Expansion with a 12 stone rider for general all round riding we recommend a final drive of 4.6 (GP200 gearbox = 20T–46T with stretched 82 chain).
RB Road Race NK Expansion with a 12 stone rider for fast ‘A’ Road riding we recommend a final drive of 4.6 (SX200 gearbox = 16T–47T with 81 chain).

Chassis, Forks, Brakes, Tyres and Suspension
A correctly set up RB25 is substantially more powerful and significantly faster than the standard Lambretta. It is prudent to ensure your Lambretta’s handling is adequate. A powerful front brake, good quality tyres, and a well-maintained machine are essential to your safe enjoyment.

Important Concerns
Clean out your petrol tank and petseal it before use, especially if you are fitting a new petrol tank or a long range tank. If it contains any abrasive it will rapidly destroy your kit. Blasting grit is the worst offender but polishing compound will have the same result. Also check your fuel flow and filter in your carb regularly for contamination. Fuel starvation will cause a number of expensive problems. Check your head cowl is the late GP type with the large air outlet, and that your flycowl is a close fit with your flywheel to ensure you have the best cooling you can. We do not recommend you set your bike up on a dyno. We did extensive dyno testing in the development stage of this kit (and build those engines with this in mind), and I expect other dealers to do similar levels of dyno work whilst they develop their own set ups to suit the kit. We do this so we can give you the correct answer and the best performance without you having to so load your engine. We have never dynoed a customer’s bike and do not intend to. We recommend jetting changes be judged by road testing.

Running in Your RB Kit
During the running in period adhere to the following:

  1. 4% (25:1) mixture, use a good quality Fully Synthetic oil. Never drop below 4%. We recommend Rock Oil Synthesis 2 and Motul 710.
  2. Try to use the bike for short journeys (i.e. less than 12 miles) or break longer journeys in to smaller chunks.
  3. Vary the rpm while riding i.e. do not just sit at 40mph all the time.
    First 100 miles: Stay below 45 mph and vary the rpm.
  4. Next 200 miles: Stay below 55 mph and vary the rpm.
  5. Next 200 miles: Stay below 65 mph and vary the rpm.

At 500 Miles give the bike a check over, 1st service & oil change.

Servicing your RB Kit
The RB Kit is significantly more powerful than the standard engine. As a result servicing is even more important in order to maintain a safe and reliable bike. We recommend the following as a minimum level of care to ensure you get the most enjoyment out of your bike. Please show this information to your servicing dealer, and make them aware of any considerations or areas of concern before the service..

500 Miles 1st Service: Re-Torque Head Nuts to 18ftlb, Tighten exhaust nuts, Check and reset / replace spark plug, Clean carb, Check and adjust carburation, Check fuel flow and carb filter, Check and adjust control cables, Change gear oil (ST90), Grease appropriate points, Check tyres and tyre pressures. Test ride, check lights and report condition of machine highlighting any areas of concern. Approx, 2.5 hours.

1500 Miles 2nd Service: Check and reset / replace spark plug, Clean carb, Check and adjust carburation, Check fuel flow and carb filter, Check and adjust control cables, Open crankcase side, Check gearbox clearance, Inspect Clutch, Check Chain and adjust Chain Tensioner, Inspect Sprockets, Replace gear oil (ST90), Check tyres and tyre pressures, Inspect Brakes, Grease appropriate points, Test ride, check lights and report condition of machine highlighting any areas of concern. Approx, 3.5 hours.

3000 Miles 3rd Service: Inspect Top End, Check Ring Gap, replace if greater than 25 thou (0.6mm) Check and reset / replace spark plug, Clean carb, Check and adjust carburation, Check fuel flow and carb filter, Check and adjust control cables, Open crankcase side, Check gearbox clearance, Inspect Clutch, Check Chain and adjust Chain Tensioner, Inspect Sprockets, Replace gear oil (ST90), Check tyres and tyre pressures, Inspect Brakes, Grease appropriate points, Test ride, check lights and report condition of machine highlighting any areas of concern. Approx, 5 hours. Note: Re-Torque Head to 20ftlb after 50 miles.

All subsequent service intervals are at 3000 miles or 12 months and should follow the same procedure as the 3rd service, taking special consideration of individual use of the bike.

Important information you should know regarding your engine set up and provide to the servicing dealer

  Typical AF Set Up Your Engine
Bore of Cylinder 70mm  
Stroke of crankshaft 60mm  
Spec of any additional porting None  
Gearbox GP200  
Front Sprocket 20 Tooth  
Crownwheel 46 Tooth  
Clutch type AF 6 plate  
Timing 17º Before Top Dead Centre  
Compression Ratio 9.5:1  
Squish Clearance 1.5mm  
Carburettor 34mm Dellorto Jetting as above  
Exhaust System NK RB Race  
Electronic Kit AF Light Flywheel