Batavus performance

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This is a list of performance modifications you can do to make your Batavus (M48 engine) go faster.

Carb Mods

The ideal float setting for an Encarwi.

Float pin and seat

  • The top of float should be 12mm to 12.5mm (with an un-worn/new tip) or 12.5 to 13mm below pin tip (with a used/worn tip). Hold the float in one hand and twist the pin at the bottom to adjust the float.

The best way I found to tune in Encarwi S25 carbs is to start with the top of the float pin tip at 12.5 mm .

Now plug chop for WOT top speed. Say a 56 works, but you are to rich at low speed 1/2 to idle throttle.

NOTE 1 mm of pin tip to float is 1.5 to 2 jet sizes at WOT, adjusting the float is how you adjust/change your 1/2 to idle throttle jetting.

So a 12.0 mm float to tip size 56 jet WOT jetting at 13.0 mm float to tip needs to be 57.5 to 58to have the same WOT jetting and with a 13.5 mm float to tip chang you need a 58.5 to 59 jet.

So you need to lower the float 0.6 mm and up jet one size to have the same mix at WOT with a leaner 1/2 to idle throttle.

You adjust top of Float to tip of pin for low speed and jet size for top speed together and replug chop.

Fast Bat out

Fuel system cleanliness

Check out the condition of the gas tank and look for rust on the inside .You need to disassemble the petcock and spray with carb cleaner. Check the petcock pickup tube and screen filter in the tank and replace the plastic tube with a 1 1/2" long brass tube (1/2" on reserve) so you don't pick up rust . Install a BIG in line fuel filter and new fuel line from the tank to carb. Then mix up some new premix and see how well it flows out at the carb.

If fuel flows okay, pull the carb and disassemble in this order:

  2. Remove the air filter assembly and spring clip.
  3. Push the choke spring up to remove the choke cable.
  4. Remove spanner nut cover and spanner nut holding slide and throttle cable.
  5. Set a drain pan under moped.
  6. Remove flange bolts holding carb reeds.
  7. Pull carb and reeds check the reeds.

If the reeds and gaskets look okay, dissect fuel system in this order:

  1. Remove the bolt holding the fuel line banjo fitting. In side the banjo bolt you will find a very small screen filter with O-ring.
  2. Remove float cover/seat housing (carb top) by unscrewing the two screws. A light tap with a screwdriver handle should free it if it's a little stuck.
  3. Remove gasket and float.
  4. Remove main jet holder.
  5. DON'T USE CARB CLEANER ON THE O-RINGS, FLOAT, BANJO, or SCREEN FILTER. With these parts SET OFF TO THE SIDE, clean all other parts with cemtoole B-12.
  6. Check the float pin and seat. Set the float to the specifications listed above.
  7. drill a 1/16 vent hole in carb above choke lever into top of fuel bowl
  8. Reassemble in reverse.
  9. Reattach assembly on moped and check for air leaks.
  10. Ride and do a plug chop to check jetting.

Exhaust Mods

This mod is for the M48 engine with the 26mm exhaust system.

Materials Needed

  • One 6mm X 1mm pitch die
  • One 1/4 inch drill bit.
  • One die grinder or dremel
  • One mig welder
  • One 6mm X 1mm pitch tee nut
  • One 6mm X 1mm pitch locking nut
  • One 1/4 inch fender wisher
  • One steel tube 1/2 inch ID X 1 inch long .
  • One steel tube 5/16 inch ID X 7/16 inches long
  • One small computer fan guard /cover.
  • One Bing jet #60


  1. Remove exhaust system.
  2. Clean de-carbonize the exhaust system. See Cleaning the muffler for suggestions on this procedure.
  3. Watch this video on YouTube.

Aftermarket Exhausts

Motion Left Mopeds (MLM) has a bolt on performace pipe for the M48 Starflight. It has been available at Treatland[1] for some time.

Link to pipe[2].

Clutch Mods/Repair

The way this centrifugal clutch works is that the crankshaft all the way to the right spins and is rotationally connected to the clutch hub (#21). The groove in the clutch hub holds the clutch spring (#18). As the engine hits higher RPMs, the hub will naturally spin with it and the clutch spring will expand as it rotates faster and "flings" outwards. Since the hub's groove has a ramped radius inside, as the clutch spring grows in diameter it will be forced in an axial direction away from the engine center. This causes contact and friction with the pressure plate (#63), which in turn captures the clutch plate (#14). The clutch plate is tooth-connected to the clutch housing (#71) (all these parts sit within the clutch housing), so that will spin and its V-belt is connected to the pulley (#74) which changes the gearing to your rear wheel. Off you go!

SOOOoo, these clutch mods are attempting to reduce the weight of the spring (by replacing the ball bearings inside with something lighter) so that the clutch will engage later (aka higher in RPMs). This is because the less weight there is in the spring, the more resistance there is to the spring expanding so you fling that spring out only if you're going faster. This way you are closer to the power band and accelerate faster when the clutch actually does engage.

When you start this bike, the clutch cable compresses the starter leaf spring(#7), which temporarily connects all these pieces together, but from the opposite side of the clutch plate (compared to the motor running). So that pedaling will drive the v-belt from the wheel, to the clutch hub, and then (with the clutch plate compressed) to the crankshaft to start the motor. This is just a simpler, reverse version of the bike running.

M48 Clutch.JPG


Batavus clutches can have a .025" to .125" gap. Any gap over .150" means it's time to replace the disk or shim.

  1. Check for missing or flat anti-rattle springs.
  2. Take the 31 7mm bearings in the spring out. Tap the spring with a small hammer where it twists together to help unseat it and untwist it in a large bucket .
  3. Remove/sand all rust from inside the spring/housing.
  4. For this step, I used 1/4"OD stainless steel ball chain because it won’t rust, but any 1/4" ball chain will work or cut plastic tubing into 5/16 inch lengths . The maximum OD is 0.265 inches to 0.245 inches. I found that 0.250 inches OR 0.255 inches works BEST.
  5. Cut plastic tubing into 5/16 inch lengths = 8 inches to 8 3/8 inches total length with 6 OR MORE BALLS into the spring!

Clutch Mod 1 High RPM With all 1/4” ball chain.

  1. Use 8 inches to 8 3/8 inches total length of 1/4”OD ball chain, with the links pushed together.
  2. When loading the spring use a old razor blade to hold the balls in (sand the cutting edge down).
  3. Twist spring backwards 2 1/2 turns bring spring ends together and twist together foreword.
  4. Lay the spring on a flat block of wood. Tap the spring with small hammer where it twists together to seat it.
  5. The total spacer’s MAX GAP should be 3/8 inches with the spring installed.
  6. Grease spring and Re-install on bike.

Clutch Mod 2 Mid RPM 1/4"ball chain and 6 stock 7mm ball bearings.

  1. Cut 1/4" ball chain into three lengths of 8 or more balls.
  2. Load the spring starting with 1 stock 7mm BALL bearing, then a length of ball chain, then 2 stock 7mm ball bearings, then a length of ball chain, then 2 stock 7mm ball bearings, then a length of ball chain, then 1 stock ball bearing.
  3. When loading the spring use a old razor blade to hold the balls in (sand the cutting edge down).
  4. Twist spring backwards 2 1/2 turns bring spring ends together and twist together foreword.
  5. Lay the spring on a flat block of wood. Tap the spring with small hammer where it twists together to seat it.
  6. The total spacer’s MAX GAP should be 3/8 inches with the spring installed.
  7. Grease spring and Re-install on bike.


Adjust the ratio of ball chain to 7mm bearings to fine tune engagement of clutch. Just be sure to evenly distribute the weight to reduce vibration and provide even pressure to the plate.

e.g. slightly lower rpm engagement with better grip: 2 BBs, 4 chain links, 4 BBs, 4 chain links, 4 BBs, 4 chain links, 4 BBs, 4 chain links, 2 BBs.

Clutch Mod 3 Just run the spring empty

  1. Some just run a empty spring and run it.
  2. Grease it and install on bike.


  • The balls won't get wedged into where spring twists together.
  • Remove all rust from inside the spring/housing.
  1. Having nothing in the spring can, over time, chatter and/or sing then kink, flatten, twist, and fry your clutch.
  2. The spacers and or balls wont let the spring kink and twist and flatten etc.

Cylinder Port Timing Mods

Batavus M48 cylinder port timing/map mods.

Power band 5,100 to 7,400+ RPMs, peak power at 7,125 RPMs

Port tuning to get a power band between 5,100 and 7,400+ RPMs, with peak power at 7,125 RPMs.

This port timing is for running OVER size piston's, it will kill all low end power with the stock bore.

  • Weaker then stock low end power with 32%+ more top end.
  • Needs lower gearing (12/50 = 4.17:1.... 11/48 = 4.36:1) or a 42mm+ over bored cylinder for strong low end power.
  • Piston speed at peak power m/s 9.12
  • Piston acceleration at peak power m/s2 14069
  • HP approx 4.7
  • Displacement 47.75 cc
  • Bore 40 mm
  • Stroke 38 mm

Exhaust port

  • Exhaust port duration 164 degrees
  • Exhaust port opens 98 degrees after TDC
  • Exhaust port opens 24.3 mm after TDC
  • Port height .................13.7 mm
  • Port width ...................25 mm
  • Polish exhaust port
  • The exhaust port should open .933" after TDC

Intake/transfer ports

  • Ports open 120 degrees after TDC
  • Ports open 30.5 mm TDC
  • Intake / transfer ports duration 120 degrees
  • Ports height:................. 7.5 mm
  • Ports width:.................. 17 mm
  • Make the transfer ports point away from exhaust port
  • Transfer ports need to open at 1.205inches after TDC.

Power band 4,500 to 7,000+ RPMs, peak power at 6,750 RPMs

Cylinder port tuning to get a power band from 4,500 to 7,000+ RPM's, with peak power at 6,750 RPM's.

General info

  • Strong better than stock low end power with 25%+ higher top end. Can run it all day at WOT.
  • HP approx 4.0+
  • Displacement 47.75 cc.
  • Bore 40 mm / 1.575 inches.
  • Stroke 38 mm / 1.496 inches.
  • Stock 26mm EX with power band modification

Carb info

  • S-25 Encarwi Carb, modded to inlet 15mm high X width 13mm, venture 12.2mm, out 12.4mm .
  • Set carb float 12.5mm pin tip to top of float
  • Carb jet S-25 #58 to #62.
  • Or mod a Dellorto SHA 14/12 Boar opening at air filter to 16mm and taper down to 13.5mm stop 3mm's from brass tube Jet #54 to #58.

Intake info

  • Stock intake modifications: 12.4 mm in 12.2 mm out. Top sped 37 MPH
  • Hand made intake for S-25 Encarwi Carb with mods to Carb, 13 mm in out mach reed block. Top sped 39.MPH
  • Hand made intake for SHA 14 /12 with mods to Carb, 14.2 mm in out mach reed block 39.MPH.

Reed info

  • Stock reed block. deburr
  • Or replace stock reeds with super thin carbon reeds by drilling out rivets to remove old reeds and taping to install new reeds, and removing the center bridge to make it a big 2 pedal.
  • 4-40 tap and 4-40 x 1/4 or 3/16" phillips pan head works perfectly. (4-40 x 1/4" stick out a little on the inside)
  • Polini yellow reed paper .25mm thick
  • make a 1/8 " shim to go in be-twine the reed block and Engine case, so the reeds can open further.

Timing/spark plug

  • 0.065" = 19deg's BTDC to 0.075" 21 degrees BTDC Max.
  • Stator plate at mid point +.
  • Spark plug NGK B8HS, or BU8H set plug gap .017" to .021”.
  • New points gap: .0155” to .016".

Cylinder modifications

  • Cut top of cylinder .024”. to .044". stopping at the top cooling fin (Note: Dome of piston should rise 1mm or .039 inches above cylinder at TDC This should give 16.6mm or 0.655 inches (NO LESS) between the top of the piston dome and the fin side/top of the cylinder head where the spark plug gasket sits on top of head at hole. (You can use a tire tired depth test to gauge it if you cut into the sides to fit. This is good for for timing too.)
  • Mill head. To set gap, mill ??mm off head -- the tip of the spark plug must be 1mm (0.039 inches) (or more) away from the top of the piston dome at TDC. You're looking for an approximate compression ratio of 9.4:1 to 9.6:1 Max. If you go higher than that, it can overheat the engine or piston, cause pinging, or make you use less spark timing (which will result in less power at 6,000+RPM’s).
  • Use one base gasket. Torque the head to 8 ft lbs (not 11 ft lbs like the manual says). Don’t break it! When torquing down the head, do it in four steps: First to 2 ft lbs, then 4 ft lbs, then 6 ft lbs, then 8 ft lbs. Also make sure to torque the bolts in a criss-cross or "X" pattern (for example, bottom left, top right, bottom right, top left) for each step.

Exhaust port

  • Exhaust port duration 152 degrees → port height is 12.5mm ovule-shaped (like Stew’s head or a football), width 24.5mm.
  • Exhaust port opens 104 degrees, 25.5mm, or 1.005 inches after TDC. (not top of cylinder)
  • Port height 12.5 mm (0.492 inches)
  • Port width 24.5 mm (0.964 inches), = to 60% of bore.
  • Polish and (taper exhaust port opening on piston side) don’t brake a ring.
  • The exhaust port should open after the piston moves 1.005”=25.5mm down from TDC (not top of cylinder) or 104.Deg's ATDC.


Intake and transfer port info

  • Ports open 121.5 degrees, 31mm, or 1.220 inches degrees after TDC. (not top of cylinder)
  • Intake / transfer ports duration 117 degrees
  • Ports height is 7 mm (.275 inches), the shape of the transfer ports is like a long long tear drop pointing away from the exhaust port.
  • Transfer ports openings must not taper or point on the exhaust port side of the transfer port opening.
  • Ports width is 17 mm (0.669 inches). Make the transfer ports point 25 degrees away from exhaust port.
  • Taper and deburr the transfer ports. Save the last .5mm (0.020 inches) of porting for hand touch-up and polishing.


Power band:

  • Power band starts at 22MPH=3,800 RPM's.
  • Kicks in at 24 MPH=4,350RPM's, peaks at 37+ MPH=6,750 RPM's.
  • Fades at 38 MPH
  • Four strokes at 7,000 RPM’s = 39+ MPH with the stock carb and reeds.

I still have strong better than stock low end power with one cylinder base gasket with a duration 152 degree or less. Over 154/156 degrees and you start losing low-end power BIG time.


Related Information

A whole series of videos on Batavus Repair. Current repair videos include:
The Batavus is Bad Video
The Tools of the Trade
The Great Batavus Clutch Mod
Encarwi Carb Tutorial
Informational page on both Batavus Laura M48 and M56 engines. Lots of stock information and modification instructions/suggestions. Other links on this page don't work, so to be safe download anything you need before it disappears.