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.

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. Reassemble in reverse.
  8. Reattach assembly on moped and check for air leaks.
  9. 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.

Clutch Mods/Repair

These mods should give better RPMs and help keep you from frying your clutch.


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

  1. Check for missing or flat anti-rattle springs.
  2. Take the 31 7mm bearings in the spring out. Open it in a large bucket .
  3. For this step, I used 1/4"OD stainless steel brake line because it won’t rust, but any steel tubing will work. The maximum OD is 0.265 inches to 0.245 inches. I found that 0.250 inches OR 0.255 inches works BEST.
  4. Cut tubing into 5/16 inch lengths = 8 inches to 8 3/8 inches total length with 6 OR MORE BALLS into spring!
  5. When loading the spring use a old razor blade to hold the balls in.
  6. Take the bearings out of the bucket .

Clutch Mod 1

  1. Make a series that goes: ball tube ball tube ball tube and ending with ball. Remember that the balls seat into the tubing, making it shorter in length.
  2. Twist spring backwards 2 1/2 turns bring spring ends together and twist together foreword.
  3. Lay the spring on a flat block of wood. Tap the spring with small hammer where it twists together to seat it.
  4. The total spacer's MAX GAP should be 3/8 inches spring.
  5. Grease and Re-install on bike.

Clutch Mod 2

Use 6 bearings and tubing or brake line

  1. Cut tubing into 5/16" lengths and install starting with 1 BALL: tubing tubing tubing tubing tubing tubing tubing tubing ball tubing ball tubing tubing tubing tubing tubing tubing tubing tubing ball tubing ball tubing tubing tubing tubing tubing tubing tubing tubing ending with BALL into spring.
  2. Twist spring backwards 2 1/2 turns bring spring ends together and twist together foreword.
  3. Lay the spring on a flat block of wood. Tap with small hammer where spring twists together to seat it.
  4. The total spacer's MAX GAP should be 3/8 inches spring.
  5. Grease 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.

General info

  • 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 over bore 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 4.2+
  • 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 16mm high X width 14mm, venture 14mm, out 14mm .
  • Set carb float 12.5mm pin tip to top of float
  • Carb jet #62 or #64.
  • Or Dellorto SHA 15.15 C COD.2045 Jet #68.

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, 13.8 mm in out mach reed block. Top sped 39+ MPH
  • Hand made intake for SHA 15.15 Carb, 14.5 mm in out mach reed block.

Reed info

  • Stock reed block. deburr

Timing/spark plug

  • 0.085 inches or 26 degrees before TDC
  • Stator plate at mid point +.
  • Spark plug NGK B8HS, gapped to .021”.
  • New points gap: .0155”.

Cylinder modifications

  • Cut top of cylinder .024”. (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 155 degrees → port height is 13mm ovule-shaped (like Stew’s head or a football), width 24.5mm.
  • Exhaust port opens 102.5 degrees, 25mm, or 0.984 inches after TDC. (not top of cylinder)
  • Port height 13 mm (0.512 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 .984”=25mm=102.5 Deg ATDC. (not top of cylinder)


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 25 MPH=4,500RPM'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 155 degree or less. Over 156 degrees and you start losing low-end power.


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