Over the years most manufacturers have made lots of improvements to Cylinder port shapes. Most cylinders can actually flow worse, if you grind /polish the area,s from under the cylinder. If you take material away, you may decrease bottom to mid power, as this effects the primary compression ratio. This can be corrected with case stuffing. Without a good small Dyno, you may not realise whats happened. The curves in the transfer ports have now a more tighter curve, increasing the slug volume speed into the cylinder, KTM have pioneered this design which works really well, and yet the ports look small! High air speed is gained. Most surface finnishes on modern cylinders cannot be changed, to gain power. Even the exhaust expansion chamber inlet is now smaller than ever, on late model pipes, keeping air speed high. However, there is still work that can be done, port windows, chrome rough edges, and the whole exhaust port can be changed that yields good power improvements over most of the rev range. Cutting through the chrome is tricky, most engine porters now using some lubrication/cooling method, so as the chrome plating is not being effected by heat and vibration. Dyno testing with many cylinder port shapes, sizes, heights etc is done to make sure the power is gained. So before you grab that file, or porting tool, call and have a chat, we can hopefully save you some time.
Some engines 85cc 125cc 150cc 250cc need to use more stronger/robust type spark plugs.
If the engine has higher internal pressures, excessive crankcase stuffing, too high compression ratios, incorrect crankshaft balance factor, Detonation, engine frequency type vibrations/pulses can harden and weaken the earth electrode and it will FALL OFF causing expensive damage.
Some modern engines are already made high performance from the factory and pressures are really high and they need better spark plugs. Some 85cc and most larger 2 strokes will need these plugs. part number NGK R6918B-9 is one of the best choices. $60.00 each. Replaces BR9EIX or BR9EG or BR9EV or BR9ES etc. ( 9 ) is the heat range ,so you can buy 8,9,10 heat ranges normally.
Be careful, if the bike comes from the factory with a R6918 type plug,(like the current model Suzuki RM125) dont put a cheaper plug in as you will reduce performance and damage the engine.
In winter you always need to increase fuel supply. Approximately 5% richer.
Balance factor is different from sizes of engines. Change of the factor will change the power curve.
The new VP Roo Fueld makes combusion and mixing temperatures cooler. Good for 2 strokes! Must change jetting approx 6% richer. Best to increase compression too. Good for 4 strokes too.
Note: Race Fuel Above 100 – No point using it. Fuel testing will be done at Australian and FIM Wold Championships.
The benefits you can achieve by having components treated include:
HGS Exhaust. VHM Cylinder Head. 185-190 main jet. Ignition timing 1.1mm BTDC.
The European exhausts are the best tested on the dyno and at the track. The scalvini produced good power in the top end, and the engine needs to be tuned for this type of power, the pipe is thin and will get damaged easily , the damaged pipe will have a loss of power. Van Hasselt exhaust is specially designed for MX, best mid range power and easy to race , and the construction is good. the HGS has the best top end power out of all the pipes, but engine must be tuned to suit this for the rider. All pipes have more power than the stock pipe. USA pipes, euro pipes included. USA pipes crack alot. Euro pipes are stronger and make more power, but are not coated and rust easily. Most of the power increase comes form the pipe, not the silencer . The silencer does change the engine noise alot, but the dyno tests show no big power changes are made.
Some pistons on the compression side do not have a wear band-ring. You can order one from the rebound side and machine the piston to fit it so you have smoothness on compression (as it should be). Common SX65. Shivers have this as standard.
Note: Only use Mazzoochi Seals (a lot less drag).
Many different production types. Example: KTM Vertex is not the same piston as Vertex after-market type. Check weights!
This Italian company is spending more time and money making and testing Exhausts than others. Some of the 2 stroke systems add a lot of power. After installing a Scalvini on a SX85, the engine feels like it has had large modifications made. Expensive pipes. Some pipes are hard to fit (SX65). They can get damaged easy, and power losses result. look great.
Development changes to expansion chamber and mufflers. Each exhaust is exactly the same from 1st stamping to the 50th stamping.
EHR products for making racing bikes go faster, but also the engines are easier to race as the power band becomes wider. The power doesn’t have a hard hit, so the kids like the set up. All the race parts have been tested on the dyno, then on bikes in Holland and we test them here. You can still make some mods yourself to improve the set ups too.
Treatment of gears for low friction and better shifting, done with a Tumbler/Vibration machine.
H.P.S.F Victoria Ph 03 9775 0898.
Make sure to use good quality wheel bearings with the extra clearance rating of C3 in the part number. They have less friction, especially when hot and particles like dirt and water etc get in, will not cause as much friction in them.
Also check your chain for any drag from guides, rollers, etc. Especially small sixe bikes as this robs power.
What a ripper! The fastest 50 made for racing so far.
The suspension is extremely good and the handing of this bike is great.
The bike comes std with Vforce reeds, a full circle racing type performance crankshaft and a insert type 2 piece cylinder head and adjustable ignition timing. The cooling system on this engine is very well designed, engine works well in sand as well as long tracks and also the ignition stator and housing is cooled really well to stop failures.
Best mods- HGS exhaust, reduce squish clearance, adjust slipper clutch tighter (70-80) use a good engine tachometer and adjust clutch engagement speed 8500rpm to 9500 for racing.
The suspension and handling is very good. The engine needs more power to compete in racing. The primary compression ratio needs to be changed, using stuffing to enhance the flow from the crankcases into the cylinder. The engine needs a better crankshaft , a full circle type race crankshaft. The powervalve is not adjustable in any way, except modifiying it , trial and error. The Carby is old fashioned round slide, and is not close to the engine induction. Comes std with Vforce reeds which is great.
Best mods – HGS exhaust works well. Crankcase stuffing required. Use the KTM type 24mm carby, and best to bore to 26mm, need to modify intake rubber boots and air cleaner boot.
The engine power is too high in the rev range for MX, ok for flat track.
Suspension,linkage change required as well as revalve and springs required for racing MX. The bike is now well out dated for racing, although very reliable and cheap to race. A great club level bike to learn skills , power needs to moved to get more bottom end for MX. Cylinder and head work required. The KTM sx65 is alot faster as the bike has a powervalve and better suspension. HGS exhaust and a vforce reed valve are the best mods for the engine, and with cylinder and head work the engine is very good for mx or flat track.
The basic bike is still a little behind the KTM sx85 in stock form but mods are easy to do on the bike.
Fitting the HGS or VH exhaust with the VHM cylinder head and special insert developed by BUD racing, the engine really performs well. Some jetting and needle changes are required for racing. Vforce reeds work better then the boysen rad valve.
Changing the suspension springs, a new linkage, and revalve to suit rider weight, the bike performs very well. The conrod weight is very heavy and needs to be reduced for top level racing. After the mods are done for racing you have a very solid bike that gives very little trouble. Overall this is probably the best mx or flat track bike as the performance is good and ongoing costs for racing can be low. This bike has much better brakes,throttle set up, rear suspension than the Ktm sx85.
Things to watch- run oil ratio at 30-1 for racing. Crankshaft oil seals wear if using some race fuels and you have to split the cases to replace them. The 2 ring piston is only good for trail riding or enduro but you can remove 1 ring for racing. The Vertex coated piston is better for racing. Overall bike weight is higher than the ktm, the gearbox is heavier than the ktm. There are areas on the bike you can reduce weight. ( gear tumbling works well for racing on the kx85) long term we will see more of KX85 on the racing seen.
Primary compression is correct (stock). HGS Exhaust, VFORCE Reeds. Reed Valve Spacer. VHM Head (stock jetting).
410main jet at 25-30deg- 420,-440 below 25deg standard needle and settings clip 3or4 and std pilot. mixture air screw 1.5
Note. This 125cc engine is very well designed and makes very good power for a 125cc.engine. Be carefull of different exhausts and silencers as different lengh can effect flow and can result in bad piston detonation (destroying piston tops) Some exhausts dont pull enough heat from the piston crown. We have found the HGS controls the temperature very well ,as in Holland they race alot in sand. You can check temperature readings at the spark plug on the Dyno when testing on maximum run up and constant/steady load- rpm. Also its best to have the piston cryogen treated, using a different brand can be better, we use Vertex, they are a little heavier than the std piston, but seem to last .
Yamaha 85s, lower the barrel to 30thou squish, raise compression with EHR head, don’t machine std head. Machine upper exhaust port upwards 15thou. Make the flywheel heavier with add on weights. We do a lot of work on crankcases to increase flow that gives more torque, we have EHR working on new parts for this engine. Scalvini Exhaust is great! Rear Swing Arm is too short for SHOCKY to function well. Hinson clutch is best on this engine.
Best Mods: VFORCE HGS – Barrell mods. VHM Head.
Fit the Scalvini Exhaust or HGS. Close squish to 28thou. Mods to cases etc. The RM85 doesn’t need a lot of work to improve it compared to other 85s.
Best Mods: RM85 use Powerband Linkage, Springs, Revalve. VFORCE reeds. HGS/Scalvini New 28mm carby.