Continuing the series of articles on two-stroke tuning, this month is the turn of the RGV250. As you know the Suzuki RGV and the Aprilia RS250 share the same engine so it applies to the RS as well.
Next month will be about tuning the Suzuki RG500 because I won’t be doing anything else this month — a customer has delivered 20 RG500 engines to overhaul and tune. Yes 20.
Any dimensions of ports I give will be in mm measured from the top face of the barrel and widths will be as measured simply across using a caliper or vernier.
By the time the RGV came out especially the RGV250M model onwards the Japanese were making the two-stroke engines more like race engines, giving the tuner less scope for large gains in performance. The tuning work has to be carried out more accurately and more carefully. One of the main reasons for this is the Nicosil plated cylinder bores. To machine the exhaust ports on most other barrels you would use a long-series cutter up the port from the outside. Whereas on the RGV I use a right-angle tool from inside the bore and use a small diamond impregnated grindstone. The reason is that with the bridges between the exhaust port and the transfer ports are so narrow that when using normal cutters you run the risk of chipping the nicosil or flaking a piece off.
For this reason i do not narrow the bridge in the middle of the exhaust port, the bridge gives enough trouble from standard. It obviously is the hottest part and with any wear or seizure it is always the bridge which suffers.
This particular RGV is being used for track days so I am doing virtually a race tune to it. First I strip the power valves and check them for loose pins or any other damage, they really are a crappy design. I prefer to use Cougar PVs if the customer’s budget will run to it. I refit the PVs and lock them open so that when porting the exhaust port I am machining the PVs at the same time. I raise the top of the ex port and the PVs to 25mm and widen each port to 32mm. The top of the standard port slopes away from the centre to the width from between 5mm and 10mm, they vary alarmingly. I continue the height of 25mm right across to within 5mm of the width and then shape the end of the port, I then narrow the bridge between ex port and transfer port and try to make the width continue down as uniformly as possible.
It’s very hard to describe but I hope you will be able to see better from the photos. With the port window carefully modified I then continue porting the exhaust port from the outside, carefully matching the PVs and continuing the widened part of the port and flowing it.
I raise the transfers to 38mm and narrow the bridge between the transfers to 2mm, once again using the diamond grindstone to cut the port windows to ensure the nicosil doesn’t chip. Without raising the transfers you would end up with a very peaky engine.
I flow the collection areas at the bottoms of the transfers but to be honest there is very little to be gained there.
I then remove the PVs again and clean them off and grease them. When the barrels are finished I carefully stone the port edges with a diamond grindstone and with a mandrel with emery cloth on it I polish the edges of the exhaust ports and the exhaust bridges.
The standard head gasket is 1.2mm thick but the SP gasket is 0.95mm thick so I use the SP gasket which tightens up the squish clearance by 0.25mm. The SP gasket is fitted as standard on the later Aprilia RS250s. As I have explained before, by raising the exhaust port you effectively lower the compression because the compression doesn’t start until the ex port closes, so there is still a little to be machined from the face of the head, I skim 0.2mm off on the lathe.
The spark plugs aren’t central or upright so I can’t use my normal simple mandrel which I screw into the plug-holes of other heads to mount into the lathe chuck. I have made an adjustable mounting which locates on the stud holes and fits in the chuck. I say adjustable because the casting of the stud holes between the RS250 and the RGV are different.
With the thinner SP head gasket the squish clearance comes out at around 1mm, which is nice and safe, on a race motor to get the squish tighter would entail skimming the tops of the barrels.
When fitting race pipes i.e. Arrows, Jolly-
Moto or Lomas and maybe others, the jetting needs to be altered, surprisingly not to a larger size but to jet down. Standard the jets are 270 in the rear cylinder and 280 in the front. They need to be lowered to at least 260 and 270.
Don’t forget next month it’s the RG500,1 can’t forget because I have got RGs everywhere! Right, me and the wife are off in the caravan.