1999 Aprilia RS421 Mutts Nuts special

2019 Simon Lee and Drive-My EN/UK

Dave Whattam’s 100bhp RS250-chassised YPVS project hits all the right notes


Bangin’ big bore YPVS’ plant in exquisite RS250 running gear. Looks great, rides better

When PS visited Dave Whattam’s Mutts Nuts workshop for a 360º garage feature a few months ago, our attention was captured by what he calls the ‘Mutts Nuts project bike.’ It’s nestling in the corner of his workshop in its understated carbon fairing with red pinstriping (like the Batmobile), and at a quick glance it just looks like an Aprilia RS250 with some smart TYGA Performance bodywork and an either-side exhaust conversion.

1999 Aprilia RS421 Mutts Nuts special

1999 Aprilia RS421 Mutts Nuts special

But then you spot the inverted forks and radial brakes, and as your attention is captured you start to soak up other differences – the chassis no longer houses a V-twin and instead has a Yamaha parallel-twin YPVS between its beams. And not just any two-stroke twin, this one’s 421cc and makes close to 100bhp.

Naturally, we very quickly organised a return visit when we could arrive with some riding gear ready for action…

“Engine note is deep at lower revs, but gives way to a delightful scream when you feed it some fossils”

“I used to run the same spec engine in an LC, which was great fun – but lethal,” explains Dave Whattam, who owns and runs two-stroke specialist business Mutts Nuts. “Under acceleration you could see the swingarm twisting, so the stock chassis was fairly obviously not up to the job, but the issue now is cost. LCs, YPVSs… everything is so expensive to buy and you’re taking a perfectly good bike and bastardising it, which isn’t very financially sensible. And even if you do decide to take this route, you are overstressing a fairly flimsy frame so you need to brace it – and at that point where do you stop? I’d stuck a few high-performance LC engines into sportier chassis such as a Kawasaki KR-1S for customers and so I decided to buy a proper chassis and use this as a starting point for my own special. From then on it was just a case of waiting for the ideal donor bike to pop up for sale, and it did.”

1999 Aprilia RS421 Mutts Nuts special

1999 Aprilia RS421 Mutts Nuts special

When an unfinished Aprilia RS250/YPVS project bike appeared in 2017, Dave’s attention was quickly aroused. A fairly typical project whose owner had basically run out of steam ticked all the boxes. “If you are talking two-stroke 250 race reps, the RS250 is the one to have,” reckons Dave. “I used to own one but while it handled fantastically, it was frustratingly gutless below 8000rpm, something my conversion could certainly cure. I contacted the owner and bought the bike off him for £8000.”

While this sounds like a lot of money, Dave had taken a calculated risk, and in actual fact he was quids in.

“The bike was already converted to run a 421cc LC engine, which was great news,” he remembers. “So I had around £4000 of engine, a conversion to form the basis of my project, and a quality chassis with a few good extras such as the CBR inverted forks and radial brakes – not to mention an all-important V5. That was the good news, the bad news was that it had been crashed on track and the wiring loom was a disaster as the owner had attempted to merge the Aprilia and Yamaha electrics, which is never a good idea. He was having problems keeping the engine cool and after a series of cracked cylinder heads, I think he just lost the love. But all the problems he was having are the kind of stuff I deal with everyday at Mutts Nuts, so I was confident it would be ideal.”

With the bike on his bench, a full stripdown revealed that the crash hadn’t bent the frame or forks. However the front wheel was dinged and would require replacing. Dave’s pile of surplus parts (that he didn’t require but which someone else certainly would) was a great way to offset the cost of the project, which were starting to mount up.

“I didn’t like the Reggiani Replica bodywork, so I got rid of that alongside the aftermarket stacked carbon exhausts, and I stripped the motor and re-used all the good parts when I rebuilt it. Along with the bike some carbon bodywork was included, which I sold, along with a few other extras odds and sods. So by the time I finished I effectively paid very little for it.”

With a rolling chassis on his bench, complete with all new bearings, and a straight RSV-R front wheel in the CBR’s forks, Dave’s fresh 421cc YPVS-based engine slotted straight in. Almost…

“The guy had done a decent job of the conversion, which involves building a new cradle to hang the motor from and adding a few rear engine mounts,” he said. “But I did move the motor about 3mm upwards in the frame to improve its centre of gravity. However this did throw up a few issues as it meant you couldn’t remove the clutch side-casing without dropping the engine, which is why it now has a window in its case. I don’t particularly like that look, but it means I can easily access the clutch and that’s been in and out a few times during the rebuild. It required a lot of beefing up to deal with 100bhp.”

Engine in and with everything all lining up nicely, Dave then turned his attention to the electrics. “The wiring was rubbish, so I removed all that and started from scratch,” he said. “I knew I wanted to run an autolube system, so I left enough room for an oil tank under the petrol tank, and used a CAN Bus system to reduce the number of wires on the bike to make it as neat as possible. The   M.Unit is a simple aftermarket CAN Bus system that is brilliant and costs less than £250 with a built-in fusebox. And it and means you can throw away 90% of an original loom. I added a prototype CDI to allow me to reprogramme the timing on the engine, fitted a Translogic dash and some lights – and that was the electrics.”

Much of Mutts Nuts’ work revolves around Dave’s dyno, so with his own design of exhaust system fitted he turned his attention to getting the fuelling sorted. “Getting it running properly has been hard work,” he admits. “It’s a road bike, not a track bike, so I wanted it easy to ride with a strong torque curve rather than an aggressive, peaky power delivery. This meant I’ve knocked back the power a bit and actually reduced the size of the carbs to calm it down and make it fuel better. I also had to increase the size of the float bowls due to the amount of fuel it was sucking as starvation was an issue.”

With a net result of a genuine, rear wheel, power figure of 99.6bhp @ 9691rpm and 54.6lb.ft of torque at 8956rpm, the Mutts Nuts special is very healthy indeed.

Especially as it weighed 146.5kg on our scales ready to go. And all wrapped up in a very stylish off-the-shelf TYGA Performance RS250 carbon fairing. However like so many projects, Dave is happy to admit that this bike is far from finished, but that’s the whole point of why he started it in the first place.

“This bike is a test bed, which I’m using to get everything sorted before I build another RS-based special for myself using all the knowledge I’ve learned,” he says. It is, in effect, a prototype. Having said that, we’ve seen so-called ‘finished’ items that aren’t even up to the standard of this rolling test bed, so take Dave’s description with a pinch of salt. “That’s why the fairing is unpainted, the screen is scratched and it’s a bit dirty. It’s a bike to be ridden, evaluated and improved upon before being sold to fund the next one. The tank is a Mk1 RS, but the TYGA carbon fairing is designed around a MkII tank, so that’s not perfect, and there are a few finger prints in the lacquer and neutral is a bugger to find, but that doesn’t stop me riding it!” Nor PS throwing a leg over it for a quick blast either…


When you first sit on the Mutts Nuts special, it feels like there’s nothing between your legs (not an uncommon sensation for me) as it is so light and well balanced. The chassis is stock Aprilia RS250, which means it’s very narrow and the clip-ons fairly low, but surprisingly the TYGA rear sets aren’t ridiculously high. Either that or my Pilates classes are finally paying off…

Flick the kickstart out and down and it crackles into life with a sharp exhaust note and real sense of occasion. The engine note is quite deep at lower revs, but this soon gives way to the delightful scream once you feed it some fossils. But more of that later.

Bimbling through the 30mph limits that lead to some of Dave’s favourite roads, the bike is remarkably civilized. Dave reckons a drop to 33mm carbs will make it even better. But this is a bike for twisty roads where it truly comes alive.

In the first three gears the 421cc YPVS engine will merrily lift the front wheel once on song on the throttle alone, but it does it in a remarkably relaxed way thanks to Dave’s work on the dyno. It doesn’t hit the power and throw the front up, it’s actually nice and torque-laden and once you hear the exhaust note change you can wind the throttle back to lift it and hover the front at will. But should you wish to simply accelerate hard, the gear ratios are spaced so that when the rev warning lights flash and you cog it up, you drop right back into the power band again, which makes it deceptively fast.

The RS250 chassis is never going to disappoint in the bends, but while you can certainly feel there is huge amounts of potential there, Dave by his own admission is no suspension guru and a bit more work is needed to hit the mark on a bumpy road.

It’s no big job, and a company such as Maxton or K-Tech allowed to work their magic would transform the bike, but as it stands it is a touch harsh on the forks and soft on the rear. That said, given a dry racetrack instead of a bumpy North Yorkshire road I’m sure I’d be saying something very different. What you can’t fix through revised damping is a fat and lardy bike and that’s one thing the Mutts Nuts special can never be accused of being as it flicks from side to side with minimal effort. It’s hard work but huge fun.

Riding a two-stroke is a visceral experience that’s all about sound, feel and sensations. On that front the Mutts Nuts special absolutely hits the spot. You can go out on this 421cc stroker and nail it for a 15-mile run through some twisty bends and return with more adrenalin running through your veins than you would get on a four-stroke on the same route. And that’s always the joy of a stroker. Yes, they require far more input and concentration than a four-stroke, and to be perfectly honest I felt a bit battered when I returned the bike to Dave, but I had a huge grin on my face because it was such an engaging ride. Not to mention the smell of two-stroke ingrained in my leathers, which is always the sign of a good day out.


SPECIFICATION Aprilia RS421 Mutts Nuts special

ENGINE Yamaha RD350LC YPVS water-cooled, parallel-twin two-stroke. 421cc (68mm x 58mm), Hotrods 115mm conrods, Wiseco pistons, Wicked billet cylinder head, Cheetah Cub 68mm barrels, custom billet manifolds. 2 x 34mm Keihin carbs and VForce 4 reeds.

CHASSIS 1999 Aprilia RS250 chassis and swingarm modified to suit RD350LC YPVS motor. CBR600RR-8 forks and Tokico four-piston radial brakes with Brembo discs, Goodridge lines and Brembo master cylinder. RSV Mille front wheel, RS250 rear wheel. TYGA Performance rearsets and RS250 carbon bodywork.


Wheelbase: 1450mm

Power: 99.6bhp @ 9691rpm.

Torque: 54.6lb.ft @ 8956rpm.

Wet weight: 146.5kg

Front lofts easily and without drama via a friendly torque curve.

Still plenty of work to do on the suspension as Dave freely admits Urry’s about to leave, Dave celebrates with a tea.


Not bad looking for a rolling test bed.

Dave’s not exactly mad keen on the clutch window, but it does mean he can gain easy access to it.

That looks like fun all the way from six-five to nine-five. With over-rev too.

Looks like they were made for each other – always the measure of a good marriage.




The Banshee quad bike bottom end (which is based On the Yamaha RD350 YPVS but carries a 3GG engine number as opposed to a YPVS with 31K, 1WT or 1UA) has a wider front casing than the motorcycle equivalent, allowing the bigger CPI Cheetah Cub 68mm barrels (up 4mm on stock) and Wicked billet cylinder head to fit with less mods. The crank is 4mm stroked with 115mm Hotrods conrods and Wiseco pistons, giving 421cc


Stock MkI Aprilia RS250 with a cradle extension. New rear engine mounting brackets are welded directly to the frame.


The bike runs between 65 and 70-degrees, however the R6 radiator is a touch small and so Dave is getting a new one fabricated to lower that.


Dave went down from 38mm to 35 to improve gas speed and reduce peakiness. A set of Dave’s own billet manifolds inject the two-stroke oil down into the airflow rather than up from the carb as the stock bike does. “If the oil drops into the air flow it gets distributed better and is also delivered when the bike is stood stationary, ”he says, “which helps with crank life.” The float bowls are also over-sized to stop fuel starvation.


The bike runs an M.Unit CAN-bus system with an aftermarket programmable CDI controlling the ignition. Two buttons on the left-hand bar operate the indicators while the other is the horn. The starter buttons waps lights from main to dip beam as well as turning them on and off if held down.

Oil tank

Built by a fabricator friend, the 2.5-litre aluminium oil tank features a quick release feed to the pump and is easily accessed viare moving the tank. It lasts 500-miles between fill-ups.

Suspension & wheels

The rear is supported by a YSS shock while the forks are 2008 Honda CBR600 RR units, which are15mmshorter than the RS’s units. The front wheel is Aprilia RSV and the rear the stock RS250 unit, which is a worry as the extra power has a habit of cracking sprocket carriers. “I’m working on a billet replacement,” said Dave.


Built by Dave, the side-by-side exhaust system is a matter of personal taste (he doesn’t like stacked pipes) and is tuned for mid-range rather than peak power. Remarkably, it is sound tested at just 103dB at 5000rpm at Cadwell Park.


The CBR’s Tokico four-piston radial calipers with high-friction pads grip standard RSV Brembo discsand Goodridge braided lines lead to a Brembo19RCSmaster cylinder. The rear is a stock RS250 set-up.


TYGA Performance RS250 carbon fairing and tail unit with a TYGA subframe.


Wet, the Mutts Nuts special weighs 146.5kg with a near 50/50 weight balance. Not bad considering the tank holds 16.5-litres of fuel with 2.5-litres of two-stroke oil and another 2-litres of coolant.


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Additional Info
  • Year: 1999
  • Type: Petrol
  • Engine: 421cc
  • Fuelling: Keihin carbs
  • Aspirate: Natural
  • Power: 99.6bhp at 9691rpm
  • Torque: 54.6lb.ft at 8956rpm
  • Drive: chain
  • Trnsms: Manual
  • Weight: 146.5kg
  • Economy: 55mpg
  • Speed: 155mph
  • 0-60mph: 4.0sec
  • Type: Petrol