Czech pointers
Ultimate power or easy to ride? Tim Britton hears an answer.
Just how do you go about building a stunning CZ Mxer? We get the scoop from John Fleming.
What do you go for when looking for a bike to ride in classic MX? Yes, that’s one of those ‘how long is a piece of string?’ questions isn’t it? There are all sorts of considerations to take into account, such as which class do you want to ride in? Is there are particular marque which holds favour with you? Do I need to go on? You get the picture...
If you’re John Fleming you arrive at your answer by doing a fair bit of research, asking questions, talking to people and building up a picture of what you actually need. “I’ve been involved in MX for a lot of years,” the 63-year-old project engineer tells me, “my dad Bill started ‘Bill Fleming Motors’ in Glasgow from nothing and built it up to be a massive business, so encouragement wasn’t in short supply.”
Now based in Cumbria, he admits he had moved away from riding a little bit while supporting his
son in modern MX – until one of those life-changing moments happened: “I had to have heart bypass surgery and it made me realise there were a few things I’d not done.”
While making light of the seriousness of what he went through, he explained that it had made him realise he hadn’t quite finished with the riding side of things, and looked at the classic scene – but what to ride?
The result of his research was a 1971 380 single port CZ. “I settled on a CZ because they’re light, strong and probably has the best spares situation of any classic Mxer around. All sorts of stuff is available, even though the factory has gone there is a cottage industry in Czechoslovakia which produces new parts for certainly the later MX models such as the single port. It had to be pre-74 so I could ride in the ECMO – European Classic Motocross Organisation – series and there are more races for this era.”
With his first event a few weeks after his surgery John took it easy. “I only did half the meeting as I didn’t want to go too daft, but it was encouraging enough to want to make the bike go better.”
Now CZS have a good reputation as a motorcycle with a strong engine which holds together but is let down a little by the handling and suspension. George Crichton, a friend from way back, suggested contacting Norrie Pattie who ran 3L Road and Race and was involved in tuning and building all sorts of motorcycles. Turns out John and Norrie had had a nodding acquaintance for years but never really talked until George put them in touch.
“A lot of people are put off when an engine builder has ‘road and race’ in his business name,” says Norrie, “they think all we do is produce a motor with on/off power all at the top end though the reality couldn’t be
further away as such a motor would be difficult to use.”
Getting together with John the pair spent the first season developing the motor to give it a broader spread of power, making it easier to ride and learning about what would need to be changed to make the bike even better. The result of the research is standing in front of me in John’s garage, where not even the terrible weather could take the shine off it, as it is truly a thing of beauty and by all accounts it goes as well as it looks.
As we waited for the rain to cease John and Norrie gave me a run-down of the machine and early on in the process they arrived at the ‘modify’ or ‘make new’ point. They went with ‘make new’ and are glad they did. “Everything we’ve done here on this machine is within class rules, no more, no less – we can even
supply a complete bike to this spec,” John tells me. Norrie added: “I’ve built five engines to this spec so far and anyone ordering one gets exactly the same as this one.”
It is as well to start the tale with the engine, as the whole project started with it, though initially in a standard rolling chassis. One of the problems with a CZ engine is also the reason why it’s so popular, it is virtually bullet-proof and works so well the few failings it has are either ignored or lived with. However, this doesn’t mean that there isn’t room for improvement and with a lot of experience at hand to back up his experiments the engine was ‘looked at’ with a critical eye. Breathing, power spread, gear ratios and gear change were areas of interest to the lads. Few people will need to be told crank seals are an
important part of the sealing process in a two-stroke engine and once they let air past then the chances of a bike running properly are remote – luckily modern seals work very well but still do wear.
Through testing and experimentation Norrie developed a much-improved crank seal arrangement and this is part of his three-stage engine tune service. Air escaping or entering other parts of the engine where it shouldn’t was also addressed. The factory exhaust port arrangement on CZ barrels was vulnerable to damage as it was a steel tube cast in place when the barrel was made. Once damaged this was a major engineering job to replace. Norrie came up with a flange adaptor tig-welded in place, to which a flange with a steel tube can be bolted, sealing the exhaust to the head. Should it become damaged then it is a simple unbolt to attach a new one.
By now you’ve probably guessed or worked out if the crank is sealed properly and there’s no air being pulled in elsewhere in the engine, then all the sucking is coming in through the carburettor. It was pulling so much air in now the airbox wasn’t big enough to cope… so a new airbox was developed.
The accepted way to produce more power involves silly compression ratios and radical ignition advances but John and Norrie wanted a better spread of power and a more flexible engine so their port work and cylinder head reprofiling reflected this search for more useable power and actually in some ways went against what had been accepted practice. “When we told people what we’d done there were scoffs of disbelief, yet it works,” John said.
In order to make use of this power spread, a better set of gear ratios were needed and in order to achieve the ideal a standard CZ cluster was modified which brings us nicely to the CZ gear shift… Did I know, they asked, of the extremely long travel on the CZ gear lever? Yes, I did but only because Peter Jerred had shown me the linkage system he’d developed to combat the vast leg-movement needed to change gear on CZS. “It is a ridiculously long travel and it feels like your knee is going to hit your chin when you change gear,” John laughs. “Japanese machines are much less involved and Norrie not only added a linkage system on the outside but made some selector cam modifications on the inside,” John adds. He continues by telling me about an on-track incident which left
him stalled and unable to find neutral so he could kick the bike over and get on the way again. Discussing this with a respected CZ parts supplier at a Dutch meeting he was met with “you don’t go forward in neutral…”
Now, from what I could gather from the lads, finding neutral on a CZ ’box is akin to trying to balance two marbles on top of each other. There is a redesigned method now which is a retrofit to the ’box. Gearbox mods are stage two of the build process and the final stage is the motor built to the exact spec of the one in this bike, which includes all the mods plus a crankshaft alteration which was developed after John frustratingly found himself off the pace against other CZS in a continental meeting.
Once he described what was happening over the phone to Norrie, the penny dropped and he knew exactly what had been done to the engines. Their experiments in this area to increase torque led to the only back-track they had to do… “Our initial measuring was slightly out so we had to go back a bit,” Norrie admits. To finish off the motor bit John said “the clutch was left as standard CZ and the bearings, bushes and seals were all renewed if there was the slightest doubt about them.”
A motor worked over to this level deserves a chassis with equal attention to detail, and again this has been the subject of much discussion. John wanted to retain the low centre of gravity but lift the bike up a bit, so adding suspension length but not travel – this would be outside the class regs – while lifting the bike up also moved the centre of gravity and affected the sharp turning CZS have.
Tucking the frame nose in will bring back the sharp turning but make the bike drop in to corners… the solution was to ship a selection of frames to Harry Stitt in Northern Ireland and tell him what the needs were, then let him try a standard CZ to see for himself what was happening. Harry did this and based on his experience of frame work produced a frame which met the requirements asked for, also did away with the standard CZ swinging arm bearing and replaced it with a much improved type which stiffened the frame up.
Instead of whipping about when the throttle is wound on, the frame stays straight, the ride stays straight and the suspension can be oh so much softer. Add in the frame is a thing of beauty and metal artistry and well, gasp in awe. The ride height is now
higher, but the centre of gravity is still where a CZ’S should be and it turns as it should. The new frame means softer suspension can be used which improves the ride too.
In the old days, as long as a suspension unit or front fork went up and down it was okay for most people, then it began to dawn on the world maybe someone eight stone in their riding kit might need different suspension to a rider nearer 20 stone when kitted up. Traditionally in British factories the tuning tip for scrambling was to fit sidecar rated springs, job sorted off we go… like a pogo stick more than likely.
These days there is much more information about things such as spring rate and damping available, and a multitude of people who will help you… as long as you tell them what you need or at least give them a clue as to how heavy you and the bike are.
For this CZ John was put towards HFS – Hybrid Fork Suspension – with their progressive cartridge system. The Spanish company have developed the retro-fit system over a number of years and it is infinitely adjustable while slipping into the standard stanchions. John built up the forks first using the CZ sliders on Marzocchi stanchions with Marzocchi damper rods inside, then the HFS cartridge at the top.
At the rear end are Falcon Units, built by Robin Packham to the team’s spec. After giving Robin as much information as they could about the bike, as well as the regulations for the class which state what suspension travel is allowed, back came the units and they’re ideal for the job.
The bodies are alloy, and the gas inside them is nitrogen which stays the same temperature throughout a race. “We hear all the horror stories about ‘this suspension is bad’, the other one is worse and so it goes on, but when you ask if they told the supplier how heavy they were in their riding kit or what mods had been done to the bike they go all coy, you’ve got to talk to the supplier, says Norrie.”
As CZ hubs and brake plates are still available new from suppliers there’s no need to look elsewhere, for the heart of the wheels and alloy rims are modern Excels and laced on by an old contact from Fife. Mitas rubber is what the bike rolls on and they’re apparently the tyres to have.
Topping off the frame is an alloy tank made by Allan Harrow for Norrie, Alan also made the airbox and sidepanels. The airbox is of bigger volume than a standard one but fits in to the standard frame. “As all of the parts are available through Norrie’s business we reckoned someone might go with all the engine mods but want to use a standard frame set-up so there was a bit of thought went in to the box design and the one on my bike is a prototype with a bit of welding here and there,” John allows.
Inside the airbox is a filter system from Sammy Doble at MD Racing Products, oh and the plastics came from there too, their UFO range. ‘Come on lads, what else is there?’
“The seat! The seat is a CZ base and foam covered by an upholster I know,” John says, “I’ve got Renthal Fat Bars on, with Magura controls and Venhill cables. The ignition is a CZ ‘Vape’ unit, standard, works.”
By now the rain had at least stopped and it was possible to take some pictures of the superb machine. As I did, John and Norrie were telling me a few more things about the bike, like the oils they use are Opus oils – Norrie is a stockist – from Glasgow.
“I use the outboard two-stroke in the mix as it is just superb, it’s also mixed at the recommended 25:1 which is a bit thicker than most people use but it works for us and we’re particular about our fuel too, 50/50 Avgas/ Super unleaded.” They go on to say when they’ve supplied an engine to anyone this is the instructions the owner gets to run it on.
How does it go?
Well it goes very well indeed, Wayne Partington has the exact same motor in his CZ and waxed lyrical on the phone about how good it was. It wasn’t a test session to try the bike when we did the statics but a test session is arranged for later in August where John’s and a few other bikes will be present.