When they were new, Royal Enfield proudly announced that there weren’t enough Interceptors to go round: not everyone can have one. So for the lucky few who own one today, getting it running right is doubly important. Paul Henshaw fettles a Mk1A and takes
Irecently worked on a Mk1A Royal Enfield Interceptor 750 – in the past I owned a Mk2 and have fettled other Interceptors over the years. This time I was asked to make a very tidy (almost immaculate) Mk1A run as well as it looked. The 750’s owner, Norman, used to work at Royal Enfield and he actually saw this very machine coming down the production line. He road-tested it and even stamped on its engine number!
I was asked to look at this Interceptor’s engine because it was using large amounts of oil and smoking badly as a result. Starting the engine was quite difficult and resulted in rough running and, as reported, considerable smoke. Stripping the top end revealed that the recently-fitted oversized pistons had been clipping the standard-sized Cross sealing rings. These rings seal the compression instead of using a conventional head gasket, and are a bit like a tapered piston ring themselves. All very good as long as you fit oversized ones when you rebore the cylinders…
There was also an issue with the piston rings, none of which looked quite right to me. The oil rings in particular were too tight in their grooves and had stuck in their closed positions. This threw more than a little light on why so much smoke was coming from the chimneys. It seemed that when the top end had been rebuilt, new valves and guides had also been fitted and (not for the first time while doing such work) I saw no real evidence of them having been ground in. A few hours in the company of the workshop radio and my favourite valve grinding stick soon put that right. I love my job, but I have a near hatred for grinding in valves. Even so, they have to be seated correctly if an engine is to give its best, so it is time well spent.
With the top end reassembled, it was time to start the engine again – but it still ran rough. Checking the ignition timing revealed it was nearly 15 degrees too far advanced, so this was put right, but the engine was still not completely happy. A look at the carburettors was called for.
The Mk1A was the first of the Interceptors to wear a pair of Amal Mk1 Concentrics. The earlier 750 used a pair of mirror-imaged Amal Monoblocs; one 389 and one 689. Investigations on this bike revealed a sticky float in one carb, thanks to a home-made float bowl gasket snagging on the float, while the other carb’s needle was in its lowest position, rather than the correct middle position as on the other side. There was also about 1/8” difference in lift of the slides when the throttle was opened, thanks to poorly adjusted throttle stops and cable adjusters. With all of these issues put in order, the machine actually started to run quite well but it still wasn’t entirely right.
More investigations revealed that the charging system was giving no more than about 11.5V, and less than that when the lights were switched on. I had my suspicions that the Boyer electronic ignition system already fitted to this machine might not be entirely happy with that. I had an old Wipac alternator rotor on one of my shelves, and its magnetism felt much stronger than that of the one which had been in situ. This rotor swap grabbed about another one and a half Volts. Improving the rectifier’s earth connection gained a little more on top of that, so we now had around 12.5 Volts with the lights on. That’s not brilliant but it qualifies as ‘OK’, and it cured any last tendencies towards spitting or spluttering and with a warm engine.
The Interceptor would now start first kick with no throttle and go straight into a steady tickover at around 1000rpm, with a steady, crisp beat emanating from the upswept silencers. With a few other things checked, such as fork and gearbox oil, tyre pressures, etc, it was time for the fun part of the job: a ride to make sure all was well and see how it went…
The term ‘musclebike’ was way in the future when the first (and even the last) Interceptors were built, but it could have been coined for these fantastic machines. Said to have a range of 100mph in top gear, from 20 to 120mph,
coping with the weight and speed of this beast. That’s even after this machine has been upgraded with an Indian 2ls front brake. The riding position was comfortable and relaxed at lower speeds, but it would be a bit of a struggle at higher speeds – definitely so when approaching the machine’s top speed. However, speeds of up to 70mph should be comfortable for bike and rider all day long, with little in the way of vibration.
My first sudden, big opening of the throttle in order to overtake was rewarded with good acceleration, accompanied by some spluttering and some clutch slip although we got past the vehicles in front of us. The next time I did this, the spluttering was gone but the clutch slip was still present. I have ridden other Interceptors with Newby belt drives and clutches, which can cope with all that torque without slipping. A useful upgrade, at a price.
Once you’re under way, this is one of those machines which can be left in top gear for much of the time. I would expect the fuel consumption to be around 50mpg unless ridden hard, and if ridden hard it would probably earn you the wrong sort of points, so a sensible right hand is probably for the best.
When parking there’s a choice of the easyto-use prop stand or the hernia-inducing item of torture commonly known as the
centrestand. Seriously, this is probably best used only for maintenance or showing off / deluding yourself that you are Britain’s strongest man / woman / fool. After parking by either means, it is just a matter of admiring the machine you’ve just ridden, before the next time it bursts into life without difficulty or fuss, ready for more effortless touring, accompanied by a deep, strong exhaust note and little in the way of mechanical clatter.
A final thought occurred on the ride home. Did Royal Enfield shoot themselves in the foot by calling this machine the Mk1A? That tag suggests that the Mk2 was just around the corner, which might’ve stopped potential purchasers rushing to buy the Mk1A. Inevitably, sales of the Mk1A fell sharply when dealers got a whiff of the Mk2 and that machine, possibly the finest old Enfield of them all, is another story…
They do look like big bikes, the later RE twins, but in fact they’re quite compact
Test rides after a rebuild demand a halfway stop to ponder … well … things
One maintenance item which is often neglected is changing – or even topping up – the fork oil. They do work better when lubricated
Neat Indian 2ls anchor helps kill the speed
It helps to assess how well a bike’s running if the rider knows the model. Here’s Paul’s old Mk1
After the Mk1s came the Series 2 Interceptor… But that is another story
Paul takes the Interceptor for a 15 minute test ride on YouTube, which you can find at https://youtu.be/nGwJzOXAhW4
Lifelong Britbike enthusiast Paul runs Performance Classics in Carmarthenshire, and offers restoration services for all kinds of old motorcycles. Rebuilds, tuning and wheel-building are all done in-house. 01550 777608 / 07909 740160 / phen[email protected]internet.com