SOME­THING SPE­CIAL

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

Real Classic - - Making Sprockets - Pho­tos by Paul Hen­shaw

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

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