Steve Cooper shows you how.

One of the quick­est ways to res­tore two-stroke per­for­mance is to de-coke the ex­haust sys­tem.

Classic Motorcycle Mechanics - - CONTENTS - cmm

Ican still re­call the end­less hours spent chas­ing my tail some­time in early 1977. My pre­vi­ously dev­as­tat­ingly ef­fec­tive Yamaha RD350 had turned into a to­tal and ut­ter slug; noth­ing seemed to be able to res­tore its per­for­mance. Carbs, tim­ing, points and the like had been checked and dou­ble checked but all to no avail. Asth­mat­i­cally crawl­ing to work one morn­ing, the ‘Light­bulb of Logic’ fi­nally il­lu­mi­nated – baf­fles – it had to be the baf­fles. Ripped out of the bike while still smok­ing they were hastily dropped into a live steam tank the en­gi­neers used for de­greas­ing. Aug­mented by some caus­tic soda the malev­o­lent black tubes sat stew­ing all day un­til re­fit­ted at 4pm. Noth­ing, ab­so­lutely noth­ing I have ever done to any bike be­fore or since has had such a dra­matic ef­fect upon a ma­chine’s per­for­mance. That one, un­in­ten­tional, wheelie out of the car park proved to me beyond doubt that two-stroke ex­hausts should never be taken for granted. On any stro­ker I fet­tle these days my pri­mary fo­cus, once I’ve had the bike run­ning, is the ex­haust sys­tem. Yes I’ll sort out the tim­ing and ser­vice the carbs but none of this is worth a tup­penny damn un­less the ex­haust sys­tem is free-flow­ing. Ev­ery­one has their own pet meth­ods but what fol­lows is a method­ol­ogy that has worked well for me over the years and as I no longer have ac­cess to live steam tanks, this is now my ‘de­fault-de­coke’ MO! Among my project bikes is a ratty Yamaha AS1 that will need some TLC to its ex­hausts. My lo­cal chromer will strip and re­plate them for me but be­tween the two pro­cesses I need to get some dam­age sorted out. I’ve a good mate who can cut and weld as nec­es­sary but, quite rea­son­ably, he doesn’t want his work­shop filled with lots of oily smuts and grey smoke. So, on that ba­sis I need to evict as much crud from the pipes as hu­manly pos­si­ble. So it’s work­shop clob­ber on, dig out the in­dus­trial gloves and gog­gles and on with a de-cok­ing ses­sion.

If you have a drain-clean­ing mole that fits your pres­sure washer this can be used to give the in­sides one fi­nal rinse cum blast just to be sure. And that’s it, your two-stroke ex­haust should be as clean as you can pos­si­bly get it within a DIY en­vi­ron­ment. Trail bike ex­hausts may not nec­es­sar­ily take too kindly to be­ing ex­posed to paint thin­ners but it’s a call only you can make: some nice satin black paint or a free-flow­ing ex­haust? Life is al­ways about com­pro­mises so why not clean the crud from the in­side, then give a nice fresh coat of paint? Chrome sys­tems will just need a good rinse in hot wa­ter fol­lowed by a clean with some sol­vent and then a bit of TLC with some pol­ish. Don’t for­get to fit the baf­fles please. Leav­ing them out doesn’t make the bike any faster what­so­ever and you are no longer 17! With the si­lencers fit­ted back on the bike you can now fire it up and marvel at just how much bet­ter it sounds. En­joy the sounds and all that ex­tra power you’ve sud­denly lib­er­ated.

87/ With the rub­ber bung re­moved this is lit­er­ally just a snap­shot of what’s been lifted out of the ex­haust’s giz­zards over 48 hours and there’s more to come! 8/ Once rinsed through the same ex­haust has de­posited this lit­tle lot in my rinse bucket. A sol­vent and caus­tic de-coke; the gift that keeps on giv­ing!


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