Mark Hay­cock on the nu­ances of this ne­ces­sity.

Classic Motorcycle Mechanics - - CONTENTS -

Ihad a ques­tion for the Q&A sec­tion from a reader re­cently where the sub­ject was ig­ni­tion tim­ing, specif­i­cally how to check it on a bike with no tim­ing marks, so I thought it might be an idea to look at the whole sub­ject in a bit more depth. Firstly, a bit of back­ground: we all know that a bike en­gine works by burn­ing a mix­ture of petrol and air and if (like me) you have en­cour­aged a fee­ble bon­fire to per­form ad­e­quately by us­ing petrol, you will ap­pre­ci­ate that the burn­ing can be very fast in­deed. It did work, by the way, but I didn’t re­peat the ex­per­i­ment! You might think that the petrol/air mix­ture ex­plodes in the en­gine cylin­der: that is not quite the case. Un­der some cir­cum­stances the mix­ture can det­o­nate, but this is very harm­ful to an en­gine and needs to be pre­vented. The burn­ing process does nor­mally take a fi­nite time, mea­sured in mil­lisec­onds (i.e. a few thou­sandths of a sec­ond). If the spark, and thus the start of com­bus­tion, were to oc­cur at top dead cen­tre (TDC), by the time the pres­sure had in­creased sig­nif­i­cantly, the pis­ton would al­ready be de­scend­ing and some of the po­ten­tial power out­put would have been lost. So the spark is set to be be­fore TDC, so that the max­i­mum pres­sure reached oc­curs a few de­grees af­ter TDC. One thing that com­pli­cates mat­ters is that in con­trast to an in­dus­trial en­gine, a ve­hi­cle en­gine’s speed varies and the tim­ing set­ting needs to ac­com­mo­date this by in­creas­ing the ad­vance at higher revs. On top of that, the air/fuel ra­tio varies in op­er­a­tion and it has been found that richer mix­tures burn quicker than weak ones, so they need less ad­vance. Apart from those things, throt­tle po­si­tion (and thus en­gine out­put), at­mo­spheric pres­sure, tem­per­a­ture and hu­mid­ity level, fuel com­po­si­tion and God knows what else have their own ef­fects, so it is not a straight­for­ward is­sue. In days of yore, ig­ni­tion tim­ing could be set by a han­dle­bar-mounted lever. It would be nor­mal to re­tard the tim­ing to pre­vent

any pos­si­bil­ity of the en­gine ‘kick­ing back’ i.e. fir­ing too soon and sud­denly re­vers­ing, vi­o­lently forc­ing the kick-start up­wards. The rider would then ad­vance the tim­ing for nor­mal use but could make fine ad­just­ments depend­ing on how steep hills

were, the qual­ity of fuel and other things. So, it was a great ad­vance (ahem) when an au­to­matic ad­vance sys­tem was in­vented. This re­lied on a cou­ple of weights flung out against spring pres­sure by cen­trifu­gal force, with greater ro­ta­tional speed pro­duc­ing more ad­vance, which was just what was needed. Photo 1 shows a typ­i­cal auto-ad­vance unit. The tim­ing changes for changes in mix­ture strength could be ac­com­mo­dated by the use of an ar­range­ment called a vac­uum-ad­vance unit. A small pipe from the in­let tract led to a di­aphragm, which was con­nected to a lever that could al­ter the tim­ing. Dur­ing cruis­ing when a weaker mix­ture was used to pro­mote fuel econ­omy, the throt­tle was scarcely opened, so the in­let vac­uum was high and the tim­ing was fur­ther ad­vanced. This set-up was fairly rare on bikes, prob­a­bly be­cause car­bu­ret­tors were not set so weak for cruis­ing. Of course, later the me­chan­i­cal de­vices were re­placed by elec­tronic equiv­a­lents that did not wear and thus were more ac­cu­rate. Later still, the con­trol came from an en­gine man­age­ment sys­tem, which in­cor­po­rated fuel in­jec­tion and ig­ni­tion pro­duc­tion and con­trol. So how do you know what is the best ig­ni­tion tim­ing set­ting to use? What­ever sys­tem you have, you can be sure that Ja­panese man­u­fac­tur­ers will have spent many hours test­ing en­gines un­der a wide range of con­di­tions to come up with the best com­pro­mise be­tween power out­put, econ­omy, ride­abilty, en­gine life, re­li­able start­ing and other fac­tors. Con­se­quently, it is usu­ally the case that it is best to stick to their rec­om­mended set­ting. How­ever, if your en­gine, car­bu­ret­tors or ex­haust sys­tem have been mod­i­fied you ought to carry out your own ex­per­i­ments to find out the new cor­rect set­tings. Un­for­tu­nately, most of us don’t have a fully-equipped

lab­o­ra­tory with dy­namome­ter in the back gar­den, so you will need to con­tract this work out. How­ever, ex­per­i­ments can some­times be car­ried out on a more in­for­mal ba­sis and I do re­mem­ber read­ing a re­port in an Amer­i­can mag­a­zine in the early Sev­en­ties about a se­ries of tests they car­ried out on a CB750. It grad­u­ally in­creased the ig­ni­tion ad­vance in steps and, af­ter each ad­just­ment, mea­sured the stand­ing quar­ter-mile times as a sim­ple way to gauge at least an idea of power out­put. The mag­a­zine found that the times did im­prove, up to a cer­tain point, but the fuel con­sump­tion in­creased cor­re­spond­ingly. I would not rec­om­mend this idea though, as the in­crease in ad­vance is ac­com­pa­nied by an in­crease in max­i­mum gas pres­sure in the cylin­der. Even­tu­ally, the mix­ture will start to det­o­nate and pro­duce a noise usu­ally called pink­ing or knock­ing. This ham­mers the top of the pis­ton and will even­tu­ally

break it, with ex­pen­sive re­sults. The dan­ger is that it is not al­ways pos­si­ble to hear the pink­ing, par­tic­u­larly at high en­gine speed and high power out­put – just when max­i­mum dam­age is oc­cur­ring. This is es­pe­cially cru­cial on high-per­for­mance two-strokes. Mod­ern en­gine man­age­ment sys­tems of­ten in­cor­po­rate a spe­cial sen­sor that, in ef­fect, con­tin­u­ously picks up the sound of com­bus­tion and there­fore de­tects the start of pink­ing. It grad­u­ally re­tards the tim­ing to just pre­vent it, which gen­er­ally pro­vides the best per­for­mance. But you don’t have any­thing like that on your old RD350 do you? That is why you need to take spe­cial care to get the tim­ing cor­rect. So now I hope you know what is meant by ig­ni­tion tim­ing and why the set­ting is im­por­tant, so next time I shall move on to the work­shop to check how the set­ting can be checked us­ing two tech­niques on a cou­ple of rather dif­fer­ent bikes. cmm

The vac­uum ad­vance is pow­ered by the sil­ver di­aphragm unit.

The Eight­ies saw solid state ig­ni­tion units: this from a VT500.

Auto-ad­vance unit: 1970s.

It’s all in the books you chucked out years ago.

You could try in­creas­ing the ad­vance on your old CB750…

Mod­ern(ish) and dig­i­tal.

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.