Cam shape

Reading David Hunter's notes on standard 52x58 tuning he speaks of a rapid then gradual to close cam shape. Does anyone have any info/blueprints of what racers (mostly aimed at ones who were around when they didn't use Motoplat etc) used about cam shape - I'm able to get cam blanks off a friend who runs a business, so it's just a means of getting the info. Any help would be greatly appreciated. 

Tim

I think Tim is asking what the profile was of those specially made contact breaker cams that mounted on a spigot on the end of the timing shaft of a set of Frank Perkins small flywheels.

Dave H's description is pretty accurate. The slow closing was to reduce contact breaker "bounce".  The only opportunity for the coil to charge was when the point were closed, so it was advisable to get the gap closed quickly, but as I say, bounce was to be avoided. To reduce bounce, I used (Austin or Morris) Cooper S contact sets because the spring was that much stronger.

Well Tim, was this the info you after?

Hi Tim ,
           George Todd produced a race spec ign cam, ostensibly, it featured rapid points opening with a longer closed period, for just the reasons Peter has outlined . For what it is worth, I used Suzuki TR250 contact breaker points together with a Bosch 12v coil .
Much more to the point, I have a small picture of the standard cam and a George cam side by side, if you care to PM me your email address I will send it to you, but there must be dozens of them out there, perhaps someone can come up trumps for you ?
Get digging fellas, there is a guy here needs your help !

Cheers,  Trevor

Yes, I think I must've worded it wrong. My bad - thank you Peter. Would it be advisable to use Cooper S sets then or? I'm only a few blocks down the road from the now defunct Downton Engineering works where the S was developed, I think Kagawa or Mikina Engineering now have some of their stock, I'll have to have a look and see what I can find. I've exchanged a few emails between the Bonneville Bantam crew and Volvo outboard engine cams can be used, only need to fit a key slot. I'm mostly aiming to replicate a mix of the championship winning bikes of the 70's/early 80's so always happy to receive advice on what's good and what didn't work. 

Trevor - I think I know about the cams you're on about. They're S1250 cams I believe, have an original magazine clipping from "Motorcycle Mechanics" I think mid-70's, Googled what I can on Bantam cam shape and nothing seems to come up trumps. It does have a plan view of the factory cam vs one of George's, it's pretty obvious in the shape it's just getting the profiling of it and I can have the cams made. For what I've researched these cams are harder to find than hens teeth! I'll PM you now. 

Regards,

Tim

Hi Tim & Trevor,

Reading you posts has jogged my memory. I discarded the Cooper S arrangement in favour of some Japanese 125cc points. They were a healthy improvement on the BMC design, having a bushing in the moving part to reduce wear of the pivot spindle. The rest of the assembly was more robust too.  Still used the same cam though. I am going to have a look to see if I have any of these cams left.  Don't hold your breath though, they would have to have been retained through 3 separate clear outs I know of!

Hi  Tim/peter/trevor

We also started off with the cooper s points system, only because Dad said a highly tuned sprint Engine cooper would rev to 10,000, but we changed them to a set off a 100cc Kart Engine that reved to 12,000. a very early BM class engine from 60's. did the job, Dad modified a std cam by first welding then grinding, and I'm sure we had the lift around 0.014 thou, sorry have no drawings, but I'm sure they have been detailed, perhaps you should face book George Todd, he may have the drawings he has sent me loads of stuff.

regards Derek

Hi Tim/Trevor/Derek,

I have searched my stuff and have no cams. I'm sure they went in a 2001 clear-out. But I did find a few new sets of Bosch points which look like they will do the job.  If I can find out how to insert a pic on this forum I'll do so. Any hints very welcome!

Courtesy & Property Of Peter Tibbitts

Hi Peter/Ed/Derek/Trevor, 

Do you know what Japanese points they were Peter? The Bosch points you spoke of and with the photo Ed uploaded I've seen before, they're off a VW Beetle but fit quite a few models. 01011 I believe, will have to check when home. No problem with the cams, I'll probably order a few standard cams then weld and regrind like Derek said. Guess I'll have to go for trial and error. Derek I think going down the kart route might help a lot, most of them still use points and are often pushing out 20-25 bhp for a 125/150cc category. Some of them use the Rotax engines I believe. I'll get in touch with George Todd however I might quickly check out his "Shoestring Racer" book in the meantime see what I can find. 

Regards,

Tim

Derek - I have a few standard cams, and with no drawings do you have any further details on the cam you and your Dad tried with? Might have to attempt the same myself, or go from scratch. 

Tim

Now I know how much of a dumb-dumb I had become during our Icarus-1 developing days of me being the Go-Fer Pilot.

  I thought contact breaker points went out when "Breakerless transitorised coil-ignítion systems" came in with either "induction-type pulse generator" or the "Hall-type pulse generator" to replace CB points. At that time people were changing over to this `something new & transitorised´ and we were sticking to our known `reliability´ of CB points and having a feeling of success at actually finishing races.  Icarus-1 was a reliable Inter-Novice finisher with one pair of tyres per three seasons...

 Of course that all changed with the 250 ABS which felt as if there was a chance of winning races compared with the Icarus-1 Bantam.


 I must say the thought of a 25 bhp at 12,000rpm Bantam is impressive and I wish we´d got to that ...

Now I am thinking -- 200times a second the CB points open & shut WITH RELIABILITY -- how is that possible?  

  It´s all magic I am thinking!

Cheers!

I.B.A. Cretin.

Hi Tim sorry not any more, im sure ive seen a drawing somewhere on here !

Good Morning Tim,

I feel you will have to cast your net a little wider than karting in order to find suitable contact breakers, I stopped karting in 2005 and there were definatley no karts using contact breakers back then, even the 210 nationls that deployed the venerable villiers engine had long since migrated to motoplat or similar, good luck in your quest.

Regards,

Mike

Hi Derek/Mike,

Derek - that's no problem, thanks anyway. Shall see what else I can find on your "weld and regrind" method.

Mike - I will have to agree there, I've had a look around and continue to do so. I think Peter and Trevor's idea of using Bosch points and coil might do. I currently volunteer at a local airfield renovating aircraft, and I might have to extend my search towards the two stroke Rotax engines, perhaps. On the other hand my Dad recommended we use Stihl contact breakers as for use in garden strimmers, as they will happily sit between 11,500-13,000 RPM for a consumable amount of time. Just a thought there that was all.

Tim

Hi Tim,

If you want me to stick a couple of these Bosch CB points in the post to you, just pm me your address.  

Re the Japanese 125cc points I used in the early to mid 1970's, I've tried desperately to remember what they were off, but nothing yet. I am sure Dave Hunter put me on to them.

But I think you will find the above robust enough. Mention of Rotax reminded me I have some ROTRAX magneto points sets, no use to anyone now I suspect.

Cheers

Pete

No answer to my query re `supposed demise´of CB points on Bantam racing engines...??

HI JB,

Please could you restate your question. I am sure I can answer it, if I knew what precisely you were asking!

Looking forward to a reply......

PJ

Interested to know why a topic of point ignition would be of any interest to any potential bantam racers given the advances in easily available and far superior electronic units ?

John and Mike,

Re-reading Tim's posts, he states he is trying to replicate a mix of the Championship winning Bantams of the early 1970's. Thus he would like to replicate their ignition systems.

Hope that explains it!

Pete

Hi Mike and Peter,

Didn't see Mike's post until this morning - another factor I chose to use points was that I can fashion an ignition that would work for my bike up to [and including] 10,000 RPM, or so I hope, for a fraction of the price of say PVL/Motoplat/Rex Caunt - I don't have to change the shaft either as well, minimising my costs. Hope this helps answer your question.

Regards,

Tim

Hello Peter!
When  "Breakerless-transitorised ignition..." hit the scene CB Points became redundant.  

 So? Did I hear it wrong -- or get it wrong,  and competitive Bantams were still using CB Points after -- say -- 1973?

 That is a thing of precision engineering -- "Profiling" a cam shape for the shutting of the points gap of 10 t0 20 thou. As far as I knew in early Seventies there was a Symmetrical shape -- open and shut shape the same;  an assymetrical where the shutting shape was `tailored´;   and a third shape where both opening & shutting were shaped to prevent bounce -- and surge -- at high speed?

Cheers!

Just a small point:     when `profiling´ a cam the system needs some sort of spring-damping if the profiling or cam-grinding is not good enough to prevent deceleration rates (gap closing)  rushing to infinity. Just a tiny bit of flop-wear on the rotor shaft cancels out the precision grinding....

The mass of the (moving) breaker-arm has to be at a minimum with a spring rate and spring characteristics (ie damping) to suit -- otherwise you might just as well go for a symetrical cam shape of short duration with the minimum CB gap at 0.010" (ten thou) and clean (or change)  the points regularly.

Having said that I must confess to having never bothered with CB cam shapes but have been involved with high-lift cams on 4 stroke engines. In the Fifties, grass track racing, some awful things were seen to have happened with exhaust & inlet valves clouting piston crowns because of faulty cam-profiling. Len Edney used to grind his own (Rudge) cams and frequently replace exhaust valves on his three Rudges...

I seem to remember Alan Brown writing -- on here-- of his ignition system which had a tremendous spark voltage and that the big publicity  thing of "Breakeless-Transitorised Ignition" -- in eraly 70´s was having 15,000  volts at the spark-plug instead of a measly 6,500 v....  

You raised an interesting subject Tim, got me thinking.

Cheers!