Topic: CB500 four, electronic ignitions, round/flat slide carb sizing

[designsincoldsteel]

Hi all,

Im a CB noob, however not a noob to choppin/ridin bikes.. I just picked up a low mileage '71 CB500 motor out of a local salvage yard for my next lil' project. I'd like to convert to an electronic ignition, and I'm wondering if anyone knows anything about or is using the Dyna S electronic ignition or Boyer Brandsen electronic ignition systems I've found for these motors..  Im also wanting to convert to two bigger mikuni round or flat slide carbs, and was wondering if anyone has suggestions on sizing for this upgrade.. I get the feeling two 30mm's or two 32mm's would suffice, however Im definitely open to suggestions/input.  Naturally the exhaust will be upgraded, most likely a 4 into 2 so that will be taken into account when choosing carb sizing.

Thanks!

[designsincoldsteel]

No one has any info for me??

Did I post this in the wrong section?

[bryanj]

Nope just a strange question, all the sohc 4 run the same ignition setup so any electronic thats fits will work its all down to your pocket depth and availability.

As to carb that is such a strange change that you are totaly on your own

[designsincoldsteel]

On the carbs; 

Strange change? xs650 guys go to round or flat slide carbs all the time, granted yes, they generally stick with two, but the only reason im looking to go from 4 to 2 is cut cost as apposed to 4, while gaining the tuning/response advantages of a modern carb design.  Im simply curious as to whether or not this has been done, but im proud to be the first to skinny dip in that pool.

Ignition;

The Dyna S ignition with new coils ends up being a little over $300, while the Boyer Brandsen is around $250. In my opinion, and compared to most CB750 parts I've seen, neither of these is too bad as far as cost, just again, wondering if they have been used, the troubles (if any) people have had, etc. But again, if I have to be the first, then more power to me! And I will keep the rest of the CB world posted.

Uh...? Thanks?

[bryanj]

Both ignition types have been used regularly by different people on the board, if you are in US Dyna is made there if UK Boyer is made there, you pays your money and takes your pike. 500 charging circuits dont like the low ohm coils apart from that whatever

XS is a totaly different beast to play with carbs on

[designsincoldsteel]

Now thats the kind of stuff i like to hear! so if i use stock coils with the dyna unit, its more likely ill have a better result? or would you say im better off to stay all stock (ignition that is)? The biggest reason im considering it is because i have just the motor right now, no coils, ignitor, nothing.

[mlinder]

cyclex makes a 2 carb setup for the 750's. Someone else did way back in the day, too, but can't remember who. 754 has one, I think.

The fabrication needed for designing CORRECT 2 to 4 runners would likely cost a lot more than throwing on some nice 4 carb rack.

As a side note, we've noticed that the carb rack from the 78gs550 is almost a perfect match up.
How are the carbs that are on the bike now?

[TwoTired]

The 500's charging system is not very powerful.  It makes 150 watts at 5000 RPM, and about 40-50 Watts at idle.

The standard dyna-s makes any coil installed draw more power than the stock points, by virtue of the dwell operation.  Stock points have a dwell of about 195 degrees of crank rotation, each.  The Dyna-S extends the coil power draw time to about 315 degrees.

The stock 5 ohm coils (or aftermarket 5 ohm), minimize increased power loads over a 3 ohm selection when using the dyna S. But, you will still have quicker battery recharge time using stock points and stock coils.

The problem with the stock coils is the wire leads.  The insulation does not age well.  And this makes it crack rather than flex.  Cracked insulation is not a good thing, and they are molded into the the stock coils, so replacement is not a simple task.

[turboguzzi]

two carbs setups are not the best ticket on fours (unless you are talking about double barrel things), the 180 spacing between cyls 1-2 & 3-4 will do strange stuff.

get a set of 26mm mikunis from a kz650 / GS 750, that'll give you plenty oomph. I race with such a set.

[designsincoldsteel]

Awesome, these are the responses i've been looking for..

There are no coils on the bike, as there is no bike yet, I'll be making my own frame, thus the only CB500 parts on the bike will be the engine, charging system, and as it looks, ignition system.

As for the carbs, looks like two might not be the answer afterall, Ill look around for some suzuki carbs...

What are some other good modifications aside from the obvious intake/exhaust mods that are available for the 500's??

[bryanj]

Head work by Mreik on here and a cam, 550 cylinder and liners with pistons---you MAY have to open the crankcase mouths to get the liners in, some do some dont, forget the 605 kits unless you are willing to do extra work with the rings or feed oil in constantly, at least thats what others on here have found

[Jay B]

fishhead, could you give me the dimensions of the kawa carb rack in the second pic? Center to center, length, and OD of both ends? If you have them laying around I'd sure appreciate it.

[designsincoldsteel]

Thanks to all for the great info/tips..

What kind of power would I expect with some head work, cam, full exhaust, and the upgraded carbs, running stock igntion and charge?

[turboguzzi]

my 500 with some very heavy mods is up to 62 rwhp @ 11,000 lately but that's a pretty racy tune, 12.5:1 CR, not so good for the street really. Another racing sohc memeber was up to 65 even (or so he says...)

if you are not bound by a 500cc limit, go big bore. you'll have much more tractable engine, need lower revs and have a longer motor life.

hats off for building a frame!

TG