750 twin and Mallory Hyfire - moved from 'Coil recommendation for '79 750 twin'

The 750 twin is a 360 degree twin, both pistons rise and fall together, and it has one set of points (or one pickup coil).
Both plugs fire at the same time, but one cylinder is at the top of the exhaust stroke (and is wasted) while the other cylinder is at the top of the compression stroke.

Four cycles equals two revolutions of the crankshaft, so yes it does see two spark signals per four cycles (or one per crank revolution).

ok so..lets say at 100 rpm of crank rotation the box actually thinks the crank is spinning at 200rpm.
then at 6000rpm the box would think the crank was spinning at 12000 rpm.
i think the only way to use the rev limiter would be to either use a 24000rpm limit to get a 12000rpm limit.

I think you went the wrong way here

ok so..lets say at 100 rpm of crank rotation the box actually thinks the crank is spinning at 200rpm.

Since a four cylinder car motor sees two spark triggers per crankshaft revolution, and the twin has one spark trigger per crankshaft revolution, the box would "think" the motor is turning at 100 rpm when it's actually turning at 200 rpm.

Keep in mind that a four cylinder car motor with a distributor has the distributor running off the camshaft which is turning at 1/2 engine rpm.

Post edited by: steell, at: 2006/08/19 10:44

Also, it doesn't matter if a single pickup is on the crank, or two pickups are on the camshaft. You still have one spark event per crank rev.

And since you mentioned it...
With wasted spark, it appears the spark has to jump double the normal spark gap. Well, in actual measured distance it does, but electrically, it does not. It jumps much less than twice the normal gap, electrically, because there is really only one gap, the other ends up insignificant.

Here's why: When you compress the gasses in the cylinder, there is more insulation in the gap. At 10:1 compression, it appears to the ignition coil that the single gap under compression has widened 10 fold (assuming you are getting full cylinder filling with atmospheric pressure, which probably never really happens). So now, if you relate it to atmospheric pressure, the gap seems a half an inch wide on the cylinder with compression. Meanwhile, the cylinder without compression is still only at a few thousandths. To the coil, it is jumping a gap with a half inch plus a few thousandths. Effectively, it only sees the one compressed gap. The other is so small, it is insignificant. That is why you need to see a consistent spark of 1/2 inch (or 3/4) in open air to test the coil.

If both cylinders fired together, the gap would appear as a full inch to the coil. It most likely won't be able to spark with stock coils. Newer coils may be able to pull it off, but probably not reliably.

Opening the gap a little on the plug means you are effectively opening it about 10 times the actual distance you move the electrode.

Post edited by: loudhvx, at: 2006/08/19 12:28

Ok, finally got to working on the 750 again. Too many other things getting in the way (Fire dept. relief association ice cream social thursday night, saw the Body Worlds exhibit on Friday, then supper, today a long day of confined space training and this evening fire relief family picnic...)anyway, it works great. We all knew that Mallory support was rude in the responses I got from them. It is now proven that they are also arrogant imbiciles who are flat out WRONG (Ok, it might be just the one person I got responses from...). this bike has never run so well since I got it 5 or so years ago. It idles smoother (although it still has some carb cleaning/tuning needed) and really comes alive from ~3500 to 8000 rpm. It is a drastic improvement. I think I will need to be looking at clutch adjustment or replacement soon too. Hopefully that can hold off till winter. The carb issues will be addressed by getting rid of them, in favor of fuel injection, again probably this winter.

If you email that guy back and say "it works great on the bike! Thanks for all the help!", he probably won't even reply back. Guys like that really hurt their business.

Glad you got it working!

Ok, so now that the impossible works, I need to ask a couple of questions. For the magnetic pickup, I was told that I need to use the advancer plate from the KZ440 instead of the one for the KZ750 twin, why is that? I did as told (I'd hate to try the impossible ) and used the advancer plate and lobe from the 440 with the springs and weights from the 750. It works well, but after a while of driving, I started to get this rather worrysome noise. At first I thought I was starting to have internal bearing noises, obviously not a pleasant thought. Then as I listened a little more, I started thinking that there was something wrong with the timing advance. After a 1 mile walk from where I left the bike to home for some tools, on returning I removed the points cover, and confirmed that the noise (kinda a loud howling sound) was coming from the advancer, and the sound changed as the advance weights changed position. I sprayed PB Blaster in there while the motor was running (nice to not have points now) and it lessened the noise some, but did not eliminate it. When I was putting it together I lubed things with some spray White Lithium grease. Could it be that that was not the right thing to use, and I just need to lube with some more PB Blaster?

Thanks all for your help and encouragement with this fun project!

I thought you need the 440 advancer because the advancer shaft was too long on the 750 unit. Was that it, Steell?

IIRC, the 440 rotor and shaft would not fit over the shaft on the advancer, I seem to recall that the shaft on the 750 advancer was bigger than the hole in the rotor shaft on the 440.

It's been awhile since I did a conversion, but I know I would have used the 750 advancer mounting plate if I could, because the 440 has a total advance of 40 degrees (limited by the tabs on the advancer plate) vs 35 degrees for the 750. It has not caused any noticable problems though.

You could bend the tabs in a little to limit the advance to 35 degrees if you think 40 degrees is too much.

I have to run into town and work for a couple of hours today (hope the boss remembers I have a four hour minimum), but when I get back I'll compare the 440, 750 electronic, and 750 points, timing advancers and report back.

I need to find another 440 wiring harness to cut the plugs off of so I can convert two more 750 twins to electronic ignition.

Post edited by: steell, at: 2006/08/20 02:33

loudhvx wrote:

If you email that guy back and say "it works great on the bike! Thanks for all the help!", he probably won't even reply back. Guys like that really hurt their business.

Glad you got it working!

I was thinking an email with video attachment of the bike running with their product. Sort of a demo video for them. Or, I could just put the video online, and provide them a link to it. Maybe I could cc the email to a a sales manager, technical folks at Mallory, a few automotive/offroad news groups, and maybe MSD (their main competitor), just for fun!

steell wrote:

IIRC, the 440 rotor and shaft would not fit over the shaft on the advancer, I seem to recall that the shaft on the 750 advancer was bigger than the hole in the rotor shaft on the 440.

It's been awhile since I did a conversion, but I know I would have used the 750 advancer mounting plate if I could, because the 440 has a total advance of 40 degrees (limited by the tabs on the advancer plate) vs 35 degrees for the 750. It has not caused any noticable problems though.

You could bend the tabs in a little to limit the advance to 35 degrees if you think 40 degrees is too much.

I first put the 440 part on the 750 advancer plate and shaft. It seemed to be fine, but I switched to the 440 plate/shaft anyway. It seems to need the choke more now to get started, but that has been a progressive thing, I think. I'll just keep an eye on how it goes from here. Any thoughts on the howling/whirring sound from the mechanical advance? Just lube the snot out of it? I suppose I could start playing withthe spark plug gap too. Might be able to go to .045-.050, to get a better spark?

That's veddy interesting

I have here at the moment, a KZ750B points advancer, a KZ750M (82) electronic advancer, and a KZ440 electronic advancer (unknown year), and the rotors interchange on all three of them (I just checked).
I know that on the first one I did (a 79 KZ750B4), the 440 rotor would not fit on the 750 plate, so I had to use the 440 plate also. So it seems to me that the only explanation is that not all 440 electronic advancers are the same, either that or my grip on reality is even more tenuous than I thought (that's always a possibility) :laugh: :laugh:

Post edited by: steell, at: 2006/08/20 14:57

Here was my reply to the friendly support folks at Century Performance. We'll see how they handle this:

I am very displeased with the lack of support for your product. I thought that I was asking a very simple question, and would have appreciated a respectful response. It may have been a mistake on my part to assume that when I sent a question to 'tech support' that I would get a response from someone who could answer a technical question, instead of 'sales support'. I realize that my question was concerning a 'non-standard' installation for one of your products, but that certainly should not be deserving of such a rude and disrespectful response.

I would like to let you know that the Hyfire 6AL does in fact work very well on a 2 cylinder engine (as I knew that it would), in fact my motorcycle has never run this well in the five years that I have owned it. The installation is straight from your own instructions (page 6), Installing the HYFIRE® VIA with a Magnetic Pickup Distributor or Crank Trigger. I did not ask about the installation of the product, I only asked about what could be expected from the rev limiter. I believe that I now know what could be expected of the rev limiter also. I believe that if I set the unit up to think it is running a 4 cylinder, I can set the limiter for 4500 and expect that it will really be 9000 for my installation. This is due to a 4 cylinder engine providing 2 trigger events for each crank revolution, while my motorcycle provides one trigger event for each crank revolution. I should also be able to accommodate for tachometer output being incorrect for use with the fuel injection I am planning to install (but I certainly will not rely on you for help with that!).

In the interest of full disclosure, I did not come to this conclusion about the rev limiter on my own. I have been discussing this issue, including your responses to my questions, with the fine, knowledgeable folks at kzrider.com , an online community of 6770 registered members. It is sad to say that a few of the members there were much more supportive of my situation than you were. If I had not already had this product on a shelf in my garage (from a previous installation on a v8 Jeep) and were basing a purchase on the response from your support organization, I would never purchase one of your products.

Before I end, as for your comments about adapting, rewiring, waving magic wands and praying to have it work... I would expect that praying over it would be the most positive approach. I shall also pray for you to understand that your approach to this issue was not appropriate, and that you would be able to use this example to improve on your customer relations, so that you may be more prosperous in the years ahead.

Respectfully,

Jeff Hanson | Network Design Engineer | GOIS
Unisys | 3199 Pilot Knob Road | Eagan, MN 55122 | 651.687.3314

Post edited by: securitygeek, at: 2006/08/22 21:49

Well said!!