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ignition curve

J

jr1967

Cruisin' Guzzisti
Joined
Oct 29, 2008
Messages
244
Who can help me with a proven ignition curve for an engine with midsize (44/37mm) valves, 41mm inlet ports, 1037cc, 1:10.5 compression ratio, Megacycle cam with Norris(I believe 382)profile. At the moment a rough running 80rwhp.
I know that max advance should be 28 degrees, but what should be the static advance, idle advance and data at 2-3-4-5-6-7-8000rpm?

I have contacted various ignition manufacturers but they all disagree on how steep and at what rpm the curve should start to climb.
Who's got real world experience with this. Once I got this tackled I can start concentrating on setting up the carbs.

Thanks, JR
 
A couple of things.

Rough running may come from your cam. 382 is an odd profile that I haven't used. Some of the Guzzi Megacycle grinds simply do not work very wel, especialyl on road bikes, and that may be one of them.

28 deg full advance seems a little bit retarded. Cant remember right now, but I remember full advance for Guzzi performance bikes being in the low 30's...32 I seem to remember. Otherwise, might cost you some top end.

Dyna is the most common electronic ignition in the US. It has a linear ignition curve. There is no curve adjustment, except to adjust idle and full advance simultaneously, advance or retard by the same amount. There is little reason to play with curves art part throttle, mid rpm range unless you are desperately trying to tune out a knock at part throttle...which you shouldn't be getting with 28 deg (mild) advance. Especially on a road bike where you are not at WOT most of the time. Silent Hektic and others offer adjustable curves, but there's little benefit to any of them except as stated above. I'd just start with a linear curve such as with Dyna.
 
Thanks for the info.
I'll try my Dyna again for now. I just found through a helpful member of the Dutch MGCN forum that Silent Hektik advises a 4 degree advance at idle and a max advance of 28 degrees for bikes with 92mm bore and compression over 1:10 , which is my case. Dyna's actually spark when the engine is turned by hand, so I have locked the advance mechanism at full advance and set it at 28 degrees, the released it and checked it for static advance. The static advance figure I just didn't know exactly until today, and like I said before every expert is right and disagrees with the rest..... The opinions varied from 0 to 2 to 5 to 8 degrees static advance... Obviously a great source of debate(shall I start an oil thread?).
I got annoyed with my Dyna since with a strobe I can not get a stable picture with the engine running; the marks go all over the place, the same as I had with my old Lucas Rita. I did shim the ignition tower, but got no imporvement. Any cures for that? Worn advancer? I assume that it means that the ignition in fact is all over the place.
Another thing: I know Dyna changed their minds about the recommended coils, it seems now that we need 5 Ohm coils to keep the ignition from frying; any thoughts since I don't feel like shelving my 3 Ohm coils.

Thanks, JR
 
The running variation may be from worn/tired advance springs. Perhaps a new set of springs would help.
 
Maybe tighten up any lash in your advance springs or distributor for the bouncing idle...low load, lumpy cam, low idle, etc, might make idle ignition imprecise. That's a good idea for checking full advance. I just set dyna-ed Guzzis by hand at static advance, with the bike not running. Then check it with a strobe at idle and full advance. Dyna updated their recommendation on the coils to the black 5 ohm one. I'd run what you have and if they blow up ~8 years down the road, you can update em. Not that big a deal.

Expert opinions should be weighted by their source. My opinion is"ignition curve doesn't matter much...just static and full advance points". SH recommendation is possibly to keep them from being sued/harassed by an owner who runs too much advance and holes a piston...all depends on gas grade, ambient temps, etc, too. 28 deg is a little too tame for my tastes. Check on a dyno to be sure.

Ed
 
I have the green 3 ohm Dyna coils on my twin plugged T3, & the black 5 ohm on my 1000S, neither has been a problem. That said, I believe the 5 ohm is preferred, much less likely to overheat & fry themselves. On my T3 with the 3 ohms, the coils are very firmly attached to the frame for best possible heat conductance, & also up front just below the steering head in the full breeze. Either way, I don't believe their us a performance difference, just a reliability one.

Also agree with others, source of your timing difficulties does not lie in the Dyna, rather in timing chain slop or advance springs/distributor. I would also trust Ed Milich re the relative importance of advance curve details, he has lots of on track experience. or you could spring for some Dyno time like Ed suggests if you gotta know for sure.....

My many years of seat-of the butt-o-meter experience on the street, esp before I ditched points for Dyna mostly for the sake of convenience is that carbed Guzzis at least are pretty darn tolerant of timing details in general.
 
jurjenratsma wrote:
I got annoyed with my Dyna since with a strobe I can not get a stable picture with the engine running; the marks go all over the place, the same as I had with my old Lucas Rita. I did shim the ignition tower, but got no imporvement. Any cures for that? Worn advancer? I assume that it means that the ignition in fact is all over the place.
Click to expand...

It could be that your strobe is mis-triggering. Not sure how your strobe is coupled to the spark, you could try reducing the coupling somehow to see if you get an improvement.

I assume you are not using the series-connected Neon strobe? These always "blurred" at higher than idle revs, I believe the random ionisation of the gas was responsible. To get a clean image you need a Xenon strobe, powered by a separate battery or mains. If you use the bike's battery, this can cause false triggering.
 
Thanks guys for all the input; I'll try to answer some of your suggestions.
Yes I can try and check my cam chain tensioner; I just forgot in what box I stored it:laugh: :laugh: :laugh: :laugh: I am running cam gears(sorry Pete R.) and will re-think their use because of what I have been reading here.
I have two strobes(1 cheap, 1 expensive) that both clamp over the sparkplug wire. Shouldn't be a problem. I should maybe just experiment with running the engine on one cylinder to see if it is not an interference thing with the other side.
With the coils, I am aware it's just a reliability/heat issue, my 3 ohm coils are bolted on with rubber insulated clamps though.....
I'll splurge for some new advance springs, unfortunately the postage will exceed the purchase(Vancouver Island).
The thing with the strobe is that I don't get a clear picture at all: at steady rpm it flutters at least 10 degrees and with changing rpms above the full advance speed it moves with the throttle movement up and down. All in all pretty frustrating. I would't bother any further if I could at least get the max advance exactly right. I have the feeling that with running motor the magnetic field of the pickups give another timing than when I crank it over by hand(different induction at speed). Not sure if it is true, but I am getting a little suspiscious about the whole thing.
Thanks again for the input.
Cheers, JR
 
FWIW,
I spent quite a lot of time a long time ago setting up race engines using Zytek's engine management box - advanced for its day, now very obsolete. Basically we sat at an engine dyno and typed in numbers at specific RPM/Loads until the engine got max power while behaving. It is a time consuming occupation as fuel and timing are interrelated. The usual drill is advance it till it knocks and then dial it back 3-4 degrees.

Here is what I learned:

The curve is 3D not a curve which is why digital ignition systems have taken over. if you don't want to spend oodles on a digital ignition then you can recurve the distributor by fiddling with weights and springs and stops. Sun made a distributor checking machine - they can be picked up for pennies if you can find one. Given that we are talking about an old donkey I would stick with the standard distributor myself.

The curve looks like a mountain and usually incorporates a double deep as the engine runs out of breath at higher RPM where you can dial in more advance

The curve more or less relates to the Volumetric efficiency profile of the engine. Interesting you can dial in fantastic amounts of advance at no idle/ tickover. Of course with a simple distributor without vac advance this is not a concern.

it can be very hard to hear detonation - a detonation sensor is critical for this kind of work especially with 10 or more cylinders.

The max advance you can run is very dependent on the fuel available so add that to the list of variables. I have found that contrary to some opinions twin spark is of great use in reducing detonation - at least with the small and mid valves. IMO 28 degrees advance is retarded given your spec. If you can't get to a dyno with a four gas meter - which given your mods is not a bad idea - then do some WOT runs and advance it bit by bit until you see no more benefits and then knock it back a bit for safety.
 
Thanks for your info; I have seen the digital programmable ignition boxes in the past, but like you say it's time consuming to set up properly. I have no direct access to a dyno on the island, it's a ferry ride away so I ordered a wide band O2 sensor kit($229 -Innovative Motor Sports). So let's see what that brings, at least I will be able to tell if the jetting is right when I change the ignition timing.
I kept the original springs in the distributor (should be ok according to Raceo UK) and I have changed the range of the advancer to 12 degrees(4 static to 28 max on the crank).
I'll post once I have tried how everything works.
Thanks, JR
 
The problem with keeping the original springs is that they fatigue over time and usage. A new set are relatively in-expensive and may help in your situation.
 
Thanks for the info.
About the advance springs; yes I will replace them, I just meant I'll keep the original type of spring. Let's see if there will be any improvement with the strobe picture after that.
I have seen that ignition of Cliff Jeffries before-it looks good, I just didn't find it on his website when I googled it lately. I'll email him once I feel the need for a new ignition, but after the replies here and elsewhere I feel that I should be able to get a Guz running properly with a Dyna, it's just a matter of too many variables now so I am eagerly awaiting my new O2 sensor kit coming week. http://www.innovatemotorsports.com/products/db.php
 
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