Ignition Module

[Jerry]

What's interesting is the amount of advance that the XJ specs you stated provide...only max 36 degrees, or perhaps that also doesn't account for any additional advance due to the vacuum boost sensor??

Specs are off the microfiche Tech Data page for the XJ. These 3 numbers are the only ones quoted.

Backing up a bit.....

Engines need to fire spark plugs BTDC (Before Top Dead Center) so the charge can burn and reach full power just as the piston goes past TDC. At the same time you don't want the burn to start to early which would rob the engine of power by putting pressure on the rising piston. The quandry is that piston speed changes but the burn rate does not. To get better performance, the point at which the burn starts needs to be varied with engine speed and load.

Most engines start with a "base timing"; in the XS it is around 15 degrees BTDC. This is known as static timing. When starting your engine the plugs will fire at 15 degrees BTDC. As soon as the engine catches and gets to idle two other factors come into play.

One of these is the curve programmed into the TCI. The other is the vacuum advance mechanism, or in the case of the XJ, the vacuum sensor.

The TCI knows your engine RPM and will produce the advance programmed into it. As RPM rises vacuum drops to the point where it has no effect, leaving the TCI to provide the timing value. In both cases this appears to be 36 degrees at slightly different RPMs, 5200 for the 2H7 and 4RO boxes, 5500 RPM for the 10M box.

So far, so good. Both the 2H7, 4RO, and 10M TCI boxes pretty much agree on the max advance. There is some difference at idle, with the XJ at 15 static + 5 advance or 20 degrees total advance vs the static 15 plus 10 advance or 25 degrees for the '81 models that TC mentions.

So why have a vacuum sensing system? Here is a quote from a Chevy performance web page:

The reason your engine is able to sustain more timing at part-throttle is because only a limited amount of air and fuel make it into the cylinder at part-throttle. Lower cylinder pressures enable the combustion process to start sooner and help improve part-throttle response by increasing torque. This additional part-throttle timing improves efficiency and torque.

Ideal ignition settings will allow your engine to run the maximum amount of timing at all engine speeds without detonation.

So, at part throttle operation, the vacuum advance system adds more timing advance to the system. It does something the TCI cannot do; and that is sense load via vacuum. That is why I am confused by the statement that the reason the 4RO box works on the XJ is that the function of the vacuum advance is programmed into the TCI. The 4RO TCI on an XJ can't sense load - no input. The 4RO depends on a vacuum-sourced signal to activate the cent advance curve programmed into it, and there is no such signal present when installed on an XJ.

I am not even convinced you would get 36 degrees in the middle rpm ranges (3000 - 4000 RPM) since I don't know when the TCI is programmed to produce the 36 value. We know max advance happens by specific RPMs or no vacuum (we think, not proven), but in the lower ranges that leads to a catch-22: if the TCI has no vacuum input, it never detects zero vacuum! Or, without inputs, it defaults to zero vacuum values and puts out the advance based solely on RPM.

The idea that a 4RO box on an XJ would default to putting out 36 degrees of advance at any RPM is unproven. We, or at least I, do not know the advance vs RPM curve. It may be the 4RO jumps to 36 at a very low RPM, but equally it is possible the 36 does not come in until much higher. After all, there is supposed to be a vacuum advance at work that provides additional advance which effect should be more pronounced at lower to middle RPM ranges. Who knows? BTW, when I say "lower to middle RPM" I am refering to 3000 - 4500 RPM, roughly. I do a lot of riding in that range, including most of my shifting. I think thats where the TCI and vacuum advance really work together. Anything over 5000 is high RPM to me.

This arguement is pretty theorhetical. Without further testing or information it is hard to say what the exact effect of running a 4RO box on an XJ is. Still, I would run a 4RO only until I could find a 10M box; I don't think the 4RO would be a good long-term solution for replacing a bad 10M TCI, nor would I promote a 4RO (or 2H7) as such.

However, it does appear that a 4RO should work on the early 11s, as the lack of vacuum-sourced input is made up by the mechanical vacuum advance.

Thge other way around, using a 2H7 on an '81, would appear to be a bad ide for the same reasons using a 4RO on an XJ: you defeat the vacuum advance control of your timing curve and go to the advance vs RPM curve programmed into the TCI. While the engines may run you would be loosing torque and efficentcy at any part-throttle operation, which is where, I suspect, most of us spend the bulk of our riding time.

Again, the loss may not be great. Without numbers to back up these theories they remain just that...theories. YMMV!

[Crazcnuk]

"However, it does appear that a 4RO should work on the early 11s, as the lack of vacuum-sourced input is made up by the mechanical vacuum advance. "

Not sure about this as mechanical and vacuum advance curves are for two completely different things, as you sort of explained.

Mechanical advance is used to compensate for the increase in piston speed to allow the combustion process to start earlier and earlier as the rpm climbs, so that the explosion has sufficient time to get to peak before the piston is too far past TDC.

Vacuum advance, on the other hand, is purely for fuel economy. The vacuum advance system changes the overall advance curve in reaction to different throttle postions, as you pointed out, to compensate for a differing explosion rate with less cylinder pressure and less fuel load.

The 4R0 and 2R7 vacuum systems soound like they do the same thing, but with a different type of process.

I think the difference, that Cat was tallking about is that the 4R0 box system uses no centrifugal (mechanical) advance, but has that part 'programmed' in, and sets the advance based purely on engine rpm, as that is a very easy calculation to make and program in.

So if you put a 4R0 box on a 2h7 bike, you should end up with double the mechanical advance as both the mechanical advance mechanism and the 4RO box would be adding thier part to the mix. This would, in effect, rob you of mid and high end power, at any point in the rpm range where mechanical advance is all-in.

Add to this the vacuum advance (which near as I can tell should work the same on either system) and you would have far too much timing, and this should seriously affect fuel economy, again in the mid or higher rpm ranges.

Putting the 2H7 box on a 4R0 bike would give you 0 mechanical advance as you would have niether the mechanical or built-in advance also causing issues with power and economy.

I am only talking about the XS systems, as I know very little about the XJ.

[TopCatGr58]

Just a FUN DISCUSSION!

I love a well mannered, fact producing, thought provoking debate that gets US to finding out truths without belittling namecalling!

Well, what I "think" I have discovered is that perhaps the CYCLE MAG guys made a mistake in their estimations/calculations of just how much timing advance is produced under low load rpm conditions! They stated that they combined the 10 degrees of static, along with the 26 degrees of Cent. advance AND the 16 degrees of Vacuum advance and came up with 52 degrees! It's in their article!!!

However, in the following diagrams taken from the XS11 shop manual and the XJ11 shop manuals, you can Kinda see where they made that assumption....remember, the article was about the NEW XS11E at the time, no XJ's! We know the XS11E has 10 degrees static/idle timing. But what they(Cycle Mag) apparently did was not realize the overlapping affect of the vac. and cent. adv. curves.....the manual states MAX adv. for Standard WITHOUT the vac. adv. is 36 degrees at 5200rpm. This I believe is the maximum travel of the timing plate assembly, and so 52 degrees is not obtainable!



The vac. curves show no advance until ~50mmHg, then 100, and Max's at 150mmHg at 16 degrees. The Cent. Adv. starts kicking in at around 2150 rpm, remember these values are added to the 10 degrees static. It rises ~5 degrees at 2500, 10 at 3k, 15 at 3500, 20 at 4k, maxing at 26 at 4750. So...the amount of total adv. WITHOUT the vac. input ranges from ~11 at 2150 to 36 at 4750 in a rather linear progression. This is in that rpm range that Jerry stated he uses most, and I would assume many other Xsives do also!

Now to look at the XJ's curves, these charts are COMBINING both the vac. and cent. adv. and Static adv. values over 3 ranges of vacuum, right at 50mmHg, greater than 50mmHg, and then ?Less than 100mmHg but I'm assuming very close to it, but surprisingly they don't show 150mmHg!!





Now, let's look at 1 rpm range, 3k. On the 2H7 XS with just Cent. Adv. it should be ~20 degrees total/Static included.

Looking at the above diagrams, for the XJ the following ranges occur:
At 50mmHg, adv. range is from ~13-23 degrees;
greater than 50mmHg: range ~13-20 degrees;
>50mmHg but less than 100mmHg: range ~23-29

It appears that under moderate loads where vacuum input to the vacuum adv./sensor would be less=50mmHg, that the amount of adv. is very close to the Cent. Adv. ONLY of the XS11. Only under very light load conditions where the vac. level is much higher do we see a considerable increase in adv, about 9 degrees more than the Cent. Adv. curve alone at that same rpm!

Again, I'm assuming here, but I would suspect that the 4RO's programmed Cent. Adv. curve is very similar to the actual mech. cent. adv. curve, and I doubt that the vacuum levels to the vac. adv./sensor gets much above 100mmHg, so the full 16 degrees of advance are not ever seen, which seems to leave about a 10 degree range of variable adv. in the lower rpm range.

I agree that the OPTIMAL configuration is using both the Vac. adv/sensor input along with the Cent. Adv. curves, but I can see how IF a person doesn't really run in the lower low rpm ranges....I know myself that I rarely run below 3500 to 4k and even in the lower gears I'm providing moderate throttle input which reduces vac. adv. input, .....that the 4RO box would work quite effectively on the XJ and not necessarily cause a considerable hit on performance or mileage.

What CrazCnuck said about mixing the 4RO box onto earlier 2H7 machines is right on. And vice versa, if you put a 2H7 TCI box on a 4RO engine, you would definitely NOT have enough timing advance, hence the need to swap out the Mech. Adv. parts from the 2H7 machine and put them on the 4RO engine....IF you used a 2H7 box..which are much more plentiful than the 4RO boxes!

Part of this discussion was to illustrate the need to use the right TCI's with the right engines, but that Variations could possibly work...albeit not the most Optimal/fuel economical way, but still work!
T.C.

[Crazcnuk]

We spent a few hours recurving the distributor on my Monte Carlo, for just this reason.

For drag racing (or any other WOT applications) You generally unhook the vacuum advance. It is not needed, and can cause trouble at the end of the track when your doing redline or better and you let off the throttle completely. The vacuum advance has nothing to do with performance, and is all about economy. The Vacuum advance is only in use during part throttle operation.

We set up my mechanical advance to put in a max of 32deg advance at 4500rpm IIRC. I should check that again, it's been a while. The Monte redlines at 6000rpm.

[pggg]

TC says "Well, what I "think" I have discovered is that perhaps the CYCLE MAG guys made a mistake in their estimations/calculations of just how much timing advance is produced under low load rpm conditions! They stated that they combined the 10 degrees of static, along with the 26 degrees of Cent. advance AND the 16 degrees of Vacuum advance and came up with 52 degrees! It's in their article!!! "

Well, with a timing light attached at 5000 revs, in neutral, no real load, my timing plate will read 52 deg every time, that's with the vac hose connected. Disconnect and plug the vac hose - the timing drops to 36 deg...

[Crazcnuk]

Yes but you would not get that under load.

Your holding the bike at 5000rpm with the throttle almost closed, or else it would climb even higher.

Under load conditions at the speed you would be going, your throttle would be open further to keep it at that rpm. Therefore the vacuum advance would be lower...

Obviously, it's possible to get that advance, but I don't think you would see it under real world conditions.

Maybe coasting down a hill at high speed, that way firing the plugs so far in advance of tdc would help brake the bike?

That is why we unhook, or plug the Vac adv for drag racing, otherwise when we let off the throttle, at the end of the run, the timing all of a sudden advances so far it puts more load on the pistons coming up than we like. This is also why engines throw a rod more often on deceleration than they do on acceleration.

[petejw]

could an xj module b fitted to an xs

This has been an interesting thread,

Im just curious,

could the xj module be fitted to the xs, perhaps the later
models that dont have cent adv.

i ask this cause of the carb mods by a few members,
that dont have a vac advance connection to the carbs,
i havent seen an xj module so im not to sure.

[Jerry]

that the 4RO box would work quite effectively on the XJ and not necessarily cause a considerable hit on performance or mileage.

Well, thanks to TC again for providing the numbers. I see now where a 4RO should work on an XJ; will include those in my search for a spare TCI!

could the xj module be fitted to the xs, perhaps the later
models that dont have cent adv.

As for fitting the 10M box to other bikes, the mounting and plugs are identicle as far as I know. I *think* the wiring pattern is also the same, but would have to check against the schematics. In the cases where people have swapped engine and TCIs they have not had to change the wiring, just plug in all the right parts.

[TopCatGr58]

Hey Pete,

The XJ uses a separate Vacuum sensor, called the boost sensor, that is connected to a vacuum source, however, I don't know if it's connected to a smaller port on the carb body like the XS's, or just one of the larger synch ports? SO....the XJ DOES use a vacuum connection, it uses that sensor to provide input for the TCI to control/adjust the vacuum adv. timing according to load!

Pggg, thanks for clearing that up. I was getting conflicting type of data from the manuals, and with the XJ's timing MAXING out at 36 degrees, was wondering IF the Cycle Mag guys might have oops!? So...now the question is WHY does the XJ max out at 36 degrees instead of 52 degrees with low load/high vacuum signals going to the TCI? Perhaps it's like CrazCnuck stated, that the 52 degrees is essentially never seen because of an unrealistic riding condition?? And so the XJ's TCI's programming controls that so that it doesn't OVERAdvance the timing, like on closed throttle high rpm no load situations, so as to NOT cause the possible damage Craz alluded to!?

You can see in the XJ diagrams that it DOES have a steeper adv. curve with the higher vacuum levels, especially at lower rpms!BUT, even with the higher vacuum level and 5k rpm, it max's out at 36 degrees! I know the earlier timing plates are labeled up to 60 degrees, and so that technique would be a good way to test the adv. module to show it's working properly, but I, too, don't think that that level of advancement is truly realized in real driving conditions.

Jerry, glad to help broaden your replacement TCI range, however the 4RO's are just about as rare as the 10Ms! Another option might be to get the 2H7 mech. timing parts, and then you can put on a 2H7 module, or even make one up out of GM ignition parts since it will work with the mech. adv. parts! JAT!

I also remember the Megasquirt or other replacement TCI from that place in NZ IIRC, that stated that it had the cent. adv. curves, but didn't have the vacuum adv. option, but we can see that the bike will run effectively with just the cent. adv. curves!

T.C.

[pggg]

TC, those vac advance units, I still haven't got them sussed properly, does not the high vacuum at low revs RETARD the timing? Low vacuum, high vacuum, throttle position, load on motor, gets me lost, but I noticed full throttle and load on the motor from acceleration, doesn't seem any difference with or without the vac unit, the vac unit seems to me more for cruising and light throttle stuff only

[TopCatGr58]

Hey Pggg,

Yeah, took me a bit to wrap my brain around it also, but here it is.

The Vac. Adv. is an advance unit. The higher the VAC, the more advance you see. But here's the tricky part. Under LOW LOADS, where the throttle plate isn't open very much...like cruising rpm, more of the vacuum pulses are being directed from the carb into the vacuum adv. port, causing a higher vac. adv. and increased timing advance which is useful for when there's little load, lean fuel/air mix at moderate rpms.

BUT...when you hit WOT, or just opening the throttle with increased load, then the vacuum level going to the adv. port. actually DROPS because with the butterflies open there's more air available to flow thru the carb throat, and so the vac. adv. unit releases it's pull which allows the timing to retard/fall back to the more powerful less advanced timing position to take advantage of the richer fuel/air charge being sucked in!
T.C.

[Crazcnuk]

Ok, now that it's almost spring, and I am back working on the bike, where do I find the numbers on the pickups to see which series I have?

The only numbers I can see in there are 3H5-10 and a s/n

Do I have to take the p/u out to see the numbers?

Also, someone mentioned the pickups should be 'set'. What is the measurement I am looking for?

[TopCatGr58]

Can't help with the PU #'s, but the gap between the PU coil and the rotor/trigger is 0.7mm. This info is at the end of the PU coil wire repair tech tip!
T.C.

[sailor202]

Can't believe I'm back here again with a bike that still doesn't work

Well I got a call from Jim the mechanic and he said, "Yep it was the pick up coil, I fixed the wires, come and get the bike". Well I went and rode it home, it was terrible, I was really pissed, still am. Half the time I was running on two cylinders, the motor cutting in and out at various speeds.
I parked the bike and haven't touched it since. I've finally decided no one has been able to fix this thing, so I'm going to have to do it myself. The bike is now officially on "the LIST", you know the list that never seems to get smaller,lol.
Well actually I've been able to get a few things accomplished, rain, cold and all. My bass boat is repaired and back in the water, the sail boat is half-way finished getting a 10hp Johnson dropped into it's well and fitted with a single lever remote control cables.
Anyway I remember someone saying that a few folks from this porum live near my marina in Rio Vista, Ca.
So here's the deal, anyone who wants to cruise my way and check this machine out and maybe offer some suggestions is more than welcome. I wouldn't mind meeting some other xs100 folks to cruise with.
I've some fiberglassing to do on the houseboat` and hopefully when that's finished in a couple of weeks I'll check in here and report what I've found. --- Sailor202

[mro]

Rio Vista is not far from Bethel Island...
Might be able to stop by in a couple weeks.
Will PM ya a few days in advance if you still would like some help on your XS.


mro