Xs / Xj Ignition Systems

[randy]

Ladies and gentlemen,

After quite a bit of research and development time on the XS and Xj ignition systems, I've come to a few conclusions and want to run them by all of you. Especially those of you who own both an XS and an XJ.

As I do not have access to an XJ, I need some help on testing out my theory:

1. The XS and the XJ share the same Crankshaft (2H7-11411-02-00).

2. The XS and XJ share the same Crankcase (2H7-15100-02-00).

3. The only difference between the two gnition systems is that the XS has a mechanical advance bolted to the end of the crank whereas the XJ has an electronic advance circuit built into the TCI box.

4. I see no reason why the pickup and advance mechanism from the XS would not work on the XJ. You would have to move the vacuum line from the XJ Boost Sensor over to the vacuum canister of the XS advance mechanism to provide mechanical advance instead of electronic advance. . other than that, it should work.

My line of thinking is this. We will probably never find a simple fix for the XJ TCI box because it is much more complicated electronically than the XS. It has proprietary circuitry that would be almost impossible to replicate, never mind repair. The XS TCI, however is a fairly simple design that lends itself to alteration, repair and, if my calculations are correct, duplicatable. (more to come on that project).

Now, of those of you who own both, are any of you up to the task of swapping the XS pickup mechanism into the XJ to test my theory? The worst that could happen is it won't run, then you can swap it back.

In the end, we may just wind up with another option for you XJ owners when your TCI fails. Think about it.

I can't wait to see the feedback on this one.

LOL,

Randy

[Jerry]

Three comments.

If you do this swap you will need the XS timing plate cover. It is taller than the XJ version to cover the additional hardware.

I belive this swap has been done, but don't remember the details. Also recall someone trying to run the XJ engine on the '81 H TCI box with poor results. Engine started and ran, but performed poorly.

The XJ ignition curve is different (I think) than the XS due to the YICS head. XJ curve specs are:

5 deg BTDC @ 1,100 RPM (Idle speed)
36 deg BTDC @ 5,500 RPM

I don't have the XS service data at hand, could check tonight but if someone has it handy look up the numbers. The important part is the shape of the curve between the 2 points, which is not in the XJ manual.

The YICS mods include a carb vacuum balancing port cast into the cylinders, which requires a special block off tool when synching carbs. Head design is also different; there is a small passage leading off the intake runners to just behind the intake valves. This is supposed to 'swirl' the incoming fuel/air charge resulting in more complete combustion, thus making the engine 'cleaner' for EPA purposes...the whole rationale for the YICS mods. Yamaha claimed that with these mods (YICS and TCI ignition) the XJ would have the same performance of the XS, which is not quite true. The XJ is also jetted leaner than the XS.

I don't have an XS to try the swap, but would be very interested in the results!

[foxjohnc]

There is a person on Ebay selling new aftermarket ignition boxes for the xj1100. New they cost $120.00. The are marketed as performance boxes with a modified timing curve and increased dwell time. I dont know if they even work on our bikes but I emailed the seller and he said that it is compatible with the '82 XJ 1100J.

[foxjohnc]

Heres the link : XJ 1100 Performance Ignition

[randy]

According to the yamaha fiche, my 79F starts out with 10 degrees retard at 1,000 rpm, added to that is 16 degrees of vacuum advance for a base idle advance of 6 degrees BTDC.

at 4,750 rpm total advance is 36 degrees centrifugal plus 16 degrees vacuum for at grand total of 42 degrees BTDC.

The 'advance curve' shown in the fiche is dead flat through the rpm range.

Does this help?

randy

[John]

Psst, hey Randy.36+16=52 But I know you know that.

[Hepcat]

http://www.erd.co.nz/

That's the site that builds these ignition units on Ebay. They say the timing curves can be pre-programmed to suit any engine. Loook at the Suzuki link.

[Underdog]

So would it be safe to assume that you could use one of those boxes on an XS if you swaped in a XJ pickup assm. ?

[randy]

John,

Subtract 10 degrees initial retard = 42, but you knew that.

Randy

[John]

No I didn't.

[Jerry]

Ignition advance - long

Hope this is not redundant, but it helps me to put things in review.

Street engines need an ignition curve because the fuel/air mix burn rate is fixed; the burn rate is the same at all engine speeds. However, the piston speed achanges.

At low speeds the spark plug can fire when the piston is a few degrees before top dead center (BTDC) because by the time the burn is in full force the piston is just starting its downward movement, making the most of the crank offset, thus delivering the most power.

At higher RPM the ignition must be advanced, that is farther before TDC, so the max effect of the burn is still just after the piston starts down.

Two systems are involved: centrifugal weights, which are RPM sensitive, and vacuum, which is engine load sensitive.

A base offset is established to make starting the engine easier.

At lower RPM vacuum is high and the timing curve is controlled by the vacuum advance mechanism. At higher RPMs vacuum drops and the weights take over more of the ignition timing. Between these points the weights establish a 'base' advance and the vacuum either advances or retards the spark based on engine load. At some point a maximum advance is established that stays in effect the rest of the RPM range.

On the XS the max advance is hit at 4,750 RPM. The XJ hits max advance at 5,500 RPM. The max advance for the XS is 42 degrees. The XJ max advance is 36 degrees.

These differences raise 2 questions: Why the differences, and what would be the effect of putting the XS curve on an XJ engine?

I don't have a background in engine thermodynamics, but I suspect the XJ, with the YICS modifications, runs leaner than the XS. This may affect the burn rate (shorter and faster) than the richer-jetted XS, requiring less advance to get max power.

What I can't figure out is why the XJ's max advance would come in at 5,500 RPM. After all, 5K RPM is about 75 MPH, so unless you shift high in the RPM range you won't get max advance. At normal crusing speeds the XJ is running quite a lot less advance than the XS. The only explanation I can think of is that at normal speeds the vacuum side is still effective, making ignition timing more sensitive to engine load. This should improve fuel economy and cut down emissions. Yamaha introduced YICS in 1982 to produce 'cleaner' engines, as the EPA was pressuring for.

My guess is that putting the XS ignition advance on the XJ would require either re-jetting or going to a higher octane, or both. Otherwise you would see pre-detonation (pinging) as the greater advance of the XS would result in max compression pressure before the piston hits TDC.

The XJ uses the same cams as the 1980 G (part number begins with 3H5) so cam timing does not appear to be a factor.

Does this make sense?

The YICS mods on the XJ include larger diameter inlet side on the carbs (54 vs 52mm) leaner jetting (112.5 across all 4, 47.5 pilot jets) all-electronic ignition, carb balancing ports cast into the cylinders (requires a special block off tool to synch carbs) and additional small ports in the head that supposedly create a swirl in the incoming air/fuel charge.

Ken Talbot's SO is running what appears to be XJ cylinders on an XS bottom end with an XS head, and she gets fuel mileage in the upper 40s (near 50) consistently. Bike runs great and uses the XS ignition system.

I would be very interested in the results if someone can try swapping the XS system to the XJ, but suspect you would have to modify jetting and try different octane fuel to get a complete picture of the viability of the swap.

[chevy45412001]

switching ignittions

why would i want to switch to something that has more problems?One being broken wires from the advance thing,in the side cover.why don't someone switch over to an xj system on there bike?wouldn't need a new cover that way either?As far as i know my bike is22 years old with all the stock ignition stuff still working proper(knock on wood) .It has always started and got me there and back no probs.Kinda like why noone runs points anymore,much easier and dependable to run electronic ignition of some type(like an xj for eXample)or an HEI in a chevy? just thinking out loud.

[John]

I had an XS ignition system, and the carbs from my 80SG on an XJ engine in my bike for many years. It ran well, and I didn't notice any problems with running rich, or lean with the stock jetting of an 80SG carbs. I would have used the XJ ignition system, but back then nobody here had a copy of the wiring schematic for me to go by, and I didn't want to try it blindly. Now that I have sold the XJ TCI units, I don't have any to work with. All I have are two sets of XJ pick up coils, and one boost sensor. They are for sale by the way.

[Lynge]

I'm lookong forward to hear more about the duplication project.

And as you see in my "no ignition an cyl 1 and 42 thread I'm curious about which transisters to use

--------------------
Jan H Lynge
jan@xs1100.dk

1978 XS1100