Rocket scientist needed

[Laptop_geek]

I believe most MAP systems work pretty much the same way as well, with the exception of the ones with knock sensors which add the ability to dynamically adjust timing based on detection of pinging. These are the engines that are supposed to be able to take advantage of higher octane fuel because they will advance the timing automatically till detonation is detected (and they can sense it long before the human ear can hear it) and then back off a little bit. It's a bit fun with my Camaro as it actually gets BETTER mileage with premium than with regular, as it will automatically adjust, and runs well on either. But I'm still not sure the price per mile isn't still better with regular than with premium even in that case.

Thank you for the explenation. My brother was wondering why his bike got better gas mileage with the premium gas, I gave him a simple explenation, but couldn't explain how this happened in acutal practice. I showed this to him and he understands it now.

[TopCatGr58]

I'd be willing to bet that the XJ uses the same method as Harley; you have two predetermined curves built into the ignition box. If manifold vacuum is below a certain level (on Harleys, about 4"hg), you get the 'mechanical' advance which is only dependant on RPM; so many revs, so much advance. But if manifold vacuum goes above 4"hg (lighter load, partial throttle opening), the switch closes and the box then switches to another curve with more advance, but again predetermined and dependant on rpm.

Hey Steve,

Here are a few diagrams from the XJ service manual that shows what looks like a variable amount of vacuum advance, not just a few preset curves/zones!? But I'm not totally sure I'm able to interpret them correctly?







This last one shows the curves on the XS11 with the mechanical vac. and cent. adv devices.


T.C.

[crazy steve]

T.C., the XJ FSM is 'less than clear' about just how it's ignition advance works...

Now, the 'vacuum switch' on the XJ does have a variable output, but the charts you posted show predetermind curves at a given vacuum; so much advance at so many rpm at so much mmHg. The first two charts appear to be identical (except for that 'bump' at 3K, which I think can be ignored), with the third chart showing a markedly different curve. The first two are at 50mmHg (about 2" of mercury), which would represent low speed/high load/open throttle operation. The third chart is at 100mmHg (about 4") and that is the 'usual' transition point between curves. Engine speed is up and/or load is down and/or the throttle is closed; all conditions where the motor can tolerate more advance without knocking. What's the most interesting to me here is the curve isn't straight; you've got that bump at 2200 rpm where the timing reaches about 28 degrees, then retards about 5 degrees on it's way to 3600 rpm, then goes back up, peaking at 36 degrees at about 4500 rpm before falling slightly as the rpms go higher.

Another very interesting thing to be noticed is max advance is only 36 degrees, no matter which amount of vacuum you have; this is surprising to me. This makes me wonder if there isn't another 'step' at a yet higher vacuum signal like CY mentioned; the manual makes reference to a certain voltage output at 200mmHg. If you compare that to the XS models, they have that amount of advance without the vacuum; under the 'right' conditions, you could have up to 52 degrees of advance with a XS. If you look at the vacuum advance chart for the XS, note the lack of a rpm reference; advance is strictly by vacuum signal.

I'd be curious to know how a stock XS and stock XJ would compare in fuel economy under the same riding conditions... I would expect the XS would do better.

[oldnortonrider]

RE timing issues and "some" differences between the 2H7/TCI and the 4R0/TCI, I thought it might be appropriate and/or interesting to insert the below information which includes an email (report) to a fellow XS11 rider, few weeks back, that I sent to him. TopCat, you may remember, some time back, I had made mention on here, that my 16 year daily ride (1981 Special) had been changed by the PO at the pickup coil timing plate to have a 2H7 plate, and the TCI had been switched also~~was a "2H7" box. But I hadn't discovered this, or 'understood' the meaning of all this, 'til about 2 years back. My fellow XS11 acquaintance had the same situation; about 2 years back he acquired a 1981 XS11 (4R0 machine), and it wouldn't go above 5000 rpm under a load; he went nuts~~put a different rack of carbs on it~~still no go above 5000 rpm~~AND we discover it had a 2H7 TCI box on it; BUT it still had the 4R0 pickup coil timing plate that one can't alter the timing with. Between the two of us we decide he's gonna need a 4R0 box; he messes around with eBay for about a month, and gets a 4R0 box (which we all know are hard to acquire for low $). In the process of all this, I decide I'm gonna need to get one , too. He installs the 4R0, and all is well; redlines easily under load; runs like it's suppose to. In the meantime, I'm trying to find a 4R0 box on eBay for reasonable $; took a year; got one about two months ago.

Following, is the email to Rick McElrea, mentioned above, that was sent to him 28 Dec.~~> I'm in shock; I have a new , "reborn" machine. Today could only be described as a day of what I refer to occasionally in life, as a moment and joyful experience of personal triumph.
Will probably have future reports, as days go by, and I've ridden it more with the 4R0. About noon I decided to see if it would at least start and run with the 4R0. Warmed it up with the 2H7 still hooked up. Had spent some time yesterday trying to "clean-up" the spade terminals on the 4R0, and fairly well saturating them with dielectric grease. Let set overnite; room temperature. After warming up with the 2H7 (seat off), I unplugged the nylon connector blocks and got some dielectric down into the terminal parts that hook up with the spades of the TCI. Plugged it in (the 4R0); the machine eagerly, and easily started. Seemed to start more easily than it usually does, even though it was already warmed up. Checked to see if it felt like it could be doing too much advance; didn't seem to have, but I knew I'd actually have to take it down the road to be able to tell. As I'd told you in the past~~to take the pickup coil plate off, to see if there was actually a mechanical advance behind it~~was something I was for now, trying to avoid, because of all those pickup coil wires are right where I want 'em for now. It's a nightmare getting them placed the way I've got 'em routed. .............so I got all suited up, and put some fancy "cushioning" over the 4R0, that was actually laying on top of the 2H7 with some cushioning between the two of them~~more cushioning on top of all this (no seat) for me to sit on, and headed south down the highway, 7 miles to the rest area. It doesn't really warm up and run like I've got it "tuned" to run, until about 3-4 miles; 10 miles is really best. Right away I could tell it wanted to stretch out and run; I stayed at fairly low rpm and kept it under 50-55, 3-4 miles ~~ meanwhile keeping my mind and focus on whether it felt too advanced; did NOT feel too advanced (famous last words, (I hope not) ) . It will idle on 80; could easily handle a 6th gear. When I got to the rest area, a solo motorcyclist on a new Harley had pulled in ahead of me; he parked and I parked aside of him. We visited for about 30 minutes and exchanged emails; he went on, and I proceeded to get buttoned up, zipped up, and buckled up, and headed back. Runs like a scalded dog. Seems like memory tells me that it did run that good about 10 years back, but has seemed to have gotten weaker and weaker over the years. Maybe when it had run this strong , years back, was if it actually has the mechanical advance, and the mechanical advance was working as supposed to. If it is actually in there, it must have corroded over the years, enough, to be locked up. Which is fine; it it would stay locked up~~can't really count on that, for it to stay locked up~~if , in fact, it is even there. I suspicion that it is, for it to have run so strong, and "proper", years back. High enough rpm, which I've not really been able to achieve, in the last few years~~would probably sling the centrifugal mechanical advance into an operating condition, again,~~or some kind of more, advanced position. Will definitely "get around" to checking to see if it's there; and will also check things with a timing light. I do remember in times past, running onto posts, in some of the threads on the XS11.com forum,~~that guys have told about locking the centrifugal advance into immovable position. I'm thinking, they didn't really say or explain what they were doing this for; best I can remember.
Probably, guys wanting to put the "adjustable" plate of the 2H7 onto a 1981 4R0 machine, so they could change the timing. My memory tells me, possibly if the 2H7 plate is used~~mechanical advance apparatus has to be in there with it~~something to do with the reluctor setup all being proper.
Yep; I'm pretty happy about the so-far results; this has been an ongoing two year exploratory-fix-project.
Tomorrow, I'll unbolt the 2H7 and bolt the 4R0 in its place, and put the seat back on. (end of email to Rick McElrea)

Will put a timing light on it in a few days.................................I've altered and tweaked the timing on this machine many times over the years~~having the benefit of 2H7 pickup coil plate with the slotted mounting screw holes~~had always done it by "feel"; no timing light. I've kept it a bit too advanced to run proper with regular gas; I use super premium, which is of course, really, only a puny 91 octane. Probably another reason I really need the higher octane: I know I've got a lot of carbon in the combusion chambers, causing higher compression~~150,000 miles on the machine. I do run COLD spark plugs in warmer, and hot weather; "7's and "8's NGK heat range(s); although recently I've switched and gone with the Denso plugs~took me a while to "de-code" their heat range numbers~~(cross referencing).

[TopCatGr58]

Hey Carlton,

Well, glad to hear your new 4RO TCI worked for your in spite of your 2H7 Timing plate assembly. It's possible and probably likely that your timing plate and the mech. cent. adv has gotten stuck/rusted frozen. The lack of your ability to reach the higher rpms with your 2H7 TCI, and now ARE able to with the 4RO TCI which has the cent. adv. curve built in pretty much proves/confirms it.

As to previous posts of folks locking their 2H7 timing plate, I don't recall anyone doing that. I do recall that folks were recommended to pull the vac. adv. hose to keep the timing plate and pickup wires from flexing...a diagnostic technique for the PU coil wires problem. Due to the $$$ of the 4RO TCI, folks running the 2H7 timing parts would NOT usually get or try to put on a 4RO TCI on one...which would require locking the cent. adv. parts if the 2H7 timing parts were still on the engine.

I also can't recall if you've been able to complete your ignition coil repair, if so, that also has helped your ignition problems.

The 4RO TCI still uses the mech. vac. adv. pot/timing plate, so you don't want to freeze that, just the mech. cent. adv. portion.

T.C.

[oldnortonrider]

Hey Carlton,

Well, glad to hear your new 4RO TCI worked for your in spite of your 2H7 Timing plate assembly. It's possible and probably likely that your timing plate and the mech. cent. adv has gotten stuck/rusted frozen. The lack of your ability to reach the higher rpms with your 2H7 TCI, and now ARE able to with the 4RO TCI which has the cent. adv. curve built in pretty much proves/confirms it.

As to previous posts of folks locking their 2H7 timing plate, I don't recall anyone doing that. I do recall that folks were recommended to pull the vac. adv. hose to keep the timing plate and pickup wires from flexing...a diagnostic technique for the PU coil wires problem. Due to the $$$ of the 4RO TCI, folks running the 2H7 timing parts would NOT usually get or try to put on a 4RO TCI on one...which would require locking the cent. adv. parts if the 2H7 timing parts were still on the engine.

I also can't recall if you've been able to complete your ignition coil repair, if so, that also has helped your ignition problems.

The 4RO TCI still uses the mech. vac. adv. pot/timing plate, so you don't want to freeze that, just the mech. cent. adv. portion.

T.C.

No, no, ~~NOT 'locking' the 2H7 slotted timing plate; was referring to they/someone having talked about, or mentioned that they had locked the centrifugal mechanical advance. Like mine is now, in the "froze" position. I'm guessing if one was doing this, it would be because they were installing a 4R0/TCI onto a '78 or '79 or '80 '2H7' machine.

Still hadn't completed the new coil and hi-tension wire project. It's in progress. Have acquired 2 of the 3 ohm Accel coils (used); am in the progress of acquiring several different type crimpers for terminals to be on ends of the hi-tension wires. And have decided to try 4 Taylor ThunderBolt50 wires , which are the 10.4mm and 50ohm per foot. Am still using the 'MikesXS black coils', that we all need to know~~they need to go; I've been lucky and gotten by with 'em for 3 years. I've really got a lot goin' on with this fine machine. Am going to get that ultra-sonic cleaner for this spare rack of carbs that came off the '79, that I have in storage. One of the carbs that are actually on the '81 right now, came off the '79 rack when I rebuilt them 14 years back. The '79 rack, after rebuild et al, will be immediately installed on my daily ride '81; and I think I will put new intake carb boots on. I'll mention; PO had also changed the carbs on my '81; not only had he switched all to "2H7", but also 1978 or 1979 carbs; ALL unbeknownst to me; .......which wouldn't have really made any difference; he had it running,.....fair.............