A little bit of tinkering today on the Monza and discovered it is fitted with a Lucas RITA ignition system. That was a new one for me so did a bit of Google mining and experimenting. Here’s the thing: only gives me about 10deg advance between idle and top end so setting the timing is a bit of a compromise. I discovered it had been set up so that the idle advance was about right at 10ish degrees which meant that I was only getting about 20deg advance at the top end versus the 35 it should be. Decided to set it up to get the full 35deg at the top end and run on the advanced side in the lower rpm range. Will be good for part load economy as there is no vacuum advance but a touch “hot” at wide open. Took it for a long test run to check out my buddy’s new man cave (very nice it was too) and all was well until both bolts securing the RHS intake stub vibrated out and it backfired all the way through an otherwise peaceful Braintree town centre. Fortunately the man cave had some very nice SS cheese head bolts of the right size and after a quick fix it was off to Maldon sea front for cockles and char.
May be the wrong kit for the Monza. (?) How are you checking the advance?
I had the same lack of advance on the rita system on my SPIII, and it was the correct unit. Did have a bit more than 10 degrees though.The cap between the rotor and pickup does affect the advance curve, I think the bigger it is the greater the advance.But you are correct in setting it to full advance.Are you using a strobe and timing disc?You might find the tickover a bit harsh, but it does improve economy.You can always back off the advance a tad to give a slightly smoother tickover if you want.
Brian UK2013-10-07 17:13:03
I’ve just decided mine may be a bit retarded too, needs rechecking. For the big blocks there’s a 5mm wide tool they give you to set the distance between the rotor tip and pick-up pole at full advance, but that’s still just a “starting point”. Think what I’ll do is in the absence of anything more obvious leaping up to slap me in the mush once I’ve looked at it is advance it a bit then see how it runs, etcetera…
So a bit more tinkering with RITA this evening (lucky RITA) and some more careful measurements and observations. Luckily I have access to a decent Snap-On strobe which allows you to dial in a required advance so that you can time it using the TDC mark. Alternatively, you can use the dial to measure advance by turning it until the TDC mark is aligned and then read off the advance from the dial (remembering to divide by 2 for a 2-cyl). This is what I observed…
- RITA starts to advance from below idle speed
- From my idle at ~1100rpm I get ~10deg of advance which reaches a peak at 4-5000rpm and is flat thereafter.
And this is what I discovered with some furious Googling when I got back in…
- There is a lot of opinion and perceived wisdom out there and some of it is incorrect so the usual law of the internet applies.
- Some great information on the Euro Spares website including diagrams and instructions specific to various MG models.
www.eurospares.com/elec.htm - Gunk on the BritBike forum has done a fantastic job reverse engineering RITA and explains how it works really clearly.
Lucas Rita notes - Britbike forum - Dyno Dave has an interesting comparison between RITA and (her boyfriend?) Boyer. This includes advance curves for both systems which align with my observations above.
dynodave/AGTS NHT ignition research
I have set my bike up with ~22deg of advance at a 1100rpm idle which increases to ~32deg at 4-5000rpm after which it is flat at ~32deg. I rides really well and doesn’t grumble at all at idle. I must now leave it alone and resist the temptation to tinker any more. All being well I shalln’t have to touch it again until a major rebuild.
Apologies if I’m telling you what you already know but there wasn’t a huge amount about RITA on this forum and I thought it might be useful to have some references all in one place.
guzzibear2013-10-09 11:20:13
And I’ll quickly point out before somebody else does that I meant “received wisdom”. Woops.
On my Rita, I was able to get an idle setting of about 14 degrees, with a fully advanced setting of 34.
This is starting to make sense with me, as originally I set mine to be at zero BTDC at idle. This is obviously where I’ve gone wrong…
Oh yay-yas!!
- Gunk on the BritBike forum has done a fantastic job reverse engineering RITA and explains how it works really clearly.> http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=32496
Oh yes saw that a while ago. The circuit diagram has a number of mistakes, here is my corrected version, transistors turned the right way round and some added notes: Note also this is based on the early AB5 amplifier, in the later AB11 there is no separate over-Voltage protection component, instead this function is incorporated into the BU941Z power transistor (as indicated by the ‘Z’ suffix) - the AB5’s transistor was a different type number altogether but can’t remember what.
Mike H2014-10-11 16:48:31
PS: the explanation for how TR2 works is wrong, I have done a LTspice simulation of this circuit and C3, D3 & D4 have sweet F.A. effect on advance, although yes there is a DC charge developed across C3 and which increases with speed, but if it is meant to turn on TR2 earlier (advance) this is patently wrong because it’s in completely the opposite polarity (negative), therefore it is much more likely to be a rev limiting means. Even so I can’t make it do much in the simulation, and ignition advance seems wholely governed by the pick-up Voltage, the greater the Voltage, the steeper the slope so that TR1 is turned on earlier.
Just for completeness here is my LTspice simulation diagram (meaning this circuit runs in LTspice, pick-up signal & power sources not shown), and is likely easier to ‘read’. Originally I traced the circuit directly from the board of an AB11. In that case TR3 should be BU941Z.
Mike H2014-10-11 16:53:14
This is fabulous and deserves much closer study than I have time for this morning. I’ll take a proper look later this evening when I get back from work. Great stuff.
Be useful to know for dead heads like me HOW you actually adjusted RiTa
Must all be done by the physical design of the pick-up arrangement. This works on the basis that it’s a kind of electro-magnetic generator that is more efficient the faster it spins. Back in the late '70’s I went with a mate one Saturday morning to the Mistral works and saw their test rig operating, a machine that they use to develop and test the rotors with variable speed, so they can then measure the degrees of angle where the spark occurs.
Which would suggest that the gap between the rotor and pickup will also have an effect.It certainly does on the V50 Bosch units.
Looking at the pic from the britbike forum the AB5 transistor (which that is a pic of) appears to be a Motorola ‘T’ something (‘U’, ‘J’ ?) 931. I’m sure though there was an even earlier version which needed a very low value bias resistor, much less than 180R, in fact 19R if memory serves and is the metal-clad so-called ‘ballast resistor’ that was screwed onto the outside of the AB5 case (but is not at all a ‘ballast resistor’, actually it provides the bias current needed to turn on the [low efficiency] power transistor. Whose number I still don’t know. Annoying because I did once). The Motorola T?931 Googled successfully as being a 400V 15 Amps darlington power transistor, but would be without the internal over-Voltage protection (no ‘Z’). (It pre-dates the BU941.) Latterly 931 acquired the Z suffix (according to a photo I have of one). Nowadays the BU941Z version is only available in plastic packaged devices (2 styles), the original metal style (as I am currently using, in an AB5 box) is non- Z, so I have had to add external protection devices. This is a combination of a 10 nF high-Voltage ceramic capacitor in parallel with a 395 Volts ‘varistor’ (aka Voltage Dependent Resistor [VDR]).
Datasheets - BU941 -http://docs-europe.electrocomponents.com/webdocs/010b/0900766b8010b6e1.pdf BU941Z -http://docs-europe.electrocomponents.com/webdocs/010b/0900766b8010b6e1.pdf Note ‘internal schematic diagram’. For the latter a Zener diode is included that will turn on the second transistor if the Voltage is exceeded. I.e. the transistor itself is used to ‘clamp’ the Voltage. This high Voltage is of course the back-EMF pulse generated by the ignition coil(s). Normally if system is working properly, plugs correctly connected and gapped etc., the back-EMF is well within maximum ratings, the over-Voltage protection is only for ‘emergencies’. Like if you leave a plug disconnected…
[QUOTE=guzzibear]Be useful to know for dead heads like me HOW you actually adjusted RiTa [/QUOTE]Here’s how I looked at it for my V50 and the settings I
decided on.My RITA installation gives me ~10 crankshaft degrees of
advance between idle at 1100rpm and where it levels off at 4-5000rpm. That is for my installation and it is very
likely to be different for others as we know that both rotor shape and gap
affect the advance characteristics (at some point I will try and post a sketch
which explains why). This means that you
will need to measure your system and determine what this figure is for your set-up. It doesn’t matter what the absolute numbers
are, just the range between idle and max rpm.The factory settings for my Monza are 10deg static (and at idle) and
35deg maximum. Immediately there’s a
problem because that is a range of 25deg and RITA only gives me 10deg to play
with. This leaves me with three choices…1. Set idle advance
to 10deg but then I only get 20deg at max rpm versus the 35deg
recommended. Fifteen degrees of spark
advance at peak power will reduce power enough to be noticeable and also significantly
increase exhaust temperature which has an adverse effect on exhaust valve
durability. It’s also not good for fuel
economy. On the plus side it will give a
smooth idle and the volumetric efficiency will suit the factory jetting for the
carburetor at idle and low throttle openings.2. Set advance at max
rpm to 35deg and run with 25deg at idle vs the 10deg recommended. That set up is good for power, good for fuel
economy, good for transient response. On
the down side the idle will not be as stable and you will be running with “too
much†advance at lower rpm. In fact
that’s not too serious as a little detonation at low engine speed should not
have any durability implications – it is certainly not rapidly destructive as
it can be when it occurs at higher rpm when the energies involved are much
higher. There’s another positive in that
at low/mid rpm and light load (lower throttle openings) you need more spark
advance than at full load. The std
ignition (at least on the Monza) has no vacuum (low load) advance so running
more advance at those engine speeds is better for both smoothness and fuel
economy at lighter loads. I bet that
most of the time you are in the mid rpm range you are cruising at part throttle
so actually you gain there. You may
(will?) need to adjust your carburetor idle mixture and throttle stop to
compensate for the compromised spark advance at idle. In the worst case you might get a back-fire
(by which I mean a proper back-fire where the mixture ignites in the intake
stub and blows out through the air cleaner, not one of those gentle pops you
get in the exhaust when a bit of unburnt fuel burns off).3. Set the advance
somewhere in between so that you are a bit retarded at max rpm and a bit
advanced at idle/low rpm. Decide where
based on how bad the idle is.I chose to set mine up with 30+deg at max rpm and
live with an idle advance of 20+deg. The
bike starts and idles fine and I haven’t
had a back-fire yet, thankfully.You situation may be different because you may get more
advance from your RITA set-up and the factory figures for idle and max rpm for
your bike may have a different range. I
can’t see there’s any substitute for having a good old fiddle with it. That’s got to be part of the fun of running an older machine, right?
[QUOTE=Mike H]
Just for completeness here is my LTspice simulation diagram…[/QUOTE]Mike, you sure have put a lot of thought in to how RITA
works. I saw some of the other work you have done on the system on your
livinginthepast-audioweb site and I’ve spent quite a lot of time trying to understand
it. Off topic for a minute, I share an
interest in audio equipment so at some time I will have to share details of my
turntable design and my current project building a combined phono pre-amp and
20W amp (with bandwidth tuned to drive tweeters as part of a bi-amped
system). All solid state with plenty of
negative feedback so I’m not sure you will approve . Anyhow, back on topic.I am no expert in analogue circuits but I thought I would
have a go at trying to understand the Mistral cct. you posted. Bear with me and correct me if I’m
wrong. I will stick with the
nomenclature you use in your LTSpice diagram of the original system.Q3 seems to be normally on so the default state is to
saturate the coils. When Q1 turns on it
pulls the base of Q2 up turning that on which in turn turns off Q3 and “firesâ€
the coils. C2 discharges through Q1
while it is on until it turns off again which turns off Q2 which turns on Q3
and re-energises the coils. Not 100%
sure on that but I’m living with it for now.Q1 seems to be triggered on by a differential voltage
between sig+ and sig-. Not sure how
exactly but instinct tells me it’s something to do with reaching a diode
forward voltage threshold and causing a voltage across R2.Anyhow, there doesn’t seem to be much in there which could
delay triggering according to input frequency so that would seem to confirm
that there is little to no advance provided by the amplifier itself. I can’t square this off with the description
by Gunk on the britbike forum.I’ve not quite figured out what C7/D3/D4 is all about. It looks like it’s something to do with
interfering with the switching of Q2 according to frequency. Doesn’t look like it’s a frequency dependent
delay though. I’m a bit puzzled by it? Did your simulation throw any light on what
that does?Anyhow, I’ve sketched
out some input waveforms based on different gaps and rotor profiles which show
how a cct. based on triggering at a simple threshold can provide advance which
is influenced by both rotor gap and rotor profile. I’ll tidy them up, scan and post them at some
point soon (when I figure out how to post pictures on the forum – url link to
web-hosted picture presumably).
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Yes, I use photobucket, and usually just copy a pic posted there and paste it into the post here.Otherwise paste the [img] code onto your post.
I think that what you were saying telegraphroad is very well explained in a clear manner to the typical layman here: Link
italianmotor2013-10-11 13:06:02