FASTdan
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| posted on 15/9/11 at 06:42 AM |
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1600 CVH - unexplained sudden timing advance?!
Here's an odd one. During the period that this car was off the road one day I came to start it and it suddenly ran like an absolute dog
misfiring throughout the rev range. I traced this to a static advance of 40degs! The timing was originally set to 10degs.
In order to get it running temporarily I had to swing the dizzy back (retard) to pretty much the end of its slots (where it normally sits about half
way) to achieve 10degs again and thats how it was left. Since getting the car back on the road I had kinda forgotten about this strange phenomenon
until I noticed its detonating at 4500rpm which it never used to - this then brought back all of the above.
To run through the basic set-up - its an XR2 engine, still running the standard late spec distributor (with small amp module bolted on top) with the
only mod being a saab turbo vac/pressure advance retard unit as opposed to the standard vac advance only unit. The dizzy is obviously run off the end
of the cam.
Now initial thought was its jumped a cam belt tooth, but to achieve that sort of advance it'd need to be 2 teeth, which would cause it to run
terribly - and the belt is tight and in good condition. Besides jumping a tooth would actually retard not advance. So I think/hope I can rule that
out. Likewise it runs a vernier pulley but this appears tight and there are no marks to indicate it has recently moved - and besides, that would have
the same (retard) effect as jumping teeth.
I have stripped the dizzy and nothing appears to be out of place (weights are all well sprung etc). I even did a direct swap for a spare dizzy (but
used original amp module) to no affect.
So, im thinking perhaps that its a fault with the ignition amp module - would I be right in thinking this little unit picks up a signal from the dizzy
internals and tells the coil when to fire? I have a spare but ran out of time to swap it yesterday and wont have access to the car for a few days. In
the meantime I thought I'd put it out there and see if anyone agrees, or has any other suggestions?
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Andy D
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| posted on 15/9/11 at 08:38 AM |
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If it's a Bosch dizzy with a four point trigger wheel, I know from experience the trigger wheel can slip round on the shaft. In my case, same
dizzy on a Pinto. It's happened a couple of times.
With the dizzy cap removed, one of the spring retaining clips can "flop" in and jam the trigger wheel, if you happen to be
turning the engine over, eg adjusting valve clearances etc.. If it has moved, you could find when it lines up at the firing point, the rotor arm is
not pointing at a plug lead contact in the cap.
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FASTdan
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| posted on 16/9/11 at 07:13 AM |
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quote:
Originally posted by Andy D
If it's a Bosch dizzy with a four point trigger wheel, I know from experience the trigger wheel can slip round on the shaft. In my case, same
dizzy on a Pinto. It's happened a couple of times.
With the dizzy cap removed, one of the spring retaining clips can "flop" in and jam the trigger wheel, if you happen to be
turning the engine over, eg adjusting valve clearances etc.. If it has moved, you could find when it lines up at the firing point, the rotor arm is
not pointing at a plug lead contact in the cap.
I see what you are saying, worth a look definitely. It is the same 4 point trigger wheel design I believe. However given that I have swapped the
complete dizzy for a spare (With exception of the amp unit) I am inclined to think that this is not a mechanical-dizzy related issue. Going to swap
the amp this weekend if I find the time, see what that does....
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adithorp
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| posted on 16/9/11 at 07:28 AM |
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I'm pretty sure the amp' doesn't control the advance in any way. It's just triggered by the dizzy.
All the alternatives I can think of would, you'd expect, cause retarded ignition but I'd check them anyway; Stranger things have
happend!
Given an alternative dizzymade no difference, start with cam timing and if thats ok check the crankshaft belt pulley; The key on the shaft has been
known to start to sheer giving sudden changes in timing. Did you use the original rotor in the new dizzy. If not check the key in that as
they've been known to fail/break.
"A witty saying proves nothing" Voltaire
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ss1turbo
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| posted on 16/9/11 at 09:45 AM |
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Maybe it was stuck (and had been timed to suit) and has now freed off?
Long live RWD...
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MikeRJ
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| posted on 16/9/11 at 09:52 AM |
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The ignition amplifier controls dwell in these, but timing is purely mechanical.
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FASTdan
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| posted on 16/9/11 at 10:17 AM |
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quote:
Originally posted by MikeRJ
The ignition amplifier controls dwell in these, but timing is purely mechanical.
If it controls dwell that could well explain a complete shift in timing? dwell time reduced = earlier firing point?
I can pretty much guarantee the weights were never stuck - it has always given the correct advance figures quoted for the stock dizzy. I used to keep
well on top of the ignition timing as I was always messing with it, the dizzy would always be somewhere in the middle of the slots.
[Edited on 16/9/11 by FASTdan]
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ss1turbo
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| posted on 16/9/11 at 10:39 AM |
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Dwell is merely the recharge time for the coil and unless its decided to reverse polarity (open = closed) then that won't affect it.
Very odd...where in the slots are the bolts now if you've had to dial that much advance out?
Long live RWD...
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FASTdan
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| posted on 16/9/11 at 10:50 AM |
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Would a reduced dwell not advance the timing though?
Slots are now right at the end - which is another indicator that the current set-up is not 'natural' 
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ss1turbo
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| posted on 16/9/11 at 11:01 AM |
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quote:
Originally posted by FASTdan
Would a reduced dwell not advance the timing though?
Slots are now right at the end - which is another indicator that the current set-up is not 'natural'
For both distributors (as you said a replacement one is the same)?
Yes, it could (dwell time)...but I can't recall how the system works (if the trigger pulse is for opening or closing the coil current path).
Long live RWD...
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adithorp
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| posted on 16/9/11 at 11:09 AM |
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quote:
Originally posted by FASTdan
Would a reduced dwell not advance the timing though?
Slots are now right at the end - which is another indicator that the current set-up is not 'natural'
It does with points but I don't think it does with the amp'. Even if it did I wouldn't expect it to account for 40degrees change.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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ss1turbo
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| posted on 16/9/11 at 11:11 AM |
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Granted - you don't have that much adjustment on the slots, i'm sure. How the heck does it run at all with 40 degree out?
Long live RWD...
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FASTdan
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| posted on 16/9/11 at 11:19 AM |
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quote:
Originally posted by ss1turbo
Granted - you don't have that much adjustment on the slots, i'm sure. How the heck does it run at all with 40 degree out?
Sorry to clarify, I swapped out the entire dizzy minus the amp unit, meaning the amp unit is the only remaining common factor. The timing advanced by
approx 30degs, from 10degs up to near 40degs. It barely ran, really bad misfiring rough as anything. Now swung right round on the slots I have 10degs
static again.
Its a bit hazy, as it was about 18 months ago when this actually happened - its been laid up 2.5 years and the unexplained occurance happened sometime
during that period. Its only now its back on the road I've got round to investigating it.
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Andy D
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| posted on 16/9/11 at 12:33 PM |
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There are two different Bosch dizzys used on Pintos, not sure about CVH. One type has springs, weights and a vacuum unit, and the other doesn't.
The advance is controlled by the ecu/ amplifier on the latter.
Easy to tell which is which. If you can twist the rotor arm against springs, it's the first type. The other one will have a solid spindle. Rotor
arm wont turn.
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FASTdan
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| posted on 16/9/11 at 12:41 PM |
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quote:
Originally posted by Andy D
There are two different Bosch dizzys used on Pintos, not sure about CVH. One type has springs, weights and a vacuum unit, and the other doesn't.
The advance is controlled by the ecu/ amplifier on the latter.
Easy to tell which is which. If you can twist the rotor arm against springs, it's the first type. The other one will have a solid spindle. Rotor
arm wont turn.
Similar on the CVH I think. Early XR2's ran a simple dizzy with large ECU/amp on the bulkhead with a 3d map, later ones ran the weighted dizzy
with vac advance and small amp unit fastened on top (which is what I have).
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Andy D
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| posted on 16/9/11 at 01:09 PM |
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I would definitely check where the trigger wheel is in relation to the rotor arm.. compare your two dizzys, see if they are the same?
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MikeRJ
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| posted on 16/9/11 at 04:41 PM |
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quote:
Originally posted by FASTdan
Would a reduced dwell not advance the timing though?
No, the coil is fired when the four reluctor points are aligned. The dwell control is a very clever analog based system that measures the peak coil
current prior to the spark. If the current was too low it reduces it's dwell timer, too large and it increases the timer. This sounds
backwards, but the dwell timer starts from the previous ignition event i.e. after a spark, the ignition amplifier then waits a certain time before
switching the coil back on.
If the dwell was wrong you'd get either a weak spark or a very hot coil. I suppose it may just be possible to have a really obscure failure
mode where the ignition amplifier is firing the coil at the start of the dwell, but it seems unlikely to me.
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