A question for older readers, younger ones will probably not have a clue what I am on about!
Anybody who has tried to set up the AC Delco distributor on a TR7 will have sympathy with this post. The distributor is at the very back of the engine, very difficult to access and impossible to see what you are doing. It is not helped by the cockeyed design of 'external' centrifugal weights which severely limit access to the points for measuring the gap.
The questions are,
1 How essential is it to get the gap correct if you can set it then measure the dwell angle?
2 The Dwell angle is specified as 39deg (+- 1). How precise does it actually have to be in practice?
My interpretation is that if the Dwell angle is measuring to low this equates to the gap being too large. Is this correct? Also that it will have a marginal effect on timing, but well if it is running ok so what!
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pmh (was peter)
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A rough initial setting by guage or eye, then finely adjusted using dwell is the way to go.
Dwell ange does affect timing quite significantly, and IME is the factor which gave old cars the "it's in need of a service" feel as the heel of the points wore down: timing retarding as the gap closes up - dwell angle increasing.
The nearest I ever got to a TR7 was an 1854 Dolomite about 30 years ago - same family of engine AFAIK.
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There is a discussion in another forum here;
messageboard.lvwc.co.uk/forums/thread-view.asp?tid...3
that sums up the position quite well.
I have never really felt comfortable with the concept of dwell angle. I understand the principle, but I prefer to measure something more tangible, such as a real gap. On the occasions when I have , out of curiosity, checked the dwell angle having previously set the gap spot on, I have, like the other poster, noticed an amazing variation from the supposedfly correct setting. Also, unless you have a very unusual distributor with an external method of adjusting the gap, you can only set dwell by repeated trial and error adjustments to the gap, so you end up effectively doing both anyway.
I think the relationship between effect of gap, and dwell, is set in manufacture by the profile of the cam lobes. These can wear, as can the heel of the contact breaker, so perhaps one method is more effective at compensating for the effects of wear than the other.
Having set the gap or dwell, you then of course have to set the actual timing. I've always done this by watching/listening for the gap to open, but maybe if you have used the dwell method this is not relevant and you really do need a strobe.
Sorry to ramble on - I kow I haven't really answered your question.
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Cliff
Like you I have only ever really bothered with setting the gap in the past, but it is a very hit and miss affair when you cannot see the gap, and the feel of drag on the feeler gauge is almost impossible to judge when stretched out full length over the front NS wing. Also gap measuring seems to change when points are tightened down anyway!
It would be interesting to see if any body has done a plot of dwell vs gap. I can appreciate that this is probably dependent on the profile of the lobes on the distributor and the state of wear of the points, since this will presumably affect the angle at which one point closes on the other anyway.
This all came about because after 3 months of inactivity it started first time ran for 15 minutes and then stopped. The clip on LT lead inside the distributor had detached itself and obviuosly was only making contact by resting on the movable contact. That took at least 5 minutes to find with a multimeter and was a zero cost fix. What would have happened with a modern car? I would still be booting the laptop.
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pmh (was peter)
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The questions are 1 How essential is it to get the gap correct if you can set it then measure the dwell angle?
If nothing's worn I'd expect the gap to be correct if the dwell is correct.
My interpretation is that if the Dwell angle is measuring to low this equates to the gap being too large. Is this correct?
Yes I'm sure I remember that increasing the gap, decreases the dwell, but it was a while ago.
Chris
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Yep been there done that and sore knuckles, The dwell angle is not that critical but if the gap is too high the coil charge time is reduced, i.e. closed contacts. and a lower HT voltage can effect higher RPM and I mean higher 5K ish and at that RPM it is difficult to detect the random missing. I assume you would like to keep this in it's tradition form and not use a 'Lumignition' electronic conversion. If the gap is too small the points will arch more and pit and spike more quickly. The other problem is the cam concentric operation, often is you set the gap on the peak of one lobe the next lope is different etc etc. The dwell measurement normalises this and gives you the best operation. I always used to set the gap on the same lobe with a feeler gage I cut out of a peice of shim with a 60 degree offset in the end to ease access to the gap. That's going back 40 years. However once you have set it and got a sensible dwell reading then that's the lobe to use. What fuel are you running in her as she was never an unleaded head. Regards Peter
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In your case I would certainly abandon originality and go for Luminition. Points erode or build up spikes, so timing drifts, and I always found on any car that I needed to sandpaper and re-set every 1000 miles or less in order to maintain peak running and economy.
In contrast, I fitted Luminition to my Triumph 2000 12 years ago, and have never touched the distributor since.
For even more fun, try a Stag with double contact breakers!
I always find the way to get my head round the effect of of any change is to imagine the situation at the precise moment when the points are opening. If you then close the points a bit, you delay opening, ie retard the timing, and also give the coil longer to charge.
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In your case I would certainly abandon originality and go for Luminition. Points erode or build up spikes so timing drifts and I always found on any car that I needed to sandpaper and re-set every 1000 miles or less in order to maintain peak running and economy. In contrast I fitted Luminition to my Triumph 2000 12 years ago and have never touched the distributor since.
Certainly not Luminition, lots of failures that I've known about, a lot of it's features are only relavant for high performance stuff anyway.
I'd go for the Aldon Ignitor, known as Pertronix in the US & very easy to fit & not yet heard of a failure.
As for the other question Dwell is the most accurate & also averages out as near as you can dizzy wear. But it's a pain to do on any of the Triumph OHC engines as access is so poor, and that comes from someone whose worked on loads of Lotus Twin cams!
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In the good old days 'when I were a lad' as an apprentice I worked for a Vauxhall agency and what you describe was fitted to the slanted VX style engines also fitted to the Bedford CF. They are as you say, a nightmare to work on. However, I always removed the ditributor (after marking it prior to removal) so you could work on the distributor in a vice. Makes for an esier life and as you will need to check the timing, removing/ refitting is slightly academic.
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These are the views of Robin the Technician with 35 years in the trade. I fix, therefore I am...
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buy a lumenition off rimmer brothers bruv
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Even in the days of BL & triumph luminition was recommended then & whilst in the early day's of luminition we had quite a few faliures it was well worth the modification. If you set by dwell the car will tend to run smother & those dizzy's were prone to wear & inacuracey.
I seem to remember we had a tool for setting the dwell & a cranked foot tool for the timing but this was in the 70's when we could fix anything they were great!
I think every stag we sold we converted to luminition as well as they benifitted over the awfull twin points system.
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The dwell angle gives a more precise measurement to use.
It does away with the impossible to measure gap on the points once one side has burned down and the up making a feeler gauge measurement practically useless.
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I used to do this regularly with Skoda Estelles and became very goood at it. Of course access was perfect as the distributor was at the back of the rear engine.
The four-lobed cam that operates the contact breaker is likely to wear slightly unevenly. So the correct gap on one lobe won't necessarily be spot on for the others.
Measuring the dwell angle with a dwell meter 'averages' the dwell for all four lobes and gets the most accurate result in terms of overall timing. An engine can be made to run with much too little dwell, but the spark will be 'thinner' as a result and power will be down.
Of course once the dwell angle is set as well as possible, the timing needs to be set by strobe light.
I still miss having to do this. It didn't need doing all that often but it was satisfying to hear the engine burst into instant life and feel it giving of its modest best.
Until the next time the idle jet got blocked necessitating removal of carb and a blow-through with an airline!
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Getting the dwell angle correct is the important factor. The points gap is designed to give the correct dwell ange, but with age, cam wear etc this becomes less accurate. Measuring dwell angle takes away any effects of worn or pitted points, slight play in the distributor etc. The dwel angle is important because the open/closed ratio of the points is important at higher revs to ensure sufficient coil HT voltage.
So, set the points to about the correct gap and then finally adjust using the dwell meter. IIRC too low a dwell angle = too big a points gap. Timing should always be adjusted after points gap or dwell angle is set.
I find the big benefit is that it's easier to clip on the dwell angle meter at regular intervals than try to measure the points gap.
JS
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The last time I had to do this, I changed the points on my son's Mazda 323. Set them to 0.015" by eye, then put on the dwell meter. Dwell figure was spot on. Hey, I thought, I've still got it! Some things you never forget.
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Used to run a Dolomite Sprint in the 80's, 16v version of same engine so similar set up, IIRC I would simply check/set the gap every couple of weeks, took 10 or 15 mins, and replaced the points every 3000 miles or so. Setting up via dwell takes much longer so perhaps better to set the gap, in time terms little and often.
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pmh
Although it's a far superior way to set the points gap, mainly through eliminating shaft slop, you needn't obsess about dwell angles. For most older cars; it was impossible to tell from the driver's seat what the points gap was, 1 thou or 50 felt the same - with the absolute proviso that the timing was correctly set to match.
The longer a clean set of points is left untouched the better they work; 60-80,000 on one set is not impossible. "Pitting and piling" dramatically increases the effective surface area; each face becomes a perfect mirror-image of the other and the indentations give a self-polishing "wiping" action to their poly-angled surfaces. Stick a feeler in there, they move and you've ruined everything.
When fitting new points; clean and set them to about the specified gap. Run them in and set the timing dynamically with a good light. To check the gap, which does have a dramatic effect on timing, just see how much the timing has moved. More than 4 degrees change - just open the gap a knat's and recheck.
As said; avoid dodgy aftermarket ignitions. It was hard enough for the OE suppliers to get one to last any sensible time. If you can find one; just carry a fully set up and alignment marked spare dizzy in the boot. Then if you break down with no spark - a quick swap and you're away. [Incidentally; forget voltmeters - get a simple test-light. Much, much, better.]
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[Incidentally; forget voltmeters -get a simple test-light. Much much better.]
I've never used either. I just listen for the click of the spark. Also with sensitive fingers you can feel when the cam touches the heel.
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You use a thin cigarette paper betwen the points and pull it gently as you turn the distributor. When the cam touches the points, out it comes. That is your starting point for dynamic strobe setting.
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Screwloose
I like the idea of carrying a spare distributor, ready set up, but the reallity of actually getting at the fixing bolts is another matter. My tool cupboard is well equipped, but I just dont see how to get at the bolts!
I actually did the job by measuring the dwell angle only by repeated trial an error, it only took 3 attempts to get to within 1 deg of specn (39).
Sequence of 21, 53 and 40. Unless you have done the job on this car with the AC Delco distributor you cannot conceive how little you can see!
If I was doing this job frequently I think I would look at modifying the distrib and cap to allow external adjustment with the engine running
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pmh (was peter)
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I don't agree with Peter D, but with only half-remembered experience to back my views up. John S has it in a nutshell.
Hawkeye
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Stranger in a strange land
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And with which bit of my posting do you not agree with exactly. Regards Peter
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"the dwell angle is not that critical".
On shiny new engines maybe with clean points, but I always understood the measurement of dwell angle accounted for normal wear in the distributor drive and pitting of the faces of the points, to name but two. So using a dwell meter for a largely self-taught amateur like me was a simple way to get effective results. That's the basis of my disagreement.
Hawkeye
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Stranger in a strange land
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">Setting points gap vs dwell angle.<"
Neither, fit Rover V8 with electronic gubbins.
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Ahh I see, no what I was implying is that the actual dwell angle is not that critical that it has to be 39 +/- 1 and the manual says then I go on to explain the effect of the gap/ dwell being wrong and where it effects the performance. Once you have a decent set up i.e. the correct gap and a good dwell angle then the function of the face of the points can be checked by a repeat dwell measurement rather than climb all over the engine to measure the gap. If the timing was set with a 40% dwell then if follows that a quick check of the dwell in 1000 miles and it is still 40 ish then all is well. Regards Peter
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