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MG MGB GT V8 Factory Originals Technical - Plug lead 'correct' orientation - longish, and possibly stupid!
I recently changed the vacuum capsule on my 35D8 distributor. (In fact, you're all bored with it by now!) I then found that I couldn't set the timing to anything sensible. The vacuum capsule collides with the thermostat housing as the dizzy is turned clockwise. The most _retarded_ I could get the ignition was 35 degrees BTDC at idle. - I'm surprised it ran at all - but it ran reasonably.
I think the new vacuum capsule has a longer lever, resulting a more anti-clockwise orientation of the distributor body for the same timing.
I had the idea that I could take the dizzy out and replace it 180 degrees out of phase, then replace the leads 108 degrees out of phase and have the vacuum capsule out of the way on the LHS of the dizzy. This works fine (except for #8 lead being a bit short for comfort).
Then I started thinking about it a bit more. Couldn't I have just moved all the plug leads round one place clockwise, then turned the dizzy 45 degrees anti-clockwise? Is there any "correct" orientation of the leads?
|108 degrees? Hmmm, think you might have trouble with that :-)|
Seriously though, I also replaced the capsule a couple of years ago and had no problems, but we may have different capsules, of course. Are you sure the distributor is correctly orientated anyway? My capsule is about 180 degrees from the thermostat housing anyway, sort-of on the coil-side of the dizzie.
This puts the dwell adjuster very close to the the LH rocker cover.
Are you sure everything is correctly hooked up inside? On the face of it I can't think of any reason why you cannot reposition the distributor and leads as you say. Just remember to swap over the middle two wires on the LH bank so that 5 and 7 aren't adjacent.
I must be dyslexic. I read that 3 or 4 times as "one hundred and eighty" and wondered what on earth you were talking about.
Seems like your capsule is where mine is now. I can't get a socket on the bolt holding the dizzy clamp as the capsule is in the way now. Thus the question about just putting it in some arbitrary position.
I'll give it a go when it's all back together.
|Nick - I use a ring spanner on mine. Not because of access limitations as I recall, just seemed easier at the time than breaking out the socket-set. Several pictures in the workshop manual show the capsule in that position.|
|Here's something I just found out - I bought a used 35D8 Rover distributor off of Ebay, spent some time cleaning it up to keep in my GT V-8 as a spare, went to install and test it and found the @#$%@% thing has the vacuum advance canister on the wrong side! It appears to be a mirror image of the GT V-8 distributor, which means that the vacuum advance is actually a vacuum retard unit. I found out this was out of a 1970 or 1971 US-spec Rover 3500S. |
Because of the revised position of the vacuum advance, the distributor won't clear the upper radiator hose on the GT V-8. It appears that this distributor was designed to mount with the vacuum advance facing more or less forward on the engine, rather than to the side as on the V-8.
Live and learn - anybody need a 3500S distributor?
|Paul, I think you'll find that this distributor should turn "backward" from the one in your car and that it really is a vacuum advance. I've never seen a single action canister that was a vacuum retard unit. Some American autos had dual action systems on their distributors about thirty years ago which advanced or retarded the ignition depending on what the engine management called for (band aid for emissions).|
|Paul - I am not sure I understand how the vacuum advance can act as a retard although I believe something like that is possible by reversing the connection to the amplifier (on later dizzies). I suppose you know that the RV8 and some RR vacuum chambers are 180 from the OE MG - as I recall' you drive out the pin holding the drive spigot (can't think of a better word) to turn it (and everything else) the way you want it.|
I don't know if this helps
|It is easy to make a vacuum retard system, simply mount the canister in such a way that it pulls the point pivot in the same direction as the cam rotation, thereby delaying onset of the point opening. Normal vacuum advance units pull the points in the opposite direction of the cam rotation, to advance the spark.|
The canister on the 3500S distributor is mounted so that its actuating arm lies on the opposite side of the distributor shaft, both the tapped holes in the distributor casing for the canister mounting screws and the large hole for the arm to pass through are repositioned to accomplish this. Ypu cannot simply reverse the canister on the distributor to mimic the configuration of the GT V-8 unit.
In addition, the points mounting plate and the points themselves are different between the GT V-8 distributor and the 3500S distributor. The V-8 points have the long pin out the bottom to engage the actuating arm of the vacuum advance, whereas the Rover points do not have this pin, the pin is mounted on the plate itself.
This is one of those situations where I DEARLY wish we had drawing capability within this forum!
Now, I am curious if the configuration of the Factory GT V-8 35D8 distributor is unique to the GT V-8, or if it was used in any other cars. I know the characteristics of the cam and the vac advance may be different across vehicles, but I wonder if the basic morphology of the GT V-8 distributor matches that of any other vehicle.
Drawing capabilities? No worries! I just nipped out to the garage and took a picture of the dizzy.
The engine number (48102659) suggests that my motor is from a 67-76 UK spec manual trans 3500s. If we assume the distributor is the original (or at least the correct one) then the US spec ones differ, as this vacuum mechanism definately provides advance: (as you point out it pulls the 'gubbins' towards the direction of rotation.)
http://www.bluebonnet.demon.co.uk/mg/images/dizzy.jpg (Sorry, the picture is not very clear. NOTE: The distributor is rotated 180 degrees from when I got the car.)
I'm puzzled that you assert that the vacuum capsule is on a particular side, as the original post in this thread points out that you can put it on 180 degrees out of phase, and postulates that you _could_ put it on in _any_ orientataion you want.
As for your final question, what are the part numbers on your two distributors? Mine is 41317.
Post a picture - go on.
|I think PaulK meant that one cannot simply reverse the position of the capsule within the dizzie to convert it from a 'retarder' to an 'advancer' rather than not being able to reposition the dizzie in the engine. I also presume Paul has the two side by side, making it difficult *not* to spot that the capsule faces the other way. We are all thinking in terms of clockwise rotation, right? The standard dizzie according to my manual is a 41394, out of interest. It also seems that my dizzie may be rotated one plug position clockwise from where it should be, as the manual shows the cap clips being fore and aft and my rear clip and the dwell adjuster are closer to the bolt on the inlet manifold than that (and on Nicks pic). There is a pic of my grubby unit at http://www.mgb-stuff.org.uk/images/dizzie001b.jpg|
|Paul H., you are correct. I could rotate the distributor in the car to get it to clear everything (and reposition the plug leads to suit), but the actuating arm of the vacuum canister ends up pulling the points in the opposite direction from what is required to give you advance. In addition, the distributor body has a notch to locate the distributor cap. On the 3500S distributor, this notch is on the opposite side of the distributor. This requires moving the plug leads 180 degrees around the cap, then repositioning them in whatever direction will compensate for the different orientation of the distributor body. |
I also surmised that if this distributor is set up with a vacuum retard canister as opposed to vacuum advance, I could hook the canister to manifold vacuum instead of ported vacuum. This would give me the proper action of the canister.
All this would be possible, but a bit messy. I kind of want a spare distributor that is close enough to the original type that it is a simple matter of pull, plonk, and twist to put in the new one, as I stand by the side of the road at midnight.
As far as uploading a picture of the two different distributor types, I have neither a scanner nor a digital camera easily available, plus no website of my own as yet. Stone knives and bearskins.....
|Paul Kile, we had quite a thread on the MGB Technical board a year or so ago regarding vacuum advances and manifold vs. ported vacuum sources. There are no "retarding" vacuum advances (boy there's a non-sequitur). Believe me when I tell you that your distributor is supposed to turn in the other direction if the vacuum capsule is as you discribe.|
Now, are you sure about what engine this Ebay distributor came from? I must admit ignorance about rover engines--but if you look at photos of V8 MGBs (conversions), the 215 Buick V8 has a different distributor (with the spark advance a "mirror image") from a 3500S engine.
|Distributor 41317 comes from the original Rover 3500 and 3.5 litre cars from 1967 to 1973. Thge MGBGTV8 as Paul H says is 41394. Of interest dual action vac units were common to most later Rover V8 applications with different vac loads being imposed on either side of the same diaphragm so providing opposing forces. One that provides advance of the timing and the other a retarding effect. The different vac signals coming from whatever throttle position was adopted. Indeed as described, 'sticking plaster' emission control, which usually fell down as the diaphragm was, and is, prone to splitting.|
The original question has pretty much been answered along the way, but at the weekend I re-oriented the distributor body to a 'convenient' position - I have the vac capsule facing the radiator - and repsositioned the plug leads to suit. Works a treat.
|At the risk of flogging a dead horse I don't think a manifold-connected vacuum retard will give the same results as a ported vacuum advance. The former would give minimum advance at idle and steadily increasing advance as the throttle is opened. The latter gives minimum advance at idle - which *is* the same - increasing advance as the throttle is opened to a certain point, then reducing advance as the throttle was opened still further. The former would probably result in pinking at full throttle - that is, if anyone ever drives a V8 MGB at full throttle for more than a few seconds ...|
|Paul H., good comments, although I think that the steadily increasing advance would be limited by the diaphragm stop in the advance unit, and if properly synchronized with the centrifugal advance could be made to work.|
I think this is what they did on my 3500S distributor, since I don't think they ever built a 3500 engine that spins anti-clockwise. Now all I have to do is find out if a US-spec 3500S has the vac unit connected to the manifold.
You're right - this horse is not only dead, but its beginning to rot and offgas as well....
This thread was discussed between 28/11/2000 and 04/12/2000
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