Post
by SHyslop » Thu Jun 01, 2023 9:20 am
Dear David, you have answered your own question with the correct answer. Looking purely at the ignition side of things, as the engine fires with greater or lesser power depending on the distributor position, the suction created on the suction stroke of the pistons creates a higher or lower vacuum which will be recorded on the gauge. Fortunately, the lubrication pump take-off provides a convenient point to attach a Gunson's Lo Gauge or similar. The answer on other vehicles used to be to drill and tap a hole in an alloy manifold using plenty of grease to catch the swarf and not have it go into the cylinders. If you'd had a Ford there would have been a similar connection powering the wiper and Saab used the same idea on the original 900s to operate the ventilation controls.
Once the gauge is connected, rotate the distributor slightly and note the readings. There should be an optimum point of highest power (greatest suction) and smoothest running. The tickover on these engines shouldn't be as slow as on some older vehicles as you don't want the engine to stop when freewheeling - I think your car might be too new to have the Startix system on it - but if you do end up finding the point of good running is also rather advanced from where you started, the throttle stop will need adjusting back a bit to slow it back to normal.
Just in the passing, and with complete and full respect to the people who have found the opposite, about 20 years ago I had a Volvo 123GT which was near perfection except that it just lacked the go I was expecting. Having been over everything, I bought a 123 distributor (the numbers were just coincidental, by the way). It made no difference, it was a lot of money for no benefit. I would accept that if there is a distributor fault or you have a seriously incompatible distributor, it is a fancy bit of kit to solve the problem but there are still a lot of cars running very nicely on comparatively crude, and cheap, setups. As with electronic ignition, just because something is electronic doesn't make it a magic bullet.
I have a 1947 12 and its distributor is also somewhat loose compared with the others. It has worked ok for the 10 years or so I've had the car but it is not the completely firm arrangement I usually associate with these cars.
I never found an answer with the Volvo, by the way, maybe my expectations were too high.
In thinking some more about this, here is something to consider. Before about 1934/35, the distributor was a fairly simple device with no weights and no vacuum connection. It had the appropriate number of lobes for the cylinders and basically that was it. To start the car, you retarded the manual control "a bit", once going you set the ignition to "normal" and when you came to a hill you advanced it "a bit". I ran Austin 7s and 12s for years with these and they worked well. If you disregard the advance mechanisms, you should be able (with a few caveats we'll leave out to save space) to fit any suitable four cylinder distributor to your Rover and it should start well, idle well and maybe with a bit of manual advance it should run generally well on the near level i.e. pull through the gears, run up to 35/40 or so. If your car were to do that, it then becomes a question of : will it accelerate better or pull better with some technology (albeit 100 years old) applied? In other words, there should be a starting point at which your new distributor works well to start and move the car and then it becomes a question of improvements to the performance in detail i.e. centrifugal advance curve controlled by the weights and weight springs.
The one gadget I have found to be useful is a Gunsons Professional gastester. An engine that is receiving too much fuel won't run well because there is a ratio (stoichometric? without looking things up) of about 15 parts to air to 1 part of petrol. More than that, the engine starts to choke itself and too weak, not enough power. The Gunsons meter does basically what your MOT station does except with a cheaper machine and shows the value of the CO2 emitted as a digital readout. Under 4.5% for a 70s car would pass an MOT and I believe is getting close to the 15:1 ratio, up to about 7% is usually the top end of what I'd be looking for on an older car but I just mention this as they made a cheaper unit in a yellow case which had a moving coil meter but it had the Morris 10 speedometer needle problem, the needle could show a wide variety of readings all at the same time whereas the green cased Gunsons device has a digital display that is pretty accurate. If you knew someone who had one it would be worth borrowing for an hour to check that the mixture is right, or maybe you have a friendly MOT station who would hook it up to their machine and just make sure. If the mixture is wrong, it can make setting the ignition timing unnecessarily difficult, particularly if the Rover has a Solex carburettor with the wrong or worn or damaged jets.
) and set it up according to the book settings.