There is a method for limiting the mechanical advance which includes welding up the slots in the dizzy plate but this sounded a bit dodgy but I happened upon another method which I prefer as its completely reversible, and I did it without even removing the distributor from the car.
In the metal strip that the rotor screws on are a some holes punched at the factory. By chance they are just over 5mm in diameter so can easily be tapped to take a homemade M6 grub screw. All I did to make a grub screw was take a regular M6 hex head machine screw and chop the top off. You can see in the photo below all you have to do is leave enough of the grub screw protruding so that when the rotor moves, the screw will hit up against the centre plate - thus limiting the mechanical advance. Lots of people have experimented with grinding the head of the screw down to various shapes and sizes to limit the travel so I did the same, then verified with the timing light.
I got the dremel out and made a slot in to get a flat head screwdriver. You can then do further grinding to the head of the grub screw to get your adjustability. I cut half the top off, retaining some of the original grooved cut so you can still get a screwdriver on it. Rotating the grub screw then gets the flat edge of the screw either closer or further from the baseplate depending on its position.
You can twist the plate that the rotor fits to by hand to see how the sweep of travel is affected and then by rotating the grub screw its easy to see how the position of the screw can alter the maximum rotation of the rotor.
It's probably best just to experiment and have a guess, then check the timing and make further adjustments. I found it quite satisfying to do it this way actually as it proved to me that I knew what I was doing!
The angle of the grub screw required for a certain amount of total advance may change depending what shape weights and centre plate you have because the starting point of the sweep of rotation of the rotor may be different. Best to experiment yourself. As long as you haven't changed the base timing position you can't hurt your engine if you start it up and run the revs up to 3000 to check the "all-in" figure, there's no load on the engine so it's no problem.
The inside of the plastic rotor housing needed a tiny bit of fettling to clear the screw but all was well. I'm now confident that the max timing is definitely the maximum and there is no extra bonus timing. We briefly ran the car up to 4000, 4500 and 5000rpm and the timing mark on the balancer was steady so I'm happy that's the true maximum.
We went for a short drive to get some fuel and to do the timing in the quiet industrial estate behind our house so as not to annoy the neighbours and it was fine.
We checked the timing: roughly 16-17 static, 30-31 all in at about 2700rpm so thats 15ish of mech advance. A quick adjustment on the grub screw and my mechanical advance was opened up a tiny bit to 16°. The final setting was 18° static and 34° all-in. This allowed me to close the idle speed screw a bit and I've now locked the dizzy down and will probably leave it there.
One important thing to note - if you limit the total advance so you can raise your static advance, you may need to swap to stronger springs because your "all-in" figure will come in sooner.
The Moroso advance kit gives the following information:
| RPM | 500 | 1000 | 1500 | 2000 | 2500 | 3000 | 3500 | 4000 | 4500 | 5000 |
| light | 0° | 10° | 20° | 22.5° | 23° | 23° | 23° | 23° | 23° | 23° |
| medium | 0° | 0° | 6° | 13° | 19° | 23° | 23° | 23° | 23° | 23° |
| heavy | 0° | 0° | 0° | 6° | 10° | 16° | 21° | 23° | 23° | 23° |
The values in the table above are added to your static timing to get the total timing for each rpm interval.
An example: say you originally had 12° initial and a a pair of medium springs. By the chart above, you would get a total of 12° + 23° = 35° advance in at 3000rpm. At 2000rpm you would have 12° + 6° = 18° mechanical advance. If you whack your static up to 18° at idle, at 2000rpm now you have 18° + 13° = 31° mechanical advance. If you already had an aggressive timing curve with light springs, you may suddenly find you have some pinging issues when accelerating from low rpm as the mechanical timing is now higher than it was before.
What I did was to check the timing at 500rpm intervals and swapped the springs for heavier ones to bring the timing curve back to what it should be. Surprisingly I ended up using two heavy springs. The top row is my original timing curve, without the mech limit grub screw, static set to 12° and two medium springs. The bottom row is my new timing curve with 18° initial, total mech advance limited to 16° and two heavy springs.
Total timing:
| RPM | 500 | 1000 | 1500 | 2000 | 2500 | 3000 | 3500 | 4000 | 4500 | 5000 |
| Original | 12° | 12° | 18° | 25° | 31° | 34° | 34° | 34° | 34° | 34° |
| heavy/heavy | 18° | 18° | 18° | 24° | 28° | 34° | 34° | 34° | 34° | 34° |
I felt that this was the best match. I tried one medium spring and one heavy spring but the advance rate was too fast and was giving me 34° timing at about 2400rpm and I was getting some light pinging when accelerating from lower revs.
Went for a drive and it felt good. Didn't notice the loss of a couple of degrees of advance in the 2500-3000rpm range. Perhaps on a dyno there might be a small difference, or maybe in a really heavy car you might possibly detect a small change but in the real world with this engine in such a light car you just can't tell. Idle screw settings don't seem to affect the cruise over 2000rpm so that suggests the idle and transition circuit is working how it should (closing down the idle speed means the transition slot is being used properly as designed) but I still get a little lean surge below 2000rpm so I wonder if the idle/transition circuit can supply enough fuel. It's on the list for things to research. EDIT - see post Idle Channel Restrictor for the solution to this! Over 2000rpm AFR's of 12.8 to 13.0:1 for cruise does seem a little rich to me but anything more than that and the lean surging gets worse so I've decided its best to give the engine what it wants, not just what the internet says it should have (mid 13's)
No comments:
Post a Comment