Thread: Mechanical advance of various Lucas distributors

I don't know about the marking
And you may already know this

But from memory it's all about the "spring weight" (tension)
You put in ......
lighter springs to make it advance early (less RPM to fully open)
Heavier spring to make it advance later (more RPM to fully open)

Buy a set of assorted springs to test up & down the scale

I remember your problem // 12:1 compression with Vacumm advance disconnected
I guess your after "Heavier springs"

That is a heck of a load of compression // Did you sort out the high & low readings you got??? I was amazed it ran

Regards Mike

Thanks for the info mike, Yes I was aware of the spring tensions determining on how the advance changed over the rev range.

There is two ways to achieve a shallower advance curve actually... higher tension springs or lighter fly weights. The springs... hmm, I don't know where I would look for them. Any suggestions as to where you get these assorted springs?

I could grind the weights down until I get the right curve also, but obviously if I stuff it up there is no going back. I suppose the first step in all this would be getting a decent timing light with an advance reading.

The compression results... hmm, I don't know about them. The engine runs fine and yeah... I have no explanation for them. Maybe I'll re-run the test next week and see if I get the same results. I'm very suspect on them.

Originally Posted by stirlsilver

There is two ways to achieve a shallower advance curve actually... higher tension springs or lighter fly weights. The springs... hmm, I don't know where I would look for them. Any suggestions as to where you get these assorted springs?

I could grind the weights down until I get the right curve also, but obviously if I stuff it up there is no going back. I suppose the first step in all this would be getting a decent timing light with an advance reading.

I wouldn't grind the weights till it was the last resort // as its too hard to reverse it

Places to get spring weights ...... This is the problem with West Ozies .. Half of us have never been over the border

Over here (Perth) i would go to "Coventry's" .... What store would be the equivalent over there???

Also ... What advance is the motor set at??? 6 / 12 degrees


Another thought ... Have you tried "locking up the mechanical advance" and just hooking up vacumm advance only
We had a race motor & locked up all advance & just set the motor up for 25 degrees (Mopar 440)

G`day Stirl

Later factory distributors have less total advance than earlier points distributors , i don`t know the general amount but as you say there are various amounts .

The carb distributors have a small plate or stamped numbers for ID and from that ID it gives the advance but only the mechanical .

At 4800rpm some have 27 - 31degs , some have 22-26 at same revs .
Some have 20 - 24 at 3600 .
Some are set at 5degs ATDC and even 8degs ATDC
Some have no advance untill 1400rpms

There are at least 6 different Carb distributors could be more .

If i have it right your running straight lpg ?

If the case .

Not sure from what you`ve said your on the right track , you may well be and i`ve not understood what you`ve said .

Compaired to petrol lpg likes a higher number early but a lower one overall .

The reason for the lighter springs/weights to to give the higher number early but there may also be a need to limit the weight movement to give the right figure .

Vac advance should only be a concern if you get pinging on light throttle pulling up a hill , ect , because the vac doesn`t suit the curve .

Our 85 has a 4.0ltr that was a bit over 10 on the factory composites it has thinner 3.9 composites now so`s probably closer to 11 has a factory elec dissy and the only time it pings is mid/light pull because of the vac and too much initial . It runs fine low to mid but loses out high with too much advance .

Ours runs a range hotter plugs and is set lean , the plugs come out white but without blisters so it`s on the limit , it could go better with more gas but don`t use the top end much .

If the white flaky stuff you mentioned on your plugs is actual blisters it`s either not getting enough gas or the curve is much to high or the wrong place .

Have you talked to the blokes at nunawading ignitions or similar about getting a curve for lpg , i haven`t but was told around $200 but that was for a windsor and was a while ago .

i`m sure you could work something out by yourself but i don`t think it`s as simple as just changing springs , though i have read that other say it is . Sounds like hard work to me because even if you get the curve for the different carb dissys , you`ll then need to go through a parts cat to work out which springs and it won`t give their tensions only a number then you need to buy an assortment and measure them yourself , all sound like alot of time and effort but good onya for having a go .

Cheers
Peter

Thanks for the info peter.
Yeah I am pretty much chasing less overall advance but that it is reached earlier... along with a high initial advance. I probably didn't explan myself too well there.

In terms of the colour of the plugs... yeah I think the engine was probably a little lean and had too much advance on the top end... I have sort of sorted it out by running the distributuor vacuum advance from manifold vacuum (instead of the vacuum advance port) which allows me to have high initial advance (since the vacuum advance is on) and reduced my overall advance. So the engine will now rev out to 4700 rpm... it's hard to tell if it's pinging at that RPM... I don't think it is. In anycase it is much better.

I had actually been to performance ignitions about a month ago asking them how much for a distributor overhaul and recurve. They quoted me $200 assuming the vacuum advance is working OK. After reading your post this morning I actually hopped in the car and did the drive to nunawading to hand in the distributor to get the work done... But they were closed... turns out they aren't opening on saturdays anymore.

I think it will be a lot less of a headache if I get it done professionally... especially since they have experience with high compression engines.

G`day ,

sorry , don`t let my opinion sway you , one of the books i have from the early 80s says that back then it was possible to buy generic kits that came with a selection of springs and limit plates , they were taking about the USA but the USA is alot closer now than it was is the early 80s .

From what i`ve read there a blokes here that reckon they can do it .


If you go with Performance igns i`d be interested to hear what they give you , i presume they will tell you figures/degrees etc and not just hand it over and say it`s sweet .

I`ve actually not long bought an engine because it came with a factory electronic dissy with the idea of having it curved for lpg .

I`ve spent some time half heartedly looking at info on curves for lpg and as i don`t know enough about it , have only got as far as this .

Somewhere around 10 degs initial ( crank ) with 9degs at dissy ( x2 at crank ) and total at around 2250 or 2500 rpms , with vac adv to be added where it works .

So total would be 28/30 degs by 2250 or there abouts plus vac in its range .

Vac adv starts at 7 inches of manifold vacuum and full adv by 10 inches which should give somewhere around 12/18 degs of vacuum advance .

Not sure if the higher 18 or lower 12 figure is going to be right but the highest would be 48 degs but of coarse this is only when the vacuum advance is doing any thing .

This is why ours is pinging at certain stages of the vac advance because it has too much/high at present .

So if you do get one curved for lpg and they do give you any figures i would be pleased to hear/read them if you don`t mind . I don`t feel i know enough about it so it would only be out of interest but it would be of much interest .

Some of the above may explain why i think that there may be a bit more to it than just changing springs but i also think it`s good for people to have a go as long as they are aware of what the consequence can be if its got wrong .

Cheers
Peter

I would say that you have done a little bit more than 'half hearted' research there peter.

One thing I ask you though is are those values specific to the RV8? Because the shape of the combustion chamber has a influence on how much total advance it will accept.

Last night I was actually thinking about making a distributorless system with coil packs... and using a microcontroller... then I could program in any advance curve I like!

Since I have an old points distributor here I could remove the guts out of it and fabricate a disc which has 8 x 4 magnets (which is all you need ot tell crank angle) and then feed the signal into a microconroller which determines how much advance is needed and then fires the necessary coil... Hmm I'll need to think about this some more.

This system would give Soooo much flexibility!

Hi,

Have had a RRC with a 350 on gas for about 10 yrs now. A friend at the time I bought this had a jeep with a worked 351 on gas running twin Impco convertors and mixers.

His advice was to take the vac for the advance from the manifold, rather than upstream of the the butterfly on the carb. Tried this and have used the set up ever since.

Have seen why this works better for gas on a pommy site some years ago. Think you get more advance at low rpm, and comparatively less at higher rpm. Still runs fine on petrol, once or twice a year when it runs out of gas.

I'm setting the advance at 10 deg at 6-700 rpm with the vac adv hooked up to the manifold. Have tried more advance, but lose mileage. Have no idea if the mech advance in the distributor is stock or not. Works well for me.

cheers, DL