Early 1.9 to 2.1 conversion - stop me if I'm being stupid!

[itchyfeet]

But where is this vacuum measured Andrew?

I've used both ports on a Pierburg, and haven't noticed any discernable difference.

Still not sure what you mean by both
The two at the back are effectively the same just below throttle ( although one is slightly closer), then there is the second throttle vac which comes from above throttle and drilled to both venturis, ( there is also the choke pull down)

P1010152 by Paul_Barr, on Flickr

2E3 2nd thrrotle vac port drilling by Paul_Barr, on Flickr

[itchyfeet]

Is there any chance that you’ve got a vacuum hose T-piece, to find out what happens when you apply equal vacuum to both spigots on your spare DJ distributor?

Good call Chris, not what I expected.
It still does not change the fact that the advance on a Digijet comes from above throttle and on a DG below this is the same full throttle but what happens part throttle under load is up for proving.


even though my spares are thoroughly organised it still took me half an hour to find the T piece, it was in the only crate not labelled

P1080561a by Paul_Barr, on Flickr

[itchyfeet]

Chris

can you please add to a graph just
my DG measured Centrifugal (mechanical) and Vac curve
my DJ Digijet C+V
your DJ with DG carb/dizzy C+V

this is the real graphs to compare, the engine never sees the centrifugal advance alone

[ajsimmo]

Yes, I meant the two at the back. When I next get a carb van in (all MV/DJ/tractor atm) I'll try the vac gauge on the secondary throttle actuator spigot and see if it behaves as you hope.

[itchyfeet]

Yes, I meant the two at the back. When I next get a carb van in (all MV/DJ/tractor atm) I'll try the vac gauge on the secondary throttle actuator spigot and see if it behaves as you hope.

there is a restrictor in the pipe, no idea if this could be a problem

2E3 2nd thrrotle vac pipe by Paul_Barr, on Flickr

[CJH]

Good call Chris, not what I expected.
It still does not change the fact that the advance on a Digijet comes from above throttle and on a DG below this is the same full throttle but what happens part throttle under load is up for proving.


even though my spares are thoroughly organised it still took me half an hour to find the T piece, it was in the only crate not labelled

P1080561a by Paul_Barr, on Flickr

Marvellous. So what this shows is that the DJ vacuum can is still essentially a vacuum *advance* mechanism, but with an extra tweak to make it retard the timing at idle. For that to work the advance signal must come from ported vacuum, which turns off when the throttle flap is closed. I suspect that tweak is not there for the engine's benefit, but for the benefit of the environment, but I need to double check that.

If that's the case, then a number of things follow:
- you could connect the two vacuum hoses the wrong way round, with no ill effect. This would mean that the retard signal switches off at idle, and the advance signal is there all the time, and the advance function would dominate even when both signals are there (from Itchyfeet's test). So the distributor would then act like a DG distributor, with no retardation. Idle emissions would suffer, but I guess the engine would run fine.

- You could leave the retard signal disconnected (but bunged up on the injection system). This would be slightly better for the environment than getting them the wrong way round, since the advance signal would still switch off at idle, so although there'd be no retardation at idle, there would at least be no advance either.

- You could use a DG vacuum can on a DJ distributor. If you use this hybrid on an injection system, you could use either vacuum port for the advance signal; use the retard port to get full-time advance, or the advance port to get advance only when the throttle opens.

- You could use a DG distributor on an injection system, with the same options as the previous point. The differences in the centrifugal advance curves at the bottom end of the range look rather small in comparison to the measured variation. I think it's possible that the steeper advance curve at the bottom end of the DJ curve *may* be there to compensate for the residual vacuum retard.

- Many people, myself included, use the DG distributor with a 2E3 on the 2.1. Obviously there's no retardation function, but as above, that is probably only meant to benefit the environment and not the engine. Connected up to the default carburettor vacuum port it provides full-time advance, even at idle, but the secondary throttle vacuum is ported, so that could be used to make sure there's no advance at idle. Equally....

I wonder if its worth trying a DJ dizzy on a dg with the advance connected to the second throttle vac port and the retard to the under throttle vac port.
Would that give results more similar to a DJ digijet?

I think it would. Using this 'ported' vacuum will switch off the advance at idle.

Chris

can you please add to a graph just
my DG measured Centrifugal (mechanical) and Vac curve
my DJ Digijet C+V
your DJ with DG carb/dizzy C+V

this is the real graphs to compare, the engine never sees the centrifugal advance alone

I agree, the total advance is the important bit. Here you go.



Above 1500 rpm, where the vacuum advance takes over from the vacuum retard, the lines are all similar if you allow for the noise in our data points. What's odd though is the offset between your DG and mine, some 7 degrees at the bottom end. Two degrees of that is caused by my timing being set (accidentally) for 12 degrees, rather than 10. The rest of it must be down to the distributor, because they'll both be at full advance with the throttle closed. Do you know if your vacuum can is a genuine part? Mine's a new genuine one from Classic parts, but I know there are other aftermarket vacuum cans, such as the one from Powerspark, which may or may not move the actuator arm by the same amount. I ran one of those on my DG for a long time and couldn't tell the difference from the genuine one.

[CJH]

there is a restrictor in the pipe, no idea if this could be a problem

My fluid dynamics isn't good, but I don't think the restrictor will matter much because there isn't much 'flow' of air - so the vacuum will work even through a small hole. That was bothering me about the retard/advance function, until you confirmed that it's the can that controls it. I couldn't see how different sized ports for advance and retard could work, since there's very little air flow.

By the way, in your test applying equal vacuum to both spigots, was the resulting advance (under maximum vacuum) as big as when you only pulled on the advance spigot?

[itchyfeet]

By the way, in your test applying equal vacuum to both spigots, was the resulting advance (under maximum vacuum) as big as when you only pulled on the advance spigot?

To be honest it was like the retard was not connected.

As far as I can see a DJ or DG advance is the same, yes I think my vac cans are origional.

Thanks for the graph, are you sure your dotted line is DG, I thought ypu did thos on your DJ with DG dizzy and carb?
the fact that my two DJ dizzies gave different graphs shows thefe is probably quite a bit of variation anyway.

[CJH]

Thanks for the graph, are you sure your dotted line is DG, I thought ypu did thos on your DJ with DG dizzy and carb?

Well it's a DG distributor and an LT carb. Differences between that and a complete DG setup can only be in the amount of vacuum advance signal, and since the vacuum advance is maxed out at idle, then the difference can only be in the rate at which it tails out at higher rpm I think. I need to do another test with it taken all the way up to 4000 rpm, but I'd say my curve is probably the same as you'd get from a full DG setup.

D'Oh - the penny's just dropped! My curve is with the centrifugal advance set to 12 degrees, whereas your DG curve is on a complete DG setup, so the centrifugal advance was set to 5 degrees I assume, hence the 7 degree offset. To see whether the distributors are interchangeable they all need to be plotted with the same initial centrifugal advance. Assuming your two DJ plots are for 10 degrees centrifugal advance, then I'll remove 2 degrees from my DG plot, and add 5 degrees to yours, and replot it.

*Correction - all three of your plots are based on 5 degrees initial centrifugal advance, so I'll just take 7 degrees off mine and replot.

[CJH]

So it looks like both the DG curves have less vacuum advance than both the DJ curves once you get above 1500 rpm. A couple of degrees of that will be the centrifugal advance, because the DJ curve is steeper at the bottom end of the rev range. The difference is bigger than that though, but it adds to my feeling that leaving mine timed to 12 degrees is a good thing to do.

[itchyfeet]

Sorryy forgot to say my DJ is at 5 degrees, runs on 95 RON

one more graph

I wonder of there is any off the shelf gauge to measure timing whipe driving?

I guess your rolling road people will know what they are doing.

[CJH]

I'm just thinking if your rolling road session is not going well because of ignition timing then having some different options with you might be worth it.
Personally I'd just get them to jet one bigger 2e3 carb and spend the extra time on playing with ignition timing, I think its pointless jetting a stock 2e3 it has to be lower power.

I've fired off a question about taking the 123Tune distributor to the rolling road session. I remember him telling me that the rolling road provides a light load (so it's ok to take a freshly built engine, although I'm past that now at 1500 miles in), and I wonder whether the load can be varied to look at vacuum advance under different loads.

But I am keen to do both carbs, and I think that's my priority. Logically the bigger one ought to be better, but using the original DG carb on a 2.1 is much more common, so I think it would be more useful to a wider audience to know whether the DG carb can be jetted differently to match the 2.1 better, and to know for sure whether there's much benefit in going to the bigger LT carb. And I'd come away with a DG carb optimised to my engine, to keep as a spare.

[CJH]

Sorryy forgot to say my DJ is at 5 degrees, runs on 95 RON

one more graph

Yep, spotted that before I plotted the graph above.

[itchyfeet]

Ah ok

[CJH]

P1080561a by Paul_Barr, on Flickr

Paul

Your vacuum pump looks to be the same as mine. I did a quick 'suck test', and the gauge on the top responds to vacuum applied to the nozzle (as well as from the trigger). If the range is appropriate, it should function as a vacuum gauge for measuring the vacuum signals. I'm planning to try and check the various vacuum points on the carb on a running engine, to see if the second throttle vacuum would work as a ported signal, and to learn about how it responds to the throttle.