Correct carb?

[CJH]

I disagree.
The DJ is just a 1.9 but with a longer stroke.
you would want the same fuel ratio heading into the cylinder for each of them.

Now listen fella, it was you that planted this idea in my head. Don't go telling me I'm wasting my time
Actually though, didn't Silverbullet hint that his cam might benefit from a bit of tuning, e.g Webers that can be re-jetted?

Have you got the pierburg figures for the Astra 1.6?

Certainly.

Astra 1.6
Idle Jet: 42.5
Main Jets: 95 and 105
Air Correction Jet: 110 and 80
Accelerator Pump Jet: 40
Needle Valve: 1.5
Choke Pull-Down (mm)(a): 1.6
Choke Pull-Down (mm)(a1): 2.05

...and to save lots of scrolling up and down:

T25 DG
Idle Jet: 45
Main Jets: 102.5 and 110
Air Correction Jet: 50 and 45
Accelerator Pump Jet: 45
Needle Valve: 2.5
Choke Pull-Down (mm)(a1): 3.3

Carlton 1.8
Idle Jet: 42.5
Main Jets: 107.5 and 132.5
Air Correction Jet: 110 and 90
Accelerator Pump Jet: 50
Needle Valve: 1.5
Choke Pull-Down (mm)(a): 1.9
Choke Pull-Down (mm)(a1): 2.85
Choke Pull-Down (mm)(a2): 3.3

[CJH]

I think if you must run on fools gold you will need a rolling road and a selection of jets, somebody like Bromyard perhaps may have some pointer as to which, no point doing all this and having a knackered carb so I'd have thought you would be going for a rebuild anyway?

My thinking was to perhaps pick up a cheap old carb and give it a service kit, get that working as standard and then have a play with different jets. If it turns out I'm wasting my time I'll just return it to standard and keep it as a spare. But I'm well aware that messing about without measuring what's happening could well end in tears, so I think you're right about a rolling road.

Or add a bit of money onto the cost of rolling road and carb rebuild LPG it, fit a lambda guage, tune it yourself
my lpg goes better than petrol now its tuned with a lambda guage (it is closed loop also but I don't think thats needed)

That's certainly crossed my mind. If only it could be done without losing wardrobe or spare wheel space....

[itchyfeet]

Cheap old carb is an unknown even with a kit.

When did you last change a tyre? Get a can of gloop, I assume you have breakdown cover and good tyres.

[MGP]

Im a numpty when trying to explain things but here goes.
Carb and jet selection is far more dependent on volumetric efficiency than anything else, its all about air speed through the venturi.
Stoichiometric for petrol is the same for all engines 14.7 to 1 and you will need it slightly richer under load.
Find a carb that has a venturi proportionately larger to the increase in displacment and it will be jetted somewhere near where you need it.

[CJH]

Get a can of gloop, I assume you have breakdown cover and good tyres.

I do, on both counts. But the other issue I've heard raised about the spare wheel space is if you need to work on the space above (gear selector box etc) then you can remove a spare wheel easily, but removing an LPG tank isn't so easy.

[itchyfeet]

True but it's just gas unions and a few wires, nuts and bolts which can be removed and replaced fairly easily for a man with your skills.

[CJH]

Find a carb that has a venturi proportionately larger to the increase in displacment and it will be jetted somewhere near where you need it.

Makes sense, thank you. The only one that's bigger than the DG's is the one fitted to the LT 2.4. That has the same secondary venturi size (26mm), but a 24mm primary venturi, which is approximately 10% bigger than the 22mm of the DG's (similar to the increase from 1.9 to 2.1). The LT's carb has main jets of 107.5 and 135 - similar to the Carlton and bigger than the DG's 102.5 and 110 (just what ARE the units here? It's not mm - must be something to do with flow rate I guess).

But presumably I can't just put on a carb with bigger venturis though - isn't that fixed by the manifold? Evidently there's some variation around the relationship you've pointed out. Looking only at the primary venturi (in twin venturi carbs) the carbs listed in the Haynes book have a variety of venturi sizes versus main jet:

20mm : 97.5 (Astra/Belmont 1.3)
20mm : 95 (Astra/Belmont 1.6)
20mm : 97.5 (Cavalier 1300)
20mm : 95 (then 92.5) (Cavalier 1.6)
*22mm : 107.5 (Carlton 1.
19mm : 95 (Polo 1.3, Golf 1.3, Jetta 1.3)
21mm : 102.5 (Polo 1.3, Golf 1.3, Jetta 1.3)
22mm : 102.5 (Transporter 1.9)
24mm : 107.5 (LT 2.4)

*E D I T - evidently you can't put 1.8 in brackets

[CJH]

True but it's just gas unions and a few wires, nuts and bolts which can be removed and replaced fairly easily for a man with your skills.

Haha - where's the 'tongue-in-cheek' smilie when you need one eh? Gas unions - hmmm.

[CovKid]

Chris, sounds like you're making this a full-time hobby. Don't forget to drive it too. Easy trap to fall into. Ask me how I know.

Now I just drive the darned thing and use it for what it was meant.

[CJH]

Chris, sounds like you're making this a full-time hobby. Don't forget to drive it too. Easy trap to fall into. Ask me how I know.

Now I just drive the darned thing and use it for what it was meant.

Very true - bit between my teeth, for a couple of days at least. My van's away having some bodywork done, so until the weekend at least my mind is free to wander. I wouldn't even be considering this if I wasn't in the process of building up a 2.1. And then Silverbullet's cam shaft post appeared. And then there was a solar eclipse. I'd be a fool to ignore all the signs.

[CovKid]

Cue Twilight Zone theme.

[CJH]

This seller has a range of main jets, from 50 to 230, and clearly they are replaceable (screw threads), so a trip to a rolling road armed with a few of these is at least feasible.



As far as I can see from the diagrams in the Haynes manual, only the main jets in the two venturis and the idle jet are screw-in replaceable. I'm sure there'd be no need to change the idle jet, but I'm struggling with the relationship between the new cam's characteristics and carb operation.

Ian quotes the timing of the new cam as 18/58/58/18, compared to the DJ's 10/50/48/0 (and the DG, for which the 2E3 is jetted, is 'more conservative' apparently). So with this change to the cam timing, am I looking to increase fuel flow in low to mid range (which I assume means the primary venturi only) or at full throttle high revs (meaning the secondary venturi)? Does the secondary venturi only come into play at high revs, or is there enough vacuum created when full throttle is applied at low revs as well?

[silverbullet]

Did I heard my name mentioned?

Of course, you could just hunt down a complete DJ (or MV) injection system & distributor, bolt it on and be done with it...
The challenge will be finding a rolling road who even knows what a Pierburg is these days. Same for Digifant. Laptops and aftermarket ecus are the way forward once you deviate from standard spec.
I have an SS 2.1 under a Weber 32/34 DMTL jetted for an LT 2.4 and it goes well, mileage no different to the 2E3 but the secondary choke opening gives a noticeable boost.
The same carb on a DJ was better, it pulled like a train, would chug up a steep (1:5 or thereabouts) incline in first at idle...all on a DG dizzy timed at around 7 degrees.
I think that the bottom line is that carbs are history, they can only deliver so much especially when you consider that a certain warmed over 2.1 from Greece (on youtube) with sequential efi & mapped ignition claims to make 150PS and 21 kg.m (work it out, its impressive!) then it all starts to look a bit futile.
Evolve and reap the benefits

[CJH]

Thanks Ian

Well I could go for injection, but I'm not going to. Sounds way too complicated, and I've heard they can be difficult to diagnose when things go wrong. Mine's an '83 bus, so it's keeping its carb.

So, a rolling road. Do they really need to know about Pierburgs? Surely they just have to measure power and torque, and monitor the exhaust gases, and while they're printing out the last run I'll be swapping the jet for the next size up.

Do I even need a rolling road - can I just do it by 'seat of the pants'? I've read varying methods - keep increasing the jet sizes until there's no improvement, or start big and decrease the jet sizes until it starts to run properly. What are the potential consequences to these approaches, if I get the mixture wrong? My uneducated guess says that the stock setup is more likely to be too lean than too rich (not enough fuel for the extra air that the longer stoke will draw). Isn't lean worse than rich? Going with bigger jets seems like it would be safer on the engine if anything.

[CJH]

My uneducated guess says that the stock setup is more likely to be too lean than too rich (not enough fuel for the extra air that the longer stoke will draw).

Let me rephrase that - obviously not too lean, given that many people run the stock carb with the 2.1 with no issues. What I meant was, making it richer feels like the right direction to go.