Correct carb?

[silverbullet]

None taken. I was just adding to MGP's comments about the 2E3 from the DG beig sized for a very specific set of parameters and the short stroke of that engine is an integral part of that.

[itchyfeet]

OMG this has all got a bit geeky

[silverbullet]

The joy of a proper technical discussion.

[kevtherev]

I love carburettors.
Even the word smells of petrol

[silverbullet]

The past is a great place to visit, but this is 2015 and we need more efficiency than carbs can deliver. The torque and power are a bonus imho

[itchyfeet]

I do 3000 miles a year in the van for leisure, I don't really give a monkeys about efficiency, or speed, I have that silly grin most of the time
a bit more power and knowing I'm not damaging the engine is my priority.

In the car I do 15k miles a year commuting , efficiency is more important as is acceleration.

[kevtherev]

I do 3000 miles a year in the van for leisure, I don't really give a monkeys about efficiency, or speed,

Ditto.
There's nothing more involving than tuning a carburettor, getting that balance, a tweek here and there.
The pierburg is a bit disappointing in that dept, twin carbs, or even better twin pairs, now your talking.

[CJH]

Well I've just paid the (very reasonable) invoice for a carb from a low mileage LT 2.4, so any day now I should have that to play with. Going back over this thread there's some very useful information about the principles for making this work on 2.1. The 24mm primary venturi is about as much of an increase over the DG's 22mm as the 2.1 is over the 1.9. It's set up for an engine that's another 15% bigger again though (2.4). So now I've got no excuse for not tackling the issue of setting it up for a 2.1.

MGP suggested in a post above that:

The lt carb will be better than the dg one however you may find that it will require smaller jets. A good place to start would probably be the jets from your dg carb with possibly a smaller air correction jet.

Since then I've also done some measurements on Ian's camshaft, which show that the inlet valve is quite similar to the stock DJ cam, and the exhaust is quite different:


But I haven't measured a DG cam, so I don't know how much difference there is. I do know from may people on here that the stock DG carb works fine with a 2.1 running the stock cam.

But I wonder if the suggestion above to use the DG jets is correct. I can see the logic if it's being used on a 1.9 DG, but is there perhaps some middle ground between the DG and the LT jets that might suit a 2.1 better? And what is the reason for the suggestion to use a smaller air correction jet? The LT has air correction jets of 110 and 100, while the DG has 50 and 45. Is the suggestion to use smaller values than the DG?

[MGP]

The air correction jet controlled how much the air flowing through the carb can act on the fuel jet. A larger air jet will allow more fuel to be pulled through a fuel jet.
The theory of using the dg jet was that increasing the choke size in proportion with the displacement increase would theoretically keep the air speed the same so the dg jet may be somewhere near right.
I will also point out that my ramblings are theoretical as I haven't done this particular conversion myself.

[CJH]

The air correction jet controlled how much the air flowing through the carb can act on the fuel jet. A larger air jet will allow more fuel to be pulled through a fuel jet.
The theory of using the dg jet was that increasing the choke size in proportion with the displacement increase would theoretically keep the air speed the same so the dg jet may be somewhere near right.
I will also point out that my ramblings are theoretical as I haven't done this particular conversion myself.

Ah, got it, thank you. So when you said 'a smaller air correction jet', did you mean smaller than the LT (110 and 100) or smaller than the DG (50 and 45)?

Theoretical ramblings - that's perfect, it's good to have you speculating intelligently on my behalf. I've not discovered anyone who's looked into this. Someone on here (Silverbullet?) I think mentioned running an LT carb, but I can't recall which capacity engine that was on, and I don't think he said whether he'd re-jetted the carb at all.

[MGP]

The Lt although I would be tempted to leave the air jets to start with stick in the dg fuel jet and go for a gentle run with an afr gauge hooked up and see what happens

[CJH]

I've bought one these:


and one of these:


I think the gauge is the same one that Itchyfeet said is annoyingly bright, but I'm only intending it as a temporary fit while I get the carburation right - perhaps mounted in the back so I can see it in the rear view mirror.

The new engine will have a Speedshop exhaust, which has a sensor boss already in place. But I'd like to see what sort of readings I get on my current 1.9DG, as a baseline, and my current exhaust doesn't have a boss.

So I'm wondering if I can just push the sensor up the exhaust to get some baseline readings. Does the sensor have to be at right angles to the gas flow, does it have to be nearer the cylinder head etc etc? And if shoving it up the tailpipe is ok, are the readings at higher rpm affected by engine load? I was hoping to just do some experiments on my driveway, without having to fix it in place and go for a drive.

[itchyfeet]

problem with sensor being up the tail pipe is it gets the exhaust way after combusion so the results are not in realtime, not going to properly detect whats happening when you throtle hard for example.

also sensor needs to be proper hot and even with it's heater it may not be hot enough.

Weld a bush in and leave the sensor so you can periodically check is my advice.

[CJH]

I was afraid you might say that. I don't weld. I'll see what my local garage will charge.

While you're on - the two white wires are for the heater circuit. Since they're both the same colour it's clear that polarity doesn't matter, but do I just need a 12V positive and an earth? Any idea what current it will draw? (The gauge hasn't arrived yet - I suppose it's possible that this part of the circuit is provided by the gauge wiring.)

[itchyfeet]

yep whites are 12v and earth non polarised, it's just a heater
can't remember current 2A ish perhaps, measure resistance
current = V/R

is your exhaust early or late type?
early is not great for lambda measurements as the four pipes never come together