When to change Carb Jets?

[silvadw]

Hi all Carb experts
I am unsure as to what changing jet sizes does to a car, or what symptoms dictate when to do it. Do you have any general advice?
I have been following the Chinese 34ADF thread, but seek a little more detail.

When is it required to change the primary and secondary jets for Idle/Main/Air correction and Emulsion tubes? The only thing I can find is a description on the movement of the mixture screw too far in/out being the idle is Rich/Lean. But is this the primary Idle or both Primary and Secondary idle?. What to do with the other jets e.g. does changing the secondary main, increase power at the top end on the revs?

I just acquired a 34 ADF and single plane manifold for my 79 spider 2000. It has the following primary/secondary jets
Idle 50/90
Main 120/135
Air correction 145/180
I didn't check the emulsion tubes.

Any assistance will be gratefully received. I visit Chicago, Austin Tx, and Northern Cali often enough to buy any respondent a beer or three (also goes for Curly in Oz).

Copied to Engine and Tuning

[majicwrench]

Carb was designed by some very smart people. Unless you know more about carb design then they do, and have access to a dyno, I would not recommend changing jets. There is a fairy tale that talks of changing jets (bigger) for more power, that is not true. Also a tall tale concerning bigger motor needs bigger jets, again, not true.

Are you having a problem with your engine/carb??

[azruss]

early 124s where jetted lean to pass emissions and many of them required moving up to a bigger main jet to run properly. My 70 would lurch from a stop regardless of how easy i was on the gas. drilled out the mains and that solved the problem. My fuel economy when from 30 mpg to 20 mpg
I also replaced that 1438 with a 79 2L carb. Took that horrible carb off and replaced it with my stock 71 carb. power was not good, so went to the next size up in main jets. power went up 20-30%, fuel economy also improved by 5 mpg.
These are extreme cases. As others have stated, Webers are very well dialed in. Stick with stock until you get a feel for your power band before you change. Brad Artigue can give you some good direction here.

[Exit98]

Well here is a reply from a completely non-Carb Expert. You will be very happy with the 34 ADF on your 79. With the help of our NJ expert Ralph (lanciahf) I put one on my 76 and it sure made a big difference. You have a very restritive exhaust header on the stock 79. If you haven't gone to a 4-2-1 yet you'll need to do that too, to really get the benefit of the new carb.

Ralph made some jet changes on my carb, and he knows what he's doing. But really, don't worry about that now, chances are the carb will be fine as is, as long as its in good shape.

Jets just meter the amount of fuel being fed to the carb. Like a doctor who uses differernt size needles to inject stuff into us. Other changes you make to your motor might benefit from a change in jet size, like adjustable cams, or a fancy exhaust header, but you don’t need to worry about that now.

[bobplyler]

You have a very restritive exhaust header on the stock 79. If you haven't gone to a 4-2-1 yet you'll need to do that too, to really get the benefit of the new carb.

My 79 came with a 4-2-1 exhaust.

[silvadw]

Thanks to all the replies.
I am yet to bolt on the Carb with the 78 intake. Wanted to see what others had to say first as i don't fancy the idea of fiddling once it is on. My 79 came from PO with a 4-2-1, but stock 28/32 lump of metal. Having said that, it runs really well as is a rebuilt motor. Just lacks some get up and go. takes ages from 60mph to 70 mph and won't pass anything on the freeway.
The bolt-on is my christmas break project, so will post the outcome here.
Thanks again, and if i end up in your neck of the woods, i'll live up to the beer

[RoyBatty]

Any assistance will be gratefully received. I visit Chicago, Austin Tx, and Northern Cali often enough to buy any respondent a beer or three (also goes for Curly in Oz).

Copied to Engine and Tuning

Hi there,
You've already been given good advice from the previous responders.
I'd wager you'll notice the biggest improvement by installing the single plane manifold if your car currently has the dual plane on it.
Be sure to let us know when you're in the Sacramento area. I'd be glad to go have a beer with you.

[131]

When is it required to change the primary and secondary jets for Idle/Main/Air correction and Emulsion tubes? The only thing I can find is a description on the movement of the mixture screw too far in/out being the idle is Rich/Lean. But is this the primary Idle or both Primary and Secondary idle?. What to do with the other jets e.g. does changing the secondary main, increase power at the top end on the revs?

I just acquired a 34 ADF and single plane manifold for my 79 spider 2000. It has the following primary/secondary jets
Idle 50/90
Main 120/135
Air correction 145/180
I didn't check the emulsion tubes.

As you're in OZ, a 34ADF on a single plain manifold is probably from a 2 litre 131, so the jetting will be close to what you need. Increasing jet size to increase performance just wastes fuel, unless the mixture is lean to start with.

[silvadw]

Thanks Mick.
Bits were from AutoRicambi in Tx. The recon motor is 131 that the PO in USA installed, the 34ADF is NOS
Ramzi is very helpful. Have spent a couple of days trawling 'net and now know a lot more about carburetors.
PS - Have outlaws in Hobart, nephew has a Volvo 240 wagon that he is turning into a turbo brick.

[Jrlovvorn]

My China 34ADF is running light brown on the plugs now, so I think I am good.

I bolted it right to the stock crappy manifold, and I swear the car is twice as fast.

I am not saying not to do the 1800 manifold, I will probably do it over the winter.

But just sticking that 34ADF on is going to change the whole car.

I am having to hold myself back to avoid massive speeding around town, and can do 65-75 on the highway with plenty of power with the secondary if I need to move out of the way.

I can pretty much leave anyone in the dust from a traffic light, with the usual exceptions.

I am driving the car 10 times as often now, because its so much better.

It took less than two hours to tear off that horrible excuse for a carb, and bolt this new one on.

Don't wait until Christmas! Open your present now.

[silvadw]

Well, after some longish travels, made it home and finally installed the 34 ADF. I think the original setup was done very well as the increase in power is marginal but still noticeable, especially in the first four gears. readily goes to 6K rpm, whereas fell over at 4.5K before.
The main issue I had was self inflicted. I shortened the linkage in anticipation of getting the throttle to fully open. the reason they were not opening fully was the floor mat was between the pedal and the rubber stop. oh well, live and learn.
Now just trying to get the cold idle set up with the water choke. Worked fine with the original 28/32 ADHA believe it or not. need to fiddle with the screw that goes into the choke body.
other thing i noticed is the the hole for the bolt at the very front of the original intake is also a passage to the exhaust. Had to put a bolt in it. Used a welsh pug for the EGR hole.
I have kept all of the original smog stuff and intake and carb if anybody is interested. All worked perfectly when removed and have cleaned the carb but not replaced any gaskets.
Thanks for reading

[johndemar]

For what it's worth, I replaced a 32ADFA on my car for a 34ADF and did not change any of the jets.

Maybe not as smooth or as quick as a FI car, but when the secondary kicks in it will bring a smile to your face.