Dual carb question, Dellorto 40s on a 1.8L to be exact Options

[terrymason]

The problem - The car will go very lean around 2500 rpm, as if the pump jets were too small.

History:
New fuel pump (3.5 PSI to each carb)
55 idle jets
55 pump jets
140 main jets
175 air correctors
These carbs are new to the car, and I'm trying to dial them in.

The car is lean when I first start it (16 / 17 on my air / fuel gauge), but then levels out to a nice 14.7 idle when warmed up. While driving, I can give the car 20% or so of the pedal, and drive up to highway speeds nicely, but when I goose it, it will go very lean (18-22 on my gauge) followed by a series of backfires and loss of power. I started off with 35 pump jets, and have moved up to 55s, which appear to be the most popular around here. I've also adjusted the pump jet linkage, from the nut fully loosened, and moved it in as far as it would go before affecting the main throttle shaft (like 60 turns or something).

What do you guys think, do I need 75 pump jets?

[terrymason]

OK, I have made some big headway on this. I pulled all the jets from the car, and blew them out with carb cleaner, along with the places they go. Replaced my 52 idle jets with 60s.

I think cranked the idle up to 2500, and used my unisyn gauge to check the flow. The passenger was a 5, while the driver was at 12. I adjusted the linkage so that they are both at 11 around 2500 rpm.

I then addressed my mixture, making all screws 2.5 turns out.

I then adjusted my pump jets. turning them about midway out. They were previously all the way tightened (I was trying to get a larger pump squirt)

I finally closed all my air bypass screws.

Now, the car idles around 10.5 a/f (probably from the 60 idle jets). It drives 100% better, but still goes lean when it starts to pull (2nd gear feels it most). It also seems that if I goose the gas it will go lean.

[BxtrBill]

I then adjusted my pump jets. turning them about midway out. They were previously all the way tightened (I was trying to get a larger pump squirt)

Ok...you got me on that one. The pump jets sit in holders that screw into the (outboard) side of the carb body.....there IS no adjustability, except to replace the jets with larger or smaller ones. Are we talking about the same jets here?

Your initial mixture screw setting is probably good for a starting point, and the somewhat "fat" idle mixture makes things less picky when you're trying to get a new carb set-up sorted out.

34mm vents still sound kinda huge to me for a 1.8, though....28mm would be more like it. An over-carbureted engine does not respond well to minor jetting changes. They sort of gasp and stumble and generally run poorly except at one magic combination of throttle setting and RPM. If it were me, I'd go to www.cbperformance.com and see what they have in the way of jets and venturis for the Dellortos. Used to be they carried a bunch of stuff, but they seem to be more into aftermarket FI these days. You may want to order up their Dellorto Book, if it's still in print. It can be helpful, especially if this is your first foray into carbs. Another possibility is European Motorworks in Hawthorne, CA...don't know their web address offhand, but Google does.. They carry a bunch of Dell (and Weber) parts.

And a couple other questions for you......have these carbs EVER worked correctly on this engine? Or, failing that, do you know anything of their history, as in, what were they used on? It sounds like they have an as-shipped set-up, so I'm just wondering. There are dang few NOS Dells floating around these days, so yours probably did time on another engine before you got them...if you bought them from a friend who used them, maybe he'll be able to shed some light on things?

One more question....what was the reason for going to carbs in the first place?
If it was to replace a cranky FI installation, was the engine otherwise in good shape? Good compression...good tune-up specs...timing, valve adjustments, etc?
I'm curious, too, to learn what you're using for a distributor. Lotsa carb guys just stick in a 009 and call it good, and suffice it to say there are whole WORLDS of things that work better than those. The stock dizzy for a 1.8 isn't a bad choice, but it might benefit from some changes to the advance curve.

Sounds like you're making progress, though. Keep us posted.

[terrymason]

I then adjusted my pump jets. turning them about midway out. They were previously all the way tightened (I was trying to get a larger pump squirt)

Ok...you got me on that one. The pump jets sit in holders that screw into the (outboard) side of the carb body.....there IS no adjustability, except to replace the jets with larger or smaller ones. Are we talking about the same jets here?

....

And a couple other questions for you......have these carbs EVER worked correctly on this engine? Or, failing that, do you know anything of their history, as in, what were they used on? It sounds like they have an as-shipped set-up, so I'm just wondering. There are dang few NOS Dells floating around these days, so yours probably did time on another engine before you got them...if you bought them from a friend who used them, maybe he'll be able to shed some light on things?

One more question....what was the reason for going to carbs in the first place?
If it was to replace a cranky FI installation, was the engine otherwise in good shape? Good compression...good tune-up specs...timing, valve adjustments, etc?
I'm curious, too, to learn what you're using for a distributor. Lotsa carb guys just stick in a 009 and call it good, and suffice it to say there are whole WORLDS of things that work better than those. The stock dizzy for a 1.8 isn't a bad choice, but it might benefit from some changes to the advance curve.

Sounds like you're making progress, though. Keep us posted.

- below the pump diaphram there is an adjustment (nut on a rod). I was under the impression that adjusting this changed the volume that was produced by a single squirt of the jet. I believe that I'll have to take the carbs off, and measure a squirt to make sure it's accurate.

As to the history of the carbs, I am replacing a single progressive carb with these dual dells. I purchased them fully rebuilt, and have been fighting a learning curve the whole time. I thougth that I would just bolt the on and go, but have ended up learning alot about the carbs. I'm currently using an 009 that came with the car.

Thanks for all the help - I'm about to go out of town for the week, so it will be a while until I can work on it again.