Carb throat popping at speed, New Problem!!! Options

[ArtechnikA]

Ok, so I timed my car by the book, and it idles a lot smoother now. However, I'm getting more backfires out of the carb throats upon revving than I was before.

There is a strange whistle when I shut of the car, right when the engine stops running, though.

But we're getting someplace.

So what are these carbs?

manufacturer,
throat size,
venturii size,
main jet size,
idle jet size

What color are your plugs ?

Does the exhaust smell stinky-rich with smoke?

[LetoAtreides]

Dellorto:
main jet 36
Venturii 140

I don't know the rest. They came with the car and worked perfectly on my old engine, so I don't know why they're acting up now.

Plugs are as follows:
1: black
2: brown
3: black
4: brown

The exhaust does smell a little rich and has some smoke when the car is first warming up.

the popping seems to come from the 1-2 carburetor.

I fixed the whistle! Just had to tighten the nuts that hold the carb to the manifold.

[ArtechnikA]

Dellorto:
main jet 36
Venturii 140

Ahhh - no.

Those aren't credible numbers for those items, sorry.

There's at least 3 sizes & flavors of Dellorto. We need the size of the throat barrel; it'll be in the casting on one of the sides of the carb.

36 might be a credible number for a venturii; with luck, you'll be able to see it as a casting number on the top - you'll need to look into the throat, under the throttle butterfly. With no visible size you have to measure. An accurate measurement involves taking off the top of the carb, but it's not that critical so if there's no number and you can't eyeball it with a millimeter ruler, don't stress.

But we do have to know the numbers for the mains and idle jets. Knowing the number on the air correction jet would be a help down the line.

I don't know the rest.

Find out. If you're not sure how to find out, ask. I last tinkered with a Dellorto-carbed 1,7 30 years ago, so there's a bunch of stuff I've forgot, like -exactly- where the jet carriers are. I could recognize them if I saw one again, but i'm not going to be able to tell you something like "right side, 10mm, near the center." I know there are guys here who've got Dellortos and maybe they'll be able to tell you exactly how to pull the main and idle jet carriers. Or get some really good pictures and we'll go exploring.

But it's important.

They came with the car and worked perfectly on my old engine, so I don't know why they're acting up now.

Since you have a '75 it didn't come with a 1,7 so right away there's one big difference. But it doesn't matter. _Something_ is wrong.

Plugs are as follows:
1: black
2: brown
3: black
4: brown

The exhaust does smell a little rich and has some smoke when the car is first warming up.

the popping seems to come from the 1-2 carburetor.

I fixed the whistle! Just had to tighten the nuts that hold the carb to the manifold.

After that, you checked all the carb and manifold hardware, yes ? It's impossible to tune an engine with big vacuum leaks...

intake popping is often associated leanness, but your other symptoms are showing incomplete combustion.

What kind of plugs - brand and heat range number?
You're -sure- you have good plug wires and the plug connectors are all on snug?
When you tested the spark you got a nice fat blue spark?

(We're making sure the primary systems are working because most carburetor problems aren't - as witnessed by your observation that they worked on a previous engine, and because you have conflicting symptoms - it acts lean but is showing obvious incomplete combustion - poor ignition can cause this...)

Do this - taking good notes:

On each carb throat, make note of -exactly- how far out the mixture adjustment screws are. Do this by gently screwing them in and counting the quarter-turns until they bottom. They -should- all be pretty close to the same. Then, back them out again to where they were, only a half-turn less. See what if any difference that makes.

What grade of gasoline are you trying to run? 1,7's - if yours is still all-original - have a little higher compression and might benefit from a midgrade - but probably doesn't require premium. Wait - you're in CA, where the octane numbers don't match the rest of the world. Try midgrade.

[LetoAtreides]

Ok, sorry about the bogus numbers. I thought the main jets were the ones that pump gas when the accelerator is pulled, and the venturii were the small tubes next to the carb throats.

I'll see if I can find my carb info today.

Plugs are Bosch, don't know the number but got them from Rich Bontempi at High Performance House, so they should be good. I'll check that number today. Wires and connectors are all good. I'll test the spark today.

I'll try the accurate mixture screw adjustment you have. Previously I set the mixture as per the DellOrto manual: Hook up an electronic tach and turn them in until the engine loses speed and then turn them out until it stops gaining speed.

Ok, The gas in my car is 87 octane, so low-grade. I didn't just guess on this one, I looked up the octane rating for the 1.7 engine online. I'll try a midgrade, though and see if that helps.

[LetoAtreides]

Ok, main jet is 140
pump jet is 35
I'll have the rest later.

[LetoAtreides]

idle jet is 55
throat is 40

sparks are nice and blue.

Tried driving it a bit today and no backfires while driving it. It only backfires when sitting still and high-revving.

[ArtechnikA]

Tried driving it a bit today and no backfires while driving it. It only backfires when sitting still and high-revving.

So don't do that.

Seriously - it's not a part of the normal driving regime. If it idles fine and drives fine, you're done.

I think you may be seeing the indications of a misfire, which is its own separate set of problems (and standard solutions.) It mught even just be residual crap burning of the plugs from being fouled earlier.

If you have to go out of your way to create the problem by artificially running the engine at no load - don't do that. You're done.

[rhodyguy]

what are the flow #s as indicated on your synchronometer? each specific throat.

being unfamiliar with dels, "throat is 40" is the size of the venturi in MM?

changing the fuel grade may have an effect on your idle speed. not the rate at which the venturis are flowing tho, as compared to one another.