Carb differences

Road-race engines and ancillaries - general discussion
MinorTC
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Joined: April 28th, 2009, 9:30 pm

Re: Carb differences

Post by MinorTC »

Interesting that you should say this, Marc. I see they are largely used on VWs, but also on the Fiat 124/131/Spyders in the US.

Here in the UK, I gather that the 32/36 DGAV is usually recommended.

I wonder what are the differences?
Maurice,
East Kent.
Urbancamo
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Re: Carb differences

Post by Urbancamo »

DFEV Weber, as the code already says, has inverted choke location compared to DGx-series carbs. Throttle also opens on different direction.
And what's easily seen on top, air cleaner mounting is totally different. It has the same mounting as found on Weber DIR carbs.

I personally prefer DGx-series always when possible.
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MinorTC
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Re: Carb differences

Post by MinorTC »

Well, after continuing problems with the original Weber 34ADF54/250, this weekend I replaced it with a Weber 34ADF15/250.

This is from a Fiat 131 Mirafiori Racing, and lacks the 'anti-stall/power' valve as fitted to the 54 version, pictured below:

Image

Can anyone confirm exactly what this part is called, and how it operates?

The 34ADF15/250 seems to be quite alot better in terms of starting and general running, though I've yet to fully bed it in and adjust it.

Are there significant differences between the Racing and normal Mirafioris? There are quite significant jet differences between the two carbs that would suggest this.
Maurice,
East Kent.
Urbancamo
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Re: Carb differences

Post by Urbancamo »

Yes it called anti-stall devive. It's more common on cars wich have automatic transmission. Of course it works with manual transmission too.
It's very simple device; hose is connected to manifold vacuum. On the other end is a diaphragm (seen in picture). If car tries to stall, manifold pressure decreases and diaphragm works just like regular accelerator pump, giving short burst of fuel preventing engine for stalling.
It won't work until engine is really stalling down...

OEM jetting for 34ADF used in 2000 Supermirafiori and Argenta with same engine is following:

1st choke / 2nd choke

Idle jets: 50/90
Mains: 122/130
Air correctors: 170/175
Emulsion tubes: F20/F5
Pump jet: 45

Hope this helps.
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MinorTC
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Re: Carb differences

Post by MinorTC »

Thanks for confirming that. I suppose I should have tried simply disconnecting it before going to the trouble of swapping the carb, but hey, ho - too late now!
Maurice,
East Kent.
MinorTC
Posts: 83
Joined: April 28th, 2009, 9:30 pm

Re: Carb differences

Post by MinorTC »

I hoped that fitting the replacement carb would sort out all my running issues, but alas, not so. Just lately, maybe due to the cold weather, it has been running very rich (sooty plugs and exhaust), and returning as little as 12mpg! Also quite hard to start sometimes, without the aid of a squirt of carb cleaner down the barrels.

It seems to be intermittent, in typical Italian temperamental fashion; sometimes it will run very sweetly and return 30 mpg, whilst on other days it runs lumpily and with poor economy.

I'm guessing that the automatic choke mechanism is jamming somehow.

Has anyone else experienced such issues?
Maurice,
East Kent.
MinorTC
Posts: 83
Joined: April 28th, 2009, 9:30 pm

Re: Carb differences

Post by MinorTC »

Well, I discovered a few 'issues' with the replacement carb: the float was dropping down too far, causing the needle valve to stick open, so I bent back the little brass tab to set the float height to the book 6mm, which has cured the flooding.

Also, an air passage on the secondary barrel was blocked, so I cleaned that through, also cleaning up the emulsion tubes, etc.

There was slight leakage of fuel from the accelerator valve and the little roller wheel on the bottom of its arm was jammed, so I swapped it over with the good one with a new diaphragm from the original carb.

This has got the car generally running very well, that is, until you get it up to speed or go up a long, steep hill, which will cause it to start 'kangarooing', as if there is interrupted fuel flow! Sometimes, it gets so bad that the engine will cut out - not good.

I'm beginning to suspect that there is a fault with the fuel tank pick-up pipe causing this...
Maurice,
East Kent.
MinorTC
Posts: 83
Joined: April 28th, 2009, 9:30 pm

Re: Carb differences

Post by MinorTC »

I think I may have finally cracked it! Today, as an experiment, I removed the aftermarket inline fuel filter between the block-mounted mechanical fuel pump and the inlet to the Weber carb, replacing with a length of plain fuel hose.

And, what a difference! No more kangarooing going up hills or along the dual carriageway.

I can only assume that under certain conditions, the fuel filter was causing turbulence, disturbing and restricting the fuel flow.

I'll just rely on the little filter built into the Weber's inlet now.

It goes to show - be wary of aftermarket add-ons, as they can severely restrict performance!
Maurice,
East Kent.
WhizzMan
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Re: Carb differences

Post by WhizzMan »

If I were you, I'd try and get a nice, big full flow modern fuel filter in the line between the pump and the tank. Good fuel filtering helps keep your carbs from wearing out. It's not just about preventing debris to clog your jets, but also the wear you get from smaller particles acting as abrasives. As you discovered, the amount of flow is important, so best get a filter that's tried and tested by others. Anybody that has recommendations here?
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Urbancamo
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Re: Carb differences

Post by Urbancamo »

Whizz: I can definately recommend the following filter between the tank and fuel pump. It's easily available, reasonably priced and good for cars with carburettors also.

Fram G3829
Bosch 0 450 905 030
Mann-Filter WK 730/1

Etc. you can find more cross-references by your self.

Found in MK3 VW Golf's, BMW E30 models etc. It's used by various car manufacturers.

It has sturdy metal casing in it so it's not gonna break at all. It's meant to be placed out of the car but it can be mounted anywhere. It has 8 mm fuel fittings and it's bigger than ordinary beer can. It's pretty much plug and forget - it last years and years because it has huge amounts of filtering media inside.
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MinorTC
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Re: Carb differences

Post by MinorTC »

Good idea, that. I know the type of filter you mean - I used to run BMW E30 320i and 325i models.

Presumably, the little filter built into the Weber carb is not sufficient, I take it?
Maurice,
East Kent.
Brit01
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Location: Uruguay

Re: Carb differences

Post by Brit01 »

Whizz: I can definately recommend the following filter between the tank and fuel pump. It's easily available, reasonably priced and good for cars with carburettors also.

Fram G3829
Bosch 0 450 905 030
Mann-Filter WK 730/1

Etc. you can find more cross-references by your self.
Are these suited for mechanical fuel pumps?

I've currently got a cheapo transparent plastic filter between pump and carbs.
I've seen this paper filter fall apart (the end piece detach from the main paper element) after some time.

I'd like to have a look for a better filter also with a better flow rate.
WhizzMan
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Re: Carb differences

Post by WhizzMan »

The modern filters are built to work with high flow pumps for injection systems with return lines, so they should give very little flow resistance. Mounting them properly and getting them hooked up in your current fuel supply is the challenge here. You don't want it falling off, or getting hit by something on the road. Also, you'd want it as low in the fuel line as possible, to make certain you don't get bubbles and vapor lock in the filter.

The little filters inside the carbs are more useful to keep elephants out than microscopic debris. They are a last defense against for instance rust particles coming loose from the inside of steel fuel lines. They are by no means a replacement for a good inline fuel filter.
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Urbancamo
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Re: Carb differences

Post by Urbancamo »

Of course it can used with carbs. It has pretty much no resistance. As said earlier, filters inside the carbs are pretty much meant for filtering only small rocks.

I've just solved one case where locally made, cheap plastic fuel filter failed. The glue used to glue filtering paper started to crack and it dissolved into fuel!

Finally it got inside the idle jet channel and blocked idle jets. It forms gellow-like, stiff residue which needs to cleaned with carburettor cleaner. Compressed air won't do a thing. There is still some residue inside the idle jet channel or in the progression ports because it needs 5 numbers bigger idle jet than before in order to run properly. And this carb is under a year old and now gone to this stage, thanks to that "filter"
Took ages to figure out, about 10 visits at my garage after I finally found the cause of all problems. Almost drove me to insanity.

So good filtering and good manufacturer - more important than anything!
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Brit01
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Re: Carb differences

Post by Brit01 »

I've just solved one case where locally made, cheap plastic fuel filter failed. The glue used to glue filtering paper started to crack and it dissolved into fuel!

Finally it got inside the idle jet channel and blocked idle jets. It forms gellow-like, stiff residue which needs to cleaned with carburettor cleaner. Compressed air won't do a thing. There is still some residue inside the idle jet channel or in the progression ports because it needs 5 numbers bigger idle jet than before in order to run properly. And this carb is under a year old and now gone to this stage, thanks to that "filter"
After this incident I will be changing my asap.
The last cheapo filter I put on, it split at the same place where the glue is. I didn't notice any blockage and idles have been cleaned and changed since then. Guess I got lucky.
I'm on borrowed time now!

I'll look for the Bosch or Mann filter.

Thanks.
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