Carb differences

Road-race engines and ancillaries - general discussion
MinorTC
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Carb differences

Post by MinorTC » Sat Apr 09, 2011 1:04 pm

Is there any advantage to be gained from going from my Fiat Twin cam's original Weber 34ADF15/250 to a 34DAT?

I am having some running issues with my current carb such as hunting idle and a severe flat spot/misfire when the second choke is kicking in.
Maurice,
East Kent.

Abarthnorway - Remi L
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Re: Carb differences

Post by Abarthnorway - Remi L » Mon Apr 11, 2011 12:47 am

Hi!


I do not think that one carb is better than the other in any way....

I suspect though, that the bolt pattern between the two is not similar. The DAT has another bolt pattern than the ADF if I remember correctly, and might also have another floatbowl orientation - meaning that it will not neccesarily fit all applications. The DAT has a more compact design if space is of an issue....

In my opinion the best option is to clean the carb and buy an overhaul kit for the carb originally fitted to the engine.

Good luck!

Best regards

Remi Løvhøiden
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MinorTC
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Re: Carb differences

Post by MinorTC » Mon Apr 11, 2011 5:01 pm

Thanks, Remi. Where's the best place to get a service kit from? Tried the local Fiat dealer but they don't do them, also a local motor factors. Are the ones on a certain well-known E-auction site any good?
Maurice,
East Kent.

vandor
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Re: Carb differences

Post by vandor » Wed Apr 13, 2011 5:20 am

I can confirm that the 34DAT is a slightly smaller bolt pattern, made for transverse applications.
Sorry, I do not know where can you get a kit in the UK. Here in the USA we only got the 32ADFA, and I am not sure if all the gaskets would be the same.

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Guy Croft
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Re: Carb differences

Post by Guy Croft » Wed Apr 13, 2011 9:48 am

If anyone can get spares it will be Troy & Steve at Northampton Motorsport. Go to links.

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MinorTC
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Re: Carb differences

Post by MinorTC » Mon May 16, 2011 11:34 pm

I found a useful firm, Autostoria, that does service parts for all classic Fiats, and their carb repair kits are more comprehensive than those found on a well-known e-auction site.

I have ordered a kit and shall hopefully rebuild my carb soon and let you know the outcome...
Maurice,
East Kent.

MinorTC
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Re: Carb differences

Post by MinorTC » Sun Jun 19, 2011 12:18 am

Well, the firm duly supplied the rebuild kit, so I got on with it over the last couple of days, stripping down the carb and boiling the body parts in lemon juice to strip off the varnish. The float chamber had quite a bit of silt/rust in, and the diaphragms had seen better days. Rebuilt it this afternoon using new jets, float valve, O-rings, diaphragms and gaskets, and the result looks quite pleasing imho:

Image
Image
Image
Image

The kit supplied main jets of 140 and 120, whereas the originals were 130 and 122. Would this make much of a difference?
Maurice,
East Kent.

Abarthnorway - Remi L
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Re: Carb differences

Post by Abarthnorway - Remi L » Mon Jun 20, 2011 12:56 am

Hi!


Congratulations on overhauling the carb - the ADF 34 is considered one of the better for Your engine setup. Its looking very good - carbs are magic in my opinion;-)

Regarding the jets, to my knowledge they don`t wear out - so no reason to change a jetting that works just fine as it is!


Best regards

Remi Lovhoiden
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MinorTC
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Re: Carb differences

Post by MinorTC » Tue Jun 28, 2011 1:21 pm

Well, I ran it with the 140 and 120 main jets, but it felt flat, as if it were overfuelling, yet a check of the spark plugs revealed this not to be the case.

So, I refitted the original 130 and 122 main jets and immediately it felt better.

However, I still get the annoying bogging down and misfiring when the second choke kicks in under hard acceleration.

Any idea what the problem is?
Maurice,
East Kent.

Guy Croft
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Re: Carb differences

Post by Guy Croft » Tue Jun 28, 2011 2:13 pm

Are you sure the pump jet is actually working?

Have you checked the ignition timing with a strobe? If not you must.

G

MinorTC
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Re: Carb differences

Post by MinorTC » Tue Jun 28, 2011 7:01 pm

Hi Guy,

Yes, I recently set the timing to 10 deg BTDC @ 850rpm with a strobe as you recommended, and it made quite an improvement as the previous owner had set it to BTDC, so thanks for that.

How does one go about testing the pump jet?

Thanks,
Maurice.
Maurice,
East Kent.

WhizzMan
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Re: Carb differences

Post by WhizzMan » Tue Jun 28, 2011 8:16 pm

You can look inside the intake at the throttle plate. If you open up the throttle, you should see a spray of fuel entering the intake. This is the result of the acceleration pump, pumping fuel through the jet.
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Guy Croft
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Re: Carb differences

Post by Guy Croft » Wed Jun 29, 2011 6:54 pm

Thanks Homme!

yes, but do that check with the engine off, OK?

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MinorTC
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Re: Carb differences

Post by MinorTC » Wed Jun 29, 2011 11:09 pm

Thanks for the info. Does anyone know of an exploded diagram for the 34ADF, and how to remove the pump jet for cleaning?
Maurice,
East Kent.

MinorTC
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Re: Carb differences

Post by MinorTC » Fri Jul 08, 2011 1:58 pm

My blasted Weber 34ADF carb is still causing a flat spot and backfire. It's severe enough that if you accelerate up to 60mph, when the flat spot occurs, you drop back to about 40mph before eventually it picks up and goes again!

I removed the pump diaghragm again, cleaned out the little holes in the carb body behind it (fuel came out, so it would appear that they're clear), and reassembled using some instant gasket to ensure a good seal; sucking on the end of the inlet manifold pipe to the diaghragm, I could feel that the diaghragm had a good air-tight seal.

Actuating the throttle with the engine off causes fuel to be spurted into the primary barrel.

However, this has made little or no difference. Otherwise, the car runs well with good power and torque, returning reasonable fuel economy, so this has me stumped!

Does anyone happen to know the original jettings for the 2-litre Fiat twin cam carb, as I have read elsewhere that the primary barrels should have the larger jets, so maybe mine are in the wrong way round?

Thanks for any advice.
Maurice,
East Kent.

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