Anyone good with Weber carbs?

macaroni · Post by **macaroni** » 15 Feb 2005, 15:34

Hi,
I have fitted a Weber 38DGAS carb to my 205xs. It makes for fantastic performance, but there is a problem. It has a nasty flatspot, off/on the throttle, which I found was caused by a missing O-ring on oneo f the idle jets. When I replaced the O-ring, the car now won't idle at all, even though the flat spot has now gone.
As far as I can see, the carb has been fully refurbished and is very clean. I have cleaned the idle jets and the idle circuit.
My only thought is that it needs bigger idle jets, but it should sitll tickover with jets that are too small, surely.
Any help gratefully received.
Cheers,
Antony

bxbodger · Post by **bxbodger** » 15 Feb 2005, 17:01

Should it not have twin webers?- mine did, but you are talking in the singular.....Check for a massive air leak at the joint between either of the carbs bodies and the manifold-this is your most likely cause, and they also need to be synchronised with a vacuum gauge, this gets rid of any flat spots.

jeremy · Post by **jeremy** » 15 Feb 2005, 17:11

If its twins you can probably improvise the gauge for setting the throttles by using a piece of pipe, one end of which is in the carb throat, the other held to your ear. The difference in flow is usually quite striking and used to work well for me with SU's and Strombergs.
If you have a single check all the various engine breather pipes thoroughly and if its like a Renault you may find that at least one of them has a restrictor in it which may block with carbon. As the pipes age they tend to become slack and may need clipping. Its also making sure all linkages work properly as you wouldn't be the first with an engine trying to idle on its main jet due to throttles not shutting properly.
Jeremy

macaroni · Post by **macaroni** » 15 Feb 2005, 20:47

No it is a single carb. It is not the standard carb, but one from a Ford v6 engine.
I have checked all the joints, replaced the gaskets and renewed all the pipes.
It is looking increasingly like there is a problem with the idle circuit/jets, but I can't figure out what.

jeremy · Post by **jeremy** » 15 Feb 2005, 21:31

longshot - are there any holes in the base which are obscured by the manifold / gasket?
If it won't idle at all it suggests that the idle circuit isn't working at all. if it was a bit rich it would smoke and if it were a little weak it would sound unstable and feeble.
I'm curious about this and presume the carburrettor is essentially the same thing as i had fitted to a Mk 111 2000 Cortina GT which to be fair functioned very well and reliably. On looking at Haynes for Mk 5 Cortina I see that some versions are fitted with an Anti Stall mechanism which is diaphragm controlled and is on the right hand side of the float chamber when looked at with the float chamber nearest you. I wonder if you have one and if so has the diaphragm split or not been installed properly. there is a hose that looks like an air pipe attached to the housing and presumably it works by increasing the mixture quantity if the inlet manifld pressure drops.
jeremy

tomsheppard · Post by **tomsheppard** » 15 Feb 2005, 22:28

If this carb is off a V6 Ford, the likelihood is that the idle jet is too large.
Opening the throttle and giving it more air makes it run fine, so I reckon you've got black plugs!

Stuart McB · Post by **Stuart McB** » 16 Feb 2005, 00:02

I must concer with what tomsheppard has said. The fact that the carb is of a non Citoren should be an idicator to the jets being different. You can get replacment sets of just jets. Try a web search for Aldon automotive as they are a webber specialist here in the midlands.

jeremy · Post by **jeremy** » 16 Feb 2005, 00:23

But isn't the idle mixture the only bit you can adjust?
I agree that a missing O ring may have admitted some air which produced the effect of a smaller jet.
Jeremy

Post by **philhoward** » 16 Feb 2005, 00:25

The DGAS is a favourite of Pinto (read Capri) owners for a few extra horses; perhaps a 1600 owner could point you in the right direction for jets, but isn't the XS a 1400? You might find it a little over-choked, even so (needs smaller chokes to get the airspeed through the carb for it to work properly).

jeremy · Post by **jeremy** » 16 Feb 2005, 00:33

I don't think the choke size will affect the idle if everything else is ok. Isn't the idle mixture output a small hole at the throttle edge in the primary choke.
Too large a choke could cause flat spots low down but the dual choke design should limit this. Too small will cause strangulation at high revs - in fact just like most cars.
its also worth checking that the throttle in the secondary (larger) is shutting.
jeremy

macaroni · Post by **macaroni** » 16 Feb 2005, 01:03

Thanks for your all your help guys.
I have had the idle jets blown through with compressed air and it now idles, albeit very roughly. The mixture jets are screwed right in and the idle speed needs to be about 1500rpm. So not ideal!
It sounds like it is too rich, so maybe the idle jets are too big, but they are only size 45, with 40 being the smallest. It was jetted for an AX GT hillclimb racer, so the power jets were replaced with smaller ones, but maybe the idle jets needs replacing too.
As for it being over-choked, once the roughness at idle is past, the car absolutely flies and the mixture is perfect (as set up on a RR).
Jeremy, the anti-stall thing you are referring to is called the LOVE device (aaahh!) This appears to be in good order.
Maybe a set of smaller idle jets is the next thing to try.
I will keep you posted and any other suggestions most welcome!
Cheers,
Antony