Can a carb be too big for a car?

macaroni · Post by **macaroni** » 17 May 2005, 20:29

I have fitted a Weber 38DAS to my 205xs. On full throttle it absolutely flies, but on part throttle there is a horrid flatspot and hesitancy.
I have replaced just about everything on the carb, to no avail. Some people are starting to tell me that the carb is simply too big for the engine and that will result in the flatspot.
Has anyone else heard of this?

ActivaV6uk · Post by **ActivaV6uk** » 17 May 2005, 21:19

I thought that could be controled by the jets, are all yours working and the right sizes? i have a mate up here who does work on cars for lots of people he it pretty good with this sort of info, message me and i will send you his number...
Andy

arry_b · Post by **arry_b** » 18 May 2005, 02:43

Yes. The bigger the venturi, the slower the air flow through it for a given throttle opening.
Slow air flows are difficult to carburate cleanly, hence the flat spot.

fastandfurryous · Post by **fastandfurryous** » 18 May 2005, 04:18

You will need to have this carb jetted for the engine. That involves both jets and air correction (emuslsion) tubes. Without getting it right, you are likely to be getting appalingly bad fuel consumption too. Jetted correctly, I'll bet it'll go even faster on full throttle.

macaroni · Post by **macaroni** » 18 May 2005, 13:25

Thanks for the response. As soon as I fitted the carb, I had it set up on a rolling road and the main jets were made smaller, but operator claimed the air correctors were OK, ie the mixture was good.
I have tried smaller idle jets and smaller accelerator pump jets and there was a slight improvement, but no real solution.
Will the flat spot be caused by the mixture being too weak or too rich? I looked at the plugs and they were black so I assume it is the latter.
The fuel consumption isn't too bad ie about 30mpg driven very hard.

fastandfurryous · Post by **fastandfurryous** » 18 May 2005, 18:01

Black sooty plugs is definitely rich mixture. Plugs should have a light grey/brown coating on them if the mixture is correct. I would be inclined to try tweaking the mixture with the correction screw (I'm fairly sure just about all carburettors have them) to make it slightly more lean.
Another way to tell is to have someone follow you while the engine is on it's flat spot. If there is visible soot from the exhaust then it is indeed very over-rich.

macaroni · Post by **macaroni** » 18 May 2005, 19:14

Good tip, but I doubt you would be able to tell the sooty smoke from the blue oil smoke!
To reduce the flat spot, I have tuned the mixture to be spot on, any deviation from that and it runs very roughly.
Thing is, the carb responds exactly as Weber suggest, ie the best running is exactly at the mixture settings suggested.

ActivaV6uk · Post by **ActivaV6uk** » 18 May 2005, 23:31

Contact cybernck on ActivaNet i know he does a lot with the XS and he is an admin on www.205gtidrivers.com and www.activanet.co.uk he is a top bloke we have been friends for years.
Andy

fastandfurryous · Post by **fastandfurryous** » 19 May 2005, 03:26

Looking at the size of carburettors fitted to different sizes of engines, and having just realised that we're talking about a 1360cc engine here, then yes, I get the feeling that this carb is just way too large for the engine.
How well does it idle? Lumpy? the airflow at part throttle through the venturi can't be anywhere near fast enough to create much of a depression!
I had a 1.9 engine in my mind for some reason, and was thinking that the carb was probably large for it, but would be OK... for a 1.4 it is massive!! You should be looking at about 26-30mm venturi's, not 38!

macaroni · Post by **macaroni** » 19 May 2005, 13:16

It idles quite well actually. Much better than the stock 32/34 SOlex ever did.
It also picks up very well, it is just after the initial pick up that the flat spot occurs. If I keep it nailed and get above about 3000rpm, when the power circuit kicks in, I guess, it really flies.
Because of the extra performance I am loath to swap it back to standard, but would like to try and eliminate the hesitation. So, if indeed it is too large, is there anything I can do to compensate for that?
Someone has suggested a progressive Nos kit to overcome the flat spot, but that might be going too far!!