Just got a new weber 48 ida and new racing beat manifold. I have a bridged 12a. I was just wondering how you know when to upgrade to bigger venturis? I know a lot of people run like 42mm's but how can you tell what you need? I think I saw somewhere if you're still in vacuum at WOT you need bigger ones. Sorry if it sounds dumb, new to carbs haha

 

bump

 

Well, I'm no expert, so this may be totally wrong, but since no one else is chiming in I will give my opinion.

What I have always done is used drivability as a guide. If you can tune any bogs or hesitation out then the venturi is not too big yet.

My thinking is based on the assumption that the larger the venturi, the weaker the signal, the weaker the signal the more apparent any tuning issues become, so if you can tune away the issues then the larger venturis are appropriate.

 

i haven't really spent time playing with this, but it seems like the venturi mostly acts like a restrictor, so you just pick the one that flows enough air for your engine and you're done.

it is perfectly valid to choose a smaller one to try and limit the peak rpm of the engine.

there is probably also a drivability component to the venturi, but it seems to be minor

 

Ill go with the second poster on this, I leave mine up to drivability, i find you can go tiny and get awsome low end off the line, or go big and leave a standing start at lots of rpm and clutch, i personally tune mine to be just a bit boggy off a standing start, this will all change with jetting.
If you set up your idle circuit rich then some off the line hesitation will be cured, but fuel economy quickly goes away, i have a few quick change jet setups that bring my large streetport from 24mpg with some minor hesitation, to 14mpg no hesitation, tires spinning, get a wideband o2 if you dont have one and start around 40 or 42, get a good afr established makes the biggest difference, i think i went with 40 on mine due to the fact all i drive is street and i dont like ot bogging out at every light

 

Same here. Go a little bigger until starting off from a stop, accelerating at low speed, or a downshift blip becomes a little tricky. Even a 2mm increase in size will make a noticeable difference.

It's a weaker signal at low rpm. Going bigger will bring more power at the upper range, but the engine will stumble on the way there.

 

So... it's not that you need to go bigger, it's finding the point where venturis are too large

Although if you are properly testing, and the top end gain is minimal compared to the bottom end losses, I'd just as soon stay with the smaller ones. This assumes that you correct the fuel curve after the venturi change.

I hem and haw on if my GTI will get a pair of Webers or injection. Injection is cheap and easy compared to spending hundreds on carburetor parts, but EFI doesn't look cool, and if you go with tried and true combinations, the expense should be minimal. It's when you try to get fancy that you start bleeding time and money. Twin 40s on a mild 1800cc engine is a no-brainer.

 

Okay thanks guys! How hard is it to change venturis in a 48 ida by the way?

 

Insanely simple. You'll need a flathead screw driver and....maybe another flathead screw driver.

-David Guy

 

if you have nuts and studs, i think is an 8mm wrench?