ITB PERFORMANCE LINKAGE TIPS
Not only will that cylinder be out of balance and trying to run faster or slower than other cylinders it will also be badly out of tune because the injectors will still be delivering the same fuel to all cylinders, the result is an engine that will miss and run rough.
Think about that for a moment that’s only a tolerance of 0.002″ (0.05mm) to be out of balance!!! How often have you been at a race track and heard a car come in with Weber carburetors and its going Flut, flut, flut, sputter, sputter, flut, flut.
Nine times out of ten that is caused by the incorrect linkage setup, and people will complain about webers always going out of tune or going out of balance but Ferrari didn't use webers on there engines for them to go out of tune every month!!!
There is no reason why a side draft carburetor or ITB set up can't be used on a daily driven vehicle for years and years, always idling perfectly and never needing adjustment
"Ferrari didn't use multiple Weber's on there engines for them to go out of tune every month!!! “
There are two basic configurations and many misconceptions when it comes to each style of linkage and its appropriate application.
The first style and most wrongly used consists of a secondary shaft usually just above the butterflies and several individual drop links to each carburetor or throttle body.
This is often seen on OEM ITB applications and so people tend to think this is the best option for their aftermarket ITB or side draft carburetor set up, but the fact is this could not be further from the truth.
OEM manufacturers use this style for easy assembly and manufacturing reasons. They also have some very important differences!. Firstly they all have a factory cast manifold that is made with many rigid supports for the secondary shaft usually supported in multiple needle bearings above each runner.
They also have a large common passage cast into the manifold that connects all the runners together so this will equalize the pressure between the runners and allow each cylinder to receive the exact same amount of air, even if the throttle plates are not perfectly synchronized they also use this common passage for introducing air from the cold start and idle control valve etc.
For this reason the butterflies of each throttle do not need to be perfectly synchronized at all times, but it also means the system will get a lot of cross communication between the cylinders which will upset the idle quality a lot when running cams with large amounts of overlap.
The reality is that this will never happen and even if this is perfectly and painstakingly adjusted to work initially, after a bit of driving and temperature cycles, the linkage components wear a little and in no time at all they are out of balance again. This might be ok for a race car where the only slow driving required is off the trailer and out of the pits, after that its mostly high opening operation.
But I even like my race cars to idle well and drive smooth at light throttle. For a road car we all definitely need something that will stay reliably balanced for years
Well quite simply because the makers of the carburetors were not involved with the fitment of their product onto the vehicle, and here I'm talking specifically about Australian Chrysler and Holden when they released limited numbers of cars for racing, such as the e49 Charger and Holden XU1.
They half got it right but at the time in Europe most vehicles used 4 cylinder engines so the carburetors all came in pairs with the correct balance coupling.
If the engines had 6 or 12 cylinder like Porsche or Ferrari then two 3 barrel carburetors would be used for each bank of 3 cylinders again with one correct balance coupling but due to the high cost and limited sizes of these 3 barrel units when the Australian manufacturers came to purchase something for their 6cyl engines they opted for 1.5 sets of a 4 cyl system.
So what they had was two units nicely linked together and one all on its own. But instead of fitting a second balance coupling like they should have they opted for the notoriously bad design of using 2 drop links.
Its only here in Australia that you will find this set up, all the European manufacturers used the correct method. In the heyday there were a handful of weber specialists who grew to hero status for their legendary ability to fix and tune side
Here is a picture of the notoriously bad Aussie special, where one pair is linked to the lonely third by a secondary shaft. Just the simple act of the cable pulling on the linkage installs all sorts of play and flex to the linkage, this could easily be as much as 0.5mm!
Now this might not sound like much but remember its about 10 times the acceptable amount for smooth synchronized running, badly upsetting the idle and low speed running quality just off idle. Then add to this any tiny amount of wear or play in the system and you can plainly see that right off the showroom floor this setup was an epic failure!!
draft carburetors, people would rave about how they had continual problems for years until they took it to said tuner and after that the car was such a pleasure to drive and never gave any more problems!….. all they would do it toss out the double drop link system and fit a second coupling while nicely removing large amounts of cash from the owners pocket for the pleasure of doing so!