Weber carb balancing "Ah-ha" moment

[Backdraft1725]

So, I've been wrestling with my Weber carbs for a few months now. This has been frustrating, but a good education. I swear that these carbs will make you want to just give up some days. I was really frustrated with my off-idle acceleration (not the transition area, just the low acceleration in general). It just felt too weak in the low rpms and I couldn't figure it out. So, I read and read and read. I've been through every Weber post here and on the VW, RX-7 and Lotus forums as well.

Finally, a Ferrari 308 balancing guide helped me out with something. I've been balancing the carbs at idle based on the idle speed set screws, which is fine for a nice, balanced idle. But, when I hit the accelerator, everything went out of whack. So, given that I have an ERA and I can't adjust a throttle cable to pull the carbs off idle (there isn't a cable), I made a jig to set the idle using the throttle mechanism. I set the idle around 900rpm using the jig and then backed the idle set screws out to make sure that they weren't influencing anything. Checking this way, my left-right carb balance was way off. So, I rebalanced with the throttle jig. Then I took it off and then balanced the carbs using on the idle set screws on each side (just adjusting the screws). This made a huge difference. The engine is much more responsive.

Sorry if this was obvious for everyone else, but it was something that I was overlooking. Moral of the story -- balance your carbs twice. Once based on the throttle mechanism and once using the idle set screws. In a perfect world, both would come out the same, but I do think that there is a tiny bit of play in the overall linkage assembly (which is even the case with Ferraris according to the 308 manual).

[PDUB]

Pics to support the explanation please. Just for clarity, you use more than one term for idle set screw in your description. Are you intending that these are the same screw or screws at each point in your process?


If I recall, there are idle synch - for front to back balance, idle mixture - on each independent runner, and idle stop screws - to set overall base idle speed. Did I miss any? Mine are packed away for now so I don't have them to look at for reference...

[Backdraft1725]

Hi Paul.

Well, I typed up a nice response, but it got sent to the admins for review (due to the picture??) and I don’t know if there are actually any admins here. I’ll retype the description with no pics and see if it goes through. [EDIT] See pic of my setup in the post below. This is in an ERA with a 427 side oiler.

Sorry for the confusion above, I’ll stick to standardized terminology here:

• Idle Sync Screws - for front-to-back balance
• Linkage Adjustment – for left-to-right balance
• Idle Stop Screws - to set base idle speed
• Throttle – throttle cable or assembly from the accelerator pedal
• Idle Mixture Screws – lean/rich idle adjustment on each runner

My “aha moment” was that the carb balance is different when sitting on the Idle Stop Screws than it is when it is when pulled by the Throttle. Everything that I read said to sync the carbs at idle, but nothing, prior to reading the Ferrari 308 setup guide, mentioned that there is going to be some play in the carb linkage, so setting on the Idle Stop Screws (which pushes the carb linkage) is different than setting based on the Throttle (which pulls the carb linkage). Based on this, my new carb mechanical balancing procedure accounts for both a Throttle balance and an Idle Stop Screw balance.

Here’s what I did:

1. Set the Idle Mixture Screws at ¾ turn out (just a good starting point).
2. Set idle using the Throttle. I used a jig to hold the Throttle open a bit and then turned the Idle Stop Screws out to make sure that they weren’t influencing anything.
3. Front-to-Back Balancing. Balance carbs front-to-back using the Idle Sync Screws. Do this on each side and don’t worry about the carbs matching up left-to-right.
4. Throttle Balancing. Using the Linkage Adjustment, balance carbs left-to-right. Try to find a balance where the left and right linkage arms are the same length (if you have separate arms). This is your Throttle Balance.
5. Idle Stop Balancing. Set the Idle Stop Screws to where they are setting the idle speed and release the Throttle (take the Throttle jig off). Here you are going to balance left-to-right using the Idle Stop Screws. I found that the balance was a few kg/h off on the syncrometer from the Throttle Balance, so re-balanced left-to-right using the left-to-right this time using Idle Stop Screws. This is your Idle Stop Balance.

With this setup, I am balanced at idle (based on the Idle Stop balance) and I am balanced on the Throttle (based on the Throttle Balance). Previously, I was balanced at idle, but unbalanced when I got on the Throttle. It might also help to set the Throttle Balance at a few different RPM settings to make sure that it stays balanced. Balance matters less and less as you open the Throttle, so getting a good balance down in the 1000-2000 range likely is most important.

I hope that this is clearer and that it helps someone out. Here is a link to the Ferrari 308 Tuning Guide that I was learning from: Ferrari 308 Tuning. Step 5 talks about setting the balance using the Throttle.

[Backdraft1725]

BTW, here's my setup:

[PDUB]

OK, thanks for the clarification! I am following you now. Which brings up another question for those in the know.

I was under the impression, but not sure, that you only use one of the throttle stop screws for setting the base idle for the four unit system, while letting the other three float based on that one, which seems would solve your issue. Perhaps a guru can chime in with some thoughts...

[Backdraft1725]

No problem Paul.

I'm definitely not an expert, but, on my Inglese system, there are two Idle Stop Screws, one on the left and one on the right. I found this odd and tried to set the system using just one of them (backing the other out). Unfortunately, it didn't solve the balance issue for me. The problem was that, by doing this, I could do a Throttle Balance or an Idle Stop Balance, but not both. I believe that this didn't work out because the Idle Stops push the linkage and the Throttle pulls the linkage. Slight play causes each to react a bit differently (I've checked the linkage and it is tight and healthy all around). I believe that this is why my system has two Throttle Stop screws. That being said, there are many different linkage system setups, so the approach may vary.

Again, not an expert on these, just someone who has now gone to the brink and back trying to get my Webers to play nice (which they are finally doing for me).

[Gaz64]

I have one idle speed screw as a positive stop ON EACH BANK.

One screw on one side only encourages linkage wear/misalignment etc.

Something to think about, for Weber IR owners.

Gary

[PDUB]

OK, guys, thanks for the clarification!