Tuning Weber’s

 I met him at Elkhart Lake . Cunningham brought him. Or he came because of Cunningham.  He spoke very broken English but as bad as his English was it was better than my Italian,  yet somehow we could communicate.  Point at something nod or shake back and forth. Look in each other’s eyes to see  if we understood. Somehow it worked  and a friendship formed. 

Anyway he taught me that  to start  with the ignition. Most carb troubles are ignition based and its best to eliminate what is easy  first.  

The next thing he introduced me to was an air density gauge  apparently tuning without one is like getting dressed in the dark. You never know how it’s going to come out.  

i was surprised to find that the throat   ( Called a choke or venturi) of a Weber is replaceable  what’s more it’s not that unusual to find the wrong size in it. 

It could be optimism of the power that engine will have or the mistaken belief that bigger is always better.  Or it’s just what was available  when they were shipped.  Whatever they should be checked. Twice I’ve found odds sizes in a set one throat one size and another or the rest of them a different size. 

More later. 

Interesting. Looking forward to more. 

In reply to TenToeTurbo :

I keep delaying because I’ve got pages and pages of notes to refer to but the only time I get to peck anything out is when I’m here in the bus waiting. 

First of all, if you're planning to set up and/or tune your own Webers, one should already have a thorough understanding of how they work, and the numerous calibrated components to be dealt with.  With any luck, the carbs were purchased with a reasonable baseline for the intended engine.  If they came secondhand, then your work should start with getting them properly configured for your application.

While they're old as dirt, the first book(s) one should read are the series written by the late John Passini; Theory & Operation, and Tuning & Maintenance (or similarly worded titles, as I'm working from memory here).  The Weber factory publication, Weber Carburatori Assistenza Technica (in English, pt. no. 95.0000.54) is also a worthwhile read and includes charts for sizing (1 choke to 1 cylinder and feeding multiple cylinders) and also specifications for all calibrated parts.

I too have been telling people for decades that most carburetor problems are electrical!  I'm also in complete agreement that "tuning the carbs" (ANY type, configuration or quantity) starts with setting the valves, ignition and timing to specifications.

Furthermore, verify that the fuel level in the float chambers is correct; here, I'm somewhat less concerned that the level is correct to the millimeter, than I am about ALL the levels/floats being exactly the same.  If these vary, then, to use the OP's analogy, it's like getting dressed in the dark, only you're in a stranger's closet!

Everybody knows the engine should be warmed up, but that IS NOT DONE by idling in the driveway and grandstanding the throttle for 10 or 20 minutes!  You want the engine to be stinking hot, like after a 30 minute run at speed down the freeway.  Have all your tools and procedures laid out ahead of time, so when you get back, you can hit the ground running.

If the carbs were recently serviced for any reason, then the balance of the throttle plate action on multiple carb installations should be verified.  I find these CarbTune units, intended for motorcycles useful for this.  While calibrated in Inches of Mercury, they are actually dry, and those are stainless steel slides shown in the manomometer tubes.  If your engine has ever sucked up a slug of water, or God forbid, (real) Mercury, you'll appreciate the innovation behind these!

While it looks like these are way off, this picture froze the moment in time of each cylinder's position in the combustion cycle.  If ALL cylinders hit BDC during the intake stroke at once, you'd see they were perfectly matched (but what an unbalance beast that would be).

Now that's a good primer, and I too am looking forward to more installments by frenchyd Dork, as you're never too old to learn new tricks, and I am certain that in those pages of notes, there are tips we can all benefit from.

In reply to Randy_Forbes :

I sure didn’t think this one through very well.  The only time I have to write is when I’m waiting for the bus to load or waiting for the team to finish the game so I can take them home.  

My notes are at home to be dug out but when I get home SWMBO has the honey-do list and she’s busy cracking her whip. ( actually I’m just as eager to get the work done myself)

I agree with everything you said. Well,  I use a Unisyn   to balance my carbs but  we are way ahead of where I’d be.  I’m still checking what I have.  

The other thing is I’ve never tuned a car with a baseline.  I’m a Jaguar guy and while they used Weber’s from time to time they were all in race cars and there never is a “baseline” for a race car, Well not a published one to my knowledge.

The factory would tune them on their dyno and send along jets and air correction meters based on their best estimate of what the weather would be.  

 I had a whole series of things about the idle jets.  There is a neat formula I stumbled on that sort of gives me a base line. But without my notes I’m useless.  

I will say never, repeat never trust the numbers stamped on the jet.  Jets are always being drilled out. 

But no, a re-drilled jet cannot be trusted either. The manuals go to a great length about how the radius of entry  effects fuel flow etc. I used to have a flow checker.   just a graduated plastic tube set up to take  a jet. You’d fill the vial up with fuel and time how long it took to flow into the receiving tank. Then it’s simple math to calculate how much richer ( or leaner) that jet was. 

I tried and failed to duplicate the factories radius so when I redrill a jet I write what number size I used on a tag and thread the string through the jet  

Please do not string your jet selection on a piece of wire.  I know all too well they are now off the factory flow numbers .  Nor should you toss them in a junk drawer to be banged and scratched out of shape. In a perfect world they stay in those little plastic bags you get them in. ( or in the carbs).  

Since I’m unable yet to post any pictures. If someone could take a picture of their Unisyn  I’d appreciate it. 

In reply to frenchyd :

Looking forward to this.  I too learned the art of Webers from an old Italian mechanic when I lived in Sicily in the late 80's. I'd bought a set to swap onto my Fiat 124 Sport Coupe, and he spent hours patiently teaching me how to tune, and care for them. His garage was just down the street from my house. spent a lot of time hanging out there.  I really like the way you describe the communication barrier.  My Italian friend also spoke very little English, and my Italian was very poor, but for some reason we were always able to communicate perfectly about cars.  I hadn't actually thought about him for years, Sergio was his name if I remember correctly.  Good guy and great mechanic.

I have some Webbers, some books, and zero working knowledge of how to tune them. 

I can, however, link a picture of a Unisym.

I have one, available through VW suppliers somewhere.

I have to say, I've been hearing horror stories about the "black art" of Weber tuning for ages. But I've never had a problem, even as a total n00b.

I've got a pair of DCO/SP 48s on my Merecdes 190E 2.3-16 race car and a pair of DGV downdrafts on my Triumph TR6 project. Both were up and running immediately. In both cases, I asked for advice about what sizes to use for venturis, jets, etc. from people who knew what they were doing. Both cars start right up on the first twist, idle well, and rev up to their redlines without any hesitation. I've barely driven the Triumph, but the Mercedes has been driven in a variety of weather conditions without any complaints.

The only issues I've had were with fuel delivery. For some reason, I haven't been able to balance the two DCO/SPs on the Mercedes: the rear carb always gets more than the front. I'm switching to a different fuel regulator from PMO Carbs that's supposed to help balance things out: http://www.pmocarb.com/parts.htm

Anyway, my point is not to be afraid of Webers (as I used to be). You can get them up and running pretty quickly. They seem to be robust. I'm sure they're tricky if you're trying to squeeze out every last HP. But you can just set them up with a reasonable base tune and have a reliable car that runs well without much hassle at all.

In reply to LanEvo :

That’s true, certain “rules” apply to Weber’s just like any other carbs and as long as you are slightly to the rich side and slightly under carbed they will work.   

The art and skill of Weber’s is to get them exactly right at peak power.  since the “chokes” ( venturies ) are replaceable and come in different sizes that is where the balancing act comes from. 

Too small a choke and top power is reduced, too big a choke and slow and mid range power and delivery is compromised. 

If the effective size is so adjustable then idle jets,  air correction meters  and shapes, acceleration pumps etc are all going to have to change to be exactly right. 

There is a whole body of knowledge on those combinations and rules to start from. I remember fighting with my first set of Weber’s  on the engine dyno. I thought it would be simple. For peak Power use the biggest Venturi you could and get everything right. Then match it up with every other cylinder that particular throat fed.  

Shipping, availability meant I used 4 different sources and started out with over $2500  1980 dollars worth of parts  probably well north of $5000 now. That does not count the venturies and jets already in the carbs. To be fair it was for 6 twin choke IDF’s 

The dyno guy had his share of parts as well. Over a week of dyno time was used try to find peak power on a Jaguar XKE  V12.  Group 44 claimed 450 horsepower with a set of 4 HD8  SU carbs. My first pull was 219  I eventually got it up to 457 but that took every combination I had and over 55 hours of dyno time. 

Group 44 didnt share their carb info. Not that I blame them, by then they were busy racing in TransAm  with their XJS and had their own interests to protect. When they weren’t flogging it at a race track and had time to talk they were nice people and helped plenty.  They just couldn’t remember things off the top of their heads and I completely understand that.