Triple Weber Mixture Screw Disaster

[z boy mn]

Hello everyone, this is going to be a long post with many pictures, but I'm at a crossroads with my triples and I'm hoping someone here can suggest the best way forward. It's a long story, so here goes...

 

A while back, a friend and I took a lot of time and tuned up my engine from top to bottom. I have triple Weber DCOE 45's, and we tweaked the idle jets until it hummed. Bottom line, when we were done the car still ran rich, but there was no hesitation, plenty of ooomph, and all was good. The mixture screws were set at 2.25 turns out when we were done, and we were happy. Very happy. (Before this, by the way, I had dialed in the float, seals, etc. and done a complete overhaul on the carbs. Before that we managed timing, valve adjustments, etc.)

 

Over that winter, I tried to go from "happy" to "perfect," and this is where I got into trouble. I decided to make sure all the mixture screws were EXACTLY the same, so I removed the springs, took off the rubber washers at the base, and turned them all in, marked the spot, and turned them all out exactly 2.25 turns and marked that spot as well. I remember feeling like it was "hard" to tell where the "bottom" was, but didn't worry too much about it.

 

Later, when assembling a new airbox, I took out one of the mixture screws and realized it was now mishapened. I took them all out and they were all damaged. Clearly, I had overtightened them. No worries, right? I just ordered 6 more. Then I wondered about whether or not I had damaged the carb bodies...so I took the carbs off...and sure enough. The mixture screws had all poked through making the hole bigger, cracking it around the edges, etc. See photos (number corresponds to cylinder).

 

 

 

 

 

 

 

I called Top End Performance (all the repair kits and jets had come from them) and asked what I should do. We talked through the function of the mixture screws, which are really just "volume screws" yes?, and they suggested that I would need to set each mixture screw separately from now on. I wasn't too worried as I could just look at the plug, adjust, and keep everything else the same as I know the carbs were tuned properly before.

 

Top End Performance also said some people drill out the damage, fill with JB Weld, and drill new holes somehow. It sounded risky to me, so I spent last summer re-fiddling with the mixture screws in order to get the plugs the "right" color. But now the screws are all wildly different. To give you an idea, here's where they sit:

 

1) 2-1/4 (about where it was before)

2) 0-3/8

3) 0-7/8

4) 1

5) 0-7/8

6) 0-6/8

 

I got to these by running the car, pulling the plugs, and repeating. It took about 30 times, mostly to get #2 to go tan-ish instead of black. Here's what the plugs look like now (from 1-6):

 

 

 

I think the plugs look pretty good overall, much better than they did when we first tuned it (they were pretty black/dark even then), but the car now hesitates pretty badly right at the 2,000 mark and it didn't do that before.

 

Over the winter, I've spoken to folks who suggested I try the repair option. Another suggested just buying new carbs, but that's out of my reach for now. Others have suggested that the new mixture screw settings are going to throw everything else off regardless of the "volume" function and that the carb body and the damaged mixture screw holes are functioning as mini, secondary jets, so the idle jet circuit won't be tunable. I've read every book on Webers I can get my hands on and posts here as well, especially this one, which is excellent: http://www.classiczcars.com/topic/41332-understanding-a-weber-side-draft-carburetor-through-a-fictional-supposition/

 

My understanding from that post is that the main circuit doesn't use the idle circuit, and the progressive circuit uses only some of it, so I'm thinking there's hope....but I'm very new to carbs...especially Webers.

 

The last part of the story includes me buying a wideband air-fuel gauge (per Top End Performance's suggestion) thinking I could at least make sure the whole system isn't going lean. It's not completely installed yet, and I know the reading will be an average, but I'm hoping I can use it and the expertise here to find a way forward without having to replace the carbs themselves. I'd love suggestions or tips. I won't be able to drive the car on the road for another few months, so I have time to consider, select, and test options if I can find some.

 

Thank you in advance for any suggestions/help you can offer!

 

 

 

[Captain Obvious]

I've messed with similar idle circuits on other carbs (other than Webers) and they're touchy. There's so little flow at idle that the geometry really matters. I feel for ya. That's no fun. 

[siteunseen]

What persistence and patience you have, my hat's off to you.  I'm hoping you'll get the suggestions and tips you need.

[Captain Obvious]

Well, unfortunately, my suggestion would be to either live with it the way it is or get replacement carbs. The ones you have are "repairable", but not without extraordinary means.

 

If you can adjust the screws so that it idles OK and the plugs look good, then that's about the best you can do at this point. I know it's eating at you just knowing the damage is in there, but unless you've got the machines to do it and your labor time is "free", it would cost way more to fix those than to replace them.

[Captain Obvious]

Here's a pic of a (non-Weber) carb where I have cross sectioned the body to illustrate the idle passageway details. This is probably very similar to what you're dealing with inside:

 

[z boy mn]

Captain Obvious, thank you for your comments and the photo. Siteunseen, thanks, too. I'm also hoping at the very worst, it'll act as a cautionary tale.

And, the photo helps a lot---it does look like a simple volume issue, and maybe if I slowly adjust from the baseline for the mixture screws where they are now, I can make it work? I'm hoping others have had/solved similar issues...I'll keep crossing my fingers!

Also, you're right, some of the worst of it is knowing the mess is inside my engine bay...and it's completely my fault. The metal there must be incredibly thin as I didn't turn the screws with very much effort....one hand, small screwdriver...it wasn't like I put a breaker bar on them!

[Captain Obvious]

Yeah, I'm sure you've thrown off the whole sensitivity of screw movement vs. mixture change because the holes are busted through. I'm guessing they're much more sensitive than they used to be.

 

A couple notes on the pic I posted... For a sense of scale, the tip of the idle mixture screw is maybe .020 and the hole is maybe .040? Also, you figured out that I didn't include the spring or sealing washers and O-ring on the mixture screw, right? Took 'em off for the photo. They're probably very similar to yours.

 

Take a deep breath, get them set as well as you can, and (if you can't get over the internal knowing they're like that) keep your eyes open for a replacement set. What's a set like that cost ballpark?

[z boy mn]

Ballpark costs for 3 carbs (if you get good deals) runs just over a grand from a place like http://www.webercarbsdirect.com/45_DCOE_p/19600.060.htm.

If you go with Top End Performance/Redline, though, the same three run more like $1,200.

http://www.racetep.com/webcarb.html#webcarb

A set that includes linkages, etc., is about $1,600, but I have all that worked out so I would just need the carbs. I called all the places to see if I could buy just the carb body, but no one sells them that way. New ones, unfortunately, come with stock jets, etc., so I'd need to transfer the ones I have over anyway.

I think you're right as I understand it...with enlarged holes, the proportion of adjusting the screws will be different for each one.

My thinking is once I get the air/fuel gauge in, I watch the mixture through the rpms. My guess is it goes lean at the progressive circuit and then runs rich once it gets to the main. If that were true, I'd try to richen up the lower end (slowly) and maybe try leaner jets in the main. More consistent means smoother, yes?

[Patcon]

Occasionally I see used ones on Ebay for reasonable money. You would want some pictures of the throats so you don't buy the same problem. On a different note I thought the flat spot was caused by a different issue, but I can't recall it right now...

Charles

[71Nissan240Z]

Sounds like you had them set as close to perfect as they're gonna get. Then you got a little bit carried away. When I first put my z therapy carbs on I tuned them and really had to resist the urge to fiddle with them again. Especially since before the new carb exchange I was messing with the old ones all the time to get it perfect. Honestly I would aluminum braze the holes and seal it closed. You may want to take an extremely small drill bit and lightly drill into each end of the cracks first to stop it from spreading. Then fill the small holes u made with aluminum then fill in the main holes. I would advise against a wideband...Since you know how to read your plugs. A wideband may further your problems with wanting an efi like tune which will never happen with triples.

[z boy mn]

Thanks for your comments everyone--I just wanted to provide a final update here with information that might help others. I met with a fabricator/welder and we looked over the carbs, the damage, and the cutaway I've attached here (it was in the Weber book). We considered JB weld, plugs, bushings, etc., but in the end decided a very light weld over the hole would do the trick as long as it didn't overheat the carb or fill the void completely so we couldn't use the original hole (from the inside) to re-drill the hole.

We measured the original hole and it is somewhere between .052" and .055". They don't make bits in the middle size, so we went with the smaller one thinking it's easier to make it bigger, holes are always a hair bigger than the bit, and the reference hole we were measuring was probably enlarged anyway. .0535" would probably have been perfect, but we went with .052".

I've attached photos of the before, the after, and the cutaway image. Hope it helps!

Edited April 1, 2015 by z boy mn

[240260280z]

Nice repair! Looks great.