Weber DCNF cleaning and tuning

As I have very little experience with these, and couldn't find an earlier thread, I thought it be good to have a thread on these carbs that are mounted on many of our cars. My set belongs to Khamsin # 435, a Euro-spec manual car

What I'd like to do is start from factory base-setting for carbs and linkage.

I currently have my intake manifold off the car, and step one is cleaning the outside. The carbs look good, all plating is excellent, but there was some caked-on dirt and a 'film' that made it all look kind of grubby. I first washed down with diesel, followed by un-diluted Clean Green. The latter did the trick, it loosened all dirt after a day, and I washed down again with diesel. They are clean on the outside.

Question 1)
Were these carbs proyected with anything from the factory, and if so, with what, and if not, what do you use?

I took the eccelleration membranes out, there are no rips, so I guess that I can just re-use them.

Step 2 will be the float settings, but I find various figures.

Question 2)
What is the correct figure for the floats?

That's where I am right now.

Best,

Jack.

 

Float settings should be in the owners manual Jack. I'd use those.

If your carb bodies look nice an clean without corrosion then leave then alone.
I'm not sure anyone knows exactly what Weber did to protect them. I've restored a set of 6 DCOEs that had to be bead blasted so I did use a protective coating process but that's not applicable here.

Replace the accelerator pump diaphragms.

 

Bob,

They look like new on the outside, the diesel leaves somewhat of a coating/film.

I hear what you're saying about the accel pump membranes, I'll order new ones. They are esy to reach even on the car.

Reason for wanting to start from scratch is, that the valve clearances were all over the place, and thus even if adjusted 'recently' the carbs will be too. If they were adjusted with the valves as they were, it's no good either. I'm in there now, so might as well do it properly.

Best,

Jack.

 

Hi Jack,

you'll find a nice write up of basic tuning on Ivan's site here: The Car Nut - Maserati Manuals and Brochures under Tech Articles, lower right hand side.

Otherwise there's the "Weber Bible", published in the early 90's. Covers pretty much everything.

Just in general to the thread, not your specific questions

 

I assume you'll be taking them all apart. Don't pull the throttle butterflys out of the shaft though. Probably not necessary and big trouble is you do. I did that. ONCE.

I'd soak them well in carb cleaner. Can you still get the good stuff (really toxic) where you are? Then blow all the passages clean. I doubt you'll need to remove the lead plugs but you might.

If you can't find a good chemical cleaner think about an ultrasonic cleaner. I've never tried one. Pierce Manifolds does theirs this way with a biodegradable detergent of some sort.

Not sure if the ball bearings are something to worry about in an ultrasonic tank.
You could also take the carb bodies to an engine shop that has one of these.

You carbs look and sound eons better than those DCOEs I did and they turned out fine. I used chemical cleaner and blew out the passages. No plugs removed.

Good luck.

 

I found this article very useful. See what you think

Carb synch tutorial: 308 Series

 

Thanks much for the input!

Yes, I read Ivan's excellent write-up. I am going a little further in though, and need all the base-settings.

Question: What is the butterfly clearnce in closed position?

Bob, I am not planning to take lead plugs out, and will leave the throttle shafts in the carbs if the bearings feel taut and without play.
I am not sure about various cleaners, but so far am quite happy Clean Green works. I use a large syringe to force it through channels, and jets can soak in it. I wash everything with diesel before putting it back together.

Julian, Birdman's write-up seems good, it covers the process of synch well. I'll read it to the end in the weekend.

I am wondering why everyone talks about the float-settings and accel pump membranes, but nobody mentions the basic throttle-valve setting. This makes quite a difference to everything that follows, and if not set correctly put adjustments down the line out of reach.

This afternoon I will take the tops off the carbs and see what I find; float leavels check and needle valve check. Most imoprtant some small containers, so everything can go back where it came from and avoid mixing parts between carbs. I once spent a day soring out the triple SU's on an E-type that were put back together all mixed up. Not fun.

I'll post some fotos later as well.

Best,

Jack.

 

I would definately replace the accelerator pump jets...seen lots of pictures of burnt Maseratis and my Merak was leaking petrol before I overhauled the carbs. Also lots of SM and Merak owners swear by the Grose jets. The 308 carb article also has a link for setting the float levels ...just check these are applicable to the Khamsin. Good Luck for the weekend. I am hoping to sell my Merak so fingers crossed!!

 

I would definately replace the accelerator pump jets...seen lots of pictures of burnt Maseratis and my Merak was leaking petrol before I overhauled the carbs. Also lots of SM and Merak owners swear by the Grose jets. The 308 carb article also has a link for setting the float levels ...just check these are applicable to the Khamsin. Good Luck for the weekend. I am hoping to sell my Merak so fingers crossed!!

 

The carbs are off the manifold, the tops off and the jets out. It all looks pretty good, no play on the throttle shafts and the throttles even look really clean.

I added some more photos to the album Carburators here:

https://www.facebook.com/media/set/?set=a.139424746239146.1073741825.119309631583991&type=3

Been looking on various web-sites to find a base setting for the throttles, but came up empty. Does anyone have the specification?

Julian, yes, Gorse jets are excellent. I haven't tested what's in there other than blowing on the pipes and moving the float up and down, but if I need to replace them that is what I'll choose.

Best,

Jack.

 

TECH HELP

I just used this site a couple weeks ago to set the float level on my carbs. Good luck.

 

Those should be a breeze to put right Jack. Don't do anything radical. Looks like they were well looked after as does most of your car.

 

Thanks Bob, yes, that is the impression I have too, and I won't do anything drastic, just making sure it is operating at 100% and clean.

Best,

Jack.

 

Hi Jack.
The khamsin hand book has just about all you need. Weber books are available in English have a look on line.
There a basic seqence to setting the carbs up. What is a huge help is a mercury filled manometer that will do 4 carbs. Bike shops use them and Alfa workshops of old had them. The key is take off the the linkage , ,dalance each one left and right and then abjust the lot to the same vacuums fit the linkage with the lock nuts backed off. Then carefully do up the lock nuts. This is an overview tke you're time.
Cheers
Graeme

 

Thanks Graeme,

The carbs are now clean, and even with this set that looked quite good from the start that was still a lot of work. I am waiting for some gaskets (bottom and float chamber level) to bolt them all up, as well as new accel pump diaphragms.

From there everything will be at base level settings, and they can go back on the car (well, after re-fitting the l/h cylinder head, of course)

Yes, the mercury column set-up works beautifully, unfortunately I don't know anyone who has one. Thinking about that, it shouldn't be all that difficult to make an electronic version.

Then remains the point of setting the mixture. I suppose the bungs on the exhaust manifold come into play here, and I am wondering who came up with the brilliant idea to place these UNDER the car. I may still re-consider my smog-pump and cut the rails off the manifolds, and weld bungs for gas analysing on top.

Best,

Jack.

 

Hi Jack
There are electronic Vacuum balancers available
Carbtune Pro Carburettor Balancer Carb Vacuum Gauges. on eBay!
The very basis of all tuning is to get the cams correct and the cam timming before takling the carbs.
Get copy of on of the Weber books ie Weber Carburetors (HP Books 774) [Paperback]
Verlon P. Braden (Author)
Link curtesy of AMAZON
Weber Carburetors (HP Books 774): Verlon P. Braden: 0075478637744: Amazon.com: Books

Follow the practiced sequence and you will find the job straight foward.
Cheers
Graeme

 

Hi Graeme,

Thanks much for the link.

Yes, of course the cam timing is essential, as well as valve clearance, which |I am addressing at the moment.

I have the usual single-throat vac device (SU) which works well, but I was also thinking about tuning my Alfa Romeo carbs 30+ years ago, which I did having full use of an Alfa offical dealer's workshop and those wonderful mercury columns. Of course the Alsa also had bungs on the exhaust manifolds, so each cylinder could really be done to perfection.

Some suggest adjusting mixture by ear, which is a n0-no to me. The Color-tune wrks of course, but tells me about combustion, not about exhaust gas compnents. Using the bungs and a proper analyser will overcome this.

I suppose I will then just use my trusted old English vac apparatus, it's worked well for me in the past.

I may also be over-thinking this, as, like you say, it is a pretty straightforward and logical sequence of steps.

The carbs themselves are really a sight to behold now. Pity the Maserati castings are rather rough and not very aluminium-like in colour. The manifold is squeky clean though, and I'm looking forward to putting it all back.

Best,

Jack.

 

I've used one of these for years.

Aldon Automotive Synchrometer Carburettor Balancer | eBay

Certainly on the Maser V8 it's easy enough to get the balance close enough.
No point in getting fanatical about this. The entire linkage, carbs and manifold expand and contract depending upon the heat range so it never remains "just so" anyway.

That gauge on a V12 Lambo with it's "lovely" linkage and those 6 DCOEs without a balance screw is another matter. No taps on that intake manifold anyway. Exhaust headers either.

 

If you havent already, it might make sense to rebuild each carb with the rebuild kits readily available off ebay, including all new gaskets, orings, float pins, filters, etc.

Here are a few videos i made rebuiding the 42DCNF's off my Lamborghini.

[ame=http://www.youtube.com/watch?v=yW8maoWVXzg]42 DCNF Carb Rebuild(1/2) - YouTube[/ame]

[ame=http://www.youtube.com/watch?v=FKvzHK5Skrs]42 DCNF Carb Rebuild(2/2) - YouTube[/ame]

 

Hi Geno,

Yes, I found those very helpful videos on Youtube, had no idea that was you.

Thanks for making them!

Best,

Jack.

 

Bob,

How do you solve that, just by bench-setting the lot and hook it up?

Best,

Jack.

 

Amateur speaking here:

You set all the throttles at about the same opening with the idle screws.
Install the linkage and get it close so that they about all open simultaneously - still crude adjustment at this point. Then get the engine started and warmed up.

Now you go back and set them all very closely with the idle screws and then make your adjustments to the linkage so that they all open simultaneously.

Even with a modified dual thread set of small linkage arms this is a pain because when you unlock the lock nuts the engine's vibrations cause the rod to rotate and either lengthen or shorten!

Meanwhile everything is getting nice an toasty! Not a summer job.

But what I find the most difficult about that Lambo linkage is that each side has it's owne VERY robust return spring on the common side. then there's the cross side linkage arms which you adjust to balance both sides with each other. But adjusting them can be tricky because in shortening an arm to open or close further you are also affecting the other side due to the two springs on each side which are now competing with each other.

I've often wondered if instead of having a fixed ball join on the center pivot arm for eahc sides linkage arm (the the throttle cable pulls on this pivot arm) a better design would be to have a slip rod type fitting, you know a square block of metal that pivots but allows the linkage shaft to slip through a round whole in it. Then the two spings ieach side would not be competing. Easier to adjust I should think.

I've also though about peening the threads on those dual threaded linkage arms so that they won't spin with vibration. Maybe nylock nuts is another answer. Of course when you go to tighten them with the lock nuts everything jumps in adjustment as well. ARGH!

Getting even hotter now!

I made that a bit easier by using some Loctite on an extra nut I added to the shaft to use as a steadying fixture while I lock the nut. But something with a micro-ratchet lock mechanism would make this task a dream by comparison.

Now it's mixture time!

Now aren't you glad you asked? LOL

The Maser V8 with it's very simple and short linkage for the DCNFs is a wet dream by comparison. No cross venturi balance screws on these old DCOEs either so it is what it is.

 

Buon giorno tutti,
I finally located the write-up on Weber DCNF carburetor tuning given to me when I first bought our Bora. This is something I've been thinking about since October when 'Warburg'/Carsten asked for help tuning the Weber 42 DCNF carburetors on his Khamsin. I am reposting it here in hopes it is more easily 'searchable' the next time someone needs to tune their DCNFs. Following is the full text I found on file, copied and pasted here with full credit to 'BartvanderWeiden' / Bart van der Weiden and his copyright, if any. Cheers, - Art


MASERATI 4.9L WEBER TUNING by Bart Van Der Weiden, www.ferrarichat.com
(Maserati) Weber Carburetor adjustments 42 DCNF
Based on my practical experience herewith a check list, point 1 to 10 must be OK before fiddling with the carb adjustment:

1. Camshaft timing and valve clearances to be to spec.

2. Compression, turn engine on starter engine with coil cable or rotor off: The engine shall rotate even with an even compressing sound per cylinder. If in doubt: Compression check with a warm engine with a tester, should be within 15% equal to each other.

3. Ignition timing static and dynamic. Ignition needs to be working well!

4. Sparkplugs, cables, plug-caps(!), and distributor-cap. Check for unwanted sparks between elements of the unit in the dark!

5. Fuel supply pressure: 1.000mm< X <2.500mm water column.

6. Float-level: 48 (European) or 50mm (USA) both +/- 0,5mm, see manual.

7. Dripping nozzles in the inlet:

a) Leaking float needle: dripping when engine is NOT running! Mix a can of injector clean and run the engine, if no improvement: change needles.

b) Dripping when running: change diaphragms of acceleration pumps. Make sure that the covers are flat! (Eventually flatten them on a piece of glass cover with fine sand paper and oil. Mine were not air tight with new diaphragms!)

8. Air-filter cleanliness: running rpm with or w/o lid shall not vary!

9. Little balance tube on the inlet manifold under No 1 & 8 carb and the air inlet box to be connected! If not: irregular running at idling.

10. Acceleration nozzle capacity: equal quantity at opening butterflies.

Note: Advise by the master Sg. Tralli: Accelerator pumps: are always too rich shorten leverage effect to 2nd hole smallest capacity!

11. Synchronize individual carbs left and right inlets by special mix screw(with lock nut); one side stays closed!

12. Synchronize carbs at idling on the adj. screws and on the accelerator pedal: all equal suction by measuring device and listening: all same “hiss”.

13. Check leakage of the axles by spraying brake cleaner on the axle: if RPM increases false air is sucked in: renew bearings and seals. Quite a precise job to do that right!

14. Run engine under load for minimum 30min to clean plugs! Also during long adjustment session! Fouled plugs do interfere with the adjusment!

15. After 30min running: Check synchronization again and adjust.

For the basic adjustment I use a product named Color Tune: a transparent spark plug which shows you the color of the burning process. I only use one set and do the cylinders one by one.

16. Color Tune at idling: constant spark and firing with white/blue ring visible in CT plug: more to the rich side as to the lean side. Change rich or lean burning by individual idling air mixture screw. I recommend to adjust the mixture at idling on the rich side: tending to a shade of orange/brown.

17. Same as 16 but “on the accelerator pedal” around 1.000-1.300RPM. I made a little device to keep the RPM stable whilst adjusting. Popping of exhaust at constant RPM means rich burning cylinder(s)! ) One can find the too rich cylinder by taking the spark plug cap off during running, wear a glove for insulation!

Check too lean burning by holding a piece of carton against exhaust pipe: if it hits the pipe: too lean cylinder(s)! Pull caps off one by one till the carton does not hit the pipe anymore; increase mixture of that cylinder ¼ turn; richer and check again.

18. Popping on the overrun (deceleration on the engine while driving the car) means lean burning cylinder(s); run car for 30min and take the plugs out, check the color”(should be coffee with milk): for the too light colored plug: increase mixture of that cylinder between 1/8 and ¼ turn; richer and check if the backfiring/popping stops.

The 1/8 turn on one cylinder may be the difference between smooth running and irregular running!!! Those little pushes in the back that you may experience while driving at 2000 rpm. That is how sensitive the adjustment is!

19. One may have to repeat the points 16, 17 and 18 a couple of times but patience pays of! It took me a while to figure out the above but now my V8’s are running like a dream! Enjoy the process and the driving!

Saluti,
Bart

 

A lot of guys in the California Maserati Club have been swearing by the Colortune for decades. The plugs and wires are so accessible in classic Maserati V8 so that makes sense. I think you need to be indoors or at least out of bright sunlight though.

Absolutely worthless on a vintage Lamborghini V12 though, as plug and wire access is pretty horrific on those engines. I had no luck on that engine with it.

 

I haven't had trouble seeing the Colortune flame in daylight conditions.. the deep sparkplug wells offer the necessary 'darkness', and the kit anticipates this with a "sighting tube" included if necessary.

That's unfortunate to hear about the Giotto V12, as attractive as it is to own one.
- Art