Tuning the Carbs on a 308

Guys,

I have done the search and did not find a complete procedure. I have found a discussion about there being a compluation of such instructions.

Please let me know if this procedure is either here on this forum, or a link to a site.

Thank you,
Dan

 

There is plenty of write ups on this. Search again.

 

http://ferrarichat.com/forum/showthread.php?t=37614&highlight=carb+synchronize

First hit.

 

Dan
I'm going to stick my neck out here 'cause I may be able to save you some time. Several months ago I rebuild my carbs on my 308 and have done much research on the subject of balancing/tuning. Now, I will bow to the expertises of the guys that know Weber carbs but I have some novice observations on the task of "equalizing" these carbs. Like skinning a cat there appears to be more than one approach to setting up your Webers. I tried at least three different ways. All three have the same starting point: Throttle valves closed with the idle stop/throttle stop screw just lightly touching the throttle shaft lever (to take pressure off the throttle valves); air bypass screws all the way closed; Air/Fuel mixture screws out any where from 2 1/2 to 5 turns out. (this depends on which A/F type screw you have); and the airflow meter reading anywhere from 3 1/2 to 5 1/2 (some feel this is irrelevant as long as all throats are equal). From here the approaches differ. 1. Adjust idle speed to 900-1100 rpm's using the idle stop screw then balance each throat with air bypass and then adjust air/fuel mixture. Then readjust idle speed with idle stop screw if necessary. 2. Use the Linkage to attain a balance...more involved but that is the main emphasis (still use A/B and A/F adjustments). 3. Throttle valve CLOSED and then set idle and balance with air bypass and A/F mixture only. Don't change throttle valve from the closed position. If idle is off then readjust air bypass to all carbs up or down then A/F again to attain correct idle.

Now all of these approaches work because I've read the different comments made on this Chat line. I think the differences may be due to conditions of the car and/or carbs (timming, dwell, compression, vacuum leaks, etc) and the differences in the types of carbs. I think the 308 may have originally had 40 DCNF 70 series but some have 40 series or 12's I don't really know. But someone suggested that maybe the progressive circuit holes are in different positions relative to the idle circuit holes thus opening the throttle plates may include more input of fuel which affects idle speed.

Now, at the risk of being a windbag there is one more comment. I have the 70 series DCNF's and I tried 1.and 2. techniques for over a month of adjustments. I would get a balance then after revving the motor I would get erratic idle speeds. Jump to 2000RPM's and hang there or even stalls. The only way I got a good balance and decent idle was the third approach. This is what they told me at Pierce Manifold when they do their set ups. I hope I haven't misquoted them.

I should say that I have not yet completed my adjustment as I am trying different jet sizes and since winter is here I haven't had the car out since the jets were changed.

Dan, I may be all wet on this and I'm open for and welcome corrections on all of the above but I just wanted for you to be aware of different theories. It could save you some time.

Good luck and you can pm me if you want to discuss any of this.

 

Thank you both for your response.

Docweed,

I really appreciate your comments. I will PM you with a little more history on what I have done, and maybe you have crossed a couple of the same bridges, with better results.

I plan to tune the car tomorrow.

Dan

 

My complete A-Z carb synch tutorial is on my site here:

http://www.birdmanferrari.com/

This is the proper, factory service manual method, with pictures. Not hard at all.

Birdman

 

Nice car. And nice website, Jonathan. That's the kind of website I bookmark.

I also have an ANSA on my 1976 GTB. There were no transitional muffler shields that year. Could you post a picture of your muffler setup and shield?

Barry

 

Using info from all I've read on FCHAT and birdman's site and listening through a rubber hose instead of a using a synchronizing tool, my 308 sounds awesome, has a broad power band free of flat spots (without rejetting), and (when warm) idles perfectly. There is no small amount of self satisfaction from tuning a Ferrari by ear. Many thanks for the information!

A few observations:

the effects of A/F mixture adjustments aren't instantaneous, I sometimes had to wait about 20 seconds

you might want to remove your distributor caps and give the rotor and cap contacts a fresh surface with some emory cloth, it can make a big difference and you never know what you might find... for example, my distributors have the rubber boots over them. these boots are great for keeping out the elements but also hide any problems that might be detected visually, one of mine had 2 of the attachment bolts loose, one was completely loose while the other was 3 or 4 turns from finger tight - a fresh surface on the contacts and tightening the bolts (fortunately the cap wasn't damaged and in fact the rubber boot did a fine job of holding it in place) made a difference in idle quality and high rpm performance