Advanced 308 carb sync question -- throttle plates and progression holes

Peter, Mike: The air bleed screws WILL DEFINITELY register on the syncrometer. Just make BIG adjustments. I had one 'hole' that needed 3 1/2 turns to match the other hole in the same carb. One measured about "4", the other about "3". Took 3 1/2 turns to equalize.

 

Peter: That's inexcusable. Don't do it again.

 

I'm sorry! Don't hurt me!

As for the air-bleed screws, I have fooled around with those before, with little registering on the meter, but I suppose there were gross differences between throats because the car drove poorly afterwards. Went back and shut them all (going solely by the already fine results on the meter and acting on the throttle-stop screws only) and it drove much better with no faults...

?

 

Peter: Did you get any significant 'side-to-side' descrepancies? I had 2 carbs (one REAL bad) that needed significant 'air screw' adjustment, as the 2 sides were hideously unbalanced.

Anyway, the damn thing runs like a Swiss watch and I'm not going to screw with it any more.

 

The Haynes isn't very clear here, but if you say the air bleed source is not external but internally from just above the butterfly, then an air flow meter or a pipe in ear should register it. My mistake ..........

Richard

 

What a great thread. Just what I love this board for.

I've been offline for a while so let me jump back in.

For Mike P:
Regarding terminology. Haynes uses different terminology in different places, sorry for the confusion. I agree that we are dealing with 3 adjustments :

(1) Throttle stop screw. On the external linkage

(2) Air Compensation screws, Air bypass screws, etc. These are the needle screws with the locking nuts on them. They are used to "ensure identical vacuum below each throttle valve [plate, butterfly], and therefore ensure identical mixture during idling and progression." Lets call them Air Bypass screws.

(3) Idle mixture screw - the knurled needle screw with the spring on it. According to the figure in Haynes, the idle mixture is actually determined by the fixed idle jets on the top of the carb near the emulsion tubes. This aireated mixture is then channeled to the "idle mixture screw" which enters the barrel below the throttle plate at rest. This aireated mixture is also delivered to the progression holes above the throttle plate at rest. So, technically, I would call this an "Idle Screw" but lets keep calling it the Idle Mixture Screw.

I don't think that Haynes contradicts the procedure you propose I think it confirms it. Provided that the progression hole directly affected by the "idle mixture screw" is not obstructed by the throttle plate.

Is it possible that the resistance you were feeling off idle was a result of manifold vacuum holding a tightly-fitting butterfly valve closed?

From my reading of the manual (see the quote above ) the air bypass screws take air from inside the air horn at the the top, then inject it below the throttle plate. If so, an air-flow meter would read it, if the air horns and rubber gaskets were in place and air tight.

As for drilling a hole in the throttle plate, I think I remember that comes from the Passini book Weber Carburettors 2. Tuning and Maintenance. He also cautions that it is difficult to "back out" this particular mod. In my humble opinion, his approach is a hack job, and reminds me of English plumbing... it works, but...

If the air correction circuit is properly working, and adjusted correctly, it should take care of this.

Keep us posted on your progress

 

The mixture screw doesn't affect the progression holes in any way. The progression holes are fed directly from the idle jet. The mixture screw determines the fuel mix when the throttle plate is closed below the progr. holes, as it "leaks" fuel into the air that passes by the nearly closed throttle plate and from the air bleed screws. As this flow is quite small, the idle jet has little (but some....) to do with actual idle mixture. Idle mixture is determined entirely by the mix screw.

Richard: The Haynes book also gave me the impression that the air bleed screws got their air from "above" - like perhaps the intake for the choke system. However, you can test this. Take out an air bleed screw. Blow into the screw hole. This is easier if done with the carb off of the car. Stick your fingers into the throttle bore and cover the big choke outlet hole - it's very low, right at the base of the bore. Now you will feel the air come out of the 2mm hole just above the progression holes. Cover this hole and you have now completely sealed off the air bleed circuit and you will not be able to blow any air into the screw hole.

I wish I had a different way of wording the above, as it sounds terribly gay.

I was the poster child for a Jeff Foxworthy joke. Picture guy sitting in underwear, living room coffee table covered with carb parts, and carb body against my face as I'm blowing into the air bleed hole. Residual carb cleaner doesn't taste good.

 

There are two types of mixture screw. US, which have a shoulder and designed for limited if any adjustment for the emissions stuff. Euros have longer taper and provide finer adjustment.
Philip

 

Progression hole sizes for the 7X series carbs are 1.50, 1.15, 1.50, 1.60. and 1.80. Float height is set to 48 mm.

Progression hole sizes for the 4x series carbs are 0.80, 1.40, 1.30, 1.60, and 0.80. Float height is set to 50 mm.

Data is from the respective OM's.

 

Here's the page from Passini that Mike recalls. Note Passini emphasizes the 'last resort' nature of these fixes.
Image Unavailable, Please Login

 

Bryan,

I have successfully used the "file a champher on the edge of the throttle plate", to fix a very specific problem I had on my Lotus Elan. The Elan was fitted with 2-Weber 40DCOE18 carbs. Each choke has a pattern of 3-progression holes adjacent to the throttle plate. One of these holes was not drilled precisely where it was supposed to be. This was easily observed on DCOE's by removing an external brass cover over the progression hole pattern. I filed a small, very localized champher on the throttle plate, immediately upstream of the affected progression hole to enable it to begin to flow sooner, like its relatives on the outer 3-chokes. The fix worked wounderfully. Note that this fix was to overcome a very specific manufacturing defect.

The best way to synchronize carbs is to do like the motorcycle guys do with a bank of manometers simultaneously measuring the vacuum in each intake manifold. I used 2 carb tuners to measure all 8 simultaneously. Very sensitive. Very easy to see idle and off-idle synchronization. Very easy to see how adjustments to one barrel immediately affects all others.

Hopefully below, is a series of pictures showing this system in action.

 

Nope, everything was flowing equal between barrels on each carb.

 

Sorry, I was unable to post the picture of the manometer system in action. I'd be happy to email it to anyone interrested, just send me your email address.

 

FWIW, when I first got my car, it idled at about 700rpm. Upon checking, the idle speed screws were fully backed out.

 

When I switched my 78 car over to an Electromotive system, initial advance was set to about 7 BTDC and idle speed could be lowered to about 900 rpm. I later fitted a vacuum advance module that wouldn't allow idle speed below about 1400 rpm. I finally cured this high idle condition by enabling a stock microswitch, normally used to switch between R1 and R2, to disable the vacuum advance signal with the throttle closed. This allowed the idle to return to 900 rpm and still retain the benefits of vacuum advance under light throttle conditions.

 

One source for Carb Tuner http://www.motorcycle-superstore.com/item.aspx?style=8005&department=646&Division=6

Photo 1: Overall system
Image Unavailable, Please Login

 

Photo 2

Base of manometer.

The vacuum in the intake manifold pulls the mercury in the reservoir up the tubes. Much more accurate than Synchrometer, but somewhat more complex to setup. Also more expensive if you buy 2 for V-8s or 3 for V-12s.
Image Unavailable, Please Login

 

Photo 3

The intake for each cylinder is tapped for just the purpose of measuring manifold vacuum.
Image Unavailable, Please Login

 

Photo 4

Shows one intake with carb tuner tubing attached and one with normal plug in place.
Image Unavailable, Please Login

 

There is also a second generation unit that is mercury free, but more expensive.

http://www.carbtune.com/

 

Unfortunately, the 308 does NOT have those manometer ports cast into the intake manifold (unlike your example, which looks to me like either 365 GTC/4, or 365/400 GT/4 2+2).

Unless the diverter valves and Carbon-canister vacuum taps are removed and modified to accept this type of device (needs to be tapped and plugged after). Lots of work (unless the engine is already in pieces).

 

Definitely 365 GTC/4 (twin scoops on the hood, instead of the 400's single grille).