Two Fours and Tunnel Ram Carb Set ups

More than likely, the total CFM of your two four barrel set up is way too big for your engine. This is the single most common mistake people make when installing a tunnel ram or dual quad induction system on their T Bucket. And it's not all your fault! The carb manufacturers have a very limited selection of carbs which would work and are generally expensive due to lack of production. So most people buy the larger and less pricy carbs. The problem is, these larger carbs can not provide proper VE for 99% of the applications they are used on. Therefore, we try to "make do" with what we have. This can, and will very difficult to overcome.

My first recomendation would be to locate some smaller carbs, something in the order of 390 or 450 CFM carbs. Even these will be "borderline" on some engines. At least these will get you closer to a good tune.

SIDE NOTE: There is no such thing as "MATCHED CARBS". If someone attempts to sell you "matched carbs", you are most likely getting ripped off!  Any carb will work with any other carb as long as the air/fuel mixture is correct...

The easiest way to tell if you are over carbed is to sit idling, then floor your car. If you are over carbed, it will bog, and stay boged until the RPM rises enough to pull a good vacuum. The lower your VE is, the longer it will take to recover your manifold vacuum and perform properly. I do not recommend running a VE lower than 70%! Even at 70% your tuning will be very difficult.

The first thing to consider here is the CFM that you will need for your engine. There's a simple formula available that will put you right in the CFM ball park. The formula is:

There are a couple of ways to overcome your over-carbed delima. You can disconnect the secondary barrels on one or both carbs, you can use vacuum secondary carbs, or use progressive linkage.

My first choice would be to disconnect the secondary barrels. This is the easiest and generally works well in most all applications. If you happen to have vacuum secondary carbs, it is most likely your secondary barrels are NOT working. Using progressive linkage helps by permitting the use of one carb for "normal" driving conditions, and supply full open capability on demand.

You can (if you like) disconnect one of the carbs completly, and run on a single four barrel while the other just sits there looking cool. This is a very common practice, and may be your best choice.        

The lower center lines are the carb throttle shafts, and the upper center lines are the progressive linkage points.

Because length "A" on Carb "A" is greater than Length "B" on Carb "B" then Carb "A" is the primary carb providing the idle and "normal" use. Carb "B" is 30% shorter than Carb "A" meaning that carb "A" needs to travel 30% before engaging Carb "B".  The easiest way to "set" progressive linkage is to open up both carbs to full throttle then "set" the progressive stop on Carb "A". Regardless what your geometry is, this will insure 100% throttle open on both carbs. I have found through years of multi-carb setup experience, it is better to have your rear carb be your primary carb.This similifies your cabling to the foot feed and makes  a cleaner looking setup. When "making" or buying progressive linkage be CAREFUL! The linkage MUST be able to slide freely on the primary carb prior to hitting the linkage stop to activate the secondary carb! If it doesn't slide freely, your linkage will bind causing the secondary carb to open too soon, and will most likely bend or break the linkage! Please be very careful! This effect could also bind while closing. If this happens... you WILL NOT go back to IDLE, potentially going out of throttle control! The most common progressive linkage is a 1/4" rod which "slides" through a swivel located on the primary carb. There is a collar on the rod which is the linkage stop to activate the secondary carb. This is the linkage I stay away from! They do bind at the swivel if not properly lubricated! My preference is linkage which uses a flat bar with a slot cut in them. They do not bind or hang up, and require no maintenance or lubrication.

If you are opting to use 1 to 1 linkage, just make sure the linkage connections on both carbs are the same distance from the centerline of the throttle shafts. Again, I would connect the foot feed cable to the rear carb. It just looks better!

On a progressive linkage setup, you will find it much easier to tune it up by "shutting down" the idle circuit on the secondary carb. To do this, simply back off on the idle stop on the carb until the carb is 100% closed, and screw in the idle fuel needles until they are closed. This carb is ready for power only. Now you can adjust your idle fuel and idle speed from the primary carb. If you have 1 to 1 linkage you can still shut down the secondary carb, but you will have to re-set the 1 to 1 linkage a little to keep the secondary carb from interfearing with the primary carb idle air flow. I recommend your keeping both idle circuits functioning with 1 to 1 linkage.

Now understand something right up front; If your carbs are not in perfect working order as single four barrels, they will never function properly as two fours! So make sure both carbs are in good condition and functioning properly as single four barrel carbs. Your tunnel ram will work just fine with either carb totally (linkage-wise) disconnected! This will allow you to "test" each carb to see if it needs work or an overhaul! I recommend you do this first, to insure your carb(s) are OK.

What to expect from your tunnel ram setup:

While you will have awesome topend power, don't expect much in the way of off-idle accelleration especially below 2500RPM. If you do a lot of "throttle dogging" while cruising you will need to have another set of spark plugs with you. Tunnel rams do tend to foul plugs. You may eliminate this problem by using Platinum Tipped spark plugs. Never pump the throttle unless you are spinning the engine over. A tunnel ram will drop liquid fuel directly into your cylinders and flood or liquid-lock your engine. I ran a tunnel ram on the street  with no problems at all. You should be able to do the same...

CBandit