Holly 4 Barrel Carburetor Conversion for the 3.0 et al.

 

By Tom Van Gunten

 

Why?

I like being different, and that goes with my cars, that is kind of why I drive Bavarias - E3s. This time being different serves a purpose. I had my choice of what to replace the original Zeniths with, and after listening to everybody bitch about always having to synchronizing their Webers and how difficult it was, I decided try one carburetor. This conversion, however, was new ground and all I did was avoid one set of problems to take on the problem of sorting out the unknown of converting to an american style carburetor.

 

Where do I get a manifold?

Euro versions of the 6 and 7 series come with a Solex 4a 4bbl on a BMW manifold. It bolts right up to the head in place of the two for the 2 x 2bbls. It opens up the area under the hood considerably and eliminates synchronizing problems.

There are a number of these that were imported by individuals and as grey market cars. More and more of them are finding their way to becoming donor cars for us trying to keep the E3s on the road.

The Solex 4a that is standard OEM on the BMW big 6 - M30 engines in Europe is like the Zeniths that came on the E3s and, just like them, make great door stops, paper weights, etc.. The 4a throttle plate footprint matches up with the Spreadbore footprint of American cars and carburetor manufacturers. The match is exact until you get to the back two mount holes. They are about a quarter inch off of the rear Holley mounting holes.

Holley models 4165, 4175, 4011, and the 4360 "Economaster" closely fit the mounting footprint on the BMW manifold. Also the Rochester, Quadrajet, the new Edelbrock carbs (read here "expensive" and "Weber knock off"), Carters, and the ThermalQuad have the same footprint.

 

Notes

 

1
  No manifold modifications. Use a spreadbore carb and rattail file out the 2 back mount holes into slots until they fit. Works fine, but voids the warranty on new carbs.
2
  Serious manifold modifications. Have the two back mount holes (or right holes since the carb sits sideways compared to American engines) on the manifold welded up and add extra material, machined flat, redrilled and taped. Cost about $100 or more and it needs to be bolted up to an old head during the welding to prevent warping
3
 

No manifold modifications. Use an adapter plate to adapt from spreadbore to square flange, there are some on the market only about 1/4" - 1/2" thick. You would need to redrill and counter sink the back mounting holes on the adapter, like the back mounting holes on the carbs mentioned above, but it will allow you to use the carb you have without modifying the carb. The additional volume under the carb should actually help. I haven't tried this method yet. Some of the thicker adapters may cause problems with room between the top of the carburetor / air cleaner and the hood.

    Number 3 will be my next experiment. The spreadbore pattern carbs are only available in 650 cfm or larger only, except for the Economaster at 450 cfm. The square flange will allow me to use smaller cfm carbs for better low end torque. That is what I'm am going to do on the Polaris Bavaria in the garage in the near future.

    And for all you folks in CA, when the OEM air cleaner is on the carb, it effectively blocks all visual inspection and looks dead stock, just like the OEM air cleaner on the Webers.

    Some Carburetors have the accelerator pump on the top rather than the side like Webers or bottom, and causes clearance problems with the bottom of the OEM air cleaner. A small space will take care of that problem and is relatively inexpensive.

    And you should be able to find CARB approved replacement carbs at your local auto store.

 

Linkage

The stock linkage found on the manifolds originally works great.  Using parts of the linkage from the Zeniths and a small length of 6 mm threaded rod, it is easy to make the link from the linkage on the manifold to the carburetor.

 

Now on to the carbs.
1
  Economaster, 450 CFM, electric choke, mechanical secondaries, Spreadbore foot print. Great power on the low end, but it had a serious bog when the secondaries open (accelerator pump on the primaries only, not secondaries), except when it was on my 728 auto, no bog(?), just real smooth. Probably be able to dial the out bog with float adjustments, etc.. All parts and gaskets available at parts stores, but no jets are available (according to the guys at Stupid Shops). I don't think that you can buy them new any more (maybe some one out there can help), they are about $100 +, used, in fair shape, more if rebuilt - and there are a lot of used ones out there.
2
  The next one I tried was a List # 9895, 650 CFM, electric choke, vacuum secondaries., a direct replacement for the 350 cid Corvette stock carburetor. Ran like a SLUG. I tried everything to dial it in, changed jetting, power valves, meter plates, float levels, fuel line pressures, on and on and on.....
3
 

Changed to the Model 4165, 650 CFM, electric choke, vacuum secondaries. Ran fairly good up to about 32-3300 RPM then seem to starve from there. One day, at 6000 the secondaries opened and threw me back in the seat. Bought a secondary diaphragm spring kit and used the lightest spring, secondaries opened at 4500, best I could get (changed carbs before really trying hard to dial it in - changed due to another unrelated problem). You could feel and hear the secondaries kicking in, it felt and sounded great! Remember, this carburetor is also made for a 350 cid GM.

4   My latest carburetor is a 4011. 650 CFM, electric choke, with mechanical secondaries, double pumper. The extra accelerator pump on the secondaries takes care of any bog when they open. Smooth from low to high RPM, no torque on the low end, but runs fantastic at high RPM and sometimes runs past red line quick if I don't catch it. It needs to be dialed in to get better power on the low end, I know it needs better jetting. I will hold off doing that until I get the new 3.5L engine in, sometime in the real near future. No sense in doing it twice.

For those in CA the idea of changing to a 4bbl with one of the many "approved replacement carburetors" on the market for American cars, may be worth looking into.

 

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