A while back, I rebuilt our SU carbs to new condition, and was gratified at the resulting improvements in performance and economy. Still, I found myself tweaking on the carbs every few days, looking for that elusive smooth idle and synchronized response to fit the air density of the day. I'm a perfectionist about mechanical things. I was therefore attracted by ipd's claim that the Weber conversion was a small price to pay for simplicity, and ordered the kit.

Installation:
The kit is very complete with all the gaskets, adapters and hoses needed to hook up whatever sort of PCV and vacuum advance/retard system one may desire, as well as manifold heating using engine coolant (useful in preventing wet-weather carb icing and keeping tuning consistent under various conditions). As with all ipd products, quality is excellent (even the air filter is the washable K&N million-mile kind), and the instructions adequate.

There is a problem, however. '67 and newer cars have a unified intake/exhaust manifold. You must remove this from the car and hacksaw off the intake portion (I wasn't faced with this, but it does not look like fun), after which the new intake bolts right up. '66 and older cars have a separate exhaust manifold which bolts on to the head with a 1/2-inch flange (ipd used to sell an excellent header, also with a 1/2-inch flange). The ipd intake has a 5/8-inch flange (matching the '67 onwards double-downtube exhaust); attempts to bolt this on with your old hardware will result in bent studs and frustration. Failing to come up with a suitable arrangement using cut washers, I resorted to grinding down the mounting points on the manifold, which worked. The most elegant solution is to have the whole manifold milled by 1/8-inch.

I then spent several hours tinkering with the linkage in order to get full throttle with the pedal to the floor. WARNING: be very careful that the throttle cannot go over center and get stuck wide open! In my installation, even using the short throw hole in the linkage, I had to extend all threaded rods as far as possible in order to get full throttle. Pedal travel is much longer than with the SUs, and the pedal sits much higher at idle, precluding any possibility of heel-and-toe driving.

For the manual-choke version, you can use one of your two existing cables until you decide to commit to the new carb. You can then easily chop off the extra cable and modify the cable mounting at the choke control end. For the electric choke, you must run one wire to the switched 12V ignition line (if your car has the armored cable ignition, there is still a switched terminal external to the cable on the ignition switch). Easy in either case.

Tuning:
My original installation, on a stock B18D, required only adjustments to the idle mixture and idle speed screws. Jetting was perfect right out of the box. A later installation, on a cammed-up high-compression B20 with a header, required considerable enrichment. ipd offers a jetting kit which makes this process quite simple. You can buy the kit or rent it from them, exchanging your old jets for new.

Rejetting SUs means taking the carbs off the car, draining the oil, removing the jets, replacing needles, centering jets, putting it all back on the car, synchronizing the linkage, and, finally, tuning. Even then, it's hard to be sure that you are using the best needles.

In comparison, the Weber is dead simple. Each barrel has an idle jet, which screws in without any disassembly of the carb, and a main jet, which requires removing the top of the carb body (with a little care, you can re-use the gasket over and over). Select the primary idle jet that gives the best idle with the idle mixture screw turned out between one and two turns. If you have to turn it more or less than this, select a different jet. Drive the car without opening the secondary. If it seems to gasp and hesitate, install a main jet one size bigger (sizes are stamped on the jets) until it's happy (too big, and you will get a burbling sound in the exhaust). Next, select a main jet for the secondary that is one size smaller than the primary main (this may not to make sense to you -- the secondary barrel being much larger than the primary --nevertheless, this will be the correct jet in most cases). If the car bogs when the secondary is opened quickly, go up one size on the secondary idle jet (and check that you are getting proper advance on the ignition timing).

ipd lets you keep rented jetting kits for several weeks. It only takes a minute to change an idle jet, and five minutes to change a main jet, so you will have plenty of time to drive the car in varying situations and experiment.

Conclusions:
The Weber carb will provide the same power and economy as perfectly tuned SUs. Its great advantage lies in its ease of tuning. After you get it set up, it stays rock solid I have put 30,000 miles on mine without ever having felt the need to mess with it. Even on the hypo engine, it gives a smooth idle at 900 RPM, and California smog checks are no problem (I keep a copy of the certifying E.O. with the registration so the technician won't fail me on the visual inspection). You can drive in normal traffic all day on the primary barrel (good economy), but opening the secondary will definitely startle the guy in the BMW next to you (good fun).

The down side: for concours, this obviously puts your car in the modified class. For racing, modified SUs (or, even better, Weber side-drafts) will give more sheer horsepower to a radical engine. I miss the on-off feel the SUs had.

Perhaps the real question is: for my daily driver, would I go back to the stock carbs? My answer is: heck, no!

The text of this article originally appeared in the Volvo Sports America Western States Magazine, November / December 1995.

Since this article was originally published, I have read other opinions about this conversion. While no one disputes the convenience of this setup, some found that top-end power suffered compared to SUs. My comparison was on a stock B18D; while the Weber runs great on a hot rod B20, it is quite possible that SUs would deliver more power in that context. This is essentially the same Weber that Volvo offered in "tuning kits" as a high-performance option on B18s in the 1960's, so I stand by my appraisal. I am going to stick with the SUs on our new 1800S project for a while, in any case.

Here is an additional tuning hint, thanks to John Chirgwin of San Antonio, TX. Many will find that the manual choke function, as delivered, is pretty strong on enrichment and light on fast idle speed. This is easily adjusted by bending a sharper "S" shape in the rod between the choke linkage and the fast idle lever.

For more info on the web:

The ipd Company