Balancing Dellorto & Weber Carburetor
by Tim Engel
The following procedure is written with balancing Dellorto carbs in mind, but the basics apply equally well to Webers and other carburetors. Many (most) Dellorto models have vacuum ports specifically for connecting a manometer, and Air Bleed screws for balancing the barrels on one carb.
Most Webers don't have these features. So it will take some ingenuity for making the manometer connections, and balancing the Weber's barrels is not a simple twist of an air bleed screw as it is on Dellortos. But if a Uni-Syn or similar airhorn mounted air flow meter is used with the Weber, the rest of the procedure will apply universally.
BALANCING DELLORTO CARBS WITH A MANOMETER
If your Dellorto or Weber equipped engine isn't running properly, try balancing the carbs before changing jetting or making other changes. You may find many "problems" go away once the carbs are balanced.
There's a screw-adjustable coupling on the linkage bar that connects the two butterfly shafts together. Folk often mistake that screw for the idle speed adjusting screw and mess up the setting. If the coupler is out of balance far enough, the engine will basically idle on one carb (ie, the two cylinders fed by the other carb won't even fire at closed throttle). When the throttles are opened enough to bring in the second carb, the other two cylinders will start firing but the engine will still run rough. The result is a fast, weak, rough idle.
Balancing the carbs will help the engine start easier, idle smoother and run stronger. Do it before taking off on some wild goose chase.
Never look down the throat of a carb on a running engine. If you must take a close look, keep your head off to the side and use a mirror. Full face backfires at close range can be nasty.
The following procedure assumes the carbs are in generally good condition, the floats are set to the correct heights (hi and low limits) and the ignition is in good condition and properly timed.
The best tool for setting the Idle Mixture screws and balancing the carbs is manometer. If a manometer isn't available (everything else is second best), you can try using a Uni-Syn or similar airhorn mounted device; but I find them problematic since they obstruct the very airflow they are trying to monitor.
Or just use a length of rubber tubing as a stethoscope to listen to the hiss in each carb barrel. Adjust the idle mixture to achieve the loudest hiss possible in each barrel, then balance the carbs using the Air Bleed screws and throttle linkage coupler so the hiss is equally loud in all barrels (use the same ear for all readings to avoid having the "bad ear" cause any balance differences).
As a supplemental instrument reading when adjusting the Idle Mixture screws, you can also keep one eye on a tach and watch for the rpm to peak slightly at the same time that the manifold vacuum peaks. A hand-held diagnostic tach is best, but the car's fascia tach will be useful too.
The stock Idle Mixture screws on emissions carbs are "tamper proof", have no conventional screwdriver slot, are deeply recessed in a boss, and sometimes covered with a press-fit cap. Most independent Lotus parts suppliers and Eurocarb, Ltd. all offer replacement, non-emissions style Idle Mixture Screws that stick up out of the boss where you can get at them easily, and have a slot for a straight screwdriver blade. Buy a set. Don't beat yourself up trying to adjust the tamper-proof screws.
Most of those same vendors also carry a very good book on Dellorto carbs titled:
"How to Build and Power Tune Weber and Dellorto DCOE and DHLA Carburetors" by Des Hammill. The title is awkward, but the book has very good tear-down/ rebuild instructions and troubleshooting guides accompanied by good, clear photos. If your favorite parts vendor doesn't have it, order it online at either Amazon.com or BOL.com.
Balancing the carbs involves making adjustments to:
the idle mixture screws (1 per carb barrel),
the Air Bleed screws (aka, balancing screws… 1 per carb barrel), and
the throttle linkage coupling (1 coupling between two carbs)
Some (most/ all?) Dellorto DHLA's have built-in, threaded vacuum ports for connecting a manometer. Every time I say "all", someone crys out, "but mine doesn't". Maybe. Or maybe they don't know where to look. I'll just say that I have yet to personally see a DHLA that did not have the vacuum ports, or a Weber that did have them (once saw a picture of a Weber with them). But I digress…
The vacuum ports are located right next to the Idle Mixture Screws and are normally plugged with brass screw-caps. Replaced the caps with screw-in spigots and press the ends of the tubes from the manometer over the spigots. Five minutes and you're ready.
Start the engine and allow it to warm up fully before proceeding.
Close all the Air Bleed screws (ie, balance screws… one per barrel, two per carb). Just seat them, don't crank on them. Webers don't have Air Bleed screws, so just move on to the next step. Some Dellorto's appear not to have Air Bleed screws. These are most often emissions carbs and there's a round, domed metal cap where the screw should be. If you are adventurous enough to pry out the cap (may require drilling first), you will find the Air Bleed screw underneath. If you aren't feeling that adventurous, you can assume the factory set the screw properly when the carb was built and it hasn't been touched since. But that ignores the possibility of the throttle shaft being twisted by someone tightening the nut that secures the nut at the end of the shaft.
Set the idle speed as low as possible consistent with a smooth idle.
The idle mixture screws are in cylindrical bosses right out on the carb's mounting flange... ie, the screws closest to the edge.
Adjust the Idle Mixture screws to produce the strongest manifold vacuum for each individual barrel (strongest vacuum, or loudest hiss, or peak rpm). Don't worry about balance, just make each one as strong as it can be. On properly jetted Dellortos, the idle mixture screws usually end up being out 3 to 3 1/2 turns from fully seated. Anything more than 4 turns gets beyond the effective taper on the needle and will make no additional change. Anything beyond 3 3/4 turns probably means larger idle jets would be better. Anything less than 2 turns probably means a smaller idle jet would be better. Some older Webers used a pretty coarse Idle Mixture screw, and the final setting could be as low as ¾ to 1 ½ turn.
The peak vacuum/ rpm will probably hold over a range of screw movement, so adjust the screw each way… in and out… until the vacuum/ rpm drops noticeably from the peak. Note the screw positions where the drop-off occurs relative to the peak. After the range is established, the final setting is made by backing the screw out a bit, then turning it in and stopping half way between the high and low limits you just found.
Re-set the idle speed if it changes significantly when the idle mixture screws are adjusted… as slow as possible consistent with a smooth idle.
Open "ONE" Air Bleed screw (ie, balance screw… one per barrel, two per carb) on the stronger barrel on each carb to weaken it's vacuum until it matches the weaker barrel on the same carb. Don't match between carbs with this adjustment. When you are done, there should be only one Air Bleed screw open on each carb.
For Webers, or Dellortos without Air Bleed screws (ie, under the metal seals), balancing the individual barrels on one carb involves twisting the throttle shaft. Purists may choke at the thought. A major imbalance is probably the result of the throttle shaft being bent by abuse (someone took a wrench to the nut at the end of the shaft) or the butterflies are not properly centered on the shaft. The proper solution is to replace a twisted or bent shaft and center the butterflies. Yeah, well, that's the correct way. But if you have an engine that's not running well due to un-balanced carbs, the shaft is twisted so it's toast anyway, and a new one is mail order time plus a rebuild away… you have little to loose. Give it a gentle twist. If it works, you're good to go. If it doesn't, you still have to order the new shaft and you're no worse off than you were before.
The barrel with the stronger vacuum is closed more… the barrel with the weaker vacuum is open more. Make note of which bore is stronger, which way the throttle normally operates to open, and which way the shaft would have to twisted to balance the carbs. Then use a wrench to grab the shaft (nut) where it protrudes from each side of the carb and gently twist. Don't go overboard. Check the manometer reading for any progress, or… oops… ya went the wrong way. Balance the barrels within 3 mm of mercury or less.
Re-set the idle speed to the slowest smooth idle.
Adjust the screw in the throttle shaft coupling between the carbs to match the weakest barrel on the front carb to the weakest barrel on the rear carb (the two barrels on each carb should be pretty closely matched now). Some linkage coupler versions have one screw working against a spring. Just turn the screw in or out as required. Some versions have two opposing screws. In that case, one screw has to be backed out before the other can be advanced; otherwise the coupler will just bend without making any significant progress toward balancing the carbs.
Before cranking on the coupler screw(s), think about which way the coupler needs to move (for many folk, it's counter-intuitive). One carb has the Idle Speed adjustment screw and it's butterfly opening is controlled by that screw. On a Lotus, that's the rear carb. The other carb is held open by the linkage via the coupler. The coupler only changes the second carb's butterfly opening with respect to the first carb (front carb to match the rear carb).
The carb with the stronger vacuum is closed more (the engine is sucking against a closed throttle). The carb with the weaker vacuum is open more (more air is getting by the butterfly to weaken the vacuum. Adjust accordingly.
Final adjustments should have all barrels within 3mm of mercury… or better.
Re-set the idle speed again if it changed during the balancing adjustments.
After completing the balancing procedure, the starting point may have changed significantly. That is, one or more carbs may now be flowing sufficiently more air at idle such that the Idle Mixture screw adjustments are no longer optimal. Go back and repeat the procedure from the start. Re-set the idle speed to the slowest smooth idle, adjust the idle mixture screws for strongest vacuum, balance the strong barrel to the weak barrel on each carb, balance the weakest barrels on the front and rear carbs. If nothing changes, you are done. If you need to make any significant changes during the second trial, you may want to repeat the procedure yet again... until it stabilizes.
Set the idle speed to 950 – 1000 rpm.
Remove the manometer and replace the vacuum spigots with the original screw-caps. Better yet, leave the spigots installed and cap them with the little rubber/ vinyl vacuum port caps available at auto parts stores. Secure them with nylon cable ties to keep them from being blown off by a backfire. A missing cap will produce a significant air leak. If the vacuum spigots are left installed, then the carbs will be ready to accept the manometer next time and checking the carb balance becomes a very quick operation.
NOTE: This procedure ignores emissions requirements. Adjusting the idle mixture screws for strongest vacuum does not produce the cleanest running engine... just the best running engine. For emissions, you will have to connect an exhaust gas analyzer to the tail pipe and set the idle mixture screws to achieve the specified CO level. The rest of the procedure would be the same.
IF you're starting the car for the first time since the carbs were re-built, the idle speed may not be anywhere near close. If you removed the screw and totally lost the Idle Speed setting, start at 2.5 – 3 turns. Back out the idle speed screw until the throttle linkage lever contacts the bottom of the rib by the idle speed screw. Then put a finger on the lever and feel for motion while you slowly turn the idle speed screw in. Stop at the slightest movement of the lever. From there, turn the screw in 2.5 - 3 turns. If you think the screw still has it's old setting... leave it alone, it's as good as anything. When you start the car, you can always keep the revs up with your foot if you need to. Once the engine has warmed up and settled down, you can adjust the idle speed.
I have a mercury manometer and it works great. However, the mercury type is less than convenient in that…
it's a couple feet long, and
has an un-sealed reservoir that will spill Mercury if tipped.
It's okay as a shop tool that never goes anywhere, but carrying it around to work on other guys cars is a problem. You must be very careful to "secure" it upright in the car. If you have small kids, store it out of reach or behind lock and key. Well, if you have small kids, it's not worth the risk of even owning a Mercury manometer.
The Morgan CarbTune II is an all mechanical, 4-tube manometer that contains no liquids… most importantly, no mercury. It's less than a foot tall and easily managed in one hand. It uses 4 accurately made steel rods sliding in close fitting glass tubes. It works great, can be stored in any position, and travels conveniently. I'm sure that, in a laboratory sense, the mercury version is more accurate; but for tuning carbs, the CarbTune II works just fine. I own one and highly recommend it. It's more expensive than a mercury unit, but the cost-to-hassle ratio tilts favorably toward the CarbTune II.
The Carbtune II can be purchased directly from it's manufacturer, John Morgan, at his website below. Or, Eurocarb, Ltd, also carries it and usually has slightly better pricing.
Contact info for both companies is as follows:
Carbtune II 4-tube
mechanical manometer (no mercury)
Manufactured and sold by:
Townsend Enterprise Park
Belfast BT13 2ES
028 9023 9007 Phone,
028 9024 7294 FAX, Domestic
+44 28 9023 9007 Phone from Europe
+44 28 9024 7294 Fax from Europe
011 44 28 9023 9007 Phone from USA / Canada
011 44 28 9024 7294 FAX from USA / Canada
<http://www.carbtune.com/> website, best place for INFO
Eurocarb Ltd (attn:
(Weber, Dellorto, Carbtune II Manometer)
256 Kentwood Hill
Reading RG31 6DR
+44 (0) 118 943 1180 Phone
+44 (0) 118 943 1190 FAX
Prices snipped from the Morgan website 19 Aug 2004:
£40 Carbtune II 2-column
£50 Carbtune II 2-column + Tool Pouch "special offer"
Okay for one Dellorto, but not easy for multi-carbs (907)
£52 Carbtune II 4-column
£62 Carbtune II 4-column + Tool Pouch "special offer"
The 4-column is best for the twin Dellorto set up on a Lotus Twin Cam or 9XX
£12 Tool Pouch
A very nice accessory. A padded Nylon carry case with three separate pockets for the Carbtune, the hook-up tubing, and the hose spigot fittings.
The Carbtune II comes with four 5mm spigots (Dellorto) and four 6mm (some motorcycles). Extras are available separately.
£ 5 Nylon Spigot
Adaptors, 4 each
£ 8 Nylon Spigot Adaptors, 8 each
£ 8 Brass Spigot Adaptors, 4 each
£12 Brass Spigot Adaptors, 8 each
To UK = free, To
Europe = £1
To Rest of the World = £3
A home made manometer
A regular manometer uses a single column per vacuum source to measure the strength of that source. Such a manometer requires either a very heavy liquid like Mercury (hazardous) to keep the required tube length short, or a lighter liquid (oil, water, etc) and a very tall column.
For balancing carbs, we really don't care what the actual manifold vacuum is, only the difference between carb barrels. This manometer uses a "U" shaped tube filled with a commonly available fluid (water, vegetable oil, engine oil, ATF... etc). Separate hoses connect each end of the "U" to adjacent carb barrels. Any imbalance in vacuum between the barrels will pull the fluid one way or the other. Balance the carb barrels until the heights of the fluid columns on each side of the "U" are the same. Balance one carb, then the other, and finally, balance the weaker barrel of one carb against the weaker barrel of the other carb.
Just be sure to shut the engine off while connecting or disconnecting the "U" manometer. With the engine running, if only one tube is connected... even briefly... the entire oil content of the manometer will be sucked into the carb. Not that it will do any real damage, but it will sure make a lot of smoke. Good for mosquito supression.
A lighter fluid (less dense, not cigarette lighter fluid) will be more sensitive to small pressure differences. A differential "U" manometer filled with oil will be about 16 times more sensitive than a mercury column manometer. A difference of 1/20th of an inch of mercury would show up as about 13/16ths of an inch of oil, allowing for even finer adjustment than possible with mercury.