Weber carburetor and ignition tuning of modified 3.0

[buster73]

After completing break-in of my rebuild 3.0 I just went through the whole carburetor and ignition tuning procedure and am quite happy with the outcome. Thought to post some data for others who might face a similar challenge. You can find lots of information on jetting and tuning carbs, but there is not one perfect solution and you will always have to find the right compromise for your specific engine.

Base engine: 1980 Euro 930/09, 188 hp, big port
Performance mods: 9.8;1 Mahle P&C, ported PMO inlet manifolds
Backdated SSI-exhaust 1 3/8“
123 ignition distributor with Bosch 3-pin CDI.
Distributor programmed with 930/10 ignition curve.
K&N air filters
Fuel: “Super Plus” 98 RON, (comparable to US 93 pump)
Carburetors: Weber 40 IDA3C with 34mm Venturies.

(for more details see 3.000 € Budget / Rookie / DIY 3.0 rebuild - successful )

Decided to start with a jetting as advised by Bruce Anderson for Weber 40 on a 3.0. Engine started right up and was running well with this initial jetting. Had – on purpose - selected this rather rich jetting to begin with for break-in to be on the safe side. Before firing up the freshly rebuild engine, carbs were preset on a DIY bench test rig to get the engine running stable as fast as possible and avoid lengthy idling to adjust carbs. Used default values for float level, acc. pump squirt volume, and idle jet screws. Idle air correction screws were opened almost to the max. to increase air flow as recommended for large displacement engines with 40 mm carbs. Once the engine started just did a quick STE-flow balancing and best lean adjustment. This initial setting was good enough for the break-in, but mileage was lousy, spark plugs were all black and AFR readings hardly jumped over 12. Now after having completed the break-in, I decided to go the long way and do a full carb fine tuning, following the excellent guide from Paul Abbott (Performance Oriented).

Started with adjusting float levels with the engine running (big difference to the bench test rig !). Then all the following steps were repeated after each change of jet size. Balancing STE-flow between all cylinders (side-by-side and within each bank). Then best lean procedure first at 1200 rpm, then at 900 rpm with STE readings between 4.5 and 5.0. Next was a test drive while observing AFR readings. Unfortunately I had installed the wiring for only one O2-sensor when the engine was out of the car and one AFR gauge in the cockpit. So I always needed two test runs with the O2-sensor mounted in the right or left bank of the exhaust. Next time will probably install two complete O2-sensor systems with dual gauges up front for left and right bank. First reduced idle jets and then main jets step by step and after each change started the test loop as described above. Target was to be “a bit” on the lean side to find a streetable compromise between performance and fuel economy. Was surprised in how small I could go on idle and main jets which is quite a bit smaller than I would have expected from my research.

Carburetors: Weber 40 IDA3C, Venturies: 34mm, Emulsion tubes: F1

Initial jetting (#1): Main jet: 165, Idle jet: 65, Air correction jet: 170

Setting#..................#1....#2...#3....#4...#5 ...#6
Main jet...................165...165..165..155..145..15 0
Idle jet......................65....60....55...55...55. ...55
Air correction jet.......170..170...170..170..170..170

Final jetting (#6): Main jet: 150, Idle jet: 55, Air correction jet: 170

Setting #5 with 145 main jets clearly was too lean with hesitation on acceleration from 2800 and 4000 and AFR readings >14.

Setting #6 seems perfect with engine in the high-12s to mid-13s under most conditions. Cold idle a bit lean with light sneezing through inlets when engine is cold, but as soon as engine warms up idle is fine between 12 and 13. Also some backfiring through exhaust and AFR reading above 14 when going of throttle. Otherwise engine idles rock solid at 900 rpm and runs smooth throughout from idle to redline. Acceleration is strong and without hesitation. Did some autobahn runs up to ca. 6000 rpm @ 210 km/h (130 mph) and AFR readings are stable between high-12s and mid-13s. Tires are still a bit “wobbly” after long winter break so going faster was not recommended. No need to touch the air correction jets yet, but will watch AFR at full autobahn speed as soon as tires will allow. Next challenge will be a 2,000 miles roadtrip to the Scottish highlands which will also deliver long range mileage data

Ignition mapping:
Because original engine was missing a dissi, I opted for a new freely programmable 123 ignition unit, which allows to adjust ignition timing to best suit your engine. You can set ignition advance/retard timing as a function of engine speed and manifold vacuum. After doing some research, I followed the general advice to stay below a total advance of 28 to 30° for CIS pistons and combustion chambers and single plug. As a starting point I used the ignition curves of a 930/10 engine (9.8:1, 204 hp). Currently maximum advance is 27° (22° centrifugal @ 2600 rpm plus 5° max. vacuum advance). Rev limiter is set to 6300 rpm but might move up to 6500 rpm later. Should be good enough for a street car and leave some margin for reliability and variations in fuel quality. See the latest ignition curve attached:



Left timing to 0° for the lower rev band to better verify timing with a strobe. Later will increase to 5° BTDC @ 900rpm.
With the 123 USB version you attach your notebook computer via USB cable and copy the ignition curves to the dissi. You can actually store two different settings and switch between both settings by powering the yellow wire (To switch from the drivers seat you need to run a wire from dissi to cockpit). Have not used this option yet, but planning to program a second setting with reduced advance in case I can not get high octane fuel (98 RON = 93 US) and have to use lower octane fuel.

123 also offers Bluetooth versions where you can use an App on your smartphone to tune and connect to the dissi via BT. So far I am quite happy with the unit. Installation and wiring is easy and it can be combined with the stock Bosch CDI and coil. Will see how reliability is holding up compared to the proven mechanical distributors.

[Dpmulvan]

Curious why you went with F1 emulsion tubes?

[buster73]

Originally Posted by Dpmulvan

Curious why you went with F1 emulsion tubes?

The F1 tubes were already installed to my 2.4 engine. Guess they were part of the CA-legal PMO conversion kit for early 73.5 CIS models back in the 70s. Was skeptical myself if they would work with the 3.0, but so far no reason to touch them. AFR readings very stable across the rev range and under load. Good for the budget anyway as emulsion tubes are among the most expensive tuning measures for our Webers

[buster73]

Just back from the first long tour on my rebuilt engine. Great road trip to the Scottish Highlands with a total of 2100 mi. We were lucky and always managed to source 97 to 99 Octane E5 gas (comparable to US 93 pump) so gas quality was not an issue. Was a perfect test for my carb and ignition tuning and included ca. 1000 mi cruising to and from the ferry port, mostly on speed limited roads between 60 to 80 mph. Rest was „spirited“ driving on back roads. Average speed approx. 50 mph, with lots of stop-and-go on the single track roads so typical for the highlands. Also some pretty fast passages going up or down remote canyons or „glens“ and some mountain passes. Engine performed flawless and total oil consumption was ˝ quart for the whole trip. Very happy with my carb setting and engine almost feels like on fuel injection. You can cruise along in 5th gear at 1200 rpm and accelerate up to red line without any hesitation or hick-up. Plenty of torque even at low revs and during the first drive in the Scottish rain I experienced wheel spin in the lower gears when hitting the throttle at 3000 rpm. Certainly needs more attention compared to the 2.4 engine. Not unexpected driving characteristic is quite a bit different from the 2.4, with strong and even pull across the range, while the 2.4 is more sluggish up to 4000 rpm but then suddenly comes to live big way.

AFR readings very stable between mid-12 to mid-13. Only when cruising between 3500 and 4000 rpm AFR climbed to high-13s to low 14s which first was a bit alarming. However AFR drops at the slightest touch of the pedal and after checking the spark plugs I see no sign of a too lean mixture. Total average mileage was 22.5 mpg (10.5 l/100km) with up to 27 mpg (8.7 liter/100km) when cruising at 60 to 70 mph. Guess nothing to complain about, especially as on some day trips my mileage was actually better than a 911 SC running a stock 204 hp ROW CIS engine.

First thing to-do after the trip was checking the spark plugs. Cyl. #1 is a bit sooty, but all other sparks look fine and just starting to get some nice brownish color. All-in-all no need to change anything but of course I will continue with some more tinkering. Will test my new colortune kit and check whether I can confirm and correct the slightly rich setting for Cy. #1. Will also finalize ignition curve with an idle advance of 5° BTDC.