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 Posted: 11-28-2015 05:33 am
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Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
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Carbs require a little black art to tune them. They are not ECU controlled with sensors and feedback loops, so it's not a simple matter of working out some ideal settings, plugging them in, and it runs perfectly. All the generic formulas, as in the Speedpro book, will get you close, but plan on doing some tuning afterwards. Also, carbs don't have the flexibility of an ECU controlled fuel injection systems, so you can tune for peak power, or best driveabililty, or best economy, but not for all at once. You need to choose what you want, and "both/ all" isn't a choice.

The best way to approach jetting of a custom installation is to take it to a shop with a rolling road dynamometer, an exhaust gas analyzer, and (hopefully) a comprehensive supply of Dellorto jets and parts. I'm fortunate to have one near me.

It's also important to keep the three-Cs in balance... Carbs, Compression & Cams. If a big set of carbs are just bolted onto a mostly stock sedan engine, then the resulting gain will probably be be less than hoped for with the stock compression and cams. For best results, the 3-C's need to be optimized together. The B30's 10:1 compression is higher than I would have guessed, but it's not "hi-performance" high. I know nothing about the cam, but expect that it's fairly mild.

I'm not familiar with Volvo 164/ B30, but I'm thinking a pair of 45mm DHLAs is a lot of carb for the existing compression & cams in a 496cc/ cylinder engine. Then add in the 36mm chokes, and I think the engine is a little over-carbureted. Now, maybe the 164 is a hotrod Volvo, and loves the carbs; but without knowing more, I'm betting that's not the case.

I suspect the Lotus 907 (493cc/ cylinder Vs B30's 496cc/cyl) is a bit more of a hotrod than the B30, and the Jensen-Healey version of that engine uses 40mm DHLAs with 35mm chokes. The Lotus version of the same basic 907 engine uses 45mm DHLAs with 36mm chokes, but Lotus also targets a more boy-racer clientelle. Flog the engine and it goes, but it's not meant for casual driving. Many J-H owners criticise the Lotus-spec 907 for having too little torque at low rpm, and not being easy to drive in commuter traffic. Big carbs just aggravate that condition.

So what are your goals for the Volvo B30 engine?

Searching the internet, I find:
2980 cc 6-cylinder (496 cc/cyl)
88.9 mm x 80.0 mm bore/stroke
145 bhp (SAE) at 5500 rpm
163 lbs-ft at 3000 rpm.

Using some standard formulas and assumptions, I get the first two columns:
............................... Generic ......... Generic .......... Stock
Engine .................... Volvo B30 ...... Volvo B30 ....... J-H 907 ........ Yours
Displacement (cc/cyl) 496 ............... 496 ................ 493 .............. 496
Power Peak RPM ...... 5500 .............. 5500 .............. 6500 ............ 5500
Tuned for: ............... Power ............ Flexibility ........ J-H Stock
Carb Type ............... DHLA 40 / 45 .. DHLA 40 ......... DHLA 40 ...... DHLA 45
Choke ..................... 36 mm ........... 34 mm ........... 35 mm ......... 36 mm
Venturi Reduction %.. 0.90 / 0.80 ..... 0.85 ............... 0.875 ........... 0.80

Main Jet ................... 145 ................ 140 ................ 130 .............. 150
Main Air Corrector .... 180 ................ 180 ................ 160 .............. 160
Ratio, Air Corr/ Jet ... 1.241 .............. 1.286 ............. 1.231 ........... 1.067
................................................................................................... 180 SpeedPro
Ratio, Air Corr/ Jet ....................................................................... 1.200

Main Emulsion Tube.. 7772-6 or -5 for both ................ 7772-5 ......... 7772.7
.................................................................................................... 7772.4 SpeedPro

Idle Jet ..................... 50 ................... 50 ................. 50 ................ 60
.................................................................... I prefer 55 ................ 50-55 SpeedPro
Idle Air Corrector ...... 7850-2 ............ 7850-2 ........... 7850-1 .......... 7850.9
................................................................... I prefer 7850-7 .......... 7850.6 SpeedPro
................................. 2nd richest ...... 2nd richest ... "normal" ......... 3rd leanest

Pump Jet .................. 40 ................... 40 ................. 45 ................. 40
Auxiliary Venturis ...... 7848.2 ............ 7848.2 ........... 7848.1 ........... 8011.1
Aux Vent sized for ... <550 cc/ cyl .... <550 cc/ cyl ... >550 cc/ cyl ... >550 cc/ cyl
Aux Venturi should be ............................................... Lotus uses ..... 8011.2 SpeedPro
........................................................................ large Aux Venturi.... a size too big
Pump Delivery in 20 Strokes ...................................... 8.0cc
Pump Lever Clearance (mm) ...................................... 0.1 +/- 0.05
Fuel Delivery Pressure (psi) ....................................... 1.5-2.5 at carb inlet

Idle Air Corrector Jets (aka, Idle Jet Holders) are not numbered sequentially, but totally randomly. Don't get caught making any assumptions about which one is the next size richer or leaner. Go by the following chart:

7850.5 .. Leaner, going down the list gets richer.
7850.10
7850.9 .. In your Volvo B30, pretty lean.
7850.4
7850.1 .. Normal
7850.3 .. Normal
7850.6 .. Normal, 6 & .7 are very similar, almost interchangeable
7850.7
7850.2
7850.8 .. Richest

That's a lot of writing to not come up with a specific recommendation, but I'll fall back on the earlier statement that you can't calculate a correct setting for carbs, and then have it work first time when you plug it in. But having looked at all the above...

Fuel pressure is important. The Dellortos like 1.5 to 2.5 psi at the carb inlet. Higher fuel pressure can over-power the inlet shut-off valve, and result in an overall rich condition. If the inlet pressure is too high, then install an inline adjustable fuel pressure regulator, and set it for 2.0 psi.

Next, the fuel level in the float bowl is critical to correct mixture control. There are three or four Float Weights available, and each has a different required Float Height setting. If you use the wrong height for the weight that's installed, then the fuel level will be wrong, and everything goes to heck from there.

The Dellorto part number for the floats is 7298-99-_ _ with two final digits for each weight:

01 = 10 gr, 16.5 - 17.0 mm Float Height.
02 = 8.5 gr, 14.5 - 15.0 mm Float Height.
03 = 7.0 gr, 16.5 - 17.0 mm Float Height.
(01 = 10 gr, 14.5 - 15.0 mm, Non-standard, Lotus sometimes used a higher fuel level to create an overall richer mixture)

Remove the carb's top cover. Use a small scoup, or an eye dropper/ turkey baster to remove a little fuel to create a low condition. Re-install the top cover. Switch on and run the electric fuel pump until it's tone changes, indicating the float bowl is full. If the pump is mechanical, then remove the coil wire, and crank the engine over (without starting it) long enough to ensure that the float bowl is full and the float has closed the inlet valve.

Switch off. Promptly remove the top cover. Using the depth gauge feature of a vernier or dial caliper, measure down from the top of the jet pedestal in the float bowl, near the middle between the two Idle Jet assemblies, down to the fuel surface. The correct level is 27mm / 1.063”. Adjust the Float Height and repeat the procedure as required to achieve that level in both carbs. If each available weight float is set to it's specific correct height, then all float weights will result in the same standard 27mm fuel level in the float bowl. Setting the Float Height is an easy expedient, but correctly setting the fuel level is the real goal.

Don't try to do any tuning until the fuel pressure and Float Height (fuel level) are correct !

IMHO, the 45mm DHLAs and 36mm chokes are on the large side for a Volvo 164/ B30 (while admitting that I know nothing about them). But given that is what you have, and that changing the sizes will be expensive, lets just roll with it for now.

The Main Jet and the Main Idle Air Corrector are both too rich. If you stay with the rich 150 Main Jet, then I think the Main Air Corrector number you got reading the SpeedPro book is going in the right direction, but maybe even 185-190. But if you reduce the Main Jet a size or two, to a more reasonable 145 or 140, then a 180 Air Corrector is about right.

I don't know what to say about the Main Emulsion Tube... that's more black magic than I can stir in my kettle.

The Idle Jet also seems too rich to me. I'd expect to go 55, 58 max, but 60 just seems like too much. And the Idle Air Corrector is the third leanest one available. The number you got from SpeedPro, 7850.6, is about right, but my favorite is 7850.7. The .6 and .7 are very close, and almost interchangeable, but the .7 is a wee bit richer. If the engine doesn't have to pass an emissions test, then a little richer is good.

The transition point between the Idle and Main circuits occurs at about 3200 rpm. The Idle Jet must be just large enough to have the flow capacity to feed the engine up to 3200 rpm, but be no larger than required. Then the Idle Air Corrector is sized to add the right amount of air for the best A/F ratio. The best way to set the Idle Jet and Idle Air Corrector sizes is to...

1) Give the ignition a tune-up.
2) Accurately balance the carbs (that's critical).
3) Set the Idle Mixture screws for peak manifold vacuum.
4) Set the Idle Speed as slow as possible consistent with smooth running. Then...

5) Open the throttle slowly enough that the accelerator pump isn't much of a factor. If the engine hesitates off-idle, then go a step or two richer on the Idle Air Corrector. If the engine does not hesitate, then go a step or two leaner until it does 'just' hesitate, then go back one step richer until the hesitation just disappears. The result will be fine sitting in the driveway, but in real world driving, you may find that the hesitation becomes noticable. If so, go a step richer with the air corrector until the hesitation just goes away.

6) Next, set the Idle Speed to normal (~900 rpm) and go for a drive. Short-shift into a middle gear, then accelerate at full throttle to well past 3200 rpm. If the engine stumbles at around 3200 rpm, then the Idle Jet is too lean, and the circuit is running out of capacity before the Main Circuit kicks in. Go a step or two larger/ richer with the Idle Jet until the stumble just goes away. If the engine does not stumble, then go a step or two leaner until a stumble just develops, then go back richer one step until it just disappears. Do not go richer than is required to eliminate the stumble.

The current Pump Jet size is close to right, but could go a step larger to 45 or so. I don't think it's the root cause of any running problems. The pump's stroke should be adjusted to deliver 8cc of fuel in 20 strokes. That sets the total volume of fuel that can be delivered. The linkage is spring loaded, so fully opening the throttle spring-pressurizes that volume of fuel, then the size of the Pump Jet determines how quickly that volume will be delivered. A fat stream that's gone in an instant, or a skinny stream that lasts a long time. If the pump linkage isn't correctly adjusted to deliver enough fuel, then changing the Pump Jet larger isn't going to cause more fuel to flow.

The only way to correctly adjust the linkage is to use two graduated cylinders (chemistry lab bits), one in each throat, then fully stroke the throttle 20 times. Measure the fuel dispensed, and adjust the linage accordingly until 8cc is achieved.

Koit wrote:
One other question, it seems these carbs don't have air bypass valves for balancing, so how should I balance them?The manometer vacuum ports appear to be present, but the Air Bleed Screws are covered with anti-tamper plugs. Those aluminum domes in the bosses. Pry out the anti-tamper plugs, and you'll probably find the Air Bleed Screws.

Failing that, an airflow meter, like a Synchrometer, will work. The problem with any instrument that is placed over the carb's inlet is that it disturbs the very airflow it's trying to measure. A manometer is best, but an airflow meter will work. A Uni-Syn is another type of airflow meter, but it's best used as wall art.

Koit wrote:
I haven't measured the exhaust yet with wideband lambda but It seems the mixture is a bit rich, the spark blugs where carbon fouled. So what do you guys think?
I think both the Idle and Main circuits are currently too rich, and the carbon fouled plugs confirm that.

Good luck,
Tim Engel