Balancing carbs

[red vtec]

I bought a carbtune as I want to learn how to balance my carbs, I found a set of instructions instructions

The carbtune shows the carbs to be a bit out of balance, mainly due to me replacing a few bits. But when I alter the mixture screws it seems to have no affect on the readings, If I screw a mixture screw all the way home the sound changes, and if I open them up the revs increase, but the reading does not seem to change, Have I got a vacum leak? Do I need a new plenum gasket?

Thanks for any help

Chris

[pete]

why not take it to the Ace tomorrow evening

[Sparky]

Hi Chris

The Carbtune isn't showing you the mixture imbalance - it's showing the difference in vacuum across the 4 pots. You alter this balance by adjusting the throttle linkage and turning the air bleed screws - NOT the mixture screws. As Pete says - get down the Ace tomorrow and I'll balance them for ya!

[red vtec]

Hi

I would come, but I'm out with a lady friend tomorrow night.....

Sparky are you saying that the air bleed screws balance the carbs? and then you would adjust mixture with say a colourtune?

Are these instructions correct? or should I wipe my rear on them?

Step by Step Procedure

1. Remove any anti-tamper seals from the idle mixture screw (6) housing (boss).

2. Check that the idle air bypass screws (7), to which anti-tamper paint may

have been applied are completely closed.

3. Dellorto DHLA's have built-in ports for connecting a manometer. The ports

are located right next to the idle mixture screws and may have vacuum tap

blanking plugs (8) fitted. The blanking plugs must be replaced with vacuum

taps / spigots before the manometer tubing can be installed -- one tube per

carb barrel. Some Dells come equipped with brass spigots already in place.

During normal operation these are capped to prevent air intake - simply

remove the caps for this procedure.

4. Connect the four-column manometer to the carburetor vacuum taps, making

sure you know which column goes with which carburetor barrel.

5. Connect a reliable tachometer.

6. Start the engine and leave it running to attain normal operating temperature.

7. Set the idle to about 950-1000 rpm by adjusting the idle speed screw (2).

8. Turn the four Idle air bypass screws (7) fully closed (in). Just seat them, don't

crank on them.

9. Adjust all four idle mixture screws (6) to give the strongest vacuum (tallest

manometer column reading) for each barrel.

10. Re-set the idle speed to 950-1000 rpm using the idle speed screw (2) if the

idle speed changes much when the idle mixture screws (6) are adjusted.

11. Adjust the carburetor throttle balance lever screw (4) to match the weakest

barrel on the front carb to the weakest barrel on the rear carb.

12. Open the idle aire bypass screw (7) on the strongest barrel of each carb

and weaken it's vacuum until it matches the weaker barrel on that same

carb. Match one barrel on a carb to the other barrel on the same carb. Don't

match between carbs with this adjustment. When you are done, there should

be only one balance screw open on each carb.

13. Re-set the idle speed to 950-1000 rpm using the idle speed screw (2).

14. All adjustments should be made to within 3mm of mercury.

15. After making the adjustments, your starting point may have been thrown out

of whack. Go back and repeat the procedure from the start. Re-set the idle

speed, adjust the idle mixture screws for strongest vacuum, balance the

weakest barrels on the front and rear carbs, balance the strong barrel to the

weak barrel on each carb. 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.

16. Remove the manometer (and any adapters) and replace the vacuum tap

screws/plugs/blanks.

Thanks

[Sparky]

That sounds about right! But it won't necessarily give you the ideal mixture. Worth checking that at a friendly workshop, where the equipment is quite accurate.

Turning the mixture screws won't make a huge amount of difference to vacuum; Lotus recommend a starting point of 6.5 turns out on mixture screws, I believe, but I generally settle around 4 - 4.5