Weber tuning oddity

[jstein]

Guys,

After a long panic ridden winter, I finally got the engine back in the car. Thanks to Chris Streit from Moto Delta for helping me beyond all reasonable expectations.

So the engine now sports some domed pistons to get higher compression (Mahle P&Cs score on ebay) and has gone through a 20 minute break in; running rough but not bad @ 2000 RPM.

Now it comes time for weber tuning

I have this document on tuning the webers I got from someone on the board that goes something like this:

Idle adjust to the linkage +1/4 turn.
Idle mix, two full turns out from the seat
Air mix, 1/8 turn out from the seat

The car won't idle like this (not surprising), so I turn in the idle screws about a full turn each side to get it idling ~1200.

Then "The procedure" says to turn in an idle mixture screw until the engine 'slows down' and then back out 1/4 turn.

NONE OF THE IDLE MIX SCREWS SEEM TO DO ANYTHING

Meanwhile the carbs are popping like crazy, a couple of cylinders in particular.

So I back the idle mix screws out to the two-turn mark and begin messing with the air-correction circuit. WOW; what a difference; the engine starts to increase in revs, so I continue to back it out until the increase in RPMs stop, and leave it there. Repeat this procedure with the other air correction screws and get the same results.

Now, re-adjust engine idle with the idle screws to ~1000 RPM.

THing sounds much better (very little popping from the carbs, occasional backfire from the exhaust which may be a leak down there).

Still, the idle mixture screws have NO effect; I can screw them in ALL THE WAY and the car idles fine. WTF?

So, I take it for a ride around the block and notice right away that when coming off the throttle, it takes the motor a long time to come back to idle; like 4 seconds or so. Before the engine rebuild the throttle response was fantastic; very quick both on and off throttle. Something is obvious amiss.


Any ideas?

[Tyson Schmidt]

The individual idle bypass screws are only there to synchronize the flow.

Disconnect one of the drop-links so that the two banks are separate. work on one bank at a time. Start by screwing the bypass screws all the way in. Now measure the airflow with a Unisyn or equivelent. The cylinder with the highest airflow is the one you will make the other two match by opening the bypass screws until they do.

Use the idle stop screw to set the idle speed, not the bypass screws.

now do the other bank. Once that side is even, you can now use the idle stop screws to balance the left and right banks, and achieve the proper base idle.

Now your idle mixture screws should have the normal effect. you should probably back them all out 2 1/2 turns from base as a default while synchronizing the airflow.

[jstein]

Thanks Tyson; one clarification - the "Idle Bypass" screw you refer to is the small ones towards the base of the carbs with the 8mm lock-nut, correct?

I'll try that tomorrow.

[Tyson Schmidt]

Yes, correct.

[2.7RACER]

This is a close-up picture of the center Weber carb, right side, that feeds #5 cylinder.



The path of the fuel at idle (true idle), is from the bowl, through the idle jet, down the small passageway directly below the idle jet, which turns left and then down past the low range ports to the idle mixture port.

If the engine is idling with all of the idle mixture screws fully closed, the fuel is coming from someplace other than the idle mixture port.

No mystery, the idle speed screw is holding the butterfly's slightly open, allowing fuel to flow through one of the low range ports.

To get the idle mixture screws in the game, back off the idle SPEED screw. This is the one opened at 1/4th turn, according to instructions. Try backing it off completely. It may need to be open slightly if backing it off completely stalls the engine.
One idle speed screw per 3 carb assembly, located at the end of the carbs.

Backing off will close the butterfly's blocking the fuel flow at idle from the low range port.

Do this with all three of the mixture screws open 2 turns. This will keep the engine running. Of course the linkage is dis-connected as well.

At this point you will be able to turn one of the mixture screws in and reduce the fuel to that cylinder. Turn it in until the motor rpm's drop slightly, then open the mixture screw until the rpm's just pickup, about 1/4 turn.

Adjust all of the idle mixture screws, then do as Tyson says with your synchrometer and the idle air screws.

This all assumes the floats and jets are correct.

The key here is to stop the fuel from all but the idle mixture screws, then they have control.

Edit: for the Weber experts out there, you'll notice something missing from the photo. The carbs were on the bench for a photo session.

[jstein]

Originally posted by 2.7RACER

Edit: for the Weber experts out there, you'll notice something missing from the photo. The carbs were on the bench for a photo session. [/B]

I'm no expert (obv.), but it looks like the main jet is missing on the right bowl there...


Thanks for the info Doug; I'll be going back over these things again tonight.