So last night was the first time I used my STE to sync my new 40s. I was pleased to find that all the carbs were initially pulling just about the same vacuum. However, when I went to make fine tuning adjustments with the air bleeds to sync each stack to the highest pulling carb I had no success. Am I correct in assuming that you would back out that screw to bring up the vacuum on the lowest pulling carb? That didnt work. I even backed out the screw on the highest pulling carb to see what would happen and nothing. What am I missing here?

Regardless I got them all relatively close and idling well but then had problems once I snapped linkage back together. The idle jumped up as one of the linkage arms was holding one carb off the stop (go figure).

Anyways, I would really like to figure out how to do the fine tuning with the air bleeds. Another thing I was confused about was the mixture screw adjustment. I started about 5 half turns out and then began to slowly turn in all the way. On some barrels I didnt notice a difference until the screw was almost seated. Usually once it was seated it would start to stumble then I could back it about 1/2 turn and it would stabilize. Based on this link I am still within range on my jets. Although, I must admit it is tough to distinguish when it is actually stumbling on some barrels.

I feel like even though this is a somewhat simple (when described) procedure, there is a significant learning curve to it, especially when it comes to tuning mixture screws by ear. I plan to try again this weekend but any help anyone can provide on air bleed adjustment and mixture screw adjustment before then is appreciated.

Kevin (rhodyguy) has been my Obi-wan through my carb tuning and he has taught me several key things. Hopefully these help you.

1) First, make sure you have valves adjusted to spec as this can have big effects on the amount of air/fuel that each cylinder pulls.
3) Then, make sure that both carbs rest on the idle screws when you are trying to get the mixture settings right, linkage off. If not, its possible the throttle shaft is twisted which can cause big headaches.
4) The bypass screws only have an effect on the idle circuit, so if the car is idling high you may tuning them accurately. You're looking for an idle somewhere between 700-900 rpms, the lower, the better.
4) Adjust the mixture screws first, the bypass screws second. Once mixture is set, you can go back and fine tune the bypass screws, then recheck the mixture screws.
5) It sounds like your idle jets are almost on the big side for your application. 1/2 turn out is not a lot of room to play with.
6) Hex bar linkages are to put it mildly, awful. I learned the hard way that there are a number of configurations of air cleaner bases, bar lengths, down rods, etc and if you don't get the right combo, you'll never keep the carbs in sync. It sucks to learn this after the fact because you've already spent the money, but please seriously consider a cable-style linkage like the one Chris at Tangerine Racing produces. It will make your life much easier. Trust me. It sounds like you have a similar situation with what I had where you feel like everything is set at idle, but as soon as the bar rotates, the geometry is not the same side-to-side. You might have the right setup with the correct geometry, but need to adjust the length of the downrods, or where the "arms" attach on the hex bar laterally. More than likely you'll just get frustrated at which point, again, consider the cable linkage.
7) Oh yeah, buy the Tomlinson's Weber Tech manual. It will improve your quality of life when owning a carburated car.

-George

It's the opposite. The air bleeds bring the air flow down, not up. You bring the highest ones down to match the lowers.

gint nailed it. you've been going the wrong direction with the air by-pass screws. think of them as adj plugs. turn them counter-clockwise and more air by-passes the throttle plates. go back, set them all at 1/2 turn off of gently seated, and start measuring again. perform the idle speed screw trick i pm'ed you about after resetting the by-pass.

the by-pass issue is one reason i've sent carbs to a company like A.C.E. . after the rebuild they flow bench test the carbs and set the by-passes. you know they are matched when you receive them. if the flow (at a reasonable idle) is off, the prob resides below the carbs. valve adj, worn guides, in need of a valve job, or something a simple as intake gaskets from maniold to carb or intake to head.

k

Thanks for all the input! Believe it or not this is all starting to kind of make sense to me which is good.

I am going to try again this weekend but after a valve adjustment and timing check. On that note, what would you recommend for timing with dual 40s on a 2.0?

Oh, and I have looked for Bob Thomlinson's book but only found it at CB Performance and they want a $20 minimum. I suppose I can find some other stuff to buy there as well

$19.98 @amazon

Sorry, I guess I missed the part about the direction of the bypass screws. That should definitely help, lol.

Mr. Tomlinson's book is phenomenal and I think he deserves your money, but for the meantime, you can look it up online if you register at archive.org. Here is the link. Don't buy from CB though, they will rip you on on shipping.

http://archive.org/details/webertechmanual00toml

Good question. Really depends a lot on what you're using and your advance curve. I have a Mallory distributor and I run 6-7 degrees at 1000-1100 idle. By 3k rpm or so, I'm fully advanced at 32 degrees. That's all from memory a couple of months ago when I got it running again. Could be off by a few degrees or rpm, but you get the idea.

I have a question about fuel supply. I am running with a new Facet pump that is 'supposedly' supplying 3-4.5 psi. What problems could a variance in pressure cause since the carbs should only take the amount of fuel needed? What is ideal fuel pressure for webers and should I get an adjustable regulator to ensure steady flow? Any suggestions on a regulator and vendor?

if you haven't done so already, can you post a picture of your linkage? hard to keep track with all of the carb threads lately. re pumps: go to CBperformance and have a look.

k

It looks like all the fuel pressure regulators are for much higher upstream/downstream pressures. Besides, I would think that I would need at least a couple pound differential across the regulator to get the regulator to operate correctly. Are those intended for FI setups mainly?

I dont have a good pic of linkage but will add one this weekend

the CBs are SELF regulated at 3 1/2 lbs. no need for an add on regulator and gauge. no provision for fuel a return line. i believe they are Carter fuel pumps. maybe avail thru your local parts store for less but you know exactly what you're getting at CB.

also, go to pelican parts and type PEL-TOL-STESK into the search feature. you're gonna need one of those too.

Linky -> CB Performance 3193 Fuel Pump - Rotary - 3 1/2" lbs. - 12 volts

Mine's been going for over 4 years.

Incorrect. Starting with the air bypass screws closed...when opening them it will allow more air to bypass the throttle plate raising the air flow of whatever carb throat ones adjusting. As long as each throat is within 1 point difference to one another I wouldn't worry about the air bypass adjustment.

That's right, except I've found that the closer they are matched the more reliably they will operate at lean mixtures for light cruise

With all the valves adjusted properly, the lowest efficiency cylinder is the one you need to come down to match.

Unless you port that cylinder head intake and exhaust, it will never be more efficient.
So, to tune the motor, lower down the rest to match.

In my experience, there is quite a bit of difference between cylinder efficiency on a stock motor...with no headwork done.

If you want to be all you can be, the heads need to be flow matched.

rich

Sorry but that's incorrect. Again, the air bleed screws bypass air around the throttle plate increasing airflow. There is no way to lower the air flow unless you either match flow the heads or lower the idle speed. Been doing this with Webers way to long to know better (30+ years). One should always start with them closed. After you loosen the lock nut open the air bleed screw and watch the carb throat increase in airflow...it happens every time.

The following explains how they should be adjusted (correctly) and how to correctly synch them together.

http://porsche356registry.org/356talk/1/15461.html

and that air flow increases at IDLE, yes?

Only at idle.

Air by pass screws in lessens flow (closed). Out from closed raises air flow. Start with them closed. From the closed position they will only increase air flow.

not wrong, just wrote it different from you and YOU misunderstood.

Been doing Webers for 30 years here also, your not the only one.

He has the screws way out already...so find the lowest flowing carb and turn the other screws in to lower them down.

OR
start with all of them in, but you will need to use the throttle linkage or idle screw to keep the car running.

Its not a good practice to test sync with the throttle linkage hooked up.

Rich

Oh, and

Hope you have some great positive value to add to all the threads.

rich

Here's some info from Paul Abbott's site. He's a master at Weber's. This is for triple throat 40 and 46 IDA types so I'm not sure if it relates but...

Adjust Idle Air Correction Screws

The following procedure flies in the face of conventional tuning procedures so please review Progression Circuit Operation for clarifications.

Reminder: The initial setting for the idle air adjusting screws is two full turns open from the closed position.

Start the engine and once it is capable of running without you nursing the throttle disconnect the drop links connecting the cross bar to the throttle arms so the idling is controlled only by the carbs themselves. Adjust the two idle speed adjusting screws (previously set to just touch the throttle arms and then turned in 1/2 turn) using 1/8 turn increments to just keep the engine running.

Use your synchrometer to measure air flow through the six air intake trumpets. Find the average reading for each bank of cylinders and adjust the high and low readings to match the average reading using the air correction screws. Then use the two idle speed adjusting screws to adjust both banks to have the same reading while adjusting idle speed to be 900 RPM.