Weber Help Please !

[michael sundler]

Hi Guys,
I would really appreciate some experienced help in getting my CAV GT40 running well. It is fitted with a 347 Boss Ford Crate engine running with 48 IDA Webers and MSD ignition and on NGK BCR8ES plugs. The symptoms that I have are that the engine runs smoothly on idle but on (very) small throttle openings the engine backfires through the exhausts - suggesting that it is starting to run lean? The top end performance seems fine.
The carbs are set with the following jets:
Idle 70F10
Main 135
Emulsion Tube F7
Air Correction 120
Choke Size 37
Can anybody tell me if these settings seem about right? Thank you!

 

[Mike Pass]

The quick fix is to get the car on a rolling road that knows Webers. From the symptoms it sounds like the transition mixture from idle is too lean or the pump jets are not squirting enough. See if there is a difference between opening the throttle very slowly or very suddenly. A stumble on slow opening indicates transition issues. A stumble on sudden opening indicates pump jet issues. Webers are very tunable but they cost because there are 8 of everything when you change something. If you go to a Weber place they will usually have a stock of Weber bits and will do you a swap of bits.
Without some accurate readings of the actual mixture throughout the range you will never get it right. Trial and error is a waste of time and money.
Alternative is to fit a fuel/air meter with a gauge in the cockpit. There are several available from AEM, Holley etc which are there on Ebay around £150. You will need a threaded plug (actually the same thread as a spark plug) welding into the end of the collector to take the wide band O2 sensor. I have the AEM one. These will give you a read out of air/fuel ratio accurate to 0.1 of a ratio as you drive. The gauges need a feed from the lambda sensor and a positive feed and an earth. Buy the complete kit which comes with all the bits. The threaded bungs are also on Ebay if there is not one in the kit.
The rolling road is easier to get a quick set up done as you can run the engine at a variety of speeds,throttle openings and loads but they can be expensive per hour. A setup should be possible in a couple of hours if they know what they are doing.
Cheers
Mike

 

[Chris Coote]

Hi Michael,
I run a similar engine with 48IDAs and a solid roller.Camshaft will have an effect on your Webers .
My jetting is the same as yours except-
60 F10 idles -74823-60
50 Pumps - 76402-050

I use Motorcraft AGSF32FM platinum plugs with the gap set at .040
Ignition is Electromotive crank trigger.
I assume you have run a "synchrometer" (carb flow meter) over them.
Carbs pop and spit a bit when cold but fine after 5 minutes.Takes a while for the temp to come up.
Other than that you should be very close for a good streeter.

A friend of mine runs same as you on a warm 351 Cleveland with no issues.

 

[Tim Kay]

Michael,

Have you ever experienced this motor running well with the current idle jetting? This would be good to know in trying to help diagnose your issue.

Webers have only two basic areas to tune (yes, it's that simple.....kinda!), idle circuit (up thru mid 2k rpm) and the main circuit (upbove 2800 rpm). And for the most part they are separate. There is a transition area too but getting the idle circuit right pretty much carries through the transition automatically.

With your symptoms at initial butterfly movement, I would start at the linkage.
Linkage\sync is most critical - gotta have that spot on to continue, can't emphasize enough. Will never, ever, ever, ever ....... come together unless linkage is of good quality parts and it's adjusted right. Do not underestimate this point!

Then:
1) Timing - Full advance 38 (my 302 is @ 40) all in by 2,400 rpm. Webers love advance. Entails changing the dizzy weight springs & stop bushing.
2) Make sure the bowl floats are adjusted correctly
3) While your adjusting the bowls, make sure your accelerator jet is clean\clear and all barrels are squirting
4) Fuel pressure 2 1\2 - 3 lbs
If all the above is taken care of then move on to tuning, it's easy pesy from here. You'll be amazed how your motor will talk to you through each and every adjustment you make, and that's probably the most rewarding time with Webers. It will literally transform the relationship between you and your motor, the two of you become one!! But if anything is off in the above five steps you will fight those babies forever and the Weber's will get blamed for something that's not their fault.

Start:
1) What's your idle holder?
My example won't be your motor, and it may sound odd, but on my 302 I ended with a re-drilled 80 holder (could not buy one under 120) and my idle jet is 60. Every motor is different and it'll tell you what it wants.
2) How many turns on your idle mixture screws? Should be 7/8 from closed, if more - it's asking for fuel, increase the idle jet & visa versa. If one step up or down in the idle jet is too much or doesn't help then reset original idle jet and change the holder (only make one adjustment at a time and increase\decrease one step at a time).

Once you get the above working you can start tuning the mains:
1) I would suggest an F11 emulsion
Seems V8 motors are all using the F11, mine actually likes the F16 a bit better but does ok with the F11 as well.

Hope this helps,
Tim

 

[Chuck]

Tim:

Nice summary. You didn't mention adjusting the linkage to balance the carbs, but suspect anyone that has a set on their SBF would know that.

I sometimes think that describing the meaning of life would be easier and subject to less debate than describing tuning Webers. Your summary pretty well nailed it.

 

[Tim Kay]

Chuck is absolutely right, the most important tool next to a blade driver (it's nice to have an electric one), to remove the jets, may be the Unisyn air volume tool. A must have to complete the linkage \ sync procedure.

 

[John Fitzpatrick]

I'm rebuilding my HiPo 289 and have a new linkage, so I'm going to have to go through the whole set-up process again. It's been several years since I've done it and I've copied and pasted your post into my notebook to have on-hand when I get to that stage. Thanks for your well written summary!

I'll add one ssue that bit me--my Webers were used, and the previous owner had drilled out a few of the jets without re-stamping them, and one of the four float jets was a 250 while the others were 300s. Frustration is too weak a word!
So, with used Webers, it's wise to dissemble and check everything before you start if you don't know the seller . If you don't have a set of pin guages and a pin vice to check the jet openings, a micrometer and a set of metric drill bits works in a pinch.
I am not a Weber expert and don't play one on TV, but trust me on this one!
John

 

[Tim Kay]

Glad to be some help here. John, your right on the mark regarding used Webers. You never know what previous work has been done and your suggestion of check everything can't be overstated (I have a set of jets in mine right now I haven't got around to marking correctly). Once you go step by step 'from the beginning' it pays dividends in the final result.

My info is nothing but a cliff note version of great info you'll find on Jim Inglese's website: Weber Tech

 

[Cliff Beer]

Michael, before monkeying too much with your webers I would HIGHLY recommend a careful read of Pat Braden's "Weber Carburetors" book. It is an excellent technical resource, and also a good read from a laymans perspective in helping understand the theory and operation of webers, including your IDAs.

Once you clearly understand the function and purpose of each component of your IDAs the tuning becomes much less black art and confusion, and much more fun and straight forward. For years I struggled getting webers to run right for me.....now I only have to half struggle with it....and spend a little less time scratching my head.

[ame]http://www.amazon.com/Weber-Carburetors-HP-Books-774/dp/0895863774[/ame]

 

[michael sundler]

You guys have all been truly amazing in both the content of your help and your willingness to provide it! Thank you all. I now have a slightly more pressing problem relating to one of the tanks not delivering fuel (despite containing much) and the other leaking fuel into the sill! (Replica) GT40 ownership is never dull! I'll let you know how I get on - but again any thoughts on these new issues really appreciated!

 

[Mike Pass]

Many of the GT40 replicas have fuel tanks which are filled with "explosafe" which is a swarf like material.This is well known to be very good at blocking lines and filters. If your filters are not efficient it will get into your carbs and block jets etc. Check the inside of your tanks, filters, pimps and carbs to see if you have this issue. Also check for water as it can get in through the filler caps as it has been known to rain in England. I have had both these issues on my GTD.

Cheers
Mike

 

[Rick]

I found when I was setting mine up initially, that Webers (or any carburetor for that matter) will pop back through the stacks when opening the throttles if they are LEAN and pop in the exhaust if its RICH. 70 F10's seem a little rich for this size engine, what idle air bleed is being used. The emulsion tube choice only comes into play when the engine is running on the main circuit, it has no influence on the idle circuit.

 

[Ron R.]

Deleted.

 

[Earl J. Castillo]

Hi Guys,
I would really appreciate some experienced help in getting my CAV GT40 running well. It is fitted with a 347 Boss Ford Crate engine running with 48 IDA Webers and MSD ignition and on NGK BCR8ES plugs. The symptoms that I have are that the engine runs smoothly on idle but on (very) small throttle openings the engine backfires through the exhausts - suggesting that it is starting to run lean? The top end performance seems fine.
The carbs are set with the following jets:
Idle 70F10
Main 135
Emulsion Tube F7
Air Correction 120
Choke Size 37
Can anybody tell me if these settings seem about right? Thank you!

This is one of the best current reads on the Weber 48 IDA for a V8 application I’ve seen in a while. It’s still being refined by the Tennessee Tim the author. Better than most printed material out there.

http://www.clubcobra.com/forums/weber-tuning/123177-weber-tuning-tutorial.html

Cheers,
Earl J

 

[David Briggs]

The fuel pickups on the CAV's are flexible, try removing the the pick up lines from the fitting on the back of the fuel tank and pushing a phillips screwdriver down thru the tube to straighten it, also make sure the pick up line is not on the vent fitting.
Dave

 

[Ron R.]

This is one of the best current reads on the Weber 48 IDA for a V8 application I’ve seen in a while. It’s still being refined by the Tennessee Tim the author. Better than most printed material out there.

http://www.clubcobra.com/forums/weber-tuning/123177-weber-tuning-tutorial.html

Cheers,
Earl J

+1,

Was going to suggest it myself.

Like the others have said...start with the linkage, webers are very sensitive to the butterflies being out of synch. Very.