Weber selection and initial jet tuning

[240260280]

The large 36mm chokes may be the challenging item for street tuning.
 

 

There are a few items you will have to determine for the resolving the flat spot:

A. Is there enough fuel from the main circuit during the transition? (Is  the choke too large and the air velocity is not enough to bring in the main circuit early enough?)

B. Is there sufficient fuel delivered from the idle circuit during the transition? (Is the choke too large or the throttle plate too far out and the idle circuit falls off too early?)

 

A. For bringing the main in early you have the following items you can tune:

1. Fuel level: higher fuel level brings the main in earlier.  Stock level is ~ 29mm but you can push it you towards 26mm if you can deal with the other side effects.

2. New E tubes biased towards more fuel in the lower range.

3. Smaller chokes (increasing throat vacuum to draw more fuel)

 

B. For extending the idle circuit further out  you have the following items you can tune:

1. Adjusting the throttle plate so that it is as close to completely closed at idle (if your carbs have idle bypass balance passages you can use these).

2. Use a richer idle jet.

3. Raise the fuel level in the bowl.

 

I would not worry about the acceleration jets until you exhaust the above.

 

FYI A typical test method (for conducting on a flat road without a lot of traffic, stops, etc.) is to remove the 6 main jets and run around on idle jets.

• This exploratory real-world driving will tell you  how well the idle circuit works and its rpm range. (critically the rpm's and loads where the idle circuit stops contributing to going forward ).  You should be able to get a good idea of how much it is contributing to the transition circuit.  You may have to tune it to attain higher rpm range to help with the transition.
• You should be able to slowly accelerate to highway cruise speeds on the idle jets.
• Do this and tune the 3 items in B above to get the idle working nicely into the transition RPM's of 2000 to 3500.

<<< Warning: running with no jets makes your car tricky to drive as you have to stay in the low rpm range at all times. Do not do this test in traffic.>>>

Drive out to a quiet country road (typically the ones alongside rivers are flat). Pull your main jets on the side of the road when at the test location... enjoy.

 

 

 

 

 

 

Edited April 24, 2017 by 240260280

[blodi]

^ thanks! Great input.  

I guess the easier (cheaper) one for me to try first is B. I already raised the float height a bit. I set the floats at ~12mm when touching the ball valve.  I can order richer idle jets (one step/ 65F9?)

 I do have the air bypass screws on my carbs...so I can try setting up the idle using those...they are all closed now.  If those steps don't work...I'll attack the main/choke. 

[blodi]

Ok, just re-synced carbs by first lowering idle to ~600rpm then using air bypass valves to bring it back up to ~900 all in sync.  Test drive.....it's 75% better!  Still a bit of a "hole" in the transition, but certainly a big improvement.

I did order the 65F9 idle jets. So I'll install those when they come later this week and report back.

 

[240260280]

Great! 

Here is an old trick to visually ensure your throttle plates are set as closed as they can be. With the engine shut off, remove the inspection covers just over the progression holes (2nd picture below).  Shine a light into the holes or throat to see where the throttle plate is parked.  It should be at the first hole and the same for  all 3 carbs.  Try and get the top of throttle plate as close to engine as possible at idle so that only the idle mixture hole is working (not always possible).  This will ensure you sweep the full extent of the progression holes:

 

[240260280]

btw you can open up your idle enrichment screws to speed up idle with the throttle plates  closed. It adds both air and fuel at the same time.

For selecting the correct idle jet, you will have to prioritize the cruising and transition circuit  first as there is no adjustment for tuning this circuit this except by changing the idle jet size and fuel level. 

Once you get a good air/fuel mixture at cruise, you can go to the idle circuit and set the idle enrichment screw and air bypass to tweak the idle. 

Finally you do some WOT runs to tune the main circuit and select the main jet and air corrector.

It is iterative and there are some interactions between all of the circuits so it takes a while to get it all under control.

One of the most overlooked aspects of tuning is to get all of the slop out of the linkage and to ensure all throttle plates are opening at the same time and the same amount.  Make sure all 3 linkage arms are fastened at the same angle to the main throttle rod and that all rod distances are the same.

 

Edited May 18, 2017 by 240260280

[sweatybetty]

thanks guys. tuning the carbs is my next step

[Zedyone_kenobi]

The air bypass is a strange cookie. It is down stream of the butterflies correct?  I have left all of mine closed since I have been tuning webers. I always thought that I never needed them. I am not entirely sure of their function. 

[240260280]

Because both throats are controlled by a single idle set screw, sometimes wear or manufacturing variations cause one throat to flow more than the other at idle.  The bypass is used to balance air flow between the two throats.

 

Normally the bypass valves are closed off but if one throat is flowing more air than the other at idle, you can balance the carb by opening the bypass on the throat flowing less air.  This procedure is only needed at idle.

 

FYI: The old way was to drill small holes in the throttle plate to balance (see photo below). (btw the holes solder-up nicely if over-drilled).

 

Edited April 25, 2017 by 240260280

[blodi]

I checked the throttle plates vs the progression holes and the plate is closed past them at idle. New idle jets show up tomorrow and I am also getting a bung welded into the collector so I can get a better reading than my current tail pipe clamp on setup. I'll report back. 

[240260280]

Great! You are in business!

[gundee]

                                   The Magical Mystical Weber DCOE

 

                                     http://www.teglerizer.com/dcoe/

[Zedyone_kenobi]

I did some reading on the air bypass. Yup, I concur with your explanation. New healthy carbs should not need them open at all. But what an ingenious thing to include on a carb to account for wear. I know for a fact my butterflies are set perfectly and I have a good idle and not even a hint of a transistion issue, but I have not checked for balance since I initially set them up.  I may have a go to see how close they are now. 

The air bypass actually lowers the amount of vacuum signal seen on a flowmeter for those of you who are interested. So you take the highest side of each weber and lower it to the lowest side to ensure balance on both barrels. Then you have to match each carb to one another. 

 

Very simple and elegant. Love me some webers stuff. Just when you think you know all you need.  More info keeps coming in.