Twin Weber Problems

[WhoIsSea10]

On 9/1/2022 at 8:00 AM, Hans said:

I meant in your diagnosis. Carl knows his stuff.

But best to learn on your own too. 

So carb #1 doesnt respond to mixture screws?

Carl is a legend. I just received my parts. Emulsion tubes, air correctors, main jets, idles jets, and venturi chokes. Going to install them today and see what I can do.

[Hans]

How are you synchronizing carbs?

[WhoIsSea10]

9 hours ago, Hans said:

How are you synchronizing carbs?

I have a common handheld synchrometer that I use to check each barrel separately. New jets and emulsion tube installed as well as a smaller 32mm choke. Definitely running better. I think I need to remap my timing still. i'm currently using a 123Ignition bluetooth distributor. Anyone have any suggestion on what it should look like?

[WhoIsSea10]

Update: Engine ran pretty bad with the new jets and venturi choke. I’ve reverted back to my original setup. I tried out a 32mm choke, but seems like the engine just wasn’t getting enough air. The theory was this would help by increasing some velocity to help mix the fuel and air.

[mcmurb]

I still think the throttle plates are open on 1 & 2.  The telltale sign was your first post reporting that the idle mixture screws were ineffective, meaning your 1 & 2 are sucking air/fuel around open throttle plates from the transition circuit, not the idle circuit.  That’s why people are suggesting checking the synchronization.  Best way I have found is to disconnect all the linkage and assure that the plates are fully closed on all barrels, then synchronize from there.  

 

[Son of Marty]

Are these brand new carbs? There are cases where the Spanish  Webers are incorrectly/incompletely drilled. The 32 mm venturis will provide enough air until your in the very top of the rpms.

[Hans]

25 minutes ago, mcmurb said:

I still think the throttle plates are open on 1 & 2.  The telltale sign was your first post reporting that the idle mixture screws were ineffective, meaning your 1 & 2 are sucking air/fuel around open throttle plates from the transition circuit, not the idle circuit.  That’s why people are suggesting checking the synchronization.  Best way I have found is to disconnect all the linkage and assure that the plates are fully closed on all barrels, then synchronize from there.  

 

I agree. It may not be a jetting problem. hence my questions. Although you did say it ran better before it ran badly.

What kind of common synchronizer? At what RPM are you synching? how are you adjusting carbs to synch?

What's between carbs and manifold?

Did you consider a fuel pressure regulator? Or at least check fuel pressure.

If I'm not mistaken, you need to just to run both idle screws in until they touch the base. Its kind of  dance.

Edited September 13, 2022 by Hans
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[Schon '02]

I recently went through the exercise of fine tuning the idle mixture/speed with dual 45 DCOEs. IIRC, the chokes are 34mm; Main Jets are 155; Air Correctors 180; Idle Jets 55F9s; cam is Schrick 292; Stahl header; throttle linkage is cable up to the balance lever which I use to synch the carbs. I'm able to get a relatively smooth idle at ~800 rpm considering the carb/cam/header set up.

 

A few lessons learned from my experience:

• Check for induction leaks at the intake manifold. My carbs do not have a support rod so they hang off the manifold. It has Thackery spring washers between the carb and manifold nuts to deal with vibration, and they do get hardened and will crack over time. So check for gasket leaks at the carb/manifold gaskets
• Check your idle speed lever(s) and set screws. The prior owner set the speed with the aft carb lever and used a return spring from this lever attached to the cowl; the front carb idle speed set screw was essentially disabled and it took me awhile to have an ah-ha moment with that one... I disabled the rear and now set the idle speed with the front and have the idle return spring pulling on the balance lever as well.
• Check that your return spring is fully closing both carbs
• Start the synch by setting the throttle plates evenly with the balance lever and observing the plates through the progression holes. Once they all hit the first hole at the same time, that is your starting bench mark. I found that my best idle/synch was with the aft carb lagging the front by just a hair. I did not use a synch meter... just good old fashioned eyes, ears and feel. The sweet spot is small.. maybe half a flat turn on the balance lever bolt, so patience is key
• I started idle mixture at 1-1/4 turns for all four cylinders and best idle is within 1/8 of that for each... again, I'm going old school by pulling plugs and checking color to fine tune as needed.

 

[Son of Marty]

Also check to see if your enrichment circuit is fully closed.

[Hans]

OP - do you have Spanish or Italian?  Where have you set timing?

Edited September 14, 2022 by Hans
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[Carlos_M]

I know how much of a PITA it is to get the carbs just right. If you want to chat send me a DM and I can tell you all the steps I went through and gotchas to finally get my carbs set up.  

 

Patients will be your virtue.

 

Carlos...  

[Son of Marty]

16 minutes ago, Carlos_M said:

I know how much of a PITA it is to get the carbs just right.

Not really, it's OK to cheat and copy what others have done, Toby Preyupy and Halboyles are worth listening to, between the 3 of them is about 100 years knowledge on fitting Webers to the m10. 

[Carlos_M]

5 minutes ago, Son of Marty said:

Not really, it's OK to cheat and copy what others have done, Toby Preyupy and Halboyles are worth listening to, between the 3 of them is about 100 years knowledge on fitting Webers to the m10. 

Those are my sources.   I appreciate all their guidance.  I've found each car is unique and can be a challenge.  Some are plug and play, not mine... it was a bit of trial and error.

[Son of Marty]

Agreed every engine will need to be fine tuned but these guys will put you in the ball park,  you need to follow their recipes exactly, nothing is close enough when it comes to tuning Webers, from the needle valve size, correct fuel pressure checked with a gauge, e-tubes to jet and both main and aux chokes, float level and in sync. But whatever the mods to your engine someone here has done it. not to mention timing and advance curve.

Edited September 15, 2022 by Son of Marty

[WhoIsSea10]

On 9/13/2022 at 2:14 PM, mcmurb said:

I still think the throttle plates are open on 1 & 2.  The telltale sign was your first post reporting that the idle mixture screws were ineffective, meaning your 1 & 2 are sucking air/fuel around open throttle plates from the transition circuit, not the idle circuit.  That’s why people are suggesting checking the synchronization.  Best way I have found is to disconnect all the linkage and assure that the plates are fully closed on all barrels, then synchronize from there.  

 

Makes sense, but do I go about ensuring they are closed all the way? Disassemble the carb? The linkage I have is really just 1 arm connected to the interlock joint between the carbs. Would i loosen the end nut and see if it shuts under the pressure of the spring?