Weber 40 DCOE Synchronization, Help!!

halboyles

halboyles

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I've got my brand new 3.5 L engine (Terry Sayther build) with 9.5:1, Schrick 284 cam, dual valve springs (@sfdon head build), . . . installed and running. 40 DCOE 151s used on the original engine for less than 3000 miles and a recent dissemble, clean and new gaskets. Valves adjusted and checked twice to 0.011". 123Tune BT distributor loaded with @Stevehose sidedraft curve, dynamically checked at 2500 rpm with 24° advance at the flywheel ball. Starts right up and runs, albeit a little less smoothly than I'd like. Idles pretty well at 1000 rpms. Now have about 65 miles on it with compression at 210-215 psi on all six cylinders. Pulls like a tiger and sounds like a jet engine!!

All well and good, but I can't synchronize the three DCOEs because the STE synchrometer shows 10 on #1 and only 3 to 5 on #2 and #3. All six butterflies line up at the same spot on the first progression hole. I'm using our M30 Triple Side Draft linkage so the only external adjustments are on the carbs (idle mixture and idle speed screws) and balance levers. If I try to match #2 with #1, I have to adjust the #2 balance lever so much that the idle speed can't be lowered to less than about 1500. Even with the main linkage disconnected, I get the same results. I have never run into this before.

So, the question is, what could cause #1 to have such a different reading than the other two carbs? Valve clearance is the same, compression is the same, jets/chokes are the same, idle mixture is the same (about 2.25 turns out), . . . The only thing that might have changed since I last used the same setup on the old engine, is that I, personally, might have aged. Just a tad.
 
Is only one idle air bleed screw per carb being used? One of each should be closed.

Get a large piece of cardboard and put it between carb #1 and the fan in order to block the fan air flow from getting to #1 venturi.

Which curve are you using? I've tweaked that over time.
 
Finally had a chance to look at this again and decided to configure and install a new 40 DCOE 151. No change at all in the synchronmeter readings. Redid a compression test and leak down test with all consistent readings and no leaking. I am beginning to think the new head with Schrick 284 is the problem though the valves are all set to the recommended clearance.



BTW, disconnecting all the linkage before the carbs and at the throttle shafts resulted in no change.
 
Last week I also faced the task of synchronizing Webers while barbaqueing in Kauai... not easy :).
 

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halboyles said:
BTW, disconnecting all the linkage before the carbs and at the throttle shafts resulted in no change.
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so by isolating the carb, and backing the idle speed screw all the way off the linkage piece, it still doesn't affect the rpm? Do you have vacuum take-offs for each cylinder to compare readings with the other carbs?
 
Are you sure all three throttle plates are in exactly the same position?
 
I really want to thank everyone for the great ideas in troubleshooting this very odd problem. Mid-America 02 Fest consumed much of my time last week and, thus, I have had no chance to test any theories.

It will take me a while to catch up on orders and correspondence, but I will once again have the car in my face to check out some of the possibilities proposed. As I mentioned in the original post, I had already tried many of the proposed solutions before writing up the request for help, but I will start over with the basic setup before proceeding with some of the other more exotic possibilities.

While down in Eureka Springs I did check with some DCOE pros and received suggestions ranging from "It's the cam!" to "Impossible!" Terry Sayther very sagely suggested I take to heart Sherlock Holmes famous adage that "when you have eliminated the impossible, whatever remains, however improbable, must be the truth."

Thank you everyone again and I'll inform the group when I have resolved the "improbable truth".
 
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