engine and drivetrain Quick Jetting Question

[Dabliz02]

Thanks William!!!

 

I hope I will have a rocket ship!  I am happy either way to get it this far within a year of starting the project (start date- June 7 2012).  I'm happy I can drive it around my city with no issues.  And I am happy to have such a great community to help!!!

 

I have ordered idle jets 40f9- 55f9 ( so I don't have to order again) just now so they will be here in a couple of days.

 

Question to grow knowledge-

 

Would it the more progression holes the better or it varies by application?  And are they changeable (From what I see I would say no).

[williamggruff]

The purpose of the progression holes is to add a mixture of air and fuel to the motor that gradually, or progressively, increases fuel as the throttle plates open.  As your throttle plate opens, it sweeps past the progression holes one by one, and a mixture of fuel and air is pulled through the progression holes, into the moving column of air, and into the motor.  The greater the number of progression holes, the more smoothly the increase in fuel can match the increase in the air flow.  

 

Your carbs have 3 little step functions to smooth out the air/fuel mixture between the idle circuit and the main circuit.  You'll note that as the throttle plate passes by one progression hole, it begins opening another. 

 

My early 45 DCOE 15/16s (carbs that were stock on the 1800 TiSA) have two large, similar sized progression holes, and there's a substantial gap between the back edge of the first hole and the front edge of the second.  So there's a portion of the progression circuit where the throttle plate is opening, allowing more air to flow through the throttle body, but there's no new progression hole being uncovered to provide a new trickle of fuel.  The DCOE 15/16 has two large, similar diameter progression holes rather than two different sized holes.  It appears that the large first progression hole is intended to feed enough fuel to overcome the gap until the second progression hole is exposed.  This gap in the progression circuit makes tuning of the 15/16s rather challenging, as there tends to be a stumble between 2500 and 3500 rpm.  Increasing the size of the idle jet will eliminate the stumble, but the motor will run at 10.x AFR when it's cruising.  Changing from an F16 emulsion tube to an F9 (richer) or F2 (richer still) hasn't resolved the issue on my carbs, but I'll see if recently replacing the throttle plates, shafts, bearings, and bushings has made any improvements.  

 

Over the years, Weber has gradually increased the number of progression holes in the 45 DCOE 152.  Some DCOEs have 4, others 5, and some have been modified to have 6.  From what I've read, there are 152's with 5 progression holes that were produced specifically for high-revving 4 valve motors.  I haven't seen a pair, but would be interested in trying them out to see just how much more tunable they really are.

 

As for changing progression holes, this should only be done with proper machine tools (like a computer controlled Bridgeport), and even then it would be rather extreme.  You shouldn't need to do anything like that with your DCOEs.  You should be able to get them tuned to give you smooth responsiveness throughout the RPM and throttle range.

 

 

 

 

[TobyB]

 

 It roughly 34-36 @ 1100-1300 rpm.  Revved it up to 3500 and it was still at 34-36.

 

What Byron said- there's no point going any further before you fix the mechanical advance in the distributor.

 

Really.

 

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