Question For Carb Guys - Drilling Hole In Carb Throttle Blades

[enigma57]

Thanks, pamotorman. I will make a note of that. I may be able to use smaller holes in throttle blades if running a smaller cam. This is a road car, so will likely run about 20 - 25 degrees less duration @ 0.050" than a short track cam. Will have to experiment a bit on hole size. Will start with 3/16" holes and increase size from there, as necessary.

Best regards,

Harry

[enigma57]

Thanks for your comments, naukkis79. Why am I running carbs designed for IR intake on plenum intake? Good question. The short answer is that I am getting on in years and this engine will likely be my last build. And I have a set of Weber 42 DCNF carbs original to an early 1980s Maserati that have been sitting here for over 20 years which I have no other plans for. So thought I'd rebuild them and use them on this engine.

I have run DCNF carbs on several plenum style intakes previously and have worked out a good baseline jetting, choke tube size and emulsion tube combination for this engine displacement.

Drilling holes in the throttle blades is necessary when running these DCNF carbs on plenum intake in order to establish the correct relationship of throttle blade to progression holes/transfer slot at idle (tickover) speed. Else the progression holes will be exposed sufficiently to begin premature enrichment of the air/fuel mixture. Jetting down won't cure it because with Weber, the 'idle' jets are really 'slow running' jets that the engine runs on until transitioning to main jets (high speed circuit) in the 2,500 to 3,000 RPM range (depending upon how carb is set up).

I have found that DCNF carbs require a hole in throttle blades of around 3/16 inch (2.35mm) diameter to allow correct throttle blade position without beginning to uncover progression holes at idle speed. This, even with stock or mild performance cam grind. For a hotter cam grinds where manifold vacuum drops below say 10" or 12" of vacuum at idle, a larger hole must be drilled (requires some experimentation on a case by case basis).

The other reason I am using these carbs is because I enjoy tinkering with one off carb setups (a hobby of mine) and this one should be a lot of fun. Engine is an antiquated inline 6 with siamesed intake ports, else I would have built a true IR intake for it. Insufficient cross section to make a port divider and isolate each intake valve in head though, so must go with plenum style intake having 3 carbs, each feeding 2 cylinders. Engine displacement is 4.8 litres, so each carb supplies 2 cylinders of 800cc displacement each (1600cc in total). I'll let you know how it works out.

Best regards,

Harry

[Geoff2]

3/16" seems huge for bypass holes for 4.8L & mild cam.

[1] I ran a pair of 48 IDFs on a home made adaptor mounted on a Holley single plane intake on a 440 Chrysler that a had a SFT cam of about 240* @ 050. I don't remember the exact size of the bypass holes, but nowhere near 3/16".

[2] Twin 44 IDFs on a 4500 Victor intake on a drag raced 455 Pontiac. Bypass holes were nowhere near 3/16" & this had a sizeable cam, about 9-11" vac at idle.

[naukkis79]

The other reason I am using these carbs is because I enjoy tinkering with one off carb setups (a hobby of mine) and this one should be a lot of fun. Engine is an antiquated inline 6 with siamesed intake ports, else I would have built a true IR intake for it. Insufficient cross section to make a port divider and isolate each intake valve in head though, so must go with plenum style intake having 3 carbs, each feeding 2 cylinders. Engine displacement is 4.8 litres, so each carb supplies 2 cylinders of 800cc displacement each (1600cc in total). I'll let you know how it works out.

Isn't that be extremely undercarbed? I see why you need to drill holes to plates, 42DCNF carbs are made for fueling something like 400cc per cylinder when used in IR setups. With 2 barrels to cylinder it might be enough for 800cc but for 1600cc those carbs are way above their design point.

If you want to use those carbs it might we worth to make common plenum for all 6 cylinders.

[enigma57]

I believe it will be enough carburettion for this engine, naukkis79. I have worked with a single DCNF carb on common plenum intake feeding 1700cc at similar redline RPMs and it was large enough. On my 4.8 litre inline 6, each of the 3 DCNF carbs will feed 1600cc. I will also run a large diameter balance tube connecting each of the 3 plenums. Plan on installing a ball valve in each interconnecting balance tube so I can experiment with how she runs with these valves closed, open all the way and partially open. Will let you know how it works out.

Best regards,

Harry

[enigma57]

3/16" seems huge for bypass holes for 4.8L & mild cam.

Right you are, Geoff. I got hung up in the math converting from metric to English measure. I meant to say 3/32" (about 2.35mm). That has worked for me in the past on 1700 cc engine running a single DCNF carb on plenum intake with a mild cam.

Merry Christmas to you and yours,

Harry