Dual DCNF jetting

ward00

ward00

True Classic
I am looking for suggestions on what a good starting point would be for jetting dual dcnfs that I am about to install on a 2 liter engine with 9.8 compression and 40/80 cams.
 
Great timing on this as I just picked up a set of 42 DCNF's and need a similar recommendation for where to start. Are your carbs 40 or 42?
 
I have a stock 1756cc engine with 40/80 cams and dual 42 DCNF that came jetted from Bayless ($25 extra!) in the early 1980s. I'll see if I have the specs written down somewhere.
 
ward00 said:
I am looking for suggestions on what a good starting point would be for jetting dual dcnfs that I am about to install on a 2 liter engine with 9.8 compression and 40/80 cams.
Click to expand...

Thanks for including your compression ratio and cam specs. Also important are your preferences for drivability. If its a 'track day' car, your tuning might favor wide open throttle WOT. If a large percentage of (80%+) your drive time or miles is daily drive or touring you should favor tuning for that use.
Once you realistically determine your intended use, then select the right size carburetor (butterfly (throttle plate diameter). Then invest in 4 chokes the right size for your use. Smaller chokes give better low rpm response. I have found these books to be helpful in selecting the correct sizes:
"Tuning Solex Carburetors for Performance" by R C Pack
"Weber Carburetors" (Owners workshop manual) Haynes Publishing
Since you are in the 'about to install dual carbs' part of your project, you should consider in investing in an A/F ratio guage and installing the required oxygen sensor in the exhaust collector after the 4 into 1 merge.
I have been fine tuning Fiat and Lancia Weber carbs for 15 years using these techniques.
Feel free to PM if you desire to explore this further.
 
johnph said:
Thanks for including your compression ratio and cam specs. Also important are your preferences for drivability. If its a 'track day' car, your tuning might favor wide open throttle WOT. If a large percentage of (80%+) your drive time or miles is daily drive or touring you should favor tuning for that use.
Once you realistically determine your intended use, then select the right size carburetor (butterfly (throttle plate diameter). Then invest in 4 chokes the right size for your use. Smaller chokes give better low rpm response. I have found these books to be helpful in selecting the correct sizes:
"Tuning Solex Carburetors for Performance" by R C Pack
"Weber Carburetors" (Owners workshop manual) Haynes Publishing
Since you are in the 'about to install dual carbs' part of your project, you should consider in investing in an A/F ratio guage and installing the required oxygen sensor in the exhaust collector after the 4 into 1 merge.
I have been fine tuning Fiat and Lancia Weber carbs for 15 years using these techniques.
Feel free to PM if you desire to explore this further.
Click to expand...
Yes, I plan to install a/f gauge. Curious where in the exhaust system you installed yours. I will mainly be doing spirited road driving.
 
Why not go with the jetting we all use on our street 1800/2000 spiders with dual 40 IDFs?

30 mains
210 air
32 venturis
F11 emulsion...I think.
 
carl said:
Why not go with the jetting we all use on our street 1800/2000 spiders with dual 40 IDFs?

30 mains
210 air
32 venturis
F11 emulsion...I think.
Click to expand...
Are the mains 130? 30 would be small even for the idle jets. For my DCNFs on the X, I'm running 1.30mm mains with 2.20mm air, 0.50mm idle, F36 emulsion tubes.
 
In light of the cost of parts, I like to look at what jetting you have first, to see what can be used as a starting point.
Then make some necessary changes for an initial starting point and do the rest when the engine is running using an A/F gauge and possibly a dyno.

Spending a bunch of money on jets that have worked for someone else is not the way to go IMO.
These carbs can be tailored to YOUR specific engine and I would make sure to do that.
 
Sorry, meant 130 mains. Air/fuel gauge is the only way to fly for setting up jetting. To defray costs you could ask for specific jets to borrow from members for set up and then return them.
 
The carbs came with a full set of jets, chokes etc, but they were uninstalled. I just need a starting point for jetting/chokes etc. From there I will know what to do.
 
Got this jetting from Facebook with 33 chokes. Thanks
 

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ward00 said:
Got this jetting from Facebook with 33 chokes. Thanks
Click to expand...

You definitely do not want 2.5 aux venturis., the number refers to the size in mm cross section of the opening where the fuel is drawn thru. Most times a 40DCNF will have a 4.5 and this size seems to work best. A smaller sectional area will bring on the main circuit earlier.

With 40DCNF's you're limited in the max venturi size you can use and still get good atomisation of the fuel. Venturi sizing ideally wants to be between about 70 and 80% of butterfly size for best results, so you're limited to around 32mm.

Everything will revolve around the venturi size you settle on.

If your carburettors came complete, then perhaps list the items you currently have.

DCNF's also respond very well to the fitment of bellmouths, consider getting a pair.

SteveC
 
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