1800cc Engine Weber 40 DCNF Jetting

[Bruce Fogerty]

Guy, love the new site and I think your rules are great for keeping everybody professional and informative with each other. I've bought a few things from you before (Red Book, full GC engine fastener kit, GC uprated clutch kit) and have talked with you on the phone, and could do so again, but I figured I'd post here so others might be able to learn as well.

I have a Scorpion/Monte with the 1800cc engine and I'd like to get your ideas on starting points for jetting my 40 DCNFs. I'll be using it for a hot street/limited track day car. My engine specs:

* 40DCNFs on NOS Alquati manifold, Monte Hospital Air Box ducted to standard air cleaner cylinder.

* Fully ported head and intake manifold, work done by a well respected Fiat race engine builder in Seattle, Washington, USA. 43mm Int, 37.5mm Ex stainless valves, triple angle valve job.

* CSC complete 4-2-1 exhaust system (bought quite a few years back just before your stainless system Chad sells came out, I was mad when I saw and heard about it too late!)

* NOS "true" 9.8 CR cast pistons (first oversize) from Mondial/Mahle, these have the 4mm high domes with a 63mm base tapering to about 43 - 48mm at the top.

* Entire engine fully balanced to race specs, inc. your clutch assy., lightened flywheel, crank, pulley, rod/piston assys. to within .2 gram overall difference.

* 40/80 Alquati cams, International Auto Parts sourced cast steel pullies with the large cast in cutouts.

* All the air pumps & other smog stuff gone.

* Right now the carbs have 32mm chokes, 125 mains, 200 air correctors, f24 emulsion tubes, 65 idles, 45 acc. pumps.

I've gotten advice from different folks on the choke sizes and jetting, I've had people insist that anywhere from 30 - 34 chokes are the way to go, I settled on 32s because the carbs came with them, but I'll change if needed. I live in Portland, Oregon, USA, which is roughly sea level and has a climate much like that of the UK.

Your learned advice is much appreciated!

[Guy Croft]

Hi Bruce!

Welcome and thank you very much for laying out that Q&A post in such a clear and detailed style! I am very pleased you like the forum.

My comments on your tuned 1800 TC Weber 40DCNF carbs:
- 32mm chokes - certainly could run with 34mm chokes, would give more top end power with sacrifice of a little bit of torque below 3000rpm. Nothing to speak of really. But if you are happy with the power over 5000rpm leave alone.
- 125 mains - probably correct.
- 200 air correctors - may be a bit lean over 6000rpm, might want to go down to 195 or 190.
- F24 emulsion tubes - not my preferred choice, I'd be on F21
- 65 idle jets - might be OK as the idle mixture screws are adjustable, so may not matter, but I'd use 55
- 45 accelerator pump jets - OK

As an off-the-shelf thing the CSC header is OK but I would get rid of their muffler, get a straight-through one made of comparable dimensions and with a 2.25" perforated bore, 3.1mm holes, and glassfibre packed. That will be more effective - no drumming and nicer note. After you've got the new one, cut up the CSC and you'll see why I don't like them.

As with all things accurate idle mixture settings and carb balance is imperative. As the engine is quite highly tuned, I would do a rolling-road proveout to assess the mixture, you should be getting 3.8-4.5% CO under load and well under 400ppm HC. At idle, 2.5-2.8%CO and 150ppm HC should be achiveable if oyu're setting up on a gas analyser, although really, if you can do it, the idle mixture screws are best set so as to give the highest idle (750-800rpm) power for the least throttle opening. Run the engine too fast when setting them and the carbs will go onto the progression drillings and you'll never get them calibrated.

That engine should produce peak torque at 5800 and peak power at about 7200rpm, quite comfortably. I would not go much over 7200 if the engine has standard valve springs.

I hope this helps for now,

GC

[Bruce Fogerty]

Hi Guy!

Thank you very much for replying in in such a clear and detailed style! What a quick response, talk about instant gratification! I thought you'd have to look specs up & calculate, etc, should have known you would have experience with this set up and can make instant recomendations.

I will get the engine set up on a rolling road, but they are not too common here in the states, and they are usually staffed by V-8 muscle car guys or Japanese import guys with knowledge of Holley 4 barrel carbs or Honda fuel injection chipping, etc.. Not many folks around with knowledge of Weber carbs. I will have to drive the car up to Seattle about 300kms away to have the engine builder meet me at a rolling road up there to set & jet the carbs. I wanted to get a good baseline set up from you to avoid over fueling or running too lean during break in and driving up there. Your advice will get me on the road and I'd bet pretty close to the optimum set up, so thanks for that.

Now you've got me thinking about your GC Monte exhaust (again!), maybe I'll have to sell the CSC and think about one of your units if you think the muffler could use changing. No use messing about with a CSC if I'm going to have to discard half the system anyway. I'll give it some thought. Freight would expensive from the UK, though.

Any (rough) guess on the torque and power output potential of this engine combo? You say about 140bhp in the Red Book, but that's on an 1800 fuelled with 45 side drafts. I was thinking I wouldn't be able to get that kind of output from the more restrictive down draft set up. Your thoughts on this would be appreciated.

I won't have the car on the road 'til next year at the earliest, but after I get it on the rolling road I'll post the results to this thread for everyone's future reference. I'm betting your guess will be pretty close if we factor in the drive train losses on the rolling road.

Thanks Again,
Bruce

[Guy Croft]

Hi Bruce

in view of the distance to a rolling road, I will talk you thru how to prove out the jetting by road testing nearer the date, you'll never need to go there then, OK?

Re your other questions, I'll refer later, if I forget, pm me.

GC