My 914/6 conversion with a 3.0, big ports, twin plug Patrick dizzy and MSD, 120/104 web cams, mahle rsr style pistons, 10.5 cr, with fresh 40 webers and a 1-1/2" MSDS headers.
I know I'm leaving power on the table and should get 46mm carbs and a bigger exhaust, but honestly on a narrow body 914 with 205 tires I can already light up the tires in 2nd and 3rd.
Just looking for some jetting advice to optimize the current set-up.

I have 34mm vents, 140 main jets, F3 emulsion tubes, 180 air correction, 55 idle, ported stock manifolds, custom phenolic spacers, K&N filters and rain hats. I have twin O2 meters on it and I'm running 30 degrees total advance. Floats are set and I have 3.5-4 lbs of pressure.

Right now it's running fantastic, almost don't want to touch it, but at 3000rpm cruise and at idle i'm getting about 12.5 AFR and if I kick it down or pull a steep hill at 3-5K I'm a hair over 14.7. I've been slowly reaming the main jets and I'm up to about 140 now. I think I'd like to get it to around 13.5 AFR?

Just keep reaming the mains?

You may or may not have seen this.

http://www.performanceoriented.com/

Check out the tech articles. I spoke with Paul several years ago about some issues I was having and he was very helpful.

Best I know of in the Weber business is Paul Abbott. www.perfomanceoriented.com

He built a pair of "42's" for my 2.7 engine. The car ran like a top "out of the box" with minimal tuning. Still haven't completely dialed in the idle mixture but the car accelerates with no transitions anywhere almost like fuel injection. FWIW; Paul did a pair of 40's for a '67 911 I own. Same story. Minimal tuning out of the box.

One of the tricks used is a set of trumpets/velocity stack/air horns purchased from Euro Carb. Ed Mayo was having difficulties with transitions during auto-x's and went to these per Paul's recommendation. Smoothed everything out and he's had no further issues from what I gather. FWIW; to those who don't know Ed, he's been around as long as I can remember, a great mechanic full of knowledge and still doing technical advice for the PCA Panorama magazine.

https://www.dellorto.co.uk/shop/car-accesso...mm-x-50mm-long/

Click to view attachment

Yes, I have his tech printed and in a book somewhere, the set up is per his recommendations, I just don't want to go reaming at the mains and upset what I have now.
Guess I'll have to dig it out and read it again over the next few days.

I hear ya, my engine is running perfect, no transition issues at all, just a bit lean for my liking at WOT and a hair rich at cruise. Plus a bit thirsty which is expected and I'm OK with.

My engine is a 2.7 with carbs and after some work by Paul is running perfectly. I have 55 idles, 145 mains, 180 air corrections, also with twin O2 sensors to help settings. my A/F's are 12 at idle, 12.5 up to high 13's mid range and cruising, and 12.5 steady accelerating. This gives me plus 20 highway and 16 around town miles/gallon. I suggest you try 145 up to 150 on your main jets.

Not exactly apples to apples but here is my experience. 3.4 twin plug running 46's and GE 80 cams. Made 290 hp at the wheels. Dyno tuned and made largest number with 185 mains and open exhaust. Recently converted track only to street hotrod and mufflers which probably cost me about 30 hp. Now using 165 mains and seems about right although I have not verified this with the dyno.

steve weiner says 26-29

my 2.7 settings with Richard Parr's help:

DC30/Mod. Solex (Dougherty) cams
Ports: int 35 mm, exh 38mm
Carbs: PMO 40
venturies 40 34
main jets 135
A/C's 185
idle jets 50
idle airs 130
E-tubes F1H

Main jets 145, I went further than the 13-13.5 I wanted, but I came out a pretty rock solid 12.5:1 AFR across the board except idle which is closer to 12.
It was interesting that my O2 meter, that was swinging wildly before, has now settled down to only swinging around a narrow point spread.

I have two meters, one is permanently wired, but the other I made a temporary cable that neatly goes over the rollbar. I will remove it, plug the bung and it will be my test cable for future tuning of other cars. As long as you have a good engine and carbs, you only really need one meter for tuning.

Thanks for everyone's help.

Doh - this thread made me realize I can't find my latest jetting notes.

Will be glad to share as well once I find 'em, tho it's a fairly extreme setup.

Along with all the other variables there are also the tall ('race') secondary venturis to consider as well as the short vs tall manifolds.

34mm vents seem small for the engine you have. Hotrod 2.7's seem to get the 36mm vents. I got a choke and jet kit from PartsKlassic for my 2.7 normal engine and they shipped me 34mm vents. Not that I know anything about carbs, I have to learn some of that black magic.

Jeff,

even for the twin plug?

yes twin plug

I actually ran 40's for a while and 38's now, but that is with 46s and pretty hot cams.

I am not an expert, but have studied the timing thing quite extensively. My understanding is that reduced advance is one of the big advantages with twin plugging. Since the flame front only has 1/2 the distance to travel you can reduce advance and get the same power and response

Less advance means lower ICP (internal combustion pressure) which means less engine stress, cooler temps, just as much or more power since the piston is not fighting so much pressure on its way to TDC

Sorry for the thread drift. 34 vents seem small. Pretty sure I am running 32's in my ported mod solex 2.4. Not sure how big you can go in a 40 IDA and still get good venturi effect to signal the mains in.

I cross posted this post on the bird and Steve at Rennsport likes 26 to 29 degrees total advance so I'll drive around with with my access hatch open and play with the timing.
Paul Abbott (Performance Oriented) chimed in about AFR and said you make the most power at 12.5 to 12.7 AFR (I'm at 12.5) he said I can play around, try to get it a little leaner and the motor will tell me what it wants. He also said it will always run a bit rich at idle (12:1) nothing you can do about it as it will upset low speed performance.

It's running real good now, beyond expectations, other than cold warm up you would swear it's FI. No bad habits at all, totally linear torque curve, no coughs, flat spots, transitions, etc., it can run like a common grocery getter, press the go pedal harder and it roars to life.
It's so freaking cool, once warm if I blip the throttle it sounds like a $500K supercar, orgasmic, I think I'll have to carry around baby wipes and an extra pair of undies.
Only bad so far is it's thirsty, but part of that is the operators heavy foot.

It's so good that I want to wait till I get my Micro 1000 head temp gauge hooked up before I play with the present setup. Then I'll try 36 vents and play with the jetting more just to see.

thanks all.

36 venturis will give you more power at the top end, and lose a little low end torque. I'm always one to err on the small side when it comes to venturis, unless you are building a true track car, a little too small will give better driveability than a little to large.

Sounds to me like you are at a good spot, and need to be wary of over wrenching on it. Log any changes you make, so you can go back to where you are now.

It's a street car, so drivability is paramount. That's why I went with the 34 vents, I'll likely leave it alone till I get the whole car sorted.

Right now the bigger issues are my tires totally suck and my brakes (front alfa brembos vented rotors, rear PMB v-calipers vented rotors, stock 17mm MC) seem a little soft although they work well.