engine and drivetrain Sidedrafts Tuning for POWER!

[MaTiaS]

Hi, I was wondering if someone could give me some advice on what kind of venturis and jets should I use to get the most juice out my engine. Im currently running on dual weber 40s, high comp pistons, schrick 292, ported 121 head w/ bigger valves. The venturies Im not pretty sure but I think they are 38s and the jets are 50 F8 and 190s. Thnx!

[M3M3]

get the SpeedoPro book on tuning webers....or any other weber tuning book. seriously. you need to understand how the carbs work before messing with all the options. understanding and changing stuff is most of the fun of having these carbs.

there is no secret method or combination of venturis/jets, it is just understanding how they work and applying that to your particular engine setup.

in general, bigger chokes move the power band up the rpm curve....up to the point where you run out of cam. jets are used to keep the right af ratio for the choke/engine combo.

[MaTiaS]

yes, a few weeks back I downloaded a pdf on dcoes and after reading it decided to use those jets instead of the old f11 and the 36 venturi that I had before, the problem is that I gained more mid range but lost torque at the lows. Bottom line the book says to swap jets and venturis until I achieve the desired curve. However before I start buying bunch of jets, tubes and venturis just to try on, I was hoping a little guidance on where to start and at least be closer in my goal to achieve good torque at low rpms, steady on midrange while the 292 kicks in and then more power after the 4000rpms with my engine setup.

[c.d.iesel]

nothing matters unless you have good, even compression readings,

and you FIRST SET THE IGNITION TIMING USING A TIMING LIGHT,

THEN leave the timing alone, then begin to alter the jetting

to suit your motor, temp, throttle operating range..... AND BUY AND READ ANY of the 4+ publications HOW TO TUNE WEBER CARBURETORS.

and venturi's of 38 are too big for you unless

your running at DAYTONA, MONZA, or, SPA.

HERE IS A STARTING POINT FOR YOU :

[Lorin]

Save yourself a lot of frustration and time and invest in a wideband 02 meter. And your chokes are too big. The general rule is to run the smallest choke size that allows maximum power.

[Lee]

The general rule is to run the smallest choke size that allows maximum power.

Small chokes = more torque and less power

Big chokes = less midrange torque and drivability - More power on top (providing you have the cam to go along).

But honestly, if the OP runs 292 cams, he should aim at torque not power. Things start changing with 304 which is still a mild track cam.

No need to go any bigger than 34-36mm chokes with a 292. And make sure you run a minimm of 9.5 pistons. I had 10:1 on a 292 and loved the torque.

BTW I even had some race-only 50 DCO-SP with big 46mm chokes and straight venturies. No workie on a torque motor.

[TobyB]

well, here's a stupid question-

where do you shift?

If the answer's 7300, then 36's are in the right ballpark.

If you're more of a 4500 sort of fellow, then the 28's will give you

a great vacuum signal and allow you to tune well.

A 38mm choke doesn't kill low end torque, it just makes it very hard

to TUNE for low end torque.

Generically, 32's are good for fast street- when I was limited to them

in the race car, it'd pull hard from 2k on up to about mid 6's.

When I went to 36's it started to pull at 35 and went up until rockers

started popping over 7500. But made best power in the 45- 65 range.

This is ALL dependant on rejetting to match. If you don't, then you don't get

as much benefit. Amusingly, a 2mm re- choke doesn't seem to move me

more than a jet or 2 either direction.

+1 on the WB02, btw.

t

[MaTiaS]

Yes, I think I made a bad choice going up too much on the venturis' size. I'll try to get the 34s on tomorrow and start from there again.

[boxy02]

I have a similar motor, freshly rebuilt. 9.5:1 piano tops 2nd overbore, 292 Schrick, IE shorty header, NGK plugs #7333, Crane XR700 on a mech adv distributor (need to find out the advance specs on it though), and dual DCOE (#151) 40mm Webers (Spanish) on TWM manifolds with individual linkage. I'm in the process of tuning them now and have read John Passini's book on Webers several times. There's alot of info on the 'net for tuning, but a book like Passini's will get you in the top part of the learning curve as far as how DCOEs work and how they can be manipulated to suit your individual needs. Although a Triumph site, this is a good link to get you started, too:

http://www.tjwakeman.net/TR/WeberDCOEinfo.htm

This is from a Pinto site:

http://reocities.com/MotorCity/9526/carbs.html

Having said that, here is what I'm running that got me in the ball park (I also have a wide-band o2 sensor that works with my LM1 AFR device):

34mm venturi, 4.5 aux venturi, 55F9 idle jets, 190 air correctors, F16 emulsion tubes, 135 main jets, 45 pump jets. My plastic floats are set to 12.5mm resting (top held vertically) on the ball of the needle valve and 25mm in full droop (held horizontally). Float specs from Centerline PDF link:

http://www.centerlinealfa.com/tips/images/weber_info.pdf

I know I'll have to go through this again if my distributor must be re-curved, but at least I can run it this way, with no spitting or backfiring, when I drive to my bodywork class.

Good luck with your tuning...can you post your successful "recipe" along with more of your engine specs to put it all in context?

Thanks!

[philv]

I have an IE 292 cam, IE header and 9.5:1 compression. Off the top of my head I do not remember what mains and air correctors I'm running but I do have 32mm ventures in my 40 DCOE's. I seem to be good to around 6200 rpms, give or take a little. Bottom end is decent and mid range is really good. Kicks in at around 3500 rpms and pulls strong.

[TobyB]

Warning on some of the Spanish carbs-

the machining is poo.

Let me rephrase- the machining is SHIT.

If you buy one or 2, measure the hell out of it. As soon

as you open the box.

I have a pair of 45's that have things off by so much that

the fuel and air passages drilled in don't even line up.

On ONE of the 4 barrels. So it had (as supplied) a huge air leak.

Which, I suppose, was why I got them effectively for free.

The castings are pretty good, the accuracy of the long boring is not.

Now, these are probably 5 years old at this point, so they may

be better now.

But I would still measure every port and passage if I bought a new one.

The old Italian castings are consistent within .05mm.

The Spanish, more like 5mm.

I'm not kidding.

t

[Lee]

The old Italian castings are consistent within .05mm.

The Spanish, more like 5mm.

One more reason to switch to fuel injection and ITBs ;-)

[TobyB]

...if your class allows it...

t

[M3M3]

Yes, I think I made a bad choice going up too much on the venturis' size. I'll try to get the 34s on tomorrow and start from there again.

are you sure you have 38's...for dcoe 40's? i thought the biggest size made for dcoe 40's was 36..... 38 is a dcoe 45 size.

[MaTiaS]

One more reason to switch to fuel injection and ITBs ;-)

I am pretty sure there are a lot of reasons to switch to fuel injection, in fact I had efi before I rebuilt my engine. However, I always found carburators, specially sidedrafts, pretty interesting... maybe the strong look it give to the engine bay, the amazing sound, or just the fact that I can customize them the way I want them with no need of computers