Crossflow Tuning

[new2 7s]

Hi,

 

I recently took my car out for a short drive and under 1/4 to 1/2 throttle application the motor stubbles.

The configuration of the motor is:

1700 Crossflow, 244 Kent Cam, 4 - 1 headers, dual 40 DCOE carbs, Dry Sump oil system, Pertronix Distributor.

I have a 4-6 psi Facet fuel pump with a pressure regulator set at 3 psi

 

Does the stubbling acceleration come from a lack of fuel or the timing being off?

 

 

[Cherik]

I used to run this exact configuration and found the setup to require a few sequential steps to address this.

Depending on the altitude you will need to correct a bit, but 1/4 throttle A/F is all set with the "idle" mix screws.

They are really for all 1/4 throttle and below settings. The progression circuit is for 1/4 to 1/2 throttle.

The main jets matter from 1/4 and up.

Smaller venturies (30mm) make for very strong low end (

32mm is probably about right for that cam and exhaust, 34 and up is large. Weber recommends 80% diameter

for good low and mid range fuel mixing.

 

1. Assuming idle timing is 14dg and climbs to around 20dg at 2000rpm and lands somewhere around 28-30 at 3500.

 

2. Set the idle with the throttle screw to around 1500rpm

 

3. check the flow in the 1/2 vs 3/4 carbs - adjust until absolutely equal - use the-hose-and-ear method (1/2" hose in each barrel and listen for equal noise)

or best is a flow meter. Avoid using a vacuum meter on these large carbs, it's not accurate.

Flow meter I used is the German STE Synchrometer model SK. Set flow to about 5kg/hr. Take several turns to do the balancing.

 

4. stop the engine and while keeping track of the turns close all the mix screws gently - note these are tapered screws, so if you overtighten them, they'll deform.

 

5. reopen them 1.5 turns careful to note 1/8 turn increments - use the slot as a guide.

 

6. restart and leave idle around where it was (1500rpm). If it's slow, crank it back up to 1500, if fast slow it down

 

7. If it's rich, it'll run "heavy" and slow, if it's lean it runs fast and "fizzy" and will spit

 

8. Settle it until it runs smooth - keep readjusting the idle back to 1500 and rebalance again

 

9. once happy add 1/16 of a turn open to ensure you stay on the rich side of things.

 

10. now set the idle back to 1000..1100rpm (Crossflow oilpumps are large clearance items and should not run

 

enjoy

Cherik

http://my.voyager.net/~quadrant19/index.html

 

[11Budlite]

Holy crap Cherik, are you sure you're not still running a crossflow!?!?!? Nobody knows that much off the top of their head!! :jester:

[Cherik]

ha! My very supportive wife thinks I'm married to the car.

I spent 28k miles with that Crossflow and the procedure was repeated until it was so well choreo-graphed, I could do it in my sleep.

 

The Zetec is so much easier, just put gas in and drive, and only tinker with the map when the season changes.

 

Cherik

 

[xflow7]

I don't have it so "procedureized" :jester: but that's about the process I go through with my 135HP 1700cc (not sure what cam), albeit typically at ~1000rpm (after balancing the carbs at 4-5 kg/hr as Cherik suggests).

[lowflyer]

I agree with everything you said, but you did leave out a couple of things.

 

First, the procedure you're using will give a balanced idle, but doesn't address the running of the car, once off the idle circuit.

 

The orginator of this thread didn't mention the altitude at which he lives, or the jetting that's in the car.

 

It sounds like the car idles O.K., but when going onto the main circuit it's running either too lean or too rich.

 

Does the car run O.K. at a steady state speed, say at 3000 rpm? If it's surging, it's probably too lean. If it's got a stumble on acceleration at that rpm, it's most likely running rich.

 

And I haven't even mentioned the pump circuit!

 

And after all of this, remember the old adage, " 90% of carb problems are electrical".

[Cherik]

all true.

I assumed he was talking about 1/4 throttle and that it ran ok past that (main jets / mix jets).

The standard jets that C'ham (Grand Prix Enterprise/London) puts in the Webers are quite good, but that can't do anything below 1/4 throttle, which is at least what I experienced.

The transition circuit (progression holes) are fine on the DCOE 40's, but not the 45's - John Hartley had to drill a few extra's to get those to run properly.

 

Cherik