engine and drivetrain DCOE main/air tuning

[LateApex]

I'm close to getting my DCOEs dialed in but I have a question.  My WOT A/F is fine (12.5-13.0) but I am rich at highway cruise (4,000-4,500 RPM).  I'm currently at 135 mains with an F16 emulsion tube and 155 air corrector jets.

 

I believe I need to down-size my mains to bring my highway cruise into an appropriate A/F, but do I use the emulsion tube or the air corrector to add more fuel to compensate for the smaller mains at WOT?

[TobyB]

Try the 'idle' jets first...

 

t

 

ps, smaller airs= richer WOT at high rpms, larger mains=richer WOT at lower rpms

 

 

[LateApex]

Toby,

 

I will admit, my idle jets are large (65s) but I was 2.75 turns out with the 60s so I upsized them.  My A/F at, say, 2,500 is fine (~15).  Shouldn't I be in the "main" circuit by 4,000+ rpm?

[Stevenola]

Which model carbs?  How rich is rich at cruise?

[LateApex]

13 hours ago, Stevenola said:

Which model carbs?  How rich is rich at cruise?

DCOE 45/152 .    At about 70mph (~4250 rpm) I am under 12 A/F ratio while at WOT I'm about 12.5 - 12.7 throughout the upper revs.

 

Chokes	34
Aux Venturis	4.5
Main Jets	135
Emulsion Tubes	F16
Air Corrector  Jets	155
Idle Jets	65F9

 

Edited June 3, 2016 by LateApex

[TobyB]

You'd be surprised how much the 'idle' circuit affects cruise, especially with big carbs like that.

It just doesn't take all that much power to maintain a steady cruise until you hit about 90.

I am not versed in the black art of the emulsion tube, but in your case, especially since you have the

60 idles, stick them in and see what happens.

Don't be afraid to let the idle screws hang out- if you're watching your ratios, the 'common wisdom'

is just a guideline.

 

I agree, your mains are large, too, and the air correctors small, but your chokes are also pretty big,

so the 'signal' at lower flows is pretty small- thus the big jets. 

Mains are pretty cheap- you could certainly toss in some 130s and see what happens.

 

13:1 is fine for power and safety- I was running the race car as lean as 13.7 and it kept making more power.

I was chicken to let it lean out more than that for any length of time.  Soft pistons, soft head, empty wallet, you know.

 

t

 

 

[LateApex]

3 hours ago, TobyB said:

...............

 

I agree, your mains are large, too, and the air correctors small, but your chokes are also pretty big,

so the 'signal' at lower flows is pretty small- thus the big jets. 

Mains are pretty cheap- you could certainly toss in some 130s and see what happens.

 

13:1 is fine for power and safety- I was running the race car as lean as 13.7 and it kept making more power.

I was chicken to let it lean out more than that for any length of time.  Soft pistons, soft head, empty wallet, you know.

 

t

 

 

I had already tried 60F8 idles and 125 mains and got ~16 A/F at an easy 2,500 cruise.  Lots of pops & hiccups, too. WOT was 13.7. In fact, both the 55 and 60 idles resulted in that hiccuping, lean cruise at, say, 35-40mph no matter which mains I tried. 

 

The fat 65 idles cured that.  The current configuration is good everywhere but that over-rich 70mph cruise.  I guess I'll try the 125 mains to see if that helps at 70mph.  If it starts to lean out nearing 6,000 I'll try smaller air correctors. 

[TobyB]

Yeah, I have run into that, too, where I was choke- limited- the smallest air correctors I could find.  Like, 110's or something.

 

Try 130 mains.  A jump of 10 can be HUGE when you're in the ballpark.

I made some '127's' once by very careful drilling.  125 was too small, 130 too large.

 

Also, how would you feel about smaller chokes?  They'd improve your metering.

 

Jus' thinkin'.

t

 

 

[LateApex]

1 hour ago, TobyB said:

.................

 

Also, how would you feel about smaller chokes?  They'd improve your metering.

 

This is not intended to be a daily driver, more of a streetable track car, and, consequently, I have been prepared to deal with certain tune issues on the street.  My goal is to get the WOT right, and the rest "as good as can be expected".  However, I would rather not foul a set of plugs every time I drive to an event.  Since this is my first crack at Weber side-drafts, ultimately, I am trying to get a handle on the approximate RPM levels of the chart below and the expected changes resizing the respective variables makes. 

 

It's completely possible I'm chasing my own tail due to having an A/F gauge, haha.  It is a time saver though.  How long would this take relying exclusively on plug reading and my nose?

[Guest_anonymous]

what does an F9 e-tube do for your situation?

[LateApex]

15 hours ago, dar_on said:

what does an F9 e-tube do for your situation?

 

Correct me if I am wrong, but isn't an F9 richer at the lower end of the main circuit than an F16?  Seems like that would worsen my "too rich at 4000 rpm" condition.

[LateApex]

I am a little embarrassed to ask this, but when setting up my carbs, I removed the cold start assembly from the back of the carb, but did not make any internal changes to jets or plungers.  Could this be a factor in my needing such a rich idle jet?.

[Guest_anonymous]

1 hour ago, LateApex said:

 

Correct me if I am wrong, but isn't an F9 richer at the lower end of the main circuit than an F16?  Seems like that would worsen my "too rich at 4000 rpm" condition.

Not if you reduce the slow circuit

[TobyB]

As long as the plungers are down and sealing, it shouldn't matter.  But I think they need the cover with the levers

to keep them down on their seats... 

 

Your numbers seem 'in the ballpark' for normal tuning issues, so I bet they're ok.

 

t