2002Scoob

DCOE 40 Folks, What size Pump-jets are you running?

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At the moment my jetting is pretty dialed on Brunhilde- Idle, progression circuit, all the way to top-end is all good. 

 

But now that I'm running higher-compression and have moved beyond break in period of the new motor and have started to drive the car harder, on occasion when from a stop and giving it a little bit of beans she'll get a lean stumble and fall flat. 

 

I'm assuming it's very likely the accelerator pump-jet circuit, as I've had the same 35 pump jets in the car since I had the stock bottom-end. According to the books, it's recommended to have #40 APJ's and #70 SV's. So... already a bit lean to start with.

 

I'm trying to avoid buying more jets than I've already got (I've got a big small-parts organizer full already!!), so I figured I'd see what other's are running or recommend. :) I know, I know. Search. I could. But I'm busy. And who doesn't like to discuss a little DCOE now and again?

 

Car currently has 32mm chokes, will be upping that to 34's in the spring-or tomorrow if someone has a good starting point to recommend for the rest of the jet recipe...And yah. I could probably search that too, haha. 

 

292 Cam.

9.5:1 pistons, but with a min. decked head.

 

*Wonders where that leaves me in real-world compression...

-J

 

 

 

 

 

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If it's going lean when you ease into the gas it's not the accelerator pump circuit, the pump on dcoe's will only squirt gas when you step on it hard if you ease into the gas the pumps bypass back into the float bowl unless you replaced the bleed jet with a blanked off jet (sealed) and this configuration is usually only found in monster v8's. 

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Hi,

Nothing against a DCOE talk, always room for change and maybe improvement. Is it going lean within the range of the idle circuit or just momentarily when you deck it?...are the carbs new (sure the pump jets are doing their thing)?

Andrew

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Nah, it's not when I'm easing into the throttle, it's when I stab it from a stand-still. Sorry to not clarify. 

 

Around town on the progressions and when driving aggressively it's perfectly fine. It's just when I give it some beans with a quick throttle jab from idle and a stop that I notice it. And only in first-gear, but my driving style doesn't really see me stabbing the throttle below 1500 rpm i'd say. Above that, for sure :)

 

 

Edited by 2002Scoob

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What I really should do is try to duplicate it while paying attention to my AFR gauge... but it's just one of those things I rarely encounter, and when it does happen I just lift off the throttle, then just more slowly ease on and she's fine. But the worry is in a low-speed emergency situation where I've gotta go quick from a standstill she'll fall flat. Or, if I ever get the urge to light the tires. 

 

But it's my assumption that I'm getting a lean stumble because not enough gas is being shot into the chokes on fast tip-in. 

Edited by 2002Scoob

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The debate in my head is I need to confirm the spill valve size, and look at at least a 40, or a 45. But i'd like to avoid buying both, or more. 

Edited by 2002Scoob

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Just looked, I'm running 40 with a 55 return but I don't floor mine much below 2500 rpm's i'm running f16 tubes the f9 tubes if I remember right like a smaller 30-35 jet and around a 70 bleed.

Edited by Son of Marty
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So, this doesn't happen when you open the throttle suddenly in higher gears at the same RPMs? You may get more bang for your buck by trying different pump exhaust valves (they are cheaper than the pump jets). Just for grins, you could put in a set of .000 pump exhaust jets to see if that makes any difference. Are you going WOT when you experience this lean condition? What does your AFR gauge read when this happens?

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8 hours ago, Chris_B said:

So, this doesn't happen when you open the throttle suddenly in higher gears at the same RPMs?

Good question, to be honestly it's rare I find myself booting it at 950-1000 rpm in the upper gears. I usually downshift. 

 

I've got a corner on the way to work I regularly slide the tail, and to do so sees me planting my foot at probably 2k before the apex, and there's no complains there. She just grunts and goes. 

 

8 hours ago, Chris_B said:

Are you going WOT when you experience this lean condition? What does your AFR gauge read when this happens?

No, definitely not WOT. just tipping in quicker than normal looking to pull away in a more expedited manner. I'm too nervous and aware of the lean condition and where/how it happens to fully boot it to risk damaging anything.

 

I've learned to drive around the condition, which mainly just involves easing slower into the throttle to pull away slowly, or blipping the revs up a bit higher and slipping the clutch if I wanna pull away snappier. 

 

What I'd like is to be able to get it right to where I can put my foot down with the clutch fully out at 1000-1300 rpm and she just goes. And maybe lights up a tire :)

Edited by 2002Scoob

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Unless you are experiencing detonation (highly unlikely at that RPM range), you are not going to damage your engine from running it too lean. What does your AFM read when this lean condition is happening? How do you run your idle circuit (RPMs and AFR ar idle)?

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Quote

it's when I stab it from a stand-still.

As the good doctor once said,

'well, then, don't do that!'

 

At anything below 3000, the DCOE wide open is not

getting enough airflow to carburate well.  You can simulate

this by just snapping the carbs open from idle in neutral-

hear that 'WhuffFFRrrrRRROOOOMMM' noise?  the Whuff is

too much air and not enough drop across the venturi.

 

You've figured out the fix- get airflow through the carbs!

 

Yes, a bigger pump jet and smaller bleed off jet will help some,

and I agree, start by ensmallening the bleed first,]

but you'll NEVER get a DCOE to launch from idle like a fuel injected car can.

The carbs rely on airflow to flow fuel, and when the air ain't flowin' there's no gas, either,

 

 

t

 

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What Toby said, but there are other factors that can influence how your motor will perform in these part throttle, reasonably high load situations. Again, what does your AFM read when this happens?

Edited by Chris_B
poor grammer

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Do you mind sharing what your current jet setup is?  And some of what you are getting on your WBO2 sensor?  

 

Not to derail the topic totally, but I have a fresh engine that is built very much like yours:  9.5:1 Tii pistons, E12 head, 292 Schrick cam, headers, USB programmable 123 distributor.

 

I put a WB02 in a few months ago and switched from the "out of the box" 30mm main venturis to 32mm. I also tried 34s and while I didn't get them totally tuned in, I felt I lost a bunch at the low end. 

 

My current jetting: 32mm venturis, 130 main, 190 air corrector, F16 emulsion tube, 45F9 idle, 040 pump jet. 175 needle valve, 45 aux venturi.

Fuel level - 25mm below machined surface.  I am seeing 13.8 to 14.2 AFR at constant speed cruise - 70MPH, 3500ish RPM; 11-12 AFR at WOT, 13.1 AFR at idle.   I had pretty good transition from part throttle to full.  I leaned out idle to about 14.5 using the mixture screws, and now my part throttle to full throttle transition is now a bit wonky.  So I have more experimentation to do.

 

I disagree with the statement that DCOEs don't respond well to quick throttle openings.  Out of the box I was amazed at the instant throttle response.  I compare that to my Spica mechanically injected Alfa - it runs well across the board, but response to throttle blip is nothing like the instant Weber response.  That is what the accelerator pump is for - to provide instant enrichment before the airflow catches up and the normal circuits take over. 

 

Also - what is your 123 curve?  Mainly what your max advance is at high RPM?  Mine is constant past 3.5K, and after a couple days of road testing and timing on a fixed course, I settled on 34 degrees of max advance.   I tested using the 123 timing function from 3.5K to 5K RPM in third gear acceleration.  Below 32 degrees and above 36 degrees my times dropped off.

 

H

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14.5 is pretty darn lean. If you had pretty good transition from part throttle to full with idle at 13.1, you may want to fatten it up again and see if wonky transition improves.

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