L31 and Weber DCOE

[lauko]

Hi guys,

 

I'm starting yet another topic on DCOE 45 jetting for a modified stroker L31 engine. The reason is that I would really like some feedback from someone else with a similar setup as after 2 years of driving / tuning / jetting it I still have not managed to get rid of the "bog" if flooring it below 2500 rpm.

 

Engine specs: L28 bored / stroked to L31, ported head & intake manifold, MSA 6-1 headers with 2.5" exhaust, Schneider "270-80F" cam (270/280 duration, .460" lift), 10.5 to 1 compression, ZX dizzy, a set of rebuilt Webers DCOE 45

DCOE jetting: 38 chokes, 155 main jet, 185 air corrector, F11 emulsion tubes, 55F9 idle jets, 60 pump jets, closed back-bleed valve

 

So the engine runs nice, dyno showed 242 HP@6600 rpm (calculated crank), AFR is around 11-12 at idle and cruising, 12-13 at WOT.

If I floor it below 2500 rpm the AFR gets really lean and all power is gone. I tried to fix that with a bigger pump jet (60) and a closed back-bleed valve and it helped a bit, but didn't really solve the problem. As the idle/cruising AFR is already on the rich side I don't want to increase the idle jets.

My theory is that the 38 chokes are a bit too big, or that the cam is too mild and doesn't produce enough flow to correctly draw fuel at low rpms. Any other ideas how I could improve the low rpm acceleration phase? Any other hints on the jetting setup?

 

Many thanks for all the help! :)

 

Edited April 11, 2021 by lauko

[jonbill]

I ran 39mm chokes on my L29 very happily. 

However,  I did have to add a progression hole between the existing standard 2nd and 3rd holes. Big chokes can mean it goes lean between progression holes.

What do you rev it to? TBH if you don't rev it, a smaller choke will fix the problem easier than finding where to put more progression holes and making them

[lauko]

I heard of drilling additional progression holes, but really hoped I will find another solution (drilling would require a precise machinist and in any case its a one-way trip). I hoped a different jetting (maybe different idle jets or emulsion tubes) could have a similar effect as additional progression holes?

My cam is quite mild so peak power is at 6600 rpm, from there on power goes down. I never rev past 7000.

What cam did you have in your engine to not restrict such a combination (L29+39 chokes)?

[jonbill]

The progression holes are the jets until the mains kick in. your idle jet is already quite rich, so although a bigger idle might cover it, it would make you way too rich everywhere else. 

a different emulsion tube might start your mains earlier. I had Dellortos so not familiar with what the early starting ones are, but I expect google knows.

Also I'd think you wouldn't loose much if you went to 36mm chokes, revving to 7k.

 

My cam is 15mm lift and 308/9 duration and revs to > 8k and wanted that extra flow. 

 

it took me about 5 years to learn enough to get them to run great. 

I now have ITBs and EFI and it takes a couple of days to get the fuelling as good.

[jonbill]

This looks pretty good for explaining how emulsion tubes work, although might be  talking about downdraft IDFs, so references to up and down etc might need some thinking about for DCOEs. 

http://www.performanceoriented.com/performance-tuning-2

[lauko]

On 4/9/2021 at 8:46 PM, jonbill said:

your idle jet is already quite rich, so although a bigger idle might cover it, it would make you way too rich everywhere else.

 

That is exactly my problem - how to make it richer at low rpm acceleration, but not elsewhere (cruising etc).

 

- Other (richer at low rpm) emulsion tubes would make it run richer in general, not depending of the throttle position - cruising would also get richer. Or am I wrong?

- Bigger pump jets are a band-aid i think; as this solution only masks the problem. This doesn't help if the acceleration phase takes long (high gear high speed acceleration uphill for example - you will run our of the pump jet "shot" before getting to 3000rpm).

- More progression holes: sounds good in theory, but would keep it as a last resort option.

- Other idle jets: here I also think I found the right ones, as idle & cruising the AFR is almost spot on. Maybe a tad too rich, but with leaner idle jets the bog / hesitation at low rpm acceleration gets even worse.

- Smaller chokes: I read a lot about the "golden rule" of having chokes between 70-80% of the carburetor diameter; with DCOE45 that is 32-36mm. If these are bigger than the recommendation the booster signal gets weak. But how would that feel while driving it? Would it also impact low rpms or only high? This is currently my best bet, as I could always return to 38 chokes if the smaller ones wouldn't help.

 

@jonbill thanks for the link; I read this a while ago but was a good thing to re-read anyway. There is also a very good FAQ on https://240260280.com/Tech/Carbs/Weber/DCOE Theory Operation and Tuning.html

[jonbill]

Idle through high speed cruise will all be on the idle jets/progression holes. the problem is with the transition from progression holes to the main circuit. the bigger the choke, the lower the air speed and the later the main circuit starts. 

There's richer and leaner emulsion tubes, yes, but thats not the only way they can differ. On the Dellortos, a .9 emulsion tube is richer than .7, but starts the main jet later. something to do with a hole at the float level lifting the emulsion earlier than without. 

if you find an emulsion tube that starts earlier, and maybe it is richer, you can make it leaner again with smaller jets, but it'll still start earlier. 

[jonbill]

the only potential downside of smaller chokes is at the higher rpm. a mild 3.1 at 7000 may get all the air it needs from 36mm chokes and there'd but no downside. 

the upside would be no or less lean hole in the transition to main jets. 

[lauko]

Well after what you wrote I think I will do the following. First I will try with some low-rpm-enrichment emulsion tubes. F7 look like the ones I need, I just hope that these will not make idle / cruise too rich. Is anyone here running F7 or at least tried them?

Second step: 36mm chokes.

[jonbill]

I doubt they will make any difference to idle/cruise. 

I'm sure you'll get there. 

[lauko]

Well, it took some time but today I finally went for a test drive. I put in the new F7 emulsion tubes and the thing now accepts WOT from 1600rpm on! This really made a huge improvement. I only hope that this change hasn't caused my rich "cruise" to be even richer...gotta borrow a wideband again.

At the same time I also ordered a set of 36 chokes but now I don't think I'll even try them!

[jonbill]

Thats encouraging.