48 Dellorto blow through hesitation at transition

[brookester]

I have been working with Dan @ Competion Engineering [great guy] on my tune and we have been trying to figure out how to get rid of a lean spot on acceleration right around the transition from idles to mains. He seems to think the turbo starts to move so much air that the carbs briefly can't catch up. The mains are on the money and I have tried larger idles[larger than 70s] which helps some but idle is compromised. I have gone up to over 100 on the pump jets to no avail. Venturis are 40s now but I have tried smaller ones without success.

The fuel system seems to be working fine and pressure rises with boost. Tank is vented and return line goes to the tank. Lines are large enough to handle it and the carbs are correctly turbo prepped, including mod rings, e tubes etc...

Right now I have 65 idles, 165 mains, 160 airs. I'm scratching my head on this one.

2332cc
t3/t4b p trim exhaust housing
7.6:1 comp
Tims stage 2 heads
can't remember cam specs right now [Clay Smith- recommended by Tims]

Any ideas? Not going F.I. at this time.

[panel]

Are you blowing through one or two carbs ?

[DerrickfromNC]

What size mod rings?

[brookester]

I'm blowing through 2 carbs. 33mm mod rings and I am using those little aux venturis that press in.

[seabeebuggy]

How do you know your right on? have you adjusted the airs? It's part of the combo.

[brookester]

That's something I was thinking about. I was using 180 airs earlier in my tuning process and went down to 160 to get 12.8 on top end. I thought I might change the airs to bring the mains in sooner and adjust the mains accordingly but I don't want to lean the top end out any more. Would bigger airs bring the mains in sooner?

[seabeebuggy]

I was told by john at aircooled.net on WEBBERS jets control or effect a range, idles are good till 2000ish mains control to 4000ish and airs control above 4000.

using a wide band to keep safe.
I don't know the dels. And it took me 2 weeks of tuning to get my carbs to run right.

[seabeebuggy]

Try larger mains and larger airs. You can rent wide bands from aircooled.net I believe.
I know when I did mine it came down to even tuning the air flow into the float bowl. It was insane!!!!

[brookester]

I do have a wideband that I use. I'm gonna test it without the turbo hooked up to compare how it acts and then I will try that approach.
It has taken weeks to get this far. I'm almost there. Then it's time to go drag racing!
I'm gonna need a roll cage.lol

[Buggsy]

Did you ever get it figured out?

[brookester]

It's still too hot, here in Phoenix, to work in the garage.

[turbobaja]

Would bigger airs bring the mains in sooner?

Yes, and also slightly leaner. Smaller airs delay when the mains kick in. If you go too large on the airs, they'll overlap with the idle jets on transition and you'll be too rich.
I wonder if those 33mm mod rings are too small for your 40mm vents?? but if you're making good power up top it's likely just fine.

I've always thought the jets relate more to throttle plate position rather than rpm. If you floor it at 1,500 rpms, there's no signal to the idle jets, so your hoping for the mains to kick in. If you're barely cracking the throttle on a deceleration from 5000 rpms, only the idle/transition ports will be delivering fuel and the mains won't have enough of a "signal" to draw fuel from the bowl with the almost closed throttle position.

As I found out way too late, my engine leaned out under load in the upper rpms with boost due to a lack of fuel pressure/flow. I found out my pump couldn't keep up. Just something to check.

[seabeebuggy]

I think the mod rings are too small. I did not see that. The rule is 3mm smaller than the vents.

[panel]

Brookester..........What fuel pump are you running? and what FPR ?

As I found out way too late, my engine leaned out under load in the upper rpms with boost due to a lack of fuel pressure/flow. I found out my pump couldn't keep up. Just something to check.

[brookester]

Redline pump and Malpassi style regulator from CB Performance.