Dellorto Help Required

[Spydaman]

I've got a mild 1776 Beetle engine with 40 DRLA's in the Bonito and am struggling to get rid of a lean area at the transition from the idle to main circuit. I've got an AFR meter so can see exactly what's going on but can't seem to fix it. Idle is 13.5 -14.0, Cruise is 16.0 - 17.0 and WOT 12.7 - 13.5. The problem is it that leans off to 19.0 at the transition when the throttle is eased open. I reckon the main circuit isn't coming in early enough which should be cured with smaller air correctors but this doesn't work. Current set-up is as follows:
Venturi 30
Idle 52
Main 122
Air corrector 160
Accelerator pump 45
Emulsion tubes 9165.2
Ignition 32BTDC all in by 3000rpm plus 10 degrees at cruise.

Any Dellorto experts out there offer suggestions?

[Pumaracing]

Tsk! The air correctors have got three fifths of FA to do with the transition period on any carb. They lean out the top end of the rev range. They have no effect on the bottom end. It's the emulsion tubes, idle jets and also the drillings in the carb bore adjacent to the edge of the butterfly (progression holes) that determine transition, not that you can do much about the latter but they differ on different model numbers of the same carb as was found necessary during OE manufacturer setup for different engines.

Try going one size larger on the idle jets and then readjust the idle mixture.

You sure you don't mean 9164.2?

9164.2s are lean at the bottom end. 9164.1s are rich at the bottom end. 9164.3s are supposed to be intermediate.

Edited by Pumaracing on Wednesday 24th June 09:58

[Spydaman]

Thanks for the reply.
My understanding was that smaller air's create more suck on the main circuit bring it in earlier. I agree they also limit the air at WOT and need the adjust the main jet accordingly.
Yes I meant 9164.2 General consensus is that they work best for Beetle engines. I've tried 9164.1 .2 and .3 but none get rid of the lean area. I've also gone up to 60 idles which just make the idle circuit very rich.

I'll give it another go with the 9164.1 airs and bigger idle jets.

[Pumaracing]

If you've tried all of the above then the accelerator pump setting might need looking at. Also the float bowl level which can drastically affect fuel pick up.

[Spydaman]

The carbs came with 35 pump jets and again the consensus is 45's work better. I've tried the 45's with the volume set I think to 2cc/10 squirts.
The float level was a mile out but is now set to 5-6mm between float and cover.
I'm beginning to suspect low fuel pressure not keeping the float chamber full but it seems ok at WOT so maybe not.

[pumaracing]

Definitely nothing to do with fuel pressure. Maybe time to strip the carbs and make sure no transition drilling are clagged up and that there are no air leaks anywhere, spindle bearings etc.

Maybe bigger main jets and increase the air correctors to lean the top end off again.

Also make sure carb balance is perfect so one carb is not opening before the other.

Edited by Pumaracing on Wednesday 24th June 10:52

[Pumaracing]

PS, what bhp do you think this thing has? The intergoogles seem to think 100 bhp is a decent target for a mild road motor. You might be a bit big on the choke size. Read my tuning article here.

https://web.archive.org/web/20110809041756/http://...

It's always a lot easier to get the fueling right with conservative choke sizes which give a better draw on the main jet. Even 26mm would supply 100 bhp.

[spydaman]

I've got some 28mm chokes which I also tried. Maybe its time to start again from first principles.
Venturi 28mm
Main 112-118
Idle 50-55
Air 160-170
Emulsions 9164.1-2

100bhp at 5000rpm would be nice and probably achievable. Carbs clean and unworn so suspect jetting rather than blockages/leaks.

[Pumaracing]

The absolutely first thing to do is determine whether this is a transient thing or steady state. If you can hold the load and throttle position up a slight incline and it stays weak then it's steady state. If not it might just need a bigger pump shot.

[Spydaman]

Definitely steady state. I can hold it at 15-16 cruise then ease the throttle open and it goes weak at 19 until the mains come in at 12.7-13.5 If I boot it it'll accelerate through without hesitation.

[Pumaracing]

Your mains are right down at the bottom end of sizing for 30mm chokes at approx 4x diameter and when chokes are a bit too big then the draw is weak and the multiplier needs to go up. Try some 130s. Obviously this will make the top end too rich until you increase the air correctors but you don't need to bother with that just to see if there's any effect at small throttle openings.

[Spydaman]

Based on your recommendations I've gone for:
Chokes 30mm
Mains 130
Idles 55
Air Correctors 180
Emulsion Tubes 9164.1

If that doesn't work I'll try 28mm Chokes

[DVandrews]

The idle jet holders govern how much air is emulsified with the idle fuelling, which ones are you using? Try going to a richer holder. The available holders are shown below, weakest on the left, richest on the right.

7850.5 .10 .9 .4 .1 .3 .6 .7 .2 .8

Dave

[Spydaman]

Better but a bit rich everywhere and it's still flat at the transition from idle to main circuit.
Idle mixture 12.5
Cruise 12.5 @ 3k rpm, 19 @ 4k rpm
WOT 11
It doesn't seem to respond as it should which makes me suspect it's something else. There's O ring seal on the auxiliary venture. It looks ok but if it doesn't seal 100% it will reduce the vacuum signal the main circuit?

[Pumaracing]

Have a read of this and see if you think it sounds like your issue.

http://www.alfabb.com/bb/forums/carburetors-fuel-i...

[Spydaman]

Thanks for that. I had a read through it and believe it refers to the progression from idle mixture to the progression drillings/idle jet whereas my problem is the progression drillings/idle jet to main jet circuit.
I found this http://www.datsun2000.com/tech/weber_tuning_method... which suggests that changing the air corrector adjusts the point at which the main circuit comes in. The consensus is that 180 AC work best with Beetle engines but I've also read that the AC should be the main jet plus 50. I'm now using 130 mains and 180 AC. I've tried 175, 160 and 200 but none have the desired effect which makes me think it's something else.
Until I fitted the AFR gauge I never new I had a problem but now I have I can see and feel it goes flat at that point.

[Pumaracing]

I think the article you mention is misleading. Yes the main air corrector jet has a very small effect at the bottom end of the high speed circuit but you can't just change it willy nilly without messing up the rest of the high speed circuit so its use as a tuning tool for your problem is about zero.

It's the idle air corrector jet that mainly affects the start of the transition point. To richen this up you want a smaller IACJ (idle jet holder). It might also be neccessary to reduce the idle fuel jet size if the low speed circuit then becomes too rich.

You still haven't said which idle jet holders you have. As you've been told above the weakest (largest air holes) is the 7850.5 and the richest (smallest air holes) is the 7850.8

http://www.dellorto.co.uk/shop/dhla-parts/dhla-idl...

Edited by Pumaracing on Thursday 25th June 14:02

[Pumaracing]

I think you might have some fundamental misunderstanding of the effect of the various jets involved here. The size of the idle jet does NOT control the idle mixture. That's controlled by the idle volume screw. When you say above that your idle AFR is now 12.5 that's nonsense. You can adjust the idle mixture to anything you like on any idle jet by adjusting the screw.

What the idle jet size primarily controls is the bottom end of the low speed circuit after the idle mixture screw is no longer in effect.

Then the idle air corrector jet controls the top end of the low speed circuit and the bottom end of the transition to main jet.

Finally the main jet controls the bottom end of the high speed circuit and top end of the transition circuit and the main air corrector controls the top end of the high speed circuit.

That's why you always tune from low rpm to high (left to right) on the chart you linked to.

[Spydaman]

Yes, I know I can set the idle mixture to whatever I want. I wasn't saying it was fixed at that just that 12.5 and maybe a bit weaker gives the most stable idle.

I think 7850 is for a DHLA. I've got DRLA's and there is only one size idle jet holder.

Edited by Spydaman on Thursday 25th June 14:23

[Pumaracing]

I believe that they're the same size, screw thread etc, so you can interchange them. However it may be that the stock DRLA one with no holes is as rich as you can get anyway.

If that's the case then the only remaining solution may be to reduce choke size to increase the draw on the main jet.

On Webers you could also change the size of the auxilliary venturi to affect the draw on the main jet but I don't see alternatives listed for your carb.