Weber Carby Genius wanted!

[jb_22]

I'm gonna jump on the bandwagon for this thread too, although I'm running Dellortos

I want to get my DHLA 40 Dellortos tuned well so I can take my 3TG powered TA22 up to a track day in the near future. At the moment it fouls the plugs under high load, high throttle & gets grumpy if it idles in traffic for a while. The numerous times I've checked them, the plugs have always been sooty black except for a small patch that faces the inlet port which is dark brown.

Handy Info:
1840cc 3TG (88222 head)
3T electronic dizzy w/ Bosch ignitor & GT40 coil
BP4EY plugs regapped to 1.0mm & indexed to the ports
Stock 2tg Mechanical fuel pump w/ adjustable fuel reg set at 2psi
Twin Dellorto DHLA 40 (N-type)
32mm venturis
148 main jets
7772.11 emulsion tubes (searching on the net led me to believe these are the 2nd leanest you can get)
195 air correctors
60 idle jets
& I haven't pulled the carbs down enough yet to check the pump jets

I was thinking a 130 main jet & 45 to 50 idle jet would be a good start, but I'm unsure if I need to change anything else (emulsion tubes, AC, PJ etc) at the same time. Also not sure how applicable weber DCOE jet sizes/tuning info is to Dellorto DHLA.

I've sent Norm an email too (thanks Ben!), but I'm just trawling for as much info/advice possible before trying different jets.

Cheers, Jess

[Deeluded]

I know they aren't quite the same but loose venturi's will produce this result in the Solex side draft carbs.
They rattle a little sideways & effectively pull the choke on.

Chased this issue for ages with my old 3TG

[jb_22]

Dellortos have a different venturi design & never have that problem as the Solexes - one of the reasons I chose Dellortos over the Solexes. Another reason is they have vacuum ports in each barrel so you can just screw in a vacuum gauge & use that to tune the carbs

[Nevakikamukau]

QUOTE=jb_22;1445054]I'm gonna jump on the bandwagon for this thread too, although I'm running Dellortos

I want to get my DHLA 40 Dellortos tuned well so I can take my 3TG powered TA22 up to a track day in the near future. At the moment it fouls the plugs under high load, high throttle & gets grumpy if it idles in traffic for a while. The numerous times I've checked them, the plugs have always been sooty black except for a small patch that faces the inlet port which is dark brown.

Handy Info:
1840cc 3TG (88222 head)
3T electronic dizzy w/ Bosch ignitor & GT40 coil
BP4EY plugs regapped to 1.0mm & indexed to the ports
Stock 2tg Mechanical fuel pump w/ adjustable fuel reg set at 2psi
Twin Dellorto DHLA 40 (N-type)
32mm venturis
148 main jets
7772.11 emulsion tubes (searching on the net led me to believe these are the 2nd leanest you can get)
195 air correctors
60 idle jets
& I haven't pulled the carbs down enough yet to check the pump jets

I was thinking a 130 main jet & 45 to 50 idle jet would be a good start, but I'm unsure if I need to change anything else (emulsion tubes, AC, PJ etc) at the same time. Also not sure how applicable weber DCOE jet sizes/tuning info is to Dellorto DHLA.

I've sent Norm an email too (thanks Ben!), but I'm just trawling for as much info/advice possible before trying different jets.

Cheers, Jess[/QUOTE]

Jess,

If your Dellortos are DHLA 40 N Series, then they are whats known as the emission series carby's. These are different to the earlier series carbies in respect to the fuel delivery circuitary and you need to know about this before you contemplate changes to the jets or emulsion tubes. The emission models are the DHLA 40 F, G, H, N, R & S models, and they must not be tuned like the earlier non-emission carburettors. These emission models differ from the earlier series carbies because they have more progression holes in the emulsion tubes and atomise the fuel mixture to a far greater extent. The fuel circuitary also differs as the idle jets in the emission series carbies feeds most of the cruise phase and low rpm / low throttle progression area of the engine at all times when the main jet isn't in operation, and it does this completely automatically. The idle jet has a very large fixed 2.2mm air feed, and you cannot tune this progression phase of the carburetor for length like you can in the non-emission carburettors.

But this is the secret to their tuning and operation.

The idle jet doesn't feed from the float bowl, it feeds directly from the the main jet stack, and this is what gives these Dellorto's the sweetest road behaviour and dead easy tuning. When the main jet starts to emulsify fuel in the tube, the idle jet is feeding from it, so the gassy airy fuel shuts down the idle jet and sucks backward (yes - backward) through the idle jet, thereby using every drip of fuel efficiently without any waste in circuit cross-over where one jet is going after the other. This happens the moment the main jet fires, so there is no need at all to tune the progression and idle phases. This is pure magic as they are automatically calibrated, you simply keep the idle jet above 59 up to 62, and not make the mistake of fitting emulsion tube numbers suited to the earlier carbies (non-emission types). The emulsion tubes in these emission series carburretors are always 8 or 10 or 11 and have to stay that way - which are really rich and have a hole straight down with plenty of air holes, and these atomise the fuel mixture to a massive degree, and have to be used because the idle jet will not run correctly using the DHLA non-emission 1, 6, 7, or 5 emulsion tubes. The idle jet needs these airy tubes to function and cut out as designed. People who don't understand the different science of the emission series carbies sometimes ram the DHLA idle holders with air holes in these carbies with the 1, 6, 5, or 7 style tubes and wonder why there is a massive lean spot off idle. These carburettors use a .3 vent which has a very small signal tube to the main jet stack, and this is because the holey tubes are basically ready to go from about 1250 rpm, so the holey tubes need holding back with a signal killing venturi. So all that needs to be done in tuning these carburettors is to keep the idle jet between 59 and 62, and tweak the main jet size to give the right AFR at cruise. They also have vacuumm tappings on each barrel to assist in sychronisation.

This information comes from my extensive research on the subject over many years, however, unfortunately I am not able to give references and acknowledgement to the original source of the information as this has been lost over time.

Hope this helps.

Cheers,

Peter

[jb_22]

Wow Peter! Thanks for that goldmine of info - perfectly amazingly what I was after I was always wondering what the difference between types of dellortos was. It's a pity I didn't check my emails sooner - I just ordered 130 main jets & 52 idle jets & it would seem I don't need the smaller idle jets, eh? I came to 130 mains from the old rule of thumb: Main Jet size = venturi size x 4 - Would this apply to the emissions type carbs where the main jets are partly responsible for the idle circuit mixture as well?

I made myself a vacuum manometer with two lengths of fishtank tube, cordial (easy to see - didn't have food colouring), two barbs to replace the blanking screws & a ruler. First, I screw the barbs into #1 & #3 to get the two carbs synced - adjust idle mixture to give max vacuum on each barrel, then adjust the balance screw to match the vacuum readings on #1 & #3. Then I match #2 with #1 & #4 with #3 barrels. I've only had limited success at the moment because as the car sits idling for a good while the plugs get sooty, making it really hard to feel if the car is running worse cuz of the mixture screw settings or the plugs. After some trial and error & dozens of spark plugs later I think I got it pretty sorted at the moment - the motor is wonderfully torquey (will pull from 40km/h in 5th with ease & no flat spots/stuttering/etc) & runs very well cruising - I just get the fouling from idling in traffic or flat stick up hills. The Dellortos have much better throttle response compared to even some modern EFI cars I've driven.

[Nevakikamukau]

Jess,

130 mains is a good starting point, but what size they should actually be may vary a few sizes either side of this. There is no absolute size that can be defined because all engines are different to a certain extent. The right size can only be determined by dyno runs with the AFR being monitored across the whole rev range. Colour of the plugs is no longer a reliable indicator. Time was when black plugs indicated a rich mixture, brown plugs represented the correct mixture setting and white plugs represented a lean setting. This was in the days of leaded fuel, but in these days of unleaded fuel the mixture burns black across the full spectrum of the mixture. In my opinion, if you are serious about your carby tuning, then you should invest in a proper 4 column manometer. I bought a Morgan Carbtune unit from Ireland, and although they are not cheap, they are very easy to use, accurate and robust, and well worth the investment.

If the idle jets are correct, the idle mixture screw will usually produce the strongest manifold vacuum at 3 to 4 turns out from seated. (a word of caution here - never overtighten the idle mixture screws when you seat them as they have a very fine thread that is easily stripped, and if this happens then the carby is knackered). Anything more than 5 turns out, and you're so far out on the tapered needle that you're not accurately metering anything anymore. In this case install larger idle jets and start over.
Cheers,

Peter

[J-M kujala]

The idle jet has a very large fixed 2.2mm air feed, and you cannot tune this progression phase of the carburetor for length like you can in the non-emission carburettors.

There have been years a go available KIT what included some different sizes jets, drill, and tap to make thread to idle airfeed "jet" so after that there is one fine tuning more..

Sometimes it will help hard cammed engine in mid range, when problem is to lean mixture... Ghetto test are they needed is drop needle to sit in the idle air jet bore.. they will stay in there, make the jet smaller etc...

im not sure if those kits are available anymore.

[jb_22]

So I got my 130 main jets today - put them in and went for a squirt without touching anything else.. Fair screamed its head off Still a few teething issues though - Had to disconnect the vacuum advance as it was giving the motor ~45-50 degrees full advance, lucky the motor isn't turbo! After fixing that and giving the motor ~35-38 degrees maximum it has a hesitation/misfire where the vacuum advance is needed - part throttle, high load, i.e. cruising up hill in 4th gear @ 60-80km/h. That I can live with, just gotta drive the car a little different with the right foot.

But my main gripe now is she won't rev over 5500rpm - my first thought was to check the timing - so I've fiddled with that. Also gave the fuel reg an extra 1psi incase it wasn't flowing enough at high RPM. Now I'm leaning to my stock mechanical fuel pump not giving the carbs enough fuel and/or the 130 jets are a little too small & they aren't giving the motor enough fuel beyond 5500rpm. Anyone got any other input to what might be causing the misfire in the top end?

[Dazza]

The hesitation/misfire could be from running too lean. Try 135 main jets.

[Nevakikamukau]

I agree. Main jets are probably a little small.

[Nevakikamukau]

Another thing JB_22,

Going back over your carby specs I see that youu are using 32mm chokes. I think these are too small for a 2 litre engine. I suggest you use 34mm chokes for a flexibility tune, or 36mm for a power tune.

[jb_22]

It's 1790cc 3TG to correct myself, I just recalculated it with my 85.5mm pistons.. dunno where I pulled 1840 from I did a little research & got some info off the net (can't remember where exactly) - It was a graph with displacement vs. RPM & different curves for venturi sizes. The graph said 32mm venturis were good for a 1800cc motor making peak power at 6000rpm, so I figured 32 venturis would be a flexible compromise.

Seeing as I now have a spare set of 148 main jets, I might get some jet drill bits & do a bit of fiddly soldering & try out a few different sizes.

How much do the idle mixture screws affect the top end mixture? The screws are about 1 1/4 turns out from full lean at the moment.

Cheers, Jess

[ToyTA22]

Jess. From your last reply i'd say your idle jets are a touch large. Mixture screws should be approximately 2 turns off seat and only 1/2 a turn either way, which 90% of weber books and original vehicle manuals will show. I've dealt with lotus twin cam and fiat engines which are weber fitted from factory.
Also you need to get softer distributor primary advance spring, usually small gauge or one with tension on it, to get more rapid advance down low since vaccuum advance can't be used

[jb_22]

ToyTA22, I was going to order smaller idle jets but on Peter's advice (few posts back) he informed me I have the emissions N-type Dellorto DHLAs, which need to have a jet size of 59-62. I have a 60 idle jet, so that seems to be in order. The emissions type Dellortos are a different ball game compared to tuning Webers. Changing the mechanical advance spring is a good idea though! Should've thought about that earlier. There is a different size spring on each weight, so maybe if I just use two of the smaller springs (got a couple spare dizzys lying around) it might get rid of that low rpm, high load flat spot.

As for the idle mixture screws, with the 148 main jets the best tune I could get was with the idle mixtures just a touch under a full turn (main jet size has more of an influence on the idle circuit in emissions type Dellortos), and I've only turned it another ~1/3 turn richer since I put the 130 main jets in. I haven't tried tuning it with my vacuum manometers yet - might give that a go on the weekend & see what setting I arrive to.

On a whim I've already soldered up the 148 jets, but I don't yet have a 1.35mm drill bit to try a 135 main jet size.

[71TA22]

Hi jb_22,

I ran a 1770cc hybrid on DHLA 40's for a while. Electronic ignition, extractors, 292 adv. duration cams, 88220 head with mild port, ported/polished intake.

Best results from carbs:

Chokes: 34
Mains: 135
Main Air's: 180
Emulshions: 7772.7
Pump jets: 45
Idle jets: 55/7850.1
Aux Vents: 7848.1

Good idle, good progression, ran to self imposed 7.5k rev limit (standard factory rods).