How To Learn Dhla 40F Trouble Shooting?

[Magnus K]

Hi again,

 

Another newbie question, I would like to learn to pinpoint carb troubles... Yeah, I already know that ignition settings and cams heavily affect the jetting but that would be more engine specific, right? I'm using Dellorto DHLA 40F's, so that's my carb world right now...

 

Simply, where to start to get my carbs running smooth? My guess is:

 

Float levels (Set it at 27 mm in the bowl as Dave's excellent sticky states!)

 

Next step?

 

Or should I start learning DHLA F's in some other way? I was looking in the "How to powertune DCOE and DHLA..." but all info I could find was mostly for the performance variants of DCOE/DHLAs?

 

Best regards

Magnus

[Magnus K]

I'm lost. Is it only the idle circuit and emulsion tube 8-10-11 that differs from jetting a emission controll vs universal performance DHLA? Can I follow this guide ( http://www.aircoolednut.com/erkson/ttt/engine/carbs/dellorto.html ) and land reasonably right for a starting point even if it's aimed ad universal performance models?

 

I feel like my 2xDHLA 40F jetting is of the scale, but I can't tell if it on the rich or lean side. What signs do I look for?

 

BR

Magnus

[GHR630]

I'm lost. Is it only the idle circuit and emulsion tube 8-10-11 that differs from jetting a emission controll vs universal performance DHLA? Can I follow this guide ( http://www.aircoolednut.com/erkson/ttt/engine/carbs/dellorto.html ) and land reasonably right for a starting point even if it's aimed ad universal performance models?

 

I feel like my 2xDHLA 40F jetting is of the scale, but I can't tell if it on the rich or lean side. What signs do I look for?

 

BR

Magnus

 

I did a lot of tuning with my Webers by consulting the plug colour. I would drive (carefully for a few miles on the idle circuits keeping the revs down and check the plugs. Then find a deserted road (impossible?) and drive in low gears at high revs for a few miles and then stop without driving slow or idling so that you can check the main circuits. RIch or lean are obvious by plug colour??

[Nick Jones]

Off the scale rich will give black smoke and you'll smell unburnt fuel. Engine feels flat and unresponsive. May be more prone to bogging on rapid throttle opening as the pump jets add even more fuel.

 

Very lean more likely to lead to hesitant, jerky feeling. May get some spitting back or fluffing from the carb throats. Might clear briefly on rapid throttle opening as extra fuel is added - but only if the pump on on the richer side of right.

 

Note these remarks are just general tuning comment - I'm no DHLA expert. Come in Dave......

 

If you are serious about tuning, may I suggest investing in a wideband O2 sensor.

 

Cheers

 

Nick

[Magnus K]

RIch or lean are obvious by plug colour??

Sounds like a cheap and easy way. I'll give that a try next time out with the GTV. *thumbs up*

 

 

Off the scale rich will give black smoke and you'll smell unburnt fuel. Engine feels flat and unresponsive. May be more prone to bogging on rapid throttle opening as the pump jets add even more fuel.

*snip, snip*

If you are serious about tuning, may I suggest investing in a wideband O2 sensor.

When really putting the pedal down, that quote fits my engine quite well. On the other hand, it acts as it's lean on idle / small throttle openings. So, there could be a mix of a lean idle jetting and a rich main.

 

I'd give GHR630's suggestion a try, then concider a wideband o2 sensor/instrument.

 

Thanks!

Magnus

[Magnus K]

If you are serious about tuning, may I suggest investing in a wideband O2 sensor.

 

So, now I got that piece of hardware. Will be exiting to see where in the world the fuel/air mix is.

 

Still, I'm struggling to decide where to start off regarding carb configuration. Twin DHLA 40F is set (no money to get a pair of universal perf DHLAs) so I'm trying to find the next step. Probably deciding if 32mm main venturis are way too small or acceptable. Was concidering to go for 34mm main vents but again money came up short...

 

Will try to install the O2 sensor stuff first, then rethink the main venturis and rest of the jetting.

 

Cheers

Magnus

[DaveNotSoSideways]

You can easily lathe/machine the I.D of the choke/vents, make them bigger.

 

Those emission carbs don't like chokes over 34mm.

 

What CC is the engine? Run as small a choke as you can.

 

I call the 30-32-34mm "vents" chokes, as they "choke" the engine, fitting a smaller choke is restricting the airflow, so to choke.

 

The venturi sits infront of the choke tube.

[Magnus K]

Those emission carbs don't like chokes over 34mm.

 

What CC is the engine? Run as small a choke as you can.

It's a 1996 cc (2 litre) Alfa Nord engine. (4 cyl inline) Twin DHLA 40F's on it.

 

I was clicking around to find tips on choke (thanks, not easy to get the terminology right!) and found a diagram that I tried to use. I got 32's now, according to the diagram at Aircooled nuts's site suggested max power output at 5100rpm or so.

 

Alfa people on the net thought 34's would be more correct. Personally I really don't know what to think except that 36's and possibly 35's (especially if Dave say so) is too large for DHLA40s... The main idea is to get a air pressue (venturi) effect to suck air/fuel from the carbs down to the air stream going into the inlet?

 

Ah, I hope someone can figure out what I mean.

 

Cheers

Magnus

[DaveNotSoSideways]

34mm would be fine.

 

32mm also ok, might restrict at peak rpm.

 

Jetting would be different for both choke setups.

 

If choke is too large for the engine/carb size the carburettor becomes harder to tune as the idle/progression/main circuits have to operate under a range of non-optimal conditions.

[Magnus K]

Jetting would be different for both choke setups.

 

If choke is too large for the engine/carb size the carburettor becomes harder to tune as the idle/progression/main circuits have to operate under a range of non-optimal conditions.

Tried it, don't know what went wrong. It idles just fine but when opening throttle a little bit (cruising/even speed) it jerks and generally don't like the situation. Opening throttle fully makes the engine pull just fine. I switched back to 32mm chokes and then the engine works fine at progression phase (I guess that's what it's at?).

 

Will run on this setup for the final track day on Sunday, then I'm off to the dark side = EFI... Going for a Alfa Romeo 75 2.0 litre TwinSpark engine, it's a mechanical bolt-on switch but will require some special fitting of accelerator wire and exhaust system.

 

Regards

Magnus

[Triumph-V8]

You have three carbs in one:

The idle carb, the part throttle carb and the full throttle carb.

 

Unfortunately they all affect each other and in addition

more or less heavy in different stages of engine performance.

 

What you did with the 34mm is first affect the full throttle behavoiur.

But also part throttle is affected and needs adjustment with the idle jets

mainly the fuel jet but maybe also the air jet.

 

The alfa is set on 32mm for the daily driver and work perfect under all conditions.

34mm would be the recommended size up to 150PS on the 2l nord engine on sporty driver.

 

You got good hints with the o2 wideband and the 34mm chokes.

Try to establish both and go the way of learning how all works together.

 

One hint: if the engine misfires the data of the o2 sensor are wrong

and too fat mixture might make the sensor fail for a time but it recovers.

 

In practice this means an engine being so fat that it bogs might show

poor mixture in the o2 due to the oxygen coming unburned through the engine.

So only smooth running engine without misfires is to be measured

and only under situations where it has stabilized for some seconds.