BDR Dellorto DHLA 45 emissions - ok but want better

[anthony1956]

Warm engine, timing right, CO2 7.5% CO 3.10% HC 1544

These are ok for our test, only just, but I had them better than that before; trying to recall what one tweeks to drop the CO and HC and raise the CO2 ?

Twin Dellorto DHLA 45s No Cat.

Available tools: Mixture per barrel, air compensation per barrel, tick-over all 4, Balance the two carbs together. 
Manometer to balance each barrel on a carb. 

An old idea also is to put hotter plugs in for this purpose, so say 5s instead of 7s. 

While operating temp is 80c I have a sneaky feeling the better results were at 85c. 

 

Here's my recollection (artistic licence there) of how to do this:

I think the mixture screws are all currently at 2.5 turns out, which is a bit rich as on p76 Des Hamill says:

3.5 turns (out) to start the engine

0.5 of a turn when engine warm

0.5 of a turn back in if all is well.

He says Minimum turns out will be 2.5 to 3 turns and Maximum 5.5 turns  - that's a lot of room to play with.

(I have rewritten this next sentence)

What I have to remember is that TURNS OUT means RICHER mixture, less air. 

AND: do we ever touch the air compensation screws again?

Des Hamill is talking about getting the engine "running smoothly at tick-over" whereas we (me) are using an emissions analyser..which b t w seems to respond real quick.

Later: Seems I haven't been correctly following the instructions: 

 

Edited September 11 by anthonym

[OldAndrewE]

24 minutes ago, anthonym said:

Warm engine, timing right, CO2 7.5% CO 3.10% HC 1544

These are ok for our test, only just, but I had them better than that before; trying to recall what one tweeks to drop the CO and HC and raise the CO2 ?

Idle test I guess. To me that says it is slightly rich so just screw in the idle screws a bit, say 1/4 turn, and retest.  You can do it while the engine is running, just give it time to settle to the new setting

[anthony1956]

Won't that make it more rich? (these screws do my head in)

I think what I find strange is that if one screws out the air compensation screws to add more air, then screwing in the mixture screws should do the opposite and it doesn't

Edited September 11 by anthonym

[OldAndrewE]

The mixture screws are a tapered needle that restricts fuel flow and screwing in will reduce fuel and weaken the mixture

[anthony1956]

so turns out will make it more rich...?

This is my reference, just for reference, Des Hamill's book

Edited September 11 by anthonym

[OldAndrewE]

17 minutes ago, anthonym said:

Won't that make it more rich? (these screws do my head in)

I think what I find strange is that if one screws out the air compensation screws to add more air, then screwing in the mixture screws should do the opposite and it doesn't

Not sure what you mean when you talk about the air compensation screws

[anthony1956]

We have 4 mixture screws and we have 4 air compensation screws about an inch further in to the body.

[OldAndrewE]

3 minutes ago, anthonym said:

We have 4 mixture screws and we have 4 air compensation screws about an inch further in to the body.

Is this what are described in the book as 'emission control' models?  Not familiar with them and how to adjust

[anthony1956]

no def not one of those - which one is yours? Mine's DHLA 45 C 

[anthony1956]

image I in this, also mentions them in the instructions on the next page where there is another diagram, perhaps more clear

the diagram on p 2 exactly matches mine

Edited September 11 by anthonym

[OldAndrewE]

Mine are DHLA 40 and the only adjustments (apart from changing jets etc.) are the idle speed (one per carb) and the idle mixture screws (two per carb).

[anthony1956]

show us a pic?

my idle speed is one shared by both

Edited September 11 by anthonym

[OldAndrewE]

[anthony1956]

I need a minute!

[anthony1956]

Looks to me like your air bypass screws are missing, while their holes are present. pic coming up

Edited September 11 by anthonym

[anthony1956]

My 45 (refurbed by Paul Hollidge) on the left your 40 on the right  - those are my air compensation/bypass screws

Edited September 11 by anthonym

[anthony1956]

and here what looks like a tick-over screw for twin carbs

[OldAndrewE]

28 minutes ago, anthonym said:

 those are my air compensation/bypass screws

Those are blind holes on mine

[anthony1956]

3 minutes ago, OldAndrewE said:

Those are blind holes on mine

YES IT SAYS IN HERE https://www.dellortoshop.com/contents/en-us/p412_8176-Air-Bleed-Screw-for-Dellorto-DHLA-carburetor.html that not every carb has them and those that do have three different part numbers 8,176 - 9,926 - 8,382

Does "blind" mean blocked? So not holes at all.

Edited September 11 by anthonym

[OldAndrewE]

Anyway, back to the original issue.  Screwing in the idle mixture screws will definitely weaken the mixture at tick over

[anthony1956]

ok. I have to go re do it all. Not for the first time 🙂. However, this time I am not having to battle the inlet manifold air leaks. Already everything is easier without extremes.

[OldAndrewE]

Surely nor re do it all!  Presumably you still have the engine as originally tested above.  Get it warm in that condition and then turn in each screw 1/4 turn, wait 10 seconds or so and see what has happened to the readings.  The other thing to see is how the engine likes the adjustment, i.e. does it sound better or worse, did it speed up a bit or slow down.  If it speeds up it seems to like it, slows down perhaps not! 

[anthony1956]

made me smile, thanks 🙂 Not sure if you have seen "the instructions"  which take ages the first time and mere moments the thousandth time. The thing is I failed to check the butterflies are truly closed using the process described, so starting over is not a huge chore, in fact it's not a chore at all. 

The largest changes occur when balancing the carbs between each other.

I also find the engine is cycling up and down, I think we used to call that hunting, but entirely forgotten what the cure is. Back then I'd have my ear to the exhaust, no fancy gadgets. The one thing I so hope it is not is more air leaks. That's the one continuous problem I have had with this old engine, starting with the cooling system and now the fuel system.It was a massive difference before I re sealed the inlet manifold, there is still a difference on the manometer between one and two, but not very much.. at least compared to previously. I think maybe a little check just in case. The obvious risk area between the carb and manifold interface. 

Edited September 11 by anthonym