Ideal Advance Curve

[Peato40]

Hello All,

So my engine is running much better and quieter now and for the first time in a few year has propelled the car for a very short journey around the square in front of my house.

In the end I decided to bite the bullet and ordered a new 123 distributor and Bosch coil. The installation was very easy and now it's time to dial it but would be good to have a known starting point if anybody has some advice to offer?

Not much of a curve but this is how I currently have the advance curve set



Now bearing in mind my engine is an L26 bored out to 2.8ltr, big valve conversion, fast road cam, Z story exhaust system and triple Dellorto 45's

Would anybody have some proven curve or suggestion for this configuration?

Currently the engine idles nicely at about 900 rpm and will rev ..... so far up to 3500 rpm but it bogs down if I stab at the accelerator. If I apply the throttle with just a little more patience then the engine will pick up very well but seems to peak at about 4000 rpm.

I have used a colortune to try and get the mixture right and plugs are beginning to indicate mixture is in and around the right area ..... at least for idle and some occasional reving.

I don't have any vac advance pipe installed so I set the mapping to zero across the rev range but I'm not sure if that's what I should do if not utilizing vac advance.

Anyway, an advice will be very welcome.

Phil

 

[jonbills]

Phil, I’d go for 22 at 1000 at 34 at 3000 and delete the intermediate points.
What chokes have you got?

 

[Peato40]

Thanks Jon, will give it a try. I believe I have 45 chokes

 

[jonbills]

I don’t think they go that big Phil[emoji847]

 

[johnymd]

It sounds like your engine is running out of energy as you rev it so either not enough spark or not enough fuel to produce the required energy. Very similar to my standard L24 before the recent tune. Ended up adding more spark but the main problem was not enough fuel. For me it was an 10min fix with lambda logging and ecu control. You may be better off spending some money on a good old school rolling road session as guessing on jets sizes could be expensive and time consuming.

 

[Peato40]

I don’t think they go that big Phil[emoji847]

Hi Jon,
I pulled one out and it says 36 on it.

 

[Peato40]

Phil, I’d go for 22 at 1000 at 34 at 3000 and delete the intermediate points.
What chokes have you got?

I tried 22 @1000 and 34@3000 but you have to also set 500 & 800 RPM, so I went with 18 and 34



And this is a link to the result

I can get it revving but I'm finding it difficult to get it instantly.

 

[jonbills]

Hi Phil I was suggesting get rid of the other points between 1000 and 3000, not all the other points. You still want those nice low advance points below 1000 to get it started.

 

[jonbills]

As John says, without lambda/afr you’re flying blind. Very hard to know what the probs are if you can’t isolate spark and fuel.

But maybe we can do some guessing - as well as the choke, do you know what emulsion tubes and main jets are? Is the video on the road or free revving?

 

[Peato40]

Hi Phil I was suggesting get rid of the other points between 1000 and 3000, not all the other points. You still want those nice low advance points below 1000 to get it started.

Thanks Jon,

So now I have:

18 @ 500
22 @ 1000
34 @ 3000
34 @ 8000

But it still bogs down when stabbing the accelerator

 

[Peato40]

As John says, without lambda/afr you’re flying blind. Very hard to know what the probs are if you can’t isolate spark and fuel.

But maybe we can do some guessing - as well as the choke, do you know what emulsion tubes and main jets are? Is the video on the road or free revving?

Wideband Fuel to Air is the next on my shopping list .... will it give enough info?

I'll take a look at the emulsion tubes and main jets and let you know.

Surely with this new coil and ignition, I must have enough Spark mustn't I?

 

[Peato40]

As John says, without lambda/afr you’re flying blind. Very hard to know what the probs are if you can’t isolate spark and fuel.

But maybe we can do some guessing - as well as the choke, do you know what emulsion tubes and main jets are? Is the video on the road or free revving?

It's free revving

 

[Peato40]

As John says, without lambda/afr you’re flying blind. Very hard to know what the probs are if you can’t isolate spark and fuel.

But maybe we can do some guessing - as well as the choke, do you know what emulsion tubes and main jets are? Is the video on the road or free revving?

Not easy to see but I believe these are the numbers:

Main jet = 148
Emulsion tube = 777 2.6
Retaining Cap = 185

 

[jonbills]

Thanks Jon,

So now I have:

18 @ 500
22 @ 1000
34 @ 3000
34 @ 8000

But it still bogs down when stabbing the accelerator

I'd go back to 10 at 500, that'll get it started nice and easy.
won't change the bogging down problem.
I'll just go check what E tubes I'm using......

 

[jonbills]

Yes to the wideband AFR - have one and it's very useful. It makes the difference between 'hmm, I wonder if it doesn't run right 'cos it's too lean or too rich?'
so - since you're free revving, I don't think you'll be even getting anywhere near you main circuit so lets forget about that for a while.

so what's the problem again? if you slowly open the throttle, it'll rev well, but if you stomp on the throttle it chokes?

 

[Peato40]

Yes to the wideband AFR - have one and it's very useful. It makes the difference between 'hmm, I wonder if it doesn't run right 'cos it's too lean or too rich?'
so - since you're free revving, I don't think you'll be even getting anywhere near you main circuit so lets forget about that for a while.

so what's the problem again? if you slowly open the throttle, it'll rev well, but if you stomp on the throttle it chokes?

Yes, it will rev if I'm gentle with the accelerator but if I stomp on the gas it will bog down and even try to cut out.

Sent from my SM-G930F using Tapatalk

 

[jonbills]

Assuming I’m right on that, The issue is that when you whack the throttle open the vacuum that was pulling fuel from the idle and progression holes collapses and yet there’s no significant airflow through the secondary venturi to trigger the main circuit so you get no fuel.
This is the situation that the accelerator pump circuit is designed for. It’s meant to squirt a stream of fuel straight at the inlet port on the head. But: there’s a check valve that sits at the bottom of the carb and can get gummed up and stick shut if the carbs aren’t used for a long time and there are o rings that seal the jets to stop the pressure leaking away that can deteriorate, and mechanical linkage that can mean altogether no fuel gets pumped the moment you open the throttle, hence the stumble.

 

[Peato40]

Hi Jon, I think you hit the nail on the head. I took my carbs off last night and found that the squirt of fuel was weak in a couple of barrels and non-existent on a third. I think the pump (diaphragm) arrangement is okay as well as the lower check valve as they are common for both barrels, so presumably will effect each in the same way. I can see fuel being pumped into the galleries with the jets removed and squirting carb cleaner through the jets indicate no blockage there, so I think O'rings may be questionable. I don't have anything that small, so will need to get some ordered up. Of course getting the pressure there is very important, so would you know how I should go about ensuring the pump linkage is properly set up? Looks like there is a lot of adjustment available on the threaded linkage.

 

[jonbills]

With the carbs off and full of fuel, a sudden 25% or more movement of the throttle should send a stream of fuel instantly and first time at least the manifold length.
Once you've got that working, you can look at the pump jet size. I'd think you should be around a 40.
It's worth noting though that free revving and stationary, it's not impossible that too much fuel is causing it to bog down. I wouldn't go too crazy changing stuff until you've got that wideband AFR installed and you can see what's happening out on the road. on on a rolling road

 

[jonbills]

If you haven’t already, take the pump jet cover off the bottom of one and have a play with it to see how the linkage actuates the diaphragm. You can see how it can have slack in it.
I go by Des Hammill’a book:
https://www.amazon.co.uk/dp/B00OM3UNGC/ref=cm_sw_r_cp_awdb_t1_D34fBbVJT1EPA
I think it says you should have about 7mm of thread below the nut on the shaft on the throttle spindle.