Triple dellortos & pump jet angles

[jonbills]

I thought I'd post this up here, since I've been looking for info like it everywhere on the Internet, and not found anything.

To get to the point quickly - I've found that the accelerator pump jets need to be modified and re-drilled at an angle of about 10 degrees left or right, and about 15 down on my L6 engine & manifold (& I expect this to be true of all others, and probably weber DCO too).
This is because the accelerator pump jet needs to land a squirt of fuel in the inlet port, and the shape of the L24 manifold prevents this with stock pump jets.

I bet now I've posted this, loads of you old skool fellas will tell me this is common knowledge. Go on then . There's probably a thread on here about it somewhere too eh?

The longer story:

I've got an L24 with triple dellorto DHLA 45s with 36 mm chokes.
It is 'over-choked' for sure, and I'd always put the stumble in the acceleration phase down to that (I mean giving it full throttle in any gear from below about 3k resulted in a complete loss of power!)

However, when reassembling the carbs on the manifold last summer, I noticed that although I got a nice squirt out of the carbs on their own, as soon they're on the manifold there's no stream of fuel coming out.

Looking at the manifold, it's obvious why - the runner curves left or right, AND down quite a bit, and the fuel stream hits the inside of the manifold and does no good at all.

I did lots of googling for this at the time, but found nothing and gave up, assuming that in a running engine, perhaps the airflow would help the fuel into the engine. After all, the Internet would surely have highlighted such a glaring design mismatch. So I dropped it.

a couple of months ago, I got round to installing an AFR gauge.

It was quite revealing - it confirmed that when I open the throttle fast from low revs, there's no fuel, just air.

so I decided to do something about it - I've now redrilled the jets and the squirt comes cleanly out the other side of the manifold and the stumble is gone in many cases. It's much better accelerating up through the gears.
I've still got more to do - I think with my large chokes I need to make the squirt duration last much longer to be able to snap the throttle open in 4th at 40 mph- currently it's less than 1 sec, and it probably needs to be twice as long, but I hope that's just a matter of finding the right spring combinations on the pump rod and diaphragm.
Much more fun than all this talk of 500 bhp hybridz eh?

 

[Alphabettispaghetti]

Can we have a forum for us with low power aspirations!?

 

[Peato40]

I thought I'd post this up here, since I've been looking for info like it everywhere on the Internet, and not found anything.

To get to the point quickly - I've found that the accelerator pump jets need to be modified and re-drilled at an angle of about 10 degrees left or right, and about 15 down on my L6 engine & manifold (& I expect this to be true of all others, and probably weber DCO too).
This is because the accelerator pump jet needs to land a squirt of fuel in the inlet port, and the shape of the L24 manifold prevents this with stock pump jets.

I bet now I've posted this, loads of you old skool fellas will tell me this is common knowledge. Go on then . There's probably a thread on here about it somewhere too eh?

The longer story:

I've got an L24 with triple dellorto DHLA 45s with 36 mm chokes.
It is 'over-choked' for sure, and I'd always put the stumble in the acceleration phase down to that (I mean giving it full throttle in any gear from below about 3k resulted in a complete loss of power!)

However, when reassembling the carbs on the manifold last summer, I noticed that although I got a nice squirt out of the carbs on their own, as soon they're on the manifold there's no stream of fuel coming out.

Looking at the manifold, it's obvious why - the runner curves left or right, AND down quite a bit, and the fuel stream hits the inside of the manifold and does no good at all.

I did lots of googling for this at the time, but found nothing and gave up, assuming that in a running engine, perhaps the airflow would help the fuel into the engine. After all, the Internet would surely have highlighted such a glaring design mismatch. So I dropped it.

a couple of months ago, I got round to installing an AFR gauge.
View attachment 10323
It was quite revealing - it confirmed that when I open the throttle fast from low revs, there's no fuel, just air.

so I decided to do something about it - I've now redrilled the jets and the squirt comes cleanly out the other side of the manifold and the stumble is gone in many cases. It's much better accelerating up through the gears.
I've still got more to do - I think with my large chokes I need to make the squirt duration last much longer to be able to snap the throttle open in 4th at 40 mph- currently it's less than 1 sec, and it probably needs to be twice as long, but I hope that's just a matter of finding the right spring combinations on the pump rod and diaphragm.
Much more fun than all this talk of 500 bhp hybridz eh?

Hey Jon, this sounds very familiar. I'm in the process of commissioning my L28 with Triple Dellorto's and with your advice I also installed a wideband AFR gauge and pretty much got the same results. Can I ask what size pump jets you have installed? and what size drills did you use to alter the squirt angle? I upped my jets from 33 to 40 and it made a healthy difference but I still have some hesitation and pinking between idle and full throttle. It seems to go very well on full throttle but getting there is a bit hit and miss.

I have one of those 123 ignitions so have been trying to improve things with that but so far I haven't achieved where I want to be. Actually on that note, I have been playing with the advance curve but as I'm not having a vac pipe installed, I haven't done anything with the vacuum advance curve .... do you know if I do right?

 

[jonbills]

Hi Phil, I'm not very good at writing things down, but I think I'm calling them 35s because I used a 0.35mm drill on them. It wasn't the end of the story though - I've learned a few more things since then.
The main thing is the transition from the idle/progression circuit to the main circuit, and that the accelerator pump can't really cover for it if there's a gap, which there will be if you have choke sizes optimized for top end power.

Remind me which size Dellortos you have? 40 or 45? And choke sizes?
Mine are 45s with 39mm chokes and I found it was going lean as the throttle plate moved from progression hole 2 and 3. Essentially this is why people recommend small chokes for 'street' use. I moved my 3rd hole nearer hole 2. I now have a small lean gap between 3rd progression hole and the main circuit.
Might be easier to explain with some pictures [emoji4]

 

[Peato40]

Hi Phil, I'm not very good at writing things down, but I think I'm calling them 35s because I used a 0.35mm drill on them. It wasn't the end of the story though - I've learned a few more things since then.
The main thing is the transition from the idle/progression circuit to the main circuit, and that the accelerator pump can't really cover for it if there's a gap, which there will be if you have choke sizes optimized for top end power.

Remind me which size Dellortos you have? 40 or 45? And choke sizes?
Mine are 45s with 39mm chokes and I found it was going lean as the throttle plate moved from progression hole 2 and 3. Essentially this is why people recommend small chokes for 'street' use. I moved my 3rd hole nearer hole 2. I now have a small lean gap between 3rd progression hole and the main circuit.
Might be easier to explain with some pictures [emoji4]

Hi Jon, I also have 45's with 36mm chokes. Yes I would really appreciate a diagram when you get a chance.

Was your engine also pinking in that mid range?

Sent from my SM-G930F using Tapatalk

 

[jonbills]

No, didn't notice pinking - what AFR are you seeing when it's pinking?

 

[Peato40]

No, didn't notice pinking - what AFR are you seeing when it's pinking?

I think the AGE was about 13 but I'll do some more testing tomorrow. With 33 pump jets the AFR went to 19, which is why changed to size 40. Yesterday I was playing with advance curve but ended coming back to 12 at 1000RPM and then a straight line to 34 at 3000RPM.

Sent from my SM-G930F using Tapatalk

 

[jonbills]

its pinking at AFR 13?
I'm sure you know but the pump jets are only active for reasonably fast throttle movements, so just cruising around they shouldn't be making a difference.
I did find on mine that it's not unusual for not all of the pump jets to fire, so it could be that although the o2 sensor is seeing an average AFR of 13, some cylinders are very rich and some are very lean so some could be pinking even though the average AFR is OK (if the pump jets are involved at the time!)

Here's a pic to help understand the progression holes stuff.

So two pics, one with the first hole uncovered and the 2nd with all 3 uncovered.
You can see there's a range of movement of opening the throttle between each hole where the throttle is getting more open and more air is going into the engine but the amount of fuel isn't increasing (so its getting leaner until it reaches the next hole)
Hopefully you can also imagine that the bigger the choke the more air flow there is for a given amount of throttle opening. When cruising around at say 60 with 32mm chokes you'll be operating perhaps between beginning of hole 2 and mid hole 3. With 36 mm chokes the cross sectional area is greater so you'll be operating in a shorter linear range of the progression holes, perhaps start of hole 2 to end of the gap between 2 and 3.
The problem then is when the mixture goes too lean on this long journey between progression holes. You can increase jet size of course but there comes a point where you cant go bigger because it'll make it too rich earlier in the movement and its still not enough later on.

And the same problem can happen between the last progression hole and the main circuit starting. And you can't really know which problem you have (without something like my throttle position display, mentioned in another thread last year!)

Which emulsion tubes do you have? They can definitely make a big difference to when the main circuit starts. I have a small range. Are you around on Sunday? I could come over and we could do some experimenting.

 

[Peato40]

Hi Jon .... I hadn't appreciated the progression holes are adjustable. My car is.pinking when I accelerate or when on load and not full throttle.

I'll be in after 13:00 tomorrow, you would be more than welcome if you can make it?

its pinking at AFR 13?
I'm sure you know but the pump jets are only active for reasonably fast throttle movements, so just cruising around they shouldn't be making a difference.
I did find on mine that it's not unusual for not all of the pump jets to fire, so it could be that although the o2 sensor is seeing an average AFR of 13, some cylinders are very rich and some are very lean so some could be pinking even though the average AFR is OK (if the pump jets are involved at the time!)

Here's a pic to help understand the progression holes stuff.
View attachment 30271
So two pics, one with the first hole uncovered and the 2nd with all 3 uncovered.
You can see there's a range of movement of opening the throttle between each hole where the throttle is getting more open and more air is going into the engine but the amount of fuel isn't increasing (so its getting leaner until it reaches the next hole)
Hopefully you can also imagine that the bigger the choke the more air flow there is for a given amount of throttle opening. When cruising around at say 60 with 32mm chokes you'll be operating perhaps between beginning of hole 2 and mid hole 3. With 36 mm chokes the cross sectional area is greater so you'll be operating in a shorter linear range of the progression holes, perhaps start of hole 2 to end of the gap between 2 and 3.
The problem then is when the mixture goes too lean on this long journey between progression holes. You can increase jet size of course but there comes a point where you cant go bigger because it'll make it too rich earlier in the movement and its still not enough later on.

And the same problem can happen between the last progression hole and the main circuit starting. And you can't really know which problem you have (without something like my throttle position display, mentioned in another thread last year!)

Which emulsion tubes do you have? They can definitely make a big difference to when the main circuit starts. I have a small range. Are you around on Sunday? I could come over and we could do some experimenting.

Sent from my SM-G930F using Tapatalk

 

[jonbills]

Most people would say they're not adjustable. It's possible to drill new holes and close old holes. But not to be done lightly since the holes have to line up exactly and the it's rather irreversible.
Gimme your address by Pm and I'll come and visit tomorrow afternoon with some emulsion tubes and jets and we'll see if we can get it any better. I won't be drilling any holes [emoji16]

 

[jonbills]

Which head do you have Phil and what sort of compression ratio?

 

[Peato40]

Which head do you have Phil and what sort of compression ratio?

Hi Jon,

PM sent I hope .... found it a bit difficult to do with my mobile. Pls let me know if you don't get my address.

I forget which head I have E88 N something. I worked out the compression ratio and have it written down at home but can't remember what it is .... relatively high though because I used flat too pistons.
I also put in a fast cam and wasted step big valve conversion.

Sent from my SM-G930F using Tapatalk