1966 Atlas Amal monoblock setting

pierodn · Jan 16, 2021

Hi All.
I am starting to overhaul the stock 389/241 and 689/241 monoblocks.
Stock setting has main 420 jet, 20 pilot, cut 3 slide, 106 jet, 3 position needle.
I have a pair of new nos cut 3,5 slides that i want replace with the original cut 3.
Question is if with a large cut i must reduce or contrary the main jets or other.
Thank you.
Piero

texasSlick · Jan 16, 2021

Piero:

Increasing the cut away decreases the mixture strength and vice versa, but ONLY up to approx 1/8 to
1/4 throttle opening. Therefore, only the pilot circuit and needle position are strongly affected by changing the cut away, the main jet affected not at all.

I have heard of improvements in low throttle engine performance simply by changing cut away, but I am an advocate of resorting to original factory settings.

Good luck with your project.

Slick

pierodn · Jan 16, 2021

texasSlick said:
Piero:

Increasing the cut away decreases the mixture strength and vice versa, but ONLY up to approx 1/8 to
1/4 throttle opening. Therefore, only the pilot circuit and needle position are strongly affected by changing the cut away, the main jet affected not at all.

I have heard of improvements in low throttle engine performance simply by changing cut away, but I am an advocate of resorting to original factory settings.

Good luck with your project.

Slick

Hi Slick.
I understand what you mean.
The problem is that i need to change the old slides because they are very worm.
Easy to buy the new 389 cut 3 but was impossible to find a 689 cut 3 while easy for 3,5.
So i must go on with 3,5 cut away.
In any case a 400 jet is not very big?.
Thank you.
Piero

dynodave · Jan 16, 2021

pierodn said:
Hi Slick.
I understand what you mean.
The problem is that i need to change the old slides because they are very worm.
Easy to buy the new 389 cut 3 but was impossible to find a 689 cut 3 while easy for 3,5.
So i must go on with 3,5 cut away.
In any case a 400 jet is not very big?.
Thank you.
Piero

Old worn slides can be sleeved.
The body is bored to clean up wear.
Then slides finished ,to fit the new body bore, and retain the original cutaway.

This is what I will have done to my 389/689 for my 66
Cost a little more than doing concentrics

pierodn · Jan 16, 2021

dynodave said:
Old worn slides can be sleeved.
The body is bored to clean up wear.
Then slides finished ,to fit the new body bore, and retain the original cutaway.

This is what I will have done to my 389/689 for my 66
Cost a little more than doing concentrics

Hi .
You can do it.
I am not able to do it.
So that i can only replace a new 3,5 cut.
I hope that +0,5 will do not a big difference.

dynodave · Jan 16, 2021

I put them in a box and mail them. If he does not want to do monobloc anymore than I will do my own work.

pierodn · Jan 17, 2021

Please a question.
What is the correct side of the stay up float?.
The side with Amal sign or the other without?.
Is not the same, because if you change side the float seats more into or out!.
Thank you.
Piero

texasSlick · Jan 17, 2021

pierodn said:
Please a question.
What is the correct side of the stay up float?.
The side with Amal sign or the other without?.
Is not the same, because if you change side the float seats more into or out!.
Thank you.
Piero

You should see "Amal" when float is seated in bowl.

Slick

pierodn · Jan 17, 2021

texasSlick said:
You should see "Amal" when float is seated in bowl.

Slick

Thank you.
Piero

pierodn · Jan 17, 2021

With slide cut 3,5 instead of 3 can i compesate with 25 pilot instead of 20?.
What change from 20 to 25?.
Sorry for my empirical question!

texasSlick · Jan 17, 2021

pierodn said:
With slide cut 3,5 instead of 3 can i compesate with 25 pilot instead of 20?.
What change from 20 to 25?.
Sorry for my empirical question!

I would first try raising needle, then resort to 25 pilot jet. Another option is to drill out 20 pilot jet by one drill size at a time. You will have to experiment.

Amal jet numbers refer to the ml/min the jet will pass under standard test conditions.

Slick

pierodn · Jan 18, 2021

Hi All again.
Please a question.
With the 389 and 689 in place is not possible to turn the throttle screws because they touche one against the other and they freeze.
Have you ever had this problem?.
I thought to late the screw heads reducing the circumference.
Thank you.
Piero

texasSlick · Jan 18, 2021

My throttle screws were not touching, but I made the adjustment easy by putting an extension on the screw.

Slick

kernel65 · Jan 18, 2021

I have a matched set 389/689-236 date stamped 2/67 that has a regular throttle screw on one and a smaller slotted head on the other. Maybe someone turned it down on a lathe?

I have a picture but cannot post here.

Scott

Bodger · Jan 18, 2021

texasSlick said:
My throttle screws were not touching, but I made the adjustment easy by putting an extension on the screw.

View attachment 21156

Slick

Elegant solution to one of the worst PIA issues with owning chopped Monoblocs. I guess it takes a genuine engineer to think of that!

pierodn · Jan 19, 2021

kernel65 said:
I have a matched set 389/689-236 date stamped 2/67 that has a regular throttle screw on one and a smaller slotted head on the other. Maybe someone turned it down on a lathe?

I have a picture but cannot post here.

Scott

I thought to do the same

pierodn · Jan 19, 2021

Hi All,

please, i need your help again.

I am setting my Monoblocks 389/689 of 1966 Atlas.

JUNCTION BOX FOR CABLES are the black plastic n. 01.9824.

Seems that this “junction box” for monoblock cables has the stroke too short; with the twist grip full opened the slides dont arrive up to the end of stroke but far..

May be Monoblocks need the steel/brass junction box may be longer?.

Or cables for monoblock are not good and have a longer free end?.

Please, let me have your advices.

Thank you.

Piero

1966 Atlas Amal monoblock setting

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