New SUs- stumble off of idle

[Zaspen]

I just put a set of  Z therapy SUs on my 72 240 after removing the Holley 4 barrel it came with and I'm having an issue with a slight stumble off of idle.  I have them balanced well and the valves and timing are good.  I have the mixture set to the highest idle as well.  I am running water in the carbs/manifold and the problem is bad at cold and still noticeable when warm.  As I rev the engine it stumbles a bit and the pistons in the carbs seem to chatter a bit when they are rising as the stumble happens.  If I richen the mixture the problem seems to get better but the idle drops way low.  I can't tell if the problem is actually better at this point or if its just so rich that it slow to rev and I can't see the stumble.  I tried using a Gunson color tune but I'll be damned if I can tell the difference between yellow/blue and white/blue.  Any words of advice related to my problem would be greatly appreciated!

[7tooZ]

Where do you have your timing set and does your vacuum advance work? What RPM is your idle showing?

 

[Zaspen]

I set the timing at ten and it advances to about 35.  Idle is around 750.

[7tooZ]

If you back off the timing say to 7 BTDC does the idle drop?

If you advance to timing does the idle change?

or

There is a push pin on the bottom of the carb that lifts the piston. Does the idle change?

[Zaspen]

If I back the timing of to 7 BTDC the idle drops a little bit.  It also increases a little bit if I advance it.  Not nearly as much as I remember with the Holly carb. 

If it raise the piston with the pin the idle drops way down an almost stalls the car.  Both carbs have the same response.  I though this indicated a lean condition so I lowered the jet in 0.5 turn steps but the response to the piston lift never changed.  I have the jets set at 3.5 turns out normally and took them all the way down to 5 turns and there was very little change to the piston lift results.  At 5 turns out the car would barely idle under normal (no piston lift) conditions.  

[siteunseen]

What type fuel pump are you using?

Stalling out with the pins is normal. It kills that carb and only runs on the other one, so basically turns to a 3 cylinder. Front carb fuels 1,2,3 cylinders. Rear 4,5, and six.

Getting the cables right was pain for me so I'd start there. Swapping from a 4 barrel back to SUs is going to fun. You'll get it figured out with all the help you will get here.

My first thought was float levels. Try sticking a WD-40 straw down the overflow bungs to make sure the floats are freed up. You should feel them moving up and down. I remember the video saying shipping can bang those up sometimes.

Are the choke cables pulling the nozzle all the way up?

Edited May 21, 2019 by siteunseen

[7tooZ]

55 minutes ago, Zaspen said:

If I back the timing of to 7 BTDC the idle drops a little bit.  It also increases a little bit if I advance it.  Not nearly as much as I remember with the Holly carb. 

If it raise the piston with the pin the idle drops way down an almost stalls the car.  Both carbs have the same response.  I though this indicated a lean condition so I lowered the jet in 0.5 turn steps but the response to the piston lift never changed.  I have the jets set at 3.5 turns out normally and took them all the way down to 5 turns and there was very little change to the piston lift results.  At 5 turns out the car would barely idle under normal (no piston lift) conditions.  

So go back to your original settings when you started this post. Set timing to 7 and if idle drops to low just turn the idle screw to increase idle. Does it still stumble? Your idle mixture  should be around the 2.5 turn mark. I found I needed to drive the car to really find what would take out stumble.

[Zaspen]

I'm using the stock manual pump.  Not sure what you (siteunseen) mean about getting the cables right.  Are you referring to setting the choke cables correctly?  I basically pushed the choke lever fully open and attached the choke cables with the cable levers fully dis-engaged.  I then made sure they opened the butterflies evenly and pushed the jets down.  Is there more to it then this?

I'll give it a shot with the updated timing and see what happens also.  I do think at 2.5 turns out the car would be very lean- bet I will confirm.

 

 

[7tooZ]

On 5/19/2019 at 6:09 PM, Zaspen said:

  I have the mixture set to the highest idle as well.  I am running water in the carbs/manifold and the problem is bad at cold and still noticeable when warm.  As I rev the engine it stumbles a bit and the pistons in the carbs seem to chatter a bit when they are rising as the stumble happens.  If I richen the mixture the problem seems to get better but the idle drops way low.  I can't tell if the problem is actually better at this point or if its just so rich that it slow to rev and I can't see the stumble. 

Sorry if I made you make big changes. From your above comments it sounds like you are really close. It is hard to know how bad the “stumble” is from here. I went through what sounded like similar issues with mine. Once I thought I had the mixture close I started making small adjustments such as move the timing and then adjust the idle speed screw. Drive the car. The more you drive the more you will understand small changes. Even when I thought I had it right I learned last year when I had it on a dyno it was it lean. Even then it was only about a quarter turn rich to improve it. 

 

 

[Mark Maras]

What weight oil are you using in the SUs?

[7tooZ]

13 hours ago, siteunseen said:

 

Stalling out with the pins is normal. It kills that carb and only runs on the other one, so basically turns to a 3 cylinder. Front carb fuels 1,2,3 cylinders. Rear 4,5, and six.

@siteunseen so I I am still learning things. I was advised that lifting the pin should not change the idle but that it is an indication of  the need to further adjust the mixture. maybe @Terrapin Z can weigh in here. I know he has more SU experience is than I do.

[Captain Obvious]

On ‎5‎/‎19‎/‎2019 at 9:09 PM, Zaspen said:

As I rev the engine it stumbles a bit and the pistons in the carbs seem to chatter a bit when they are rising as the stumble happens.

What Mark said... What kind of oil did you use to fill the damper tubes on the SU's when you put them on? It sounds like a classic case of too thin of an oil or a damper dashpot not working properly.

Can you elaborate a little on the "chatter" you mentioned with the pistons? Any chance you can take a video of that?