New SUs- stumble off of idle

[Terrapin Z]

9 hours ago, 7tooZ said:

@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.

I had found in a couple of SU tuning instructions about lifting the pins to check the mixture. Haynes, Chitons, and even the FSM all only mention this pin as a way to determine if the piston and/or needle is aligned correctly and not sticking for any other reason. 

The other publications I found had, not only the method above, but referred to lifting the pin very slightly (1/32 the web page says "good luck" about this) to help determine the idle AFR to some degree. If lifted any more than that they will stall that carb out like "site" says ^

This one from the web :  http://www.sucarbs.com/?page_id=20 Under "test fuel mixtures"

And the attached Tuning SU Cabs book. I am sure I read about the pins in another book, but can not find that reference right now. (page 25 I think)

I'm no expert by any means, but mine a running pretty good.

Tuning_SU_Carbs.pdf

Edited May 21, 2019 by Terrapin Z

[Zaspen]

So I believe Z Therapy uses ATF in the dashpots so that is what is in there. 

I attached a video that shows the 'chatter".  Perhaps its more of a hesitation.  The first time I rev in the video is from idle and it happens, the second rev I don't let it get down to idle completely and it seems fine and then the third is again from idle and it hesitates again.  I feel this hesitation when I take off from a stop or in between "slow" shifts.

 

I also tried the lifting the pistons again and it dies with any amount of lift <1/32"

[Terrapin Z]

I know you said these are fresh from Z therapy. I did a swap with them years ago. I just bolted them on and did the basic tune. I never could quite get it to run right. always seemed lean and uneven.

Recently I was going back through all the adjustments again and this time added the float level check. I found the rear to be way off. Once I reset the levels with the Float Sync. it ran much better.

It was running mostly on the front three just fine, but the rear three were much leaner due to the float height.  Your video sounds lean to me, but it's hard to hear clearly on my computer speakers.

Just something to at least rule out. No ill will against ZT, it just might have been missed. It did trouble me for some time thinking they had to be set up correctly.

[madkaw]

Have you run the engine enough to pull the plugs and look? Terrapin has a good point. Had the same issue of one carb being lean( equals 3 cylinders) . Not sure if you are running a vacuum advance or not, but if she’s real lean and not enough timing she’s going to balk. 

 

[Zaspen]

I need to do a plug check soon. I think I'll take it to work tomorrow and try on the way home. 

 

 

[Captain Obvious]

3 hours ago, Zaspen said:

I attached a video that shows the 'chatter".

Good video. It looks to me like the pistons are rising too rapidly the first time when you goose it and they overshoot. Then they recover where they should be and rise with the engine RPMs as they should. That overshoot situation results in a short lean burst when you goose it. Just like your what you describe with your stumble off idle. In fact, that's the exact reason the piston dampers exist in the first place.

There are typically two things that can typically cause that:

1) The first thing to look into is... Are your damper dashpots working properly? Because if they aren't then it won't matter WHAT kind of oil you're running in the carbs. So when you stick a finger in and manually lift up on the piston. Is it hard to lift up, or can you push it all the way up easily? Does it leave a dent in your finger?

2) The other thing to think about is the oil (or ATF if that's what you are using). If the dashpots are working properly, then maybe the oil is too thin allowing the piston to rise too quickly. Simple thing to try is running some heavier oil in there. I used to run ATF, but recently switched to 20W-50.

But you have to make sure the dashpots are working correctly before you start trying other oil weights.

[siteunseen]

Could the springs in the pistons be stretched some to add resistance? Mine are pretty "heavy" feeling when i raise them. Using 20wt 3in1 oil. Car runs great.

Captain O, "dent your finger" is a great description.

As a quick and inexpensive way to check fuel levels I used this first and saw how low mine were then went deeper, pun intended.

 

Edited May 22, 2019 by siteunseen

[Zaspen]

I believe the description of the piston rising too quickly and then becoming to lean was spot on. I made a big time rookie mistake and assumed the dashpot oil level was correct and it was low. I never looked into the correct level and since oil was touching the bottom of the "dipstick" I thought all was well. I put in the correct amount and the chatter is gone! But now without the chatter when I take off its like a bucking bronco. It's as if I'm just learning to drive a manual. If I give it gas after the clutch is out it starts bucking. Not like a loss of power I don't think but I'm not sure. It does it as I shift in 2nd also. Any thoughts? Thanks to all for the help with this! 

 

 

[Captain Obvious]

10 hours ago, siteunseen said:

Captain O, "dent your finger" is a great description.

Yeah, it's tough to describe how a properly working piston should feel because you have to use subjective words. If you've felt a properly working one then you know what to look for, but if the only thing you have to convey info is words, then it's all subjective. And don't stretch the springs unless you understand the implications. You shouldn't have to do that.

OP, the pistons should be "hard to lift with a finger" and they should "leave a dent in your finger" when you lift them. It should be "hard" to get them started and you'll have to "pry them up with a finger tip" at first when they are fully down.

So here's a less subjective way to check to see if the damper pistons are working properly.... Take the damper stalks out completely and lift the pistons. Then put them back in and see how hard it is to lift the pistons. It should be "much" harder to lift them when the dampers are installed.

If there isn't "significant" difference between the forces required to lift the pistons with and without the dampers installed, then there's potentially an issue there.

[Captain Obvious]

Oops. We were posting at the same time. Glad to hear that the pistons are rising smooth now. It's pretty much impossible to get "too much" oil into the damper stalk hole. If you put too much oil in the hole, Archimedes will displace the excess when you insert the damper check valve jiggly bits into the carb. And that excess will just run down into the carb and be burned by the engine.

So I'm not sure at all about the current problem now. Not sure what you are trying to describe. But good progress so far!  

[Mark Maras]

 

 

1

1 hour ago, Zaspen said:

 But now without the chatter when I take off its like a bucking bronco. It's as if I'm just learning to drive a manual. If I give it gas after the clutch is out it starts bucking. Not like a loss of power I don't think but I'm not sure. It does it as I shift in 2nd also. Any thoughts? Thanks to all for the help with this! 

 

 

 I believe the bucking is caused by a lean mixture. Too rich would just result in a lack of power but not bucking. The float levels are the primary mixture adjustment. The mixture screws are the fine tune secondary adjustment. I'd pull the floats and check the float adjustment and or the fuel level. How many turns down are the mixture screws when it bucks?

[Zaspen]

3.5 turns is the highest idle. I tried 4 turns but had the same issue. I will check the floats tonight.