Valve check

[Hap Waldrop]

anybody thought about the rings not seated yet

Hondo

The plugs would be wet with oil.

 

[Member 10617]

Hap, that's what I thought. The plug in question was never wet or oily, only black sooty.

Success or not, I do plan to do a leak down test when I get the chance. In the meanwhile, things seem to be back to "normal" (whatever "normal" is with these cars.

Right now the #1 plug is showing no build-up of soot, the exaust is a fairly steady flow (not much "puffing"), the exhaust is clear (no traces of smoke), the idle is steady, power is great and pick-up is outstanding, no problem going up steep grades... etc., etc.

This is all pretty new to me. But it looks OKI.

One small thing I noticed. The piston in the back carb falls freely and stops with a loud click; the piston in the front carb falls a little slower and stops with more of a clunk than a click. Doe that mean anything significant?

 

[Andrew Mace]

One small thing I noticed. The piston in the back carb falls freely and stops with a loud click; the piston in the front carb falls a little slower and stops with more of a clunk than a click. Doe that mean anything significant?

It means that the carburetors likely are not quite working together. It could be as simple as different oil levels or even different weights of oils in the two dampers, or some accumulated varnish/gunk is slowing the front piston. Worth a quick clean to find out, and that could still have a bit of effect on 1 & 2 cylinders.

 

[TR3driver]

Perhaps because there's that balance tube on the manifold (isn't there? I'm working from feeble memory and it's late at night) that -- for the #2 cylinder -- would help negate some of the imbalance between the two carbs? #1, being further away, wouldn't benefit so much.

My thought exactly. Front carb was closed off and not admitting enough fuel/air. #2 is both closer to the crossover tube (which is fairly small), and draws before #1, so it was still getting enough to run well enough. But #1 was pulling so much vacuum that it could not burn completely, leaving the film of soot at idle. Then when the throttle was opened, the soot burned off and it ran fine.

 

[TR3driver]

It could be as simple as different oil levels or even different weights of oils in the two dampers, or some accumulated varnish/gunk is slowing the front piston. Worth a quick clean to find out, and that could still have a bit of effect on 1 & 2 cylinders.

Another possibility is that someone has inadvertantly swapped the pistons between domes. The domes/pistons are a matched set, so interchanging them may lead to light dragging in one dome.

Anyone got the link to John Twist showing how to do the drop test?

 

[poolboy]

When I run into a situation like that, I remove the screwed in damper. Then do your test without the "piston" submerged in the oil and dampening the air valves' movements.
Sometimes it's just a slightly cockeyed piston shaft that's riding the side of the damper well's wall.
Eliminate that possibility first.

 

[Member 10617]

Randall,

In reference to the balance tube idea. I ran the car down the hiway, with no stops except to turn around and return home. No stops, and no idling, except the moment I got home and turned the car off. Soot on #1.

Yesterday, I did the same thing and got no soot. So although I know that running down the road will burn some of the soot off, it didn't work the first time.

And as for swapping the pistons... I wouldn't know. I had the carbs completely rebuilt when the engine was rebuilt... and a shop that does this almost exclusively did the work. I would think they would not have made the mistake, but who knows. Is there some test to determine this?

Poolboy. The front piston falls more slowly than the rear piston, but that is only in relation to the rear piston. It falls freely and makes a clunk at the bottom of the fall.

When you say "do your test," what do you mean: run the car down the road without oil in the dashpot? Or lift the piston and see if it falls? If it is a cockeyed piston shaft, can I just ignore it?

 

[poolboy]

I meant just lift the air valve (piston) and see if they both fall the same without the damper piston, Ed.
If the damper piston is on a cockeyed shaft (rod), just straighten it so the damper piston doesn't rub on the on the walls of the damper well (the oil container or reservoir) as the air valve rises and falls.
Terms may be confusing. I hear air valves being called pistons,.. dashpots called dampers, etc, etc.
Just unscrew the thing on top and remove it, lift the air valves and compare, is the simplest way I can think of to describe it.
https://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601d.htm

 

[TR3driver]

it didn't work the first time.

Er, no, all you know is that there was soot when you looked. It might have burned off on the highway, and been replaced when (nearly) idling as you pulled back into your driveway.

I would consider the difference in piston action to be important. The position of the piston has a profound effect on mixture, as it controls both the velocity of the air through the venturi (which in turn controls the depression or partial vacuum that pulls fuel through the jet), and the size of the effective jet opening.

'Poolboy' is referring to checking again that the pistons fall at the same rate with the damper rod removed. If they do, that indicates some problem with the damper, like different oil or a bent rod, etc.

The test for mis-matched domes is similar to what you've been doing, except you remove the domes & pistons from the carbs, and plug the air holes in the bottom of the piston (and the vent at the top if necessary). That way the piston falls entirely based on air leakage between it and the dome. If one falls faster than the other, try switching them and see if that evens it out. If not, it may be time to replace the piston & dome.

I don't have the book in front of me, but there is a specified time for the piston drop with the holes plugged. There's also a good video on the net somewhere, showing John Twist doing the test.

 

[Darrell_Walker]

Here is the John Twist video:

https://www.youtube.com/watch?v=IfU47Oqq9wA

 

[CJD]

Good stuff! I thought SU's were magic!

John

 

[Member 10617]

Randall and Darrell,

Terrific video! Many, many thanks. I can do that! John Twist is great!!!

But, first, I have to take the domes off. How is that done without doing damage to anything else? Specifically, I know there is a needle somewhere inside there and I don't want to damage it in any way.

Also... dumb question ... do I take the oil out before doing this, or can I pour it out when I get the domes off?

Poolboy,

Let me see if I understand you correctly, when you speak of the damper piston, you are talking about the small piston at the bottom end of the damper rod (yes?). So you are saying that I should look at the damper rods to make sure they are not bent or cockeyed (yes?). My rods are straight as... well... rods. So if something is bent or cockeyed, it is something else.

BTW: I am still mystified by the fact that #1 plug, and only #1 plug was sooty. The #2 plug, same carb as #1, has shown no soot at all. Which leads me again to question (a) the valves for #1 cylinder (although there was no oil on the plug), or (b) someting electical (although I tried three new plugs, checked the lead, ran a continuity check between distributor and the cap on the plug.

When I had the spring Tuneup, the mechanic removed the old plugs and said they were all normal. When I got home and looked at the new plugs, #1 was sooty. Something had changed. The only thing they did in the shop was to reset the valve gaps, install a new distributor cap, install new points and rotor, and reset the mixture.

Whatever the heck was going on, it isn't happening any more. #1 remains clear of soot.

Gremlins, no doubt.

 

[Darrell_Walker]

But, first, I have to take the domes off. How is that done without doing damage to anything else? Specifically, I know there is a needle somewhere inside there and I don't want to damage it in any way.

1. Remove the dampener rod
2. Remove the 3 or 4 screws that hold the dome to the carb
3. Carefully lift the dome off. The piston should stay on the carb, but may want to come up as well. There will be a spring in there, too.
4. Carefully lift the piston (with needle) off of the carb body.
5. There is a set screw that holds the needle in the piston. I'd take out the needle before doing much more, just to keep from damaging it. Note the installed height of the needle before removing it
6. Installation is the reverse, as they say

Also... dumb question ... do I take the oil out before doing this, or can I pour it out when I get the domes off?

No dumb questions! Dump it out after you remove the piston.

 

[Don Elliott]

You remove the domes (one at a time and keep them sparate so the front parts all go back onto the front carb) then you lift off the dome, then lift up the piston and pour out the oil if you are planning to replace the oil - or if you place the piston on its side and the you lose the oil. If you can manage to clean the labyrinth seal grooves on the pistons with a clean soft cloth without turning the piston up-side down, you can use the same oil if you want to.

 

[Member 10617]

Darrell and Don,

I should be able to do that. While I have access to the needle, is there some adjusting to be done with it, or do I return it exactly to the same height?

And another question. What if the needles don't match -- front and back? I recall that about 6 months ago the mechanic said something about the carbs being too rich, or something like that, and changed one or both needles. I had no idea what he was doing. If the needles are not the same, and the pistons have been switched, do I leave the needles with the pistons or do I make sure that the rear needle remains in the rear piston, and vice versa.

The plot thickens....

And what determines the hight of the needles?

 

[poolboy]

Yes you understood me correctly, Ed.
As long as the rod isn't tilted even the slightest amount, the damper piston should remain in the center of the bore and not hinder the air valves' movement.

 

[Member 10617]

Poolboy: I added another question to the previous posting while you were answering. Could you please take a look at it? Thanks.

 

[Darrell_Walker]

I should be able to do that. While I have access to the needle, is there some adjusting to be done with it, or do I return it exactly to the same height?

It is possible that you will need to center the jet, if the needles aren't perfectly straight. I've never had a problem, but something to be aware of.

And another question. What if the needles don't match -- front and back? I recall that about 6 months ago the mechanic said something about the carbs being too rich, or something like that, and changed one or both needles. I had no idea what he was doing. If the needles are not the same, and the pistons have been switched, do I leave the needles with the pistons or do I make sure that the rear needle remains in the rear piston, and vice versa.

The plot thickens....

The needles should be the same. If they aren't, the carbs will be supplying differing mixtures.

And what determines the hight of the needles?

I'd look it up in the manual, but as I recall, having the wider shoulder of the needle flush with the base of the piston.

 

[poolboy]

Lord, Ed before you know it we'll have you taking your carbs apart trying to make sure everything is copasetic.
I'm gonna pull out of this before that happens, but before I do, let me say this,
'Leave the carbs alone now and go for a drive.'

 

[Member 10617]

Poolboy... Ha, ha.... you see what you guys have created!

Actually, I would enjoy knowing how to take the carbs apart to make sure everything is copasetic. Not that I don't have lots of other things to do, but I find all this very challenging and enjoyable. I have the SU Cabrurettors DVD and have watched it once, but too much detail for me then. Now I think I would have a better handle on it, thanks to the Forum.

As for driving... Even with winter hereabouts, I've put on about 1000 miles since 1 January... I have an allotment of 4000 @year because of antique plates requirement, and I'll probably have that by the end of summer... with a few miles to spare for a beautiful Virginia fall.

Wish I could buy you all a beer!