A moment of apprehension

[Grumblebuns]

For those who don't know, the damper oil delays the rise of the piston giving a momentary richer mixture when the throttle opens.  It fulfills the same function as an accelerator pump in a non-CV carb.

Hmm... Interesting.  Unless there is a difference in theory of operation between the SU and Stromberg or I'm missing something (never to be discounted), that seems counter intuitive. Assuming they operate under similar principles, that would imply that oil would be required or at least beneficial in a race car since I would think you would certainly want a richer mixture on acceleration otherwise you might get detonation which would be extra bad for a race car. I had always assumed the oil was to dampen out the fluctuations in the motion of the piston rather than adjust the mixture. I also assumed (or maybe someone all those years ago told me) that it also enhanced throttle response. What am I missing, John?

BDA, just a guess on my part but this is the reason on why I think the ZS carbs need damper oil for the air valves. Each carb needs to feed two cylinders through a siamesed  intake with each cylinder at a different part of the combustion cycle. Without dampening oil, the air piston would be cycling all over the place for each pulse of the piston with the most vacuum and may not be able to respond for the other cylinder for it's turn for fuel mixture. The dampening oil evens out the pulses and reduces the extreme cycling.

Motorcycle carbs generally have CV type carbs for each cylinder and as far as I know, do not have oil. But since each carb is responsible for one cylinder, the air valve operates for that one cylinder only.

As I said, this only a wild ass guess on my part.

Joji Tokumoto

[pboedker]

The damper oil is for dampening the air valve in only the opening direction. When you unscrew the damper rod you will see that it has some seemingly loose shims and tube parts hanging down into the tube with oil. This is in fact a very clever oneway oil valve, restricting the air valve from moving up, but allowing it to drop rapidly.

This is the dampening that you, Joji, felt with your finger. And also the dampening that John has mentioned as necessary for the fuel enrichment when accelerating. 

[jbcollier]

Race cars are usually jetted quite rich and may not require the extra enrichment provided by the oil in the dampers.  It's also very common to disable accell pumps on motorcycles tuned for racing.  Cars/bikes tuned for the road, however, try to eke out as much economy as possible which will leave them rather flat footed when the throttle is suddenly opened.

The oil in the damper provides enrichment by slowing the rise of the piston.  This causes a temporary increase in airflow across the jet which provides the necessary enrichment.

[BDA]

Joji, That seems like a reasonable enough theory.

Thanks for the explanation about rich jetting in race cars, John. I don't have any way to verify it but it sounds plausible.

All this talk is reminding me of how I can think I understand the principles behind a design but only get the obvious part and miss critical aspects. I don't remember what the internals of the Stromberg look like, but it may have a similar design to the SU. It has a jet that sits in a sort of an upside down 'V' shaped rise that goes across the entire throat and was probably 1/3 the diameter of the throat (see crude text picture):
    __
    | |
   /   \
  /      \
 /         \
At the top of the 'V', there was a squared off section where the jet opening was. It was common to give a slight radius to the piston to increase flow and some tuners would hog out the 'V' shaped part and just put a ramp the width of the jet in front of and up to the jet. Of course, that let in a lot more air and the ramp was supposed to allow increased air flow to pull the fuel out of the jet. Well, I got the bright idea to radius that 'V' taking off the square part on top figuring that would give more flow and that flow wold suck up the required fuel. Well, it didn't work, I burned a few pistons and to trying to get around that, I had to make it so rich, it wouldn't run (thinking about it, it might have worked with the correct needles but I didn't have ready access to a dyno or a practice track). I think the reason it didn't work is that the square part induced turbulence that sucked out more fuel and properly mixed it up. I don't know how the carbs with the ramps worked.

(thanks for indulging my short walk down memory lane!)

[Grumblebuns]

BDA, I was just thinking out loud with my explanation, probably way off base with that one. Also, trying to improve or modify fuel/air mixture without years of experience and access to a flow bench or dyno is probably pretty difficult with just seat of the pants impressions.

I just got back from a 20 mile run this morning on the TCS and I'm pretty happy with how the car performed. Was able to go through the gears even into fifth. Everything seems to work fine except for the feeling in the steering during turn in, very little input from the tires. The PO installed a set of semi economy Nexen tires. I would say that the TCS is now "on the road".

This morning I discovered another landmine left by the PO. I had been meaning to check the gearbox oil before the first road test but got overlooked. I remembered to check it this morning and discovered that the box was dry. Been trying to figure out why he would drain the oil if no work was been done on the gearbox. It's been refilled and wondering what damage, if any, was done running without oil. One saving grace is that I only put on 12 miles running 30-35 mph during that time. I don't expect any catastrophic failure of the gearbox, but I'm sure that there is some wear. I'll change out the oil when I re-torque the head in a few hundred miles.

At least I found out why the shifting was so vague and sloppy before. It feels a lot more crisp now. Also attached a couple of pictures heading down the mountain into town.

Joji Tokumoto
Fallbrook, Ca     

[pboedker]

Nice pictures! I wish we had that sort of roads where I live.

I've been meaning to ask you how your Nexen tires performed, since I saw the photos in your 'Picked up TCS' thread. I am always interested in finding new brands in the 13" size, for use with the Lotus wheels and for road use. It's not so good, that they seem to perform not so well. 

In your photos it looks like the front wheels are swapped, so that the thread direction is the wrong way. Or is it just me, that need to polish my computer monitor? 

[Roger]

Even in single-carb installations, SU and Z-S carbs had dampers. According to SU themselves, dampers were introduced as a means of ensuring a quick pickup with rapid throttle openings.
Anyone who has experienced the flat spot you get with pre-WW2 SUs when you open up too quickly will understand!

[BDA]

Things keep getting better and better for you and this car! Glad to hear the comfort level is getting pretty high! I hope it continues!

...trying to improve or modify fuel/air mixture without years of experience and access to a flow bench or dyno is probably pretty difficult with just seat of the pants impressions.

You're not kidding! A rather expensive lesson of youthful arrogance and idiocy!

[Grumblebuns]

Nice pictures! I wish we had that sort of roads where I live.

I've been meaning to ask you how your Nexen tires performed, since I saw the photos in your 'Picked up TCS' thread. I am always interested in finding new brands in the 13" size, for use with the Lotus wheels and for road use. It's not so good, that they seem to perform not so well. 

In your photos it looks like the front wheels are swapped, so that the thread direction is the wrong way. Or is it just me, that need to polish my computer monitor? 

Peter, sharp eyes you have to pick up the arrows pointing in the wrong direction they are indeed mounted wrong. In regards to the Nexen tires, I don't have enough miles on the car to get a real feel for them. But for the short drive that I had this morning, they felt OK. I need to put more miles and push them a bit harder to see how they really stand up to my mountain test road. I should have an evaluation in a few weeks. Also, all four tires are 185/60R/13s. That may make a difference in the handling.

[jbcollier]

Change the oil again in 100 miles.  Carefully road test:

- Is it quiet on and off throttle at high speed?

- Noisy in any gears? (especially 5th)

- Drained oil clear and clean?

Yes to all of the above?  You're OK.

- Shifts clean