View Full Version : Early ZS Carb Needles

05-03-2012, 09:01 PM
In later versions of ZS carbs, with needles that are adjustable through the top of the damper, what direction is the needle moving when:

Adjustment screw turned clockwise: Needle moves up or down?

Adjustment screw turned anti-clockwise: Needle moves up or down?

I can physically/manually reposition the needles in my early carbs (C3150) by removing the top cover, pulling out the air-valve/piston, loosening the set screw and then pushing the needle up/in or pulling it down/out.

Obviously more of a PITA operation compared with "tool adjustable" needles and I'd just like to know which direction does what in terms of enriching or leaning.

05-03-2012, 10:26 PM
Clockwise raises the needle, enrichening the mixture
There are only 2-1/2 complete rotations of adjustment available. From fully rich, any more than 2-1/2 Counterclockwise turns will unthread the adjuster from the needle carrier and although you will still be able to rotate the adjuster the needle will not respond until it's rethreaded.

05-04-2012, 07:11 AM
I am curious. Would your TR250 not have the CD series carbs like my GT6 as opposed to the later CD-2 carbs with the adjustable needle?

The original CD carbs did have fixed needles but the jet/mixture was adjustable from below like on an HS series SU carb. The original Stromberg CD carb is tuned very easily and had none of the emissions components found on CD-2 carbs.

The link below shows some of the CD carb components. Note items 34 through 36 (bottom of the scree) that show the bottom adjustable jet.

The link below may help with some of your adjustments particularly if you switch to the later "top down" adjustable needle.

05-04-2012, 07:55 AM
Nope. No adjustment from below is possible. The Buckeye Triumph tutorial even has a sperate procedure for converting carbs like mine to "top down" adjustable like later versions.

Here's a link from TRF showing exactly what is in my car (69 TR6 is the same): https://trf.zeni.net/TR6bluebook/30.php

The float chamber plug is just a slotted, threaded screw with a sealing ring.

05-04-2012, 08:17 AM
So with each needle pressed upwards into the air valve as far as they will go. It appears I still have a very lean condition, particularly in the front carb. Or do I?

With the front carb, raising the air valve just slightly causes a sudden decrease in engine speed which can and does lead to stall. This means much too lean correct?

Rear carb also decreases idle but not nearly as severely and the car will not stall.

I've pulled the plugs and found those in the cylinders 1-3 to be black and sooty. From what reference photos I have seen, they appear to be the dictionary definition of an overly rich mixture, not lean. Plugs from cylinders 4-6 were only slightly sooty.

On background, the head has recently been completely rebuilt, surfaces were machined as were both manifolds. New gaskets and new hardware was used throughout. Carbs have been completely rebuilt including everything in a usual rebuild kit. In addition, new by-pass valve diaphragms, and new needles were installed.

Any ideas why I would have the carbs indicating a lean condition while the plugs indicate the opposite?

05-04-2012, 08:21 AM
Wow! Just when I thought Strombergs couldn't get any more confusing.

I have never worked on adjusting the needles from the top. I converted our Spitfire from that series of Strombergs to an HS4 SU. I like the jet adjusting method on our GT6's Strombergs as it is nearly identical to what you do with SUs. However, the o-ring seals between the float bowl and jet assembly only seem to last a couple of years. One of these days I will try Viton o-rings and see if they last longer.

Out of curiosity, would it be possible for you to convert to the adjustable jet (from below) instead of the adjustable needle (from above)? Which represents the least investment in new parts?

05-04-2012, 09:15 AM
Any ideas why I would have the carbs indicating a lean condition while the plugs indicate the opposite?
A vacuum leak could do that. By admitting a relatively fixed amount of air with no fuel, it would make the mixture lean at idle. But the mixture adjustment affects the entire range, so when you try to richen up the idle, it takes the mid-range mixture too rich.

And at idle, it's not hot enough to burn off the carbon left under load.

Dunno if this would apply to a TR250, but on a TR3-4 it is easy to not get the intake manifold seated over the locating pins, which can cause a leak. I've even seen manifolds where the misaligned pin turned up a burr inside the hole that would cause problems.

Another possible source is the brake booster, if you have one.

Having the valves too tight can do strange things to mixture.

I've also read that a severely restricted exhaust (like a muffler baffle that has collapsed or fallen over the outlet) can cause strange mixture problems.

05-04-2012, 09:56 AM
The volumn of fuel in the float chamber can also affect the mixture.
.650" float height is a pretty good height for most situations.
Also, the "lift the piston" test is subject to interpretation. The results may not be a valid indicator if the lift is too high and/or you wait too long. You want about 1/8 to 3/16 max lift and you want to evaluate the engine's immediate response.

05-04-2012, 07:38 PM
OK. Carbs have been pulled and float levels checked. Both are spot on at 16-17mm. Thanks for the photo but it looks like you are measuring to the top of the float just before the beveled edge leading up to the flat 'top face" of the float. Maybe it is the photo angle but I have measured from the carb body to the highest point of the float (the flat "top face"), which is very slightly higher than the bevel between the float's side and top faces.

I may have discovered something in this repeat exercise... carb markings on the bottom of each read C3150, which I already knew but there is a letter following each set of numbers. One is clearly an "R" with the other being much more difficult to make out. It could be an "F" or an "L"

Would "R" denote right, as on one's right if standing on the passenger side of the engine looking down at the carbs? Or does "R" denote rear, as in firewall side of the engine?

Stupid question I know... but I think I had them on backwards. DOH! Awaiting confirmation of such.

05-04-2012, 08:06 PM
It's Right and Left as opposed to front and rear, but because of the throttle shaft lengths I don't think you could mount them just any old way.
3150 series are fixed needle carbs, so if you have adjustable needles either they have been converted or the ID tags were switched.
My picture is just an illustration, mainly to show that the gasket is not in place when measuring

02-02-2013, 01:06 AM

Fancy finding you here. Why yes you can - I've done it. You need the adjustable jet assembly and then you need to drill out the carburetor main case to accept it. Not too big a problem. Of course it will be a LONG time before all this gets tested!

Jim M

02-03-2013, 11:37 AM
Jim, I'll be watching for those triple Stromberg old threads (and new ones) to surface.