Slightly annoyed at carburetters

[mole42]

I bought a couple of complete carburetters to restore - they are the same 3382 tag number as the ones already on the car. I bought the correct rebuild kits from Burlen and I asked them for the spec sheet for the 3382 so that I could rebuild them and set the metering needle orifice correctly. The carbs are working well except that I can't get them to run rich enough. Did a 200 mile run the other day and had to pull the choke about 1/4" out to compensate. Both needles are adjusted fully rich - I suspect the orifice height setting is wrong. But the engine pulls beautifully once the richer mixture gets through!

[wilf]

I predict an orifice-height reset in your near future............

[marshman]

I refurbed my Strombergs earlier this year. Fitted new jets and searched around for the correct height. Came up with a figure of 0.1" below the bridge. So to ensure I got them spot on I turned up a press tool with a step at the end which bottomed out on the bridge ensuring that the jets were both set at 0.1". I verified with a vernier afterwards.

Once refitted to the car I found like you that I ended up with both (new) needles wound in to maximum richness and still felt the engine was running a tad lean. I checked for air leaks and found none. I have since pressed the jets down another 35 thou (I turned the step 35 thou deeper on the press tool). My reasoning being that from flush to fully recessed (rich) I measured the adjustment (travel) on the needle to be 75 thou. So in theory this should put the lean starting position in the middle of the adjustment. Seems to have worked as I can now adjust the idle mixture from too lean to too rich.

Not sure why the difference from "factory" setup as the carbs were generally in good condition with new spindle seals, jets, needles diaphragms etc. What I forgot to do was to check the jet height before I pressed out the old jets. All parts were genuine "new old stock" Stromberg parts as well. In the absence of anything else I am going to blame modern fuel

This is on a newly rebuilt engine with +40 pistons and heavily skimmed heads (still just in minimum spec) and "thick" head gaskets. Timing is spot on, new distributor and aldon ignitor electronic ignition set to static advance of around 13 deg BTDC, standard B1AQ needles on early carbs (no vents, poppet valves in butterflies or temp compensators). Engine now idles and runs fine and also runs nice and cool.

[flying farmer]

I do wonder if it is the ethanol in petrol that is the problem. It does generally make the engine run more lean. It might not be a problem with worn jets and needles as it helps correct the problem. Put everything back to factory spec and the need for a little “correction” arises

[mole42]

I refurbed my Strombergs earlier this year. Fitted new jets and searched around for the correct height. Came up with a figure of 0.1" below the bridge. So to ensure I got them spot on I turned up a press tool with a step at the end which bottomed out on the bridge ensuring that the jets were both set at 0.1". I verified with a vernier afterwards.
<snip>
static advance of around 13 deg BTDC, standard B1AQ needles on early carbs (no vents, poppet valves in butterflies or temp compensators). Engine now idles and runs fine and also runs nice and cool.

That's interesting - 0.1" is 2.54mm which is within tolerance for the 2.4mm ħ0.3mm quoted in the manufacturers spec sheet. I think I'll go another 35thou (0.9mm for younger viewers) as you suggest since it would be nice to have the jets operating in the centre of the range. I'm running Lumenition (I have two and they both work the same) and these are 1971 spec carbs, B1AQ needles, temp compensators from the old set (they were tested and verified last year), with the micro vents closed so should run pretty much the same as yours. I did notice on the 200 mile run with the choke out that the engine temperature was 5 degrees cooler than previously so the mixture must be fairly close, and fuel consumption was normal.

[gbb483]

I have the same problem. I rebuilt my carbs over the winter, and had watched all the youtube videos I could find before I started. The intention was to set the orifices to whatever they were originally. Suffice to say I discovered that the carbs were not a matched pair. One had the orifice proud of the threshold, the other had the plunger stuck to the threshold with old oil and the cut-off valve jammed shut (so all the cylinders were using one carb via the cut-out in the manifold - no wonder it was always a pig to start before).

I have set the orifices to about 75thou, and feel that I cannot get the mixture rich enough; the plugs (all new set to 25thou) are all a beige colour with the metering needles wound fully up.

The idea that the ethanol might be the culprit is reassuring - so an extra 30thou on the orifices looks like it is in order. But as it requires almost a complete strip down (so I can put a support under the threshold) I am going to wait until the winter lay-up, and just apply a little choke whenever the engine stutters.

[marshman]

Having thought about it some more and done a bit more searching I think the "error" may be that I measured to the top of the actual jet "orifice" which is recessed by around 20 thou so in effect I had set the jets 20 thou too high.

The real problem is that the jet was not intended to be a "service" item. The original Zenith service bulletin (SB216) makes several references to the jet assembly being factory set and "MUST NOT BE CHANGED" and "MUST NOT BE ADJUSTED" so there are no original manufacturers figures quoted that I can find - all the measurements banded around come from other sources of unknown providence. The Zenith bulletin says the whole sub assembly must be replaced if there is a problem.

[mole42]

" so there are no original manufacturers figures quoted that I can find - all the measurements banded around come from other sources of unknown providence.

I asked Burlen for the correct height and they sent me the manufacturers spec sheet for the 3382 model carburetter so the correct information is available if not easy to find! My comment now is that, taking on board Neil’s comments about ethanol in fuel, the manufacturers specification might need tweaking….

[mole42]

But as it requires almost a complete strip down (so I can put a support under the threshold) I am going to wait until the winter lay-up, and just apply a little choke whenever the engine stutters.

This is my everyday car so I’m not happy if any part of it doesn’t work properly. I suppose that’s the downside of using a 50-year old car as a daily!

[DJT]

My Strombergs were obtained a few years ago from Alan Davis at The Gauge Shop. I have just been out and measured one of the jets and it is set at 0.095" below the bridge, measured to the rim of the jet. Measuring to the recessed face it is 0.125". The car runs fine on E10 at all temperatures, even on the occasional use of full throttle kick-down. Standard needles set with the Delrin washer flush with the bottom of the piston.

[mole42]

My Strombergs were obtained a few years ago from Alan Davis at The Gauge Shop. I have just been out and measured one of the jets and it is set at 0.095" below the bridge, measured to the rim of the jet. Measuring to the recessed face it is 0.125". The car runs fine on E10 at all temperatures, even on the occasional use of full throttle kick-down. Standard needles set with the Delrin washer flush with the bottom of the piston.

Thank you Dave. That's much appreciated information - especially as the data from Zenith doesn't differentiate between the two levels in the jet tubes. 0.095" is 2.4mm - exactly as per the spec sheet. I set mine at 2.4mm to the recessed face, so they are too high in the carb body - no wonder I can’t set them rich enough!

[marshman]

Thank you Dave. That's much appreciated information - especially as the data from Zenith doesn't differentiate between the two levels in the jet tubes. 0.095" is 2.4mm - exactly as per the spec sheet. I set mine at 2.4mm to the recessed face, so they are too high in the carb body - no wonder I can’t set them rich enough!

Yep, exactly what I did, hence as you say no wonder they wouldn't adjust rich enough. Mine are probably 10 thou too far down now, but as I have adjustment both ways that is OK.

[richardthestag]

Does it run fine with super unleaded, need to check but believe that shell are not adding ethanol..

If it runs great on super but not standard then maybe a tweak to the jet height. I work to 80thou from the bridge to the outer step of the jet. I need to make up a tool to simply knock them in, currently using an ancient Moore and Wright depth gauge which is more or less the exact diameter of the jet. hence measuring to the step and not the recess.

I am no expert on strommies but have learned a lot over the years. So excuse me for teaching granny to suck eggs.

Lean mixture can be caused by many things;

My first port of call would be to check for any an air leak around the carb body or pedestal between the jet and the inlet manifold, throttle shaft seals go hard with age and must be replaced to prevent any air being drawn in through. throttle shaft itself should show no signs of wear to the thrust side, if you can feel a step with your finger nail then replace it. Throttle disc with the damned poppet? solder them closed. Be sure that the throttle disc is installed the right was around, seen many that are not!

Fine tune screw? wind the stupid thing right in tight.

Temp compensators, be sure that the plunger is seated in the end of the unit nice and tight. be sure that you are using the correct o-ring in the hole that the temp compensator goes into and also around the neck of the unit where it seals up against the carb body. I tend to wind them closed and ambient temperature and then give the screw another turn so that they do not open. Easy to get stroms to run at 4% co without these adding to the complication. Heretic I hear you screech! But face it, 50 year old temp compensator operated by a bi-metalic strip that is very often bent due to some prior ham fisted gibbonry and a plunger that is often wide open even at room temperature is not going to help.

Be sure that gaskets are used either side of the insulators where the carb mounts to the pedestal.

Use the correct spec o-ring between the pedestal and the inlet manifold. As I understand it is designed to swell slightly when contaminated with petrol. Use a fibre washer, under a plain flat washer under the nut that secures the pedestal to the inlet.

Isolate the air filter flap vacuum feed from the forward face of the pedestal with a small fixing wedged into the end of the vacuum pipe.

Float vent needs to work else the float chamber will not fill to the correct level, if fuel in the float chamber is not at the correct level then it will run too lean if the level is too low and rich if the float chamber is flooding.

Float height gets set without the gasket and measuring from the face of the carb body to the corner of the float nearest the needle valve. Be sure that the float is the right way up. seen that before too.

Meter needle, seen far too many that are either not connected to the adjuster or are missing the star washer that holds the adjuster down. Remove the piston and set the shoulder of the needle holder flush with the face of the piston. then wind it right out, be sure that it doesn't fall off the thread and that you can then wind it right back in again. Adjustable needle on the strom can only go in one way around, be sure that the grub screw is in place to prevent the needle holder from rotating as you rotate the adjuster.

While you have the piston out check that the diaphragm tabs ore located into the slot on both carb body and piston. The needle is sprung and the piston need to be installed in the correct location. those diaphragm tabs ensure that this is correct.

Next look at the underside of the dashpot, there should be just one step on the outer lip, earlier dashpots have two steps and these are for the much thinner diaphragms that are NLA. easy to file off the second step. this then allows the dash pot to secure to the carb body and provide the essential seal for the diaphragm.

Everything can be set up beautifully and your fuel supply could be no up to scratch. I have found a pump supplying less than 2psi fails to fill the float chambers to the correct level. One carb will fill but the other will not etc

Ideally get a CO meter or ask your friendly garage owner if they can shove their probe up your exhaust fnarr, you are looking for at least 3.5% CO but not more than 4.5%

[marshman]

Agree with all of the above BUT the basic issue I had, and I believe Richard (Mole32) had, was when we fitted new jets we took the 0.1" measurement to the jet face at the bottom of the recess rather than the the rim around the edge, thus the jet was set approx 25 thou too high.

On the subject of diaphragm thickness, I have come across some diaphragms, unknown origin, that are ridiculously stiff - always check the piston assy drops and stops with a satisfying "plop" or "clack" after fitting the new diaphragm. The stack of "stiff" diaphragms I had prevented this and gave a sluggish piston movement. Needless to say I binned them.

I made a brass insert tool for pressing the new jets in (now modified to press the jet in to 0.125" (to the jet face) and use my small bench press to gentle squeeze them in.

tool-4.jpgpress tool - made of brass, step on the end is (now) 0.125".

tool-5.jpg

jet insert-1.jpgjet pressed in using small hydraulic bench press, gentle and controllable.

[MikeParker]

I did a bit of research on Stromberg jet heights when I rebuilt my carbs and came up with 2.1 to 2.7mm from bridge level to the land around the jet (not the rim). That is around 95 thou. I have an AFR meter fitted and can adjust the mixture from an AFR of 11 through to 19, so it does seem about right. That is with new jets, new B1AQ needles, temp adjusters, mixture screw closed off and E10 fuel.
Mike

Edit: Looking on the web, there all sorts of statements about how you measure the height of the jet in a Stromberg carb. The majority seem to say (wrongly) that you measure to the rim. My original source is a Burlen phone call from 2021. I have just phoned Burlen again to check and they came up with exactly the same info, so that gives me some confidence.

A bit more info from Burlen is that the numbers comes from a Zenith/Stromberg "Build Details" publication dated 1975 for a European spec Stag with a type 3833 carb.

I have to confess, I do not understand Richard (mole42) and others who find that the 0.5mm difference between measuring from the land around the jet and the rim makes such a big difference to the mixture. There is after all a much bigger (2mm?) adjustment movement for the needle?