Reopus Ignition

Dedicated to the Lucas OPUS Ignition amplifier originally fitted to Jaguar V12 (pre 1982)
Aston Martin V8, and Cosworth V8 engines.

 
 

Stromberg carburettors

 

I decided to overhaul the 4 x Stromberg CD-2SE carburettors on my E type as they were virtually untouched apart from new diaphrams and mixture tuning since 1982 when I purchased my car.

I have a Stromberg manual I purchased in the 1980's plus I have researched as much information as I could find.

I purchased 4 kits to replace the gaskets and seals plus 4 new floats and 4 new Grosse type float valves.

4 new Needles and Jets.

I have assumed you have some experience or have a repair manual for the Stromberg carburetters, the parts pages from the V12 parts manual are at the bottom of this page for your reference.

I have purchased Stromberg parts from Jeff Carter at Gower and Lee UK. I have found they reply to your emails and give excellent service.

email:sales@gowerandlee.co.uk

Web site: www.gowerandlee.co.uk

Stromberg  CDSE and CD-2SE Removing and refitting new Jets 

The emission experts say the Needles and Jets on Biased needle carburettors should be replaced every 50,000miles. It is difficult to measure the wear on the Jets and needles.

I have read some information on removing the Jets from Stromberg Carburettors, some saying you needed a 2 ton press. I looked at the Alloy carburettor body and Brass jet and doubted this amount of pressure is necessary. I found a small bench press or a bench Lath can do the job.  I suggest you don’t use a hammer to remove or refit the jets as bruising/stretching the carburettor body will cause problems.

I purchased new needles and jets, removed the carburettors. Measured the distance from the Top of the Jet to the carburettor bridge they were 0.072 ± 0.004 below the bridge. Possibly Stromberg set the jet height for each carburettor in an air flow tester hence the variation?

Using a 2.5 in long x ½ in alloy tube in the bottom of the carburettor body around the Jet against the chuck and a 6in x 5/16in steel rod I pressed the jets out the bottom using pressure from tailstock of my bench lathe.

I then machined from a 5/8 x 1.5in x 3 inch long alloy bar as per the photo. Note the raised circle for my 4 carburettors the average of the 4 Jet heights below the bridge was 0.072 so the raised part is 0.072 high x 0.36 diameter, so as to have all the Jets pressed into the carburettor body to the same height below the bridge.

I drilled a 7/64 x 1inch hole in the end of a 5 inch long x ½ inch steel rod.

I then put the steel rod in the tailstock, positioned the new Jet in the drilled end, carefully positioned the alloy block against the bridge with the raised in the jet hole so it sets the jet height below the bridge and then screwed the jet into the carburettor body.

Then checked the float heights, reassembled and refitted the carburettors. After installing new needles and jets, the idle mixture was easily corrected and the idle smoothness was much improved.

Stromberg CD needle Type (mnm) Data copied from Haynes Stromberg CD carburettor Owners Workshop manual.

 Stromberg temperature compensator BiMetal adjustment

The purpose of the Temperature compensators is to provide a small inrease in air flow past the diaphram so as to lean the fuel mixture when the carburettors warm up this maintains a more eve idle speed if they are setup correctly.

I did some research on setting up the bi-metal bypass valve operating temperature.

It is now some 40 years since the carburettors were made and service time was long overdue. I purchased my E type in 1982 and I have never touched the carburettors apart from new diaphrams, resetting the float levels and tuning this is the first time the carburettors have be dismantled by me.

 I have found there were many experts even in the first 10 years of my cars life who had adjusted the nyloc nut and bent the Bi-metal. Your manual will say “under no circumstances adjust the nyloc nut”. Service for this item involves dismantling cleaning and therefore re-setting of the bimetal strip. The factory have never supplied the information on how to setup the bi-metal operation? They simply state by new temperature compensators.

There are a number of articles on the internet about the temperature opening settings with opening values between 28°C and 43°C.

I have tested the temperature of the bimetal chamber and found the temperature to be about 39°C when idling and about 60°C after a 15 minute run. The radiator (top tank) temperature was 86°C.

I have seen a few suggestions on how to do this on the internet but one must remember the Bypass valve is normally in vacuum operation, it is  difficult to test under vacuum conditions without a vacuum pump so I used the method below which worked well.

If you do find any factory information about the setup for the Bi-metal opening temperature for various engines I am sure it would be helpful to many.

I dismanttled and cleaned the tremperature compensation valves and carefully cleaned the burrs from inside the plunger chamber with a 9mm Drill bit "by hand", then I carefully cleaned up the seating surface. I polished the taper on the plastic plunger in my lathe. I straightened the Bi-metal strip where necessary and reassembled.

Using a length a clear plastic tube about 500mm long with a 12mm bore and a small hose clip I pressed this onto the valve leaving  the holes on the side clear. It then sucked on the end of the plastic tube whilst adjusting the 6BA nut until a restriction in air flow was noticed I then tightened the nut by a further 25degrees I then filled a 1 liter container with water at 40°C  and a container of cold water I then put the end of the tube in my mouth and applied a bit of negative pressure (suction)  with the valve submerged in the 40°C water. I then checked the Bi-metal performance and adjusted the bi-metal as necessary until it was obvious that the valve was closed as only a very small flow  was evident (due to leakage)  I then decreased the bimetal pressure to the point where a large flow was obvious I then gradually wound in the adjusting nut until the small flow was restored so the valve was just closed.  You will note the valve operates very quickly when submerged in water

Setting the float level on Stromberg CD-2

Setting the float levels accuratly is very important as the level sets the base fuel mixture level.

You should make sure all floats operate the same, invert the carburettors with the float chamber removed and set the float levels as below.

I replaced the floats as I noticed the original floats "white plastic" had bulged, possibly due to modern fuels and age?

Dimension B should be 16 to 17mm, dimension A should be 13mm this ensures that the float arm is straight and all floats are in the same position when the valve is closed. Both floats on each carburettor must be set up to the correct tolarences.

Grosse float valves. My car is 1972 V12 E type. The thread on the original Stromberg Carb Float valves are 11mm x 1.5mm and the flange for the filters is 8mm dia. I had to run a die nut down the thread of the Grosse valves to fit the Stromberg carburettors and machine small sleeves 8mm x 6mm x 1.5mm long to fit the original filters. I have recently found some replacement float valves have a 6mm sleeve.

It is very important to make sure the seal betwen the float valve and carburetor body is a perfect seal, this is often a source of carburettor Flooding. I found the alloy washer supplied with the Grosse float valve was not satisfactory as the hex was not machined off at the seal face. I machined up a brass washer 16mm diameter with a 12mm hole and 1 mm thick and purchased 17mm OD x 1.5mm Viton O rings to make a good reliable seal

Do not under any circumstances fit the float valves without filters.  It is very important to refit the Stromberg filters to the float valves, the close tolerances in the float valves may cause flooding and therefore a rich mixture and the possibility of a Fire if small particles prevent the valves from seating properly. I found the Grosse float valves were very good, definately worth fitting provided it is done correctly.

Carburettors Flooding. Recently I had a customer who said the engine started running rough and cutting out. and then it would not start. I first checked the ignition was switching, all apeared OK. I removed the spark plugs and found some of the plugs sooty black ie running very rich. I found the problem was flooding carburetters. I replaced the float valves with the GrosseType see note "Stromberg carburettors"

Do not fit a replacement fuel pump that has a output pressure of more than 2.5PSI without also fitting a pressure regulator set to not more than 2 PSI. I have found some pumps have a very high flow and the return valve cannot cope so the fuel line pressure may be higher than you think when the ignition is ON and the fuel usage is low. The Stromberg carburettor float valves will not work reliably at Fuel pressures of more than 2.5PSI. The carburettors will possibly flood  causing a rich mixture which will cause poor starting when the engine is hot, plus you may also have a fire to put out.

Heat problems with the Jaguar V12

The Inlet manifolds on the carburettor V12 are 11 inches long and there are long lengths of fuel pipe in the engine bay. See my notes about why the SU pump on a V12 (carburettor) should never stop when the ignition is ON.

The under bonnet Temperature of a V12 E Type can exceed 95°C. The carburettor bowls are close to the exhaust manifolds and the inlet manifolds are bolted directly to the cylinder head, the carburettors are also heated by the engine coolent water, so what is the Fuel temperature in the carburettors and manifolds?. From the information I have researched Fuel Boils at 95degrees C.

I have measured the outside temperature of the inlet manifolds after a 20 minute run. When I turned off the engine the inlet manifolds were 87°C and after 10minutes the temperature had increased to104°C, the top tank water temperature went from 86°C to 99°C over the same time period. The ambient air Temperature was only 18 degrees C.

The fuel in the inlet manifolds will vapourise at these temperaturers, and when it is parked for a short time (15min) it can take about 10 to 15 seconds on the starter for combustable fuel to reach the combustion chamber, this is a common fault with the V12 carburettor engine, naturally it becomes worse if the ignition spark is weak due to a faulty amplifer.

Starting a hot Jaguar V12. Remember the old days when carburettors iced up on light throttle, this was due to the change in air flow speed in the Carburettor. If the air/fuel speed is increased (passing the throttle butterfly) and is then slowed down again (entering the inlet manifold) the air temperature is cooled and this naturally helps with the vapourisation problem, so if the trottle is closed the fuel/air flow change is maximised and therefore the fuel/air temperature is reduced by the maximum. Starting your V12 when warm or hot try light throttle until the engine fires.

TUNING THE ENGINE

Spark plugs must be clean and in good condition.

Set Plug Gaps to 25thou before starting to tune the carburettors.

My experience is that NGK spark plugs BPR6XXX are a good choice. Below are the NGK options.

Premium  (Iridium)

BPR6EIX

Super (platinium)

BPR6EGP

Regular

Standard BPR6ES.  V-Power BPR6EY

 

Check ignition timing.

1. Remove vacuum retard connection from distributor and block off the pipe end.

2. Start engine reduce idling speed to 500/600RPM.

3. Using an ignition strobe set the ignition timing to 12 Degrees on the timing plate.

4. Reconnect vacuum retard pipe.

5. Reset the engine revs to 650RPM.

 

Carburettor tuning

Balancing the Carburetors

Set the Air balance of the four carburettors using a carburettor balancing tool. If the carburettors won't balance or you cannot lowwer the revs to 650RPM check for air leaks.

Now when the engine is warm and idling.

I suggest you remove the top two throttle shafts so they do not interfere with your carburettor balancing.

Adjusting the balance of the 4 carburettors requires patience. 

Loosen the front carburettor balance screws and adjust the front carburettors using the balance screws on the shaft/coupling between the front and rear carburettors.

When you change the carburettor balance on a carburetor you also change the idle speed and airflow slightly so you will change the balance of all the other carburettors.

You must make small adjustments to each carburetor until all carburetors have the same balance (air flow) without any change to the throttle / balance screws.

The two throttle shaft bell cranks should also be positioned at the same angle so the right bank and left bank throttle opening is equal at all throttle positions.

Refit the throttle shafts after checking their length when the carburettors are balanced making sure they start to open the throttles equally.

Adjusting the mixture.

V12 E type Stromberg carburetters have an adjustable needle the mixture can be adjusted using the correct adjusting tool.

 Raise the piston 2 mm.

If the engine speed increases the mixture is Rich. Turn the mixture screw Anti clockwise to lean off the mixture. (lowers the metering needle)

If the engine speed remains constant or increases slightly and then slows to revs as before the mixture is correct.

If the engine speed decreases the mixture is Lean . Turn the mixture screw clockwise to richen the mixture. (raises the metering needle)

If you cannot get the mixture correct check for other faults like Float levels (mixture too rich), Air leaks mixture too lean, faulty By Pass valves, temperature compensation valves.

The specified CO measurement for a Stromberg V12 @ idle = 4-6%

Check Fuel pressure. The Fuel pressure should be 1 and 2 PSI if you change the fuel pump make sure the output pressure is not more than 2PSI as the float valves are unreliable at more than 3PSI.

After setting the Temperature compensators and fitting the correct seals. I have just taken my car for a run and it certainally has improved the engine smoothness.

Dashpot oil.

The Dashpot oil is there to provide a more rich mixture on inical acceleration.

Tha Manual states Engine oil in the dashpots. this would be 30/50grade.

I have found lighter oil provideds better acceleration.

I believe the problem with using engine oil in the dashpots is the mixture is over rich on acceleration?

I have found that my car as do some others, if Auto transmission fluid is used in the dashpots the cars acceleration is much improved.

However if your car is running lean perhaps the thicker oil is required?

 

I have scanned the Stromberg Carburetter pages from the Jaguar V12 E Type Parts manual for your reference.


 

 

 

 

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