Tuning Your Carbs with an Exhaust Gas Analyzer

... from Fred Sisson's "Bedside Reader"..
with an EFI Addendum by Lorne Goldman


Did you know that there are 354 needles listed for  (the most common) .090 jet SU! To further compound the problem of choice- your particular car will probably run lousy on 40%  of them, OK on 30%,  feel great on 20%, run really great on 5%  and really kick butt on 5%.  No wonder so few people
change needles. Where do you start?

Other people's recommendations, even "factory" recommendations are probably off somewhere in your driving range... maybe WAY off! Just remember "different is not the same". Different car (and EVERY one is different) - different requirements- different needles.... No one can "tell" you what is the really proper needle for your own car. No matter how experienced they are, it will only be an educated guess at best. We all know that  the big-bucks racers have their engines checked on a dynamometer, simulating race conditions, with lots of very expensive instrumentation. This is a bit beyond my modest means and... expertise. Still- there is a way to sort through the haystack and find the right SU needle for your own particular car. I guarantee that it will improve your engine's performance, it is not too costly and it is very rewarding.


This is not about "thinking"- this is about "knowing". How do you know?   As far as I am concerned, the ONLY way to tune SU carburetors is with an on-board exhaust analyzer ($100 bucks).  This way, you can check your car while you are driving it, on your roads (or track).  Anything else is groping in the dark.  Many companies make exhaust analyzers today. They run somewhere around $100.00. They use an O2 sensor that fits into your exhaust system and are described  in the book. Racer Wholesale now offers an analyzer that will read two sensors (and there is a decent one in the J.C. Penny catalogue for $120.  GoMoG Webmaster). This is the really hot setup for twin carbs as you can exactly tune each carb (not just the resultant combined mixture of the two.)

The analyzer itself is a 'bitty thing. Mine is about 1 ½"x 2". It has a series of LEDs that instantly read the fuel/air ratio- as you drive! Real world stuff here.....   I doubt that any standard carburetor will give the absolute perfect ratio at all times. However, you can get darn close and the reward for your time and  effort might just be a very dramatic improvement in performance. At the very least, you will have peace-of-mind, knowing that your engine is not being hurt from the effects of a drastically rich or lean mixture.

Many of the hot sports racers have the analyzer permanently mounted in their car. They "tune for the course". Exhaust analyzers are available as in-dash instruments also. I personally use the analyzer with the temporary mount  as the in-dash units are a bit out of place in a vintage car. There is also a choice of O2 sensors. I have been using the cheap kind which runs around $30.00. The 70 buck O2 sensor is better as it is electrically heated and comes to the initial readings quicker, but the cheapy  works fine once it gets hot. I tape the gage to the windscreen with "racer tape" so that I can see it at all times while driving/testing. It reacts instantly so you can even use it to set the power valve for a Holley carb. Pretty neat. When I am done- remove the gauge and O2 sensor. There is a blank plug that screws into the sensor hole.

by Lorne Goldman

Fred's article points to one of the inexpensive devices that the home mechanic can purchase to fine tune there engine as well or better than most roadside professional garage. An exhaust gas analyzer takes the guess work out of carburetor tuning.

It is also the only method of precisely tuning the Plus 8  Rover "flapper" EFI system that was optional between 1984 and 1986 and standard between 1986 and 1990. The later "Hotwire" system has the ability to be "hooked-up" to a diagnostic unit but the flapper does not.  Tuning is done by attached a exhaust gas analyzer to the tail pipe and turning the idle air bypass screw on the airflow meter. until the analyzer LED shows the correct CO2 level. (normally 1.5 to 2.5% for a Plus 8)

Plus 8s 1977-2004 & Rover Plus 4s 1987-2000)

A Special Note  offered by Andrew Green
BEWARE! Unless the replacement clutch slave was supplied by a Morgan source, it will require some filing at the rear to fit. This part should be carefully compared with the one it replaces before installation as the positioning of the part and the care necessary to place and fit it make it a wise idea to be certain that no modification is necessary before you begin the careful process. You will save much time and frustration with a little caution. The Morgan catalogue indicates the same slave cylinder was fitted to LT77 and R380 +8s and +4s from July 1984. My original Girling item has the following on the underside :- 64673467  A BS (or might be 5 ?) GIRLING  D10.

(2021) Andrew's part # refers to the later clutch used at the bottom of the sd1 bellhousing. See below. Most of that crowd use the parts #s indicated here. Other possible numbers are 64068329 or
64674336. Don't get anal about this. The only criteria is that it fits!!!


The background is that the Sd1 bellhousing (the all-important part that mates a transmission to the engine) used to fit the LT77 or R380 gearboxes to these cars for more than a quarter of century (1977-2004) is not compatible with a Morgan trad, frame or body. It and/or the frame HAS to be altered. Here is the offending part of the bellhousing. It is the part that was designed by Rover to accommodate the clutch slave.

The Morgan Company accomodated the clutch slave in two different 
configurations which I have named after the two MMC design chiefs respondible for them. Both the Plus 8s (1977 - 2004) and the Plus 4s (1987-2000) used the LT77 or its later swap-in replacement the R380.
The Maurice Owen Solution:
1977-1995 Sd1 bellhousings
The Bill Beck Solution: 1992/3 - 2004 Sd1 bellhousings


Maurice sheared the offending section for the Rover clutch slave off the bottom of the bellhousing (left) and welded the section off (MMC metal work shop). He then opened the TOP of the bellhousing (right), and moved the release bearing pivot from its normal position on the gearbox to the top of the bellhousing. He then found a tiny non-Rover generic much shorter clutch slave to replace the now too long original from Rover. 

Bill Beck's case, he had the chassis crossmember altered to allow a bit more bellhousing egress under the bulkhead, giving the Plus 8 engine some badly needed room at the front and allowing the engine to be a bit and level, meaning that it cleared the bonnet hinge and cutting the hinge or smashing down the SU air filter box was no longer necessary!  However, the outer side of the regular slave's protection had to be cut off to allow sufficent spacing and debris easily enters these bellhousings. On the other hand, production was MUCH faster and the factory could save as the clutch slaves were free with the engine/transmission purchase.


1. It solved the fit problem without requiring a modification of the frame or the firewall or the engine or its components. Earlier models forced the hammering flat of the SU air filter housing and later models a scoop out of the right side valence for the alternator. Early EFI models required a cut out of the bonnet hinge. 

2. It created a bit more clearance under the bellhousing.


A. The shorter a clutch slave it the more susceptible it is to wear. (as noted these little non-Rover/LR slaves are easily confused and don't last long is the car is used at all).

B.  The placement at the top means a releatively short push rod which usually prejudices the entry angle and path which causes the slave cylinder to wear faster as well.

C. As a general rule, it is unwise to place any hydraulic cylinder BELOW its reservoir. This too accelerates wear and makes bleeding more difficult.

D. Access to the slave became much more of a hassle in later EFI cars because of the complexity of the hoses and loom above it.

These cons makes the normal life span of this clutch slave of this configuratrion decidedly short. You can expect only 12,000 to15,000 miles and must carry an extra one with you at all times. They can be changed roadside.  However, I discovered a permanent solution 20 years ago.  A used slave sent to an expert to be sleeved in stainless steel lasts forever. The cost is the same as buying a new one with its limited lifespan.

Bill Beck's solution is smarter and saved the MMC even more in cost and labor.  Bill made two changes so that the stock LR/Rover
slave can be used an no welding or patching is needed.  Bill sliced the bellhousing section housing the stock slave open.


1. It allows the standard Sd1 clutch slave to be used which elminates parts confusion and makes sourcing easier. Much cheaper in labour and parts than the Maurice Owen solution. No welding involved.

2. It is easily accessible..

3. It is longer and allows for greater longevity

4. It is below its reservoir which increases longevity


A. It required a modification of the crossframe piece below. The piece arcs downward eliminating precious ground clearance at this point; 

B. An open bellhousing is unwise for obvious reasons.  (Though I have seen much worse design modifications in mating bellhousings to gearboxes! See the US D&D's to the right!! [shiver])  An open bellhousing collects debris which inevitably effect the components inside and accelerates rust. 

N.B. To date, the best bellhousing to mate a non-standard gearbox to a Rover V8  I have seen in the Morgan world is that made by J.E.  Developments of the UK to fit a Tremec T-5 to a Plus 8. It is bespoke and totally enclosed. It uses an internal concentric clutch release which eliminates the issues created by an external setup. 


I have tried/used them all. I have come to the conclusion that the best option for me is the Maurice Owen solution with a stainless sleeved slave. It works and last forever while leaving a pristine interior for the bellhousing.

Clutch Pedal Adjustment (LT77 and R380 Plus 8s (1977 to 1996) and Plus 4s (1987-1996)

Replacing the Clutch Slave
(LT77 and R380 Plus 8s (1977 to 1996) and Plus 4s (1987-1996) A local garage will not have an advantage here. The area is Morgan-configuration only. Part of the neverending odd modifications needed to keep fitting later motors to an engine bay designed in the Thirties for a 36bhp tractor engine. ;) That task constantly exhibits a GoMoG Law I call the Deadly Domino Effect. In this case, the bellhousing doesn't quite fit far enough into the car because of the:
1. the width of the area for bellhousing,
2, the crossmember at the bulkhead underneath
3. and the bellhousing casting placement for the regular LR clutch slave.

Two Morgan designers, Maurice Owen and the Bill Beck, (1967-2002) addressed the issue differently. However, both designs have er... sad issues. The mechanical priority, begun when Peter took over at the MMC, has been for lowering costs.

You and I could have come up with a better design. But these these designers were given priorities and instructions and attitudes from on high. You would be wrong to think that they were incompetent. The sad pity is that, as these cost priorities are indulged and such designs are currently multiplying, the cars are becoming less and less reliable. (Do not confuse aging and poor maintenance with bad design They are separate issues.)

We will be deal here with replacing the slave in the Maurice Owen system. Look at the area atop your engine. I have found some piccies (above If that is what you
have on your car, go pump your clutch vigorously a bit and then go check if you can see/feel any brake fluid. (If you get clutch fluid on your hands, wash it off quickly. I use spray brake cleaner.)

To change the clutch slave shown above.

1. with a 7/16 spanner, remove the line to the clutch master. I find this makes the rest of the job easier as you can tighten the line at the slave without twisting the line.

2. Remove the fluid line from the clutch slave.

3. Now the big bolts. These often are unwilling as they are steel and the bellhousing is aluminium. That creates a galvanic reaction (aka metal rot) that seizes these bolts. Use the correct socket and if the two bolts seem reluctant, take some time and use a penetrating oil, even many times over a few days.

4. When they loosen, do not remove them right away.

WATCHPOINT: the only annoying danger in this task is losing the cylinder's actuating rod...which can be a pain in the butt. What I do is grab it with a small needlenose vice grips as you are removing it then lock it the vice grips and secure the grips at the side.

 5. Once the rod is secure, remove the big bolts by hand, and slowly draw the slave towards the engine and, if necessary, a bit upward until it is free from its actuating 

6. Check the new cylinder. You do not need a rod (assuming it came with one). You should use the old rod as it has likely been adjusted to the right length for your car. Additionally, this little cylinder, even if it is the correct one, must have had its rear ground/filed to fit a Morgan.

7. Now you must reverse the procedure with the new slave. Carefully position the slave on the engine side and angle/slide it onto the rod, only removing the vicegrips when the rod slides in and is captured into the cylinder and the rubber!

8. Now (after covering the bolts with a little silicone to prevent the same galvanic reaction mentioned above), thread in the big bolts. Then tighten them firmly with your socket and ratchet. You can now breath.

9. Now you must reconnect the line to the master clutch cylinder. Use the bottom fitting on the slave for this as you need access to the top one. (there is no difference between the two but easy access to the top is necessary for bleeding the system which is next up). 

10. Remove the top fitting to allow you access to tighten the line and tighten the line.

11. Thread on the fitting into the master clutch cylinder and tighten after you are assured it is not cross-threaded.

12. Now you must refill and bleed the system. We all have different systems for that. I use any glass jar that happens to be around... with a tube connected between the bleeding nipple and with the other end submerged in brake/clutch fluid. Make sure the reservoir never empties or you will pumping air rather than fluid into the system.  On which brake fluid to use, CLICK HERE.