Tuning Your Carbs with an Exhaust Gas Analyzer
... from Fred Sisson's "Bedside Reader"..
with an EFI Addendum by Lorne Goldman
PICK A NUMBER BETWEEN ONE AND THREE HUNDRED...
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.
ARE YOUR NEEDLES CORRECT FOR YOUR ENGINE?
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.
AN ADDENDUM FOR EFI (FLAPPER SYSTEMS)
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)CLUTCH SLAVE CYLINDER (a special note)
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.
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:
|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.|