I’m in the proces of finetuning the 140 and working on the carbs now.
I use a synchrometer for flow and a gunson “glass” sparkplug to determine how rich/lean the mixture is while running. I also check the plug colour after each adjustment run.
The engine is equipped with a “type C “ head, pancake aircleaners. CR is 8:1. Carbs are H6, The needles used are WO2 and according to the literature those are the correct ones.
These are my findings until now:
At first the sparkplugs where deep black… my conclusion: too rich.
In four steps i tried to lean out the car while checking the plugcolour and with help of the “glass plug” the colour of the combustion running idle.
At this point the jets are in the highest position.
According to the combustion colour (not as yellow as it was but still not blue) it is leaning out but running still (a bit) rich. Combustion colour in nr 1 cyl is about the same as in nr 6 cyl.
According to the spark plugs, the porcelain bit is getting brownish black. the metal bit is getting less black. All 6 plugs showing more or less the same colour.
I concider trying different needles but would like your opinion.
Try thicker oil in the dash pots too. Are the jets and needles old (worn)?
When I finished an MGB engine rebuild I could not get the carbs to balance, ran like crap, etc. I found that SU had drilled a batch of jets the wrong size/marked wrong. I had one .090 and the other .100. That pairing was doomed.
Now, I never put anything in until I know it’s to spec.
Torsten, it is my understanding that SJ needles were originally specified for your application.
For what it’s worth, my XK120 has a C-Type head, pancake air cleaners, 8:1 compression ratio, and H6 carbs - but with RF needles. Below are the results when tuned on a Dyno. The lower graph shows the air/fuel ratio under load.
“For what it’s worth, my XK120 has a C-Type head, pancake air cleaners, 8:1 compression ratio, and H6 carbs - but with RF needles. Below are the results when tuned on a Dyno. The lower graph shows the air/fuel ratio under load.”
That is really interesting, thanks! I am going to compare those two needles. On my internet search i found a SU manual regarding tuning . It contained a table with desciptive needle sizes…Willing to share if interested.
The Jaguar Service Manual indicates that WO2 is the correct needle for the C-type head with 8:1 compression and disc air filters. This was changed from SL needles specified prior to engine number G3250, with the recommendation that the earlier engines be changed to WO2 if they exhibited certain symptoms of spitting back on full throttle acceleration. SJ needles were recommended for a standard 8:1 cylinder head and standard air filters. Of course, as fuel these days bears little resemblance to that available in the 1950s, none of these recommendations may have much value. I have WO2s fitted to my XK140 with C-Type head, and suspect that the mixture is a little rich at low revs, but don’t mess with it as I fear the consequences of lean running at high revs. I will be interested to hear what your investigation comes up with.
Then there is the post from Mike Balch.
His xk120 has the same engine configuration as my car and runs nice with RF needles. Thanks for sharing that!
My idea is to check the enrichment valve (see if closes completely) and then do some tests with RF needles.
Btw there is a booklet concerning SU carbs it can be helpful comparing needles for example.
Most of you probably have it…
Some Screenshots:
What you have not said is what plugs you are using.
I use NGK BP6ES
Also note virtually all the needles are the same thickness at the top, which is the position where the needle sits on idle.
Are you sure your needles have the shoulder flush with the underneath of the piston and both the same length. Hold them both horizontally needle to needle to see if both needles touch the opposing piston together.
Both jets are in the uppermost position… but it looks like the height of the brass bit in which the jet is sitting differs if one compares left and right carb… Besides, top of both are not flush with the carburetor housing. Shouldn’t they be flush?
Put steel ruler down the middle, to on top of carb housing, so the end flat is against the base of the carb in the picture. Then wind up the jet until it just touches. Now wind down the two jets by the same amount. Measure drop with digital callipers. Try 0.06ins then go up or down to get idle mixture right. I don’t think the outer part of the brass matters too much, it is the height of the jet that needs to be the same over both.
I’ve working on the car and made some progress so it’s time to report back.
The reason for running very rich turned out to be the enrichment valve. Somehow the spring was on the wrong side of the valve, pushing it open instead of closing it.
According to plug colour i can get it to lean out nicely.
I’m gonna use the method below to set the mixture the same both carbs… Thanks John.
