I’m hoping someone here can advise me if I’ve made the right choice regarding carb needles and spark plugs. The engine is freshly rebuilt, as are the carbs.
The specs of the engine are as follows: 3.4 block bored +.040", XK150 B Type head in standard trim with 8:1 compression, H6 carbs with standard AC air cleaners, points ignition with Lucas HS12 sports coil. Basically, everything is standard XK120 except for the B Type head.
The jets are AUC 8189 (.100") and the needles are TL. I chose these as they were the standard needles for 3.4 XK150 with 8:1, and I thought they would match the B Type head. But, of course, the XK150 uses HD6 carbs, not H6, and I’m wondering if I should have chosen RG as fitted to XK120 with 8:1, or SJ or SR, as fitted to XK140 with 8:1 (although both these heads would have been A Type). Are the HD6 needles different to the H6 needles…?
I’ve noticed the plugs are very sooty, although I haven’t really run the car more than up the (city) road yet.
The plugs are currently NGK BP5ES.
I’d be grateful for any advice!
My 120 is a similar spec but… 3.4 bored to 3.8. 9:1 compression. Free flow exhaust. Btype head with rob Beere SU cams (Pretty close to standard).
I stayed with the original 120 needles and jets. I set it up with a colour tune. Pulls like a train! And I’m using points! Same spec plugs as you. 16 inch wheels and stock diff and gearbox.
Are these NGK BP5ES what most are using in their 120’s?
I have the same and have fouling issues.
I’d like to confirm these plugs are correct before I mess with the fuel mixture.
That’s interesting, Phil. You engine is further from standard that mine is. I have standard single exhaust system, standard B Type cams and valves, and I too have 16" wheels, original gearbox and axle.
But, if the needles don’t matter very much, as you seem to suggest, why are there so many different ones available? For instance, the XK140 had 8 different needles listed for H6-equipped cars, depending on whether OTS, FHC or DHC, compression ratio, A or C Type head, remote air cleaner, etc. And then there were sometimes standard, weaker, or richer choices as well…
I’m wondering which of the variables - weight of car, air cleaner type, compression ratio, head design, has the most influence on needle choice? I’m also wondering if the jet position relative to the needle is different for H6 and HD6 carbs - should I have chosen a needle specified for the H6 carb?
I have a Colourtune and might give it a try again. Last time I couldn’t really decipher the results, but that could have been due to the air leaks around the then worn throttle shafts. I assume that the best plugs to use the Colourtune on are number 2 and 5?
Graham, there has been discussion on this in past posts, and the general consensus seemed to be that the BP5ES was the most popular. But, some swear by Champion, others favour modern high-tech platinum or iridium types… I was thinking I might try one range hotter BP4ES, as I tend to do a lot of low speed tootling about, but then the worry would be holing a piston on a high speed motorway run…
Holing a piston is not the fault of a hotter plug: that falls to a too-lean mixture, and/or too much high speed advance.
I used BP5ES plugs in many cars and had no issues. A hotter plug might take care of the fouling issues, but you need to determine the why, and what kind of fouling: is it oil or fuel fouling?
im not suggesting the needles make little difference, indeed on the saloon forum I am asking for assistance with a modified Mk2 I have just completed. Using 2 inch Su I can get the car to accelerate with the standard UM and currently trying various alternative (UB, UE and UU) IF the weather ever improves I can road test and report back on that particular issue.
All I was trying to convey is that with the mods I have made the standard needles are working well. I have NOT had it on a dyno or used any other instruments on the road. Plug colour after a run seems perfect though. the vast range of needles obviously allows the professional tuners to get things more precise.
Yes, you are quite right, of course, Paul. My plugs look like they are fuel, rather than oil, fouled. I’ve been doing a lot of idling the engine or just backing the car in and out of the garage without driving anywhere (filthy wet UK winter is to blame!) so it’s little wonder the plugs have sooted up. The engine is freshly rebuilt (under 1000 miles on it). Because there are no intake valve seals on the old-style guides (they weren’t changed as they were OK) it burns a certain amount of oil after starting up, which settles down after a minute of so. I really need to take it for a decent run and then see how the plugs look.
I need to give the car a decent run and then check on the plugs, too, Phil. Let’s hope this crappy wether improves soon!
A good–and enjoyable!-- Italian tune-up will likely set it all right.