Hello all! I’m proud to say that I’ve just bought a (my!) 1953 XK120, and I couldn’t be more excited!!!
The very first thing I know I need to do is get it prepped for altitude. The car is coming from California, and I live in Denver. I suspect 75% of my driving will be at about 5000ft, but some decent portion will be playing around on mountain twistys (twisties?) at 7000 to 9000 ft.
Please forgive what are some very beginner-esque questions, but here goes:
I know I’m gonna need to rejet the carb. How do you know what size jets to get? And does anyone have a link to good instructions or a video for rejetting and adjusting these carbs? I’ve rebuilt a Edelbrock 4bbl on a 454 before, but this: dual single-carbs, etc is all new to me. Any tips/tricks, etc are appreciated. (It’s ok to assume that I know nothing about how to do this…)
What else might I want/need to do to prep the car for tooling around at altitude? From a previous post, it sounded like getting a Pertronix distributor was a very common upgrade, although I’m familiar with adjusting points. And I suspect I’ll want to either re-core the rad or get an alloy rad and electric fan (those are very long hills on the way out of Denver). Anything else I should think of?
As soon as she comes home, I’ll get some photos up. Yay!
Before you do ANYTHING to your car ‘for altitude’, get ahold of someone on the list who lives in your area and has done a lot of work on their car (Wiggie maybe). Physically visit with them and spend as much time with them as you can. As far as I know, you don’t really need to ‘rejet’ your SU carbs. I live at sea level and have driven my car well over 5k feet and never had to do anything special with the car. The XK engine and SU carbs are a whole different creature than the 454; I like to think they are like a Vacheron & Constantin compared to a Timex. The Timex may ‘take a licken’ and keeps on ticken’, but the Vacheron is a piece of art.
Some one will come along soon this is a world wide forum, I’m currently sitting in my hotel room in the South Island of New Zealand on holiday, but as has been alluded to there will be no reason to change jets, needles are probably what might be needed but thats a simple job of removing the bell and replacing the needle in each one and making sure that the bell is seated properly before tightening the retaining screws.
I went with the matching numbers one. It had much better numbers on the compression and leak down test, was less money, and the body was in better shape. I figure with the difference in cost, anything small I might want to or need to do can be fitting under that budget. And, it gives me a chance to really get to know the car exactly as it was. So I’m very excited about all that.
Welcome to the hobby/madness.
When you say S block and head, I presume you mean there is a suffix S to the block and head serial numbers?
It looks like you have the standard SU type H6 carbs.
These are nothing like Edelbrock etc. They are variable venturi, which means there is a piston inside that thing that looks like a hand bell, and it lifts up with vacuum and pulls a metering needle through the jet.
There was a standard needle WO2 specified for FHC/DHC with the remote air filter in front of the radiator. There was also a weak needle WO3. The weak needle may have been for fuel economy. These numbers are stamped on the shank of the needle, where it fits into the piston. You might want to pull out your pistons and see what needles are in there. There is oil in the piston shafts so don’t turn them upside down unless you can catch the oil.
When putting them back, note that the shank should be flush with the bottom of the piston.
I’ve lived in Johannesburg at over 5000ft all my life. Most of the cars I’ve had had SU carbs. The main thing with altitude is ignition timing. We used to go by an old rule of thumb, ignore the first 2000ft then advance by 1deg for every 1000ft rise in altitude. Strangely, in my experience, the carbs are not greatly affected because there’s less vaccum to raise the pistons, thus they run a little leaner which compensates for the reduced oxygen. Just follow the normal procedure for setting the mixture correct at tick over. Otherwise go for Co analysis to see what’s happening throughout the range. I’m sure others will have experience too.
I grew up in that area and remember overheating the drum brakes on our big American cars in the mountains. In faster mountain driving here in New England my stock brakes noticeably fade at times (3000-4000ft elevation changes). I imagine it would be even worse in the Rockies. Suggest you consider getting a set of high temperature brake shoe linings (ex Porterfield R4-1, but there are others). I don’t think they advertise shoes it their catalog but just call them and inquire about relining, and be prepared to arc the new shoes if they come back a little thick.
Any good vintage race shop will be able to arc the shoes. A good drum brake place will also have that capability (think commercial truck places). IIRC, porterfield can if you ship them your drums or give them a spec lining thickness.
I stand to be corrected, but as far as I recall, in my readings on SU carbs, especially Des Hammills Guide to SU carbs (well worth owning), the basic design of SU carbs automatically compensate for air pressure/density, and require no alteration
My experience isn’t directly on point, but I have a 69 E-Type with Strombergs that lives at 6,000 feet and regularly is driven to 9,400 feet. My experience is that the car runs rich at those elevations even though the Strombergs, like the SUs, are supposed to compensate for elevation. If I only occasionally drove to those elevations I would do nothing. If you are always up high, I would see how your car reacts to altitude first, and then play with leaner needles if it runs rich, like mine.
