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Spitfire Carb Adjustment 78 Spitfire

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J

Joe_Pinehill

Jedi Hopeful
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I changed my plugs over the weekend. The plugs show the classic look of rich mixture, black, carbon coated. Also my mileage is not the best.

But the car runs great, always starts, idles right at 900 to 1000rpm.

Is it worth a tweak of the mixture with the carb tool?

If I do, can you recommend the number of turns or partial turn to start? How do I know the Im properly adjusted? Or do nothing? Ive had the tool in my box for 2 years, and Ive resisted the temptation to use it.
 
Adjust the carb by removing it and fitting a pair of SU's! But if that is not an option, then yes you can adjust the mixture. Its been awhile since i did a spit carb, but its pretty standard. You do need the tool so you dont tear the diaphram and it sounds like you have the tool. With the car warmed up, lift the piston just a touch, and listen to the sound of the engine..........does RPM increase and stay there? If so, you may be running a bit rich. I can't recall which way to turn on a Stromberg, so others will have to give you that information.....but its a pretty simple job. Adjust the carb, clean the plugs, go for a run and then pull the plugs again and see what they look like.....if they are a nice tan color then congradulate yourself and have a nice frosty beverage of your choise!

cheers,

mitch
 
A little rich is better than too lean. But if it concerns you, then adjusting mixture is pretty easy, given everything else is working properly.

I would first check the air cleaner, make sure its not too dirty etc. I think you have a manual choke, so also check that the cold-start valve is fully disengaged when the choke cable is pushed in all the way. You might want a quick check of timing as well, make sure its advancing properly on acceleration. And check that the dashpot oil in the carb is topped up, and not running out.

To lean the mixture, turn the adjusting tool counter-clockwise. I would start with one half-turn. Clean the plugs, drive it, and check again. If they're still fouling, you may go for one more half-turn. Thats quite a bit, and if it doesn't make a significant difference, you may have other problems.

If you can get ahold of a Gunson ColorTune, its a great tool. Allows you to check the color of the burn at different throttle settings as well.
 
Also remember to do the plug cut at speed. That is don't slow down and idle the engine before you check the plugs. Instead get the car up to temperature and then run it at speed and kill the engine while at speed and coast to a stop and check the plugs then. That will give you a better indication. At idle the plugs always show a bit too rich. Also remember you are looking for the colour deep down the cntre insulator.
 
Joe, sometimes you will find that the adjustment screw is stubborn and may not move. When I had this situation, be it from the O-ring or the star like retaining clip, by removing the air valve, diaphram and needle assembly as a unit and grasping the air valve firmly in one hand while turning the allen wrench with the other, I was able to break free whatever was in a bind, without fear of slipping and tearing the diaphram.



You really don't want an overly rich mixture washing down the cylinder walls
 
Before you pulled the plugs to find they were black, how had the car last been run? If you started it on choke and let it idle a little bit and shut it off, the plugs will be black.
Before you mess with the carbs, make sure the car has been driven at temperature for a distance to clean up the carbon build up from running on choke.
You want to adjust fuel mixture as a last step. These things don't change over night, but gradually over several years.
Any time you change your mixture, keep track of what you did, because eventually you will find yourself having to put it back to where it originally was.
 
Sorry I'm late with this, but before I got my TR-7 Spider, I ran a '78 Spitfire for 9 years during the '90's and '00's. I used mine year round, and that included snowy and cold weather here in Maine.

This is an island town so I could complete drives in short distances, and not really get the engine warmed up fully. That meant that the plugs would foul up as you described. The choke [which is a water temp automatic one in the '78] worked fine; the car would start, but the plugs needed changing during the winter

Did the plugs do the same thing in the winter? If not, then the mixture is probably correct. There's not much adjustment in those Strombergs.

Jeff Aronson
Vinalhaven, ME
 
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