View Full Version : Spitfire Spitfire carb problem

05-16-2005, 10:52 PM
Hello everyone...OK, I am really really new here and as a Spitfire owner too. I will have many questions and I hope you are a patient group. I dollied this 1972 Spitfire 400 miles home two days ago. The owner had an old shop and took the drive shaft out for me so there wouldn't be any towing problems. Well, it was a good thing because I noticed that it need a u-joint. Four hours it took my son and I to put the drive shaft back in place. I am learning...My question is that this little motor has a single carb (a SU stamped into it) and it is dripping gas down on the manifold and gas it going to the vacuum advance and dripping down on the starter too. So, I have two problems, gas leaking directly from the carb and gas traveling thru the vacuum hose. Do I purchase a carb kit or need a new carb or what? Any help is most appreciated. I have other questions, but I'll post them seperately. Thanks everyone!

05-16-2005, 11:25 PM
Welcome to the forum!
I strongly suggest reading a quick primer on SU carburetors... SUs are constant depression, or constant velocity variable venturi carbs, which means they are totally different than your standard carter, weber, or holley. Essentially, the fuel pump pushes fuel into the bowl on the side until the float reaches the top. The fuel in the bowl goes through a hole to the other side, and as the throttle is pushed -- the butterfly throttle opens, causing the piston to raise. On the piston is a little tapered metering needle -- so as the piston goes up, fuel is allowed to pass around the taper and up in the venturi area and pass the butterfly valve into the manifold.

SO...if the fuel is spilling, likely it is from the float bowl because the grosse jet is stuck open.

Now this all probably seems really confusing, so the first thing to do is get yourself a Moss Motors catalog, which has an exploded view of the carb (or go to www.mossmotors.com (https://www.mossmotors.com) -- they have it online.)

But in actuality, a 10 year old can rebuild SU carbs. They are extremely simple in terms of number of parts.

So simply unbolt the float bowl top (the small bowl thing on the side.) Pull up and see what is inside, and if pushing the float up closes the jet. The other thing is that the float might be set too high, so the bowl overfills.

It is also possible that the seals on the jet itself (what the needle is surrounded by) are gone, so the fuel is leaking. If fuel leaks from the bottom of the main section of the carb, that's probably the problem.

Although, I could be wrong, and someone might have a different idea.

The best way to understand SUs is to simply take them apart, clean them, and put everything back together! That's what I did with my TR3 and I was amazed that I could do it with such novice mechanical skill!

By the way -- were you the one I suggested join the forum from the Ebay listing?

05-16-2005, 11:30 PM
The most typical reason for gas leaking in an SU carb is a stuck float. This float is similar in concept to the float in a toilet tank...it keeps the fuel in the float chamber at a constant level. Often, just disassembling the carb and cleaning out the float chamber will "unstick" the float.
You can buy a carb rebuild kit from Moss Motors of Victoria British....it's unlikley that you need a new carb.
By the way, older Spitfires usually have two SU carbs and newer single-carb Spitires use Zenith-Stomberg carbs. I am not aware of any single-carb Spitfires that came standard with an SU carb. Replacing the Z-S carb with an SU carb is a popular modification (since many folks feel the Z-S carb is troublesome). In other words, it sounds like your car has a non-standard carb. This is not a bad thing...just something to be aware of.

05-16-2005, 11:40 PM
Good advice above.
BUT before you tear the carbs apart (which you will likely have to do eventually) just check the fuel supply lines. There's always some bits of rubber hose near the carb that will likely have perished, replace it with a new chunk from NAPA or some such supply place and stick a couple small gear clamps on the ends as well for security.
Oh, and do it soon before you have to use that new fire extinguisher you bought for the car! /ubbthreads/images/graemlins/eek.gif

05-17-2005, 12:12 AM
Hello,...I am at the right place. I can tell from the response that this is the place to be for me. Yes Sam, I believe it was you that made the suggesting (sbarer) that I join here and Graham....how in the heck did you know I got a fire extinguisher for that car!!!!? You been looking over my fence? I have limited mechanical experience altho I did take Auto Mechanics in high school (early sixties) for two years. I know what a left handed monkey wrench, lightbulb grease and a skyhook is! haha. Thanks guys, I really feel at home here and I'll get that carb apart in no time!....Bob ~ ps... thanks for the invite to this forum.

05-17-2005, 12:06 PM
If it isn't _pouring_ out of the float then all the rubber and cork is probably shot and you just need to rebuild them. It isn't difficult at all since all of the pieces that fall apart are basically one assembly of tubes, nuts, gaskets and springs.

There are a ton of resources on the web for SU carbs. Here are just a few:






05-17-2005, 02:07 PM
Also check to see if it has an electric fuel pump that could be overwhelming the carb float. These cars only need like 1.5 psi on the fuel.

05-25-2005, 03:21 AM
I completely disassembled the carb, cleaned it thoroughly, installed a new vacuum hose and a new fuel pump to carb fuel line (added an in-line filter which it didn't have) and cranked her up. Not a sign of a fuel leak anywhere. Thanks everyone....but I still have the ole fire extinguisher handy!.......Bob