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SU Carbs Dilema

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M

mgtildth

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I am a 1972 MGB GT with HIF4 carbs. I am using a maniflow intake manifold with K & N's. The HIF4 are the emission carbs which take biased needles. They are not supposed to be swapped but I was told that maniflow intake manifold will flow much more air into the head than the stock manifold. I bought a conversion kit for Joe Curto in NY that converts biased needles to fixed needles. The car runs well, but too rich.
I got about 16 mph on the last tank of gas which I think is outrageous. My problem is I cannot swap back the old needles. The brass bushing that goes into the piston which allows for a fixed needle will not come out. I am basically SOL.I am thinking about a weber down draft, although I am not all that confident that this carb set up is going to work. Opinions??....A new set of SU's is about $600.00.
Does anyone know where I could get a rebuildable used pair?

That is the dilema. I want to improve gas mileage.

Cheers, N /ubbthreads/images/graemlins/nopity.gif
 
Welcome to the forum although I am not really an official welcomer. A newbie myself.

Check eBay all the time. There are usally carbs there. I use twin Webers on one of my cars and think they are great. Many have one or two Webers on B's and I would consider it seriously.

Just one question. You said "I am a 1972 MGB GT with HIF4 carbs". Just curious if your owner knows you are posting here? /ubbthreads/images/graemlins/grin.gif
 
I have a '72 roadster with HIF-4's and get 21MPG around town and 24 to 28MPG on the road w/o OD. They sell parts for those carbs at both Moss and Victoria British, LTD.
mossmotors.com or victoriabritish.com Also there are several guys on here that are Moss distributers.
Bob
 
[ QUOTE ]
I am a 1972 MGB GT with HIF4 carbs. I am using a maniflow intake manifold with K & N's. The HIF4 are the emission carbs which take biased needles. They are not supposed to be swapped but I was told that maniflow intake manifold will flow much more air into the head than the stock manifold. I bought a conversion kit for Joe Curto in NY that converts biased needles to fixed needles. The car runs well, but too rich.
Cheers, N /ubbthreads/images/graemlins/nopity.gif

[/ QUOTE ]
N,
One of the reasons that the fixed needles are used is to get a much wider range of available needles. I assume that you have the needle bushings (SU #WZX-2003) plus the older style jet bearings to permit jet centering. A second benefit is that it reduces wear on the needles & jets. If you have not also replaced the jets at this time they could be worn enough to cause rich running. As would worn needles.

One thing that could cause very very rich running would be to use .090" needles with .100" jets. Make sure that the JETS are the correct diameter to go with the needles. The NEEDLE first step diameter (the step just below the shoulder) should be just a bit less than the jet diameter.IE. .099" for a .100" jet. or .089" for a .090" jet. I think your carbs should have .090" jets & needles.

To measure the jets, a #39 drill shank (.099") should just barely slip into a .100" jet, & a # 43 drill shank (.089") should just slip into an .090" jet.

if the above is ok, it is very easy to exchange your fixed needles for fixed needles that have a leaner profile. If you give me the the letters marked on the needle bases, the jet diameters, & what engine load range it is running too rich - idle, mid range, full throttle, I can get you in the ball park with the correct needle type. I'm sure that Joe C. can do the same.

I certainly see no need to change carbs or do any radical changes, a few small checks & adjustments should fix things.
D
 
I'm about to get a set of SU's ten I'm gonna rebuild them. I don't know what kind but it will probably be awhile. I'm working on new seats, carpet, rust, trunk carpet, sound system, and cleaning and organizing all of my parts. Post a want add in the classifieds and see what people have for sale. And BTW Bruce your not a "Newbie." You left that stage when you made your second post.
 
The problem is I cannot get the brass bushings out of the pistons. One comes out, the other one does not. I'm going to need to new pistons or new carbs.
 
What did I just say. Leave the brass bushings in, verify the jet sizes, & select appropriate fixed needles. You don't need to get the other bushing out or replace the carbs or pistons. There is plenty of help available on this forum if you wish more details.

Or did I miss something somewhere?
D
 
The little brass bushing that holds the needle into place will not come out, nor will the needle come out. The problem is getting the needles out, and have tried with no success.
I will get back to you on what type of needles I have.
I believe they are #7(rich).

Cheers, N
 
[ QUOTE ]
The little brass bushing that holds the needle into place will not come out, nor will the needle come out.

[/ QUOTE ]

Take the piston out of the carb, clean it up with some degreaser and water - and dry it...then stick it in an oven for a few minutes (not too long!) at 325° or so, a toaster oven will work fine. Then see if it will come out.
 
That's the only way I can get bearings into model airplane engines by the way - put the bearings in the freezer overnight then put the crankcase in the oven for 5-10 minutes. The "seemingly impossible to install" bearings drop right in without any effort, it's like magic /ubbthreads/images/graemlins/smile.gif To pull them out I just use the oven, hopefully it'll work with your piston.
 
Ok, I have #6 metering needle(rich). I thought I could find my old Biased needles but have had no luck. I will wait until I get several different needles before I attempt to get the brash bushing out of the piston. Do you sell needles?
 
[ QUOTE ]
Ok, I have #6 metering needle(rich). I thought I could find my old Biased needles but have had no luck. I will wait until I get several different needles before I attempt to get the brash bushing out of the piston. Do you sell needles?

[/ QUOTE ]
OK - I understand now, please excuse my density.

I have always been able to get stuck needles out by using penetrating oil & or heat as suggested & lightly clamping the piston in a padded vise, grabbing the needle with a padded plier, & twist, push, pull, or otherwise convince it to come out. There is a risk of scarring or bending the needle but since it is the wrong one, it is not likely to be reused.

If it is really stuck, a good machinist with vertical mill can reduce the needle to a stub & very precisely drill out the stub.

You are correct, the #6 is pretty rich. I would work down from #5,4,3 to see which works best.

No, I don't sell any parts, but I'm sure that Joe C. can provide parts, good advice, & even remove stuck needles. Make very sure that the needles will go back in the holes easily before final assembly. A frequent cause of stuck needles is to overtighten the set screws which raises a burr on the needle shank. Tighten the screws to no more than a couple of inch pounds. BTW - don't mix the two carb piston chambers, & pistons in this whole process. These parts are carefully matched at original assembly.

If you can find no other way, send the pistons to me & I will fix them. I've got all of the necessary equipment to do it. No charge except you cover the shipping. Don't give up, these things frequently happen.
D
 
Alright, sounds good. I pulled all four plugs this morning and they all looked beautiful. They were a nice light brown color.
The threads were black, sooty. This must be were the problem is. What I don't understand is why the plugs look as good as they do. I would have thought they'd be sooty all over. I'm going to Call Joe Curto in the morning and order some needles. When I get the needles I will experiment with baking the piston for a short while in the oven at 350f.
I will keep you informed.

Cheers, N
/ubbthreads/images/graemlins/cheers.gif
 
[ QUOTE ]
What I don't understand is why the plugs look as good as they do. I would have thought they'd be sooty all over.
/ubbthreads/images/graemlins/cheers.gif

[/ QUOTE ]
If the plug heat range is correct the insulator nose itself should run much hotter than the surrounding shell, threads, & combustion chamber. As yours is apparently doing. This is what keeps the plugs from fouling when the mixture is too rich. When you get the carbs set correctly, the shell should burn off the soot from excess fuel & be a lighter color.
D
 
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