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Author Topic: Carb leaks and floor boards  (Read 6372 times)

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Carb leaks and floor boards
« on: February 19, 2009, 07:54:45 PM »
Two more questions. I started with both of my SU side draft carbs leaking gas from the over flow tubes when I turned on the electrical system but not starting the ignition. I cleaned the float needle valve as suggested by the shop manual I have with no improvement. I bought and installed new float needle valve assemblies, floats, and gaskets. The front carb no longer leaks but the rear carb still leaks. The original floats and the replacement floats are all plastic and do not have the brass(?) tab riveted on as a hinge as shown in the manual so I can see no available adjustment. The only thing I can think of is installing a spacer when I screw the float needle valve assembly into the top of the float valve chamber. This would lower the needle closer to the float and may seat the needle better into the valve seat. Do you have a better suggestion?

Second issue: I removed several panels and the rubber floor mats so I could replace the rubber covering the side rail in the interior of the car. I discovered that I have a significant rust problem with my floor boards. The Roadster Factory has a "replacement floor panel" for sale as well as an original floor panel available for each side. I plan on having a body shop do the work. Which floor panel (replacement or original) would you suggest? Any advise on doing this project? How much of the rusted floor panels should be removed? How about cutting them away from the supports underneath? How should the seat mounting holes/nuts be located in the new floor boards?

Mitch Sussex
« Last Edit: February 19, 2009, 07:59:10 PM by amgba »


  • chfwrench
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  • MG information: '73 red B roadster
Re: Carb leaks and floor boards
« Reply #1 on: February 19, 2009, 07:56:15 PM »
On the SU's, turning on the ignition (power only, not engaging the starter), the pump does activate and builds pressure to fill the lines and float bowls.  You should still have the HS-4 (side float) series.  The replacement floats I have seen usually have the brass tab to the pivot pin on top that you can adjust them, though I have seen the completely plastic nonadjustable types for other carbs.  The rebuild kit for the carb usually comes with fiber washers that serve as shims for the needle valve and seat to adjust travel, as you suspected.  You can see the washer in the picture of the later HIF-4 carb in the Victoria British catalog where it is shown, but not numbered.  I do not think the washers are sold separately by the catalogs, but the needle valve seat is much the same as used on American carbs and those gaskets may be available from NAPA, JEGS or other performance outlets.  Also, I have always preferred the performance of the "Gross Jet" replacement for the needle valve and seat.  Moss sees to be the only seller of this, but the captive ball design seems to seat better, wear longer and, the last time I bought one, the kit also came with the extra washers.

Replacement metal panels come from a variety of sources and many steer clear of them from a history of poor quality and fit.  These days, if the bigger houses like TRF, Moss and V-B are carrying them, chances are they are of good quality.  Overall, my preference is always for the BMH (OEM) style replacement as the accuracy of the match points and bolt holes of the pieces is generally worth the extra cost.  My experience with hoods (bonnets) in particular temper that opinion.  With floor pans, there is so much fitting, cutting and welding necessary, that this may not be the best deciding criterior.  I'd cross-check the catalogs and ask the tech people at TRF their history with the aftermarket type before purchasing.  I'll be posting this on the club site ( which you can check and see if anyone else has any experience or strong opinion.  You can also check out the "MG Experience" website and see if someone posted their method.  Also, talk to your shop about their best method and the costs involved. 

As far as the installation goes, the replacement floor pans, whether OEM or aftermarket, are purposely made with oversized flanges to ease installation and maximize the area that can b repaired without getting into the next panel (like the inner sills or battery box area).  Your shop might even find it less expensive to replace the entire pan than dealing with creating a new seam across the mid of the floor.   As long as you having the work done, it is best to replace as much of the damaged panel as possible.  The captive nuts have to be placed where the old ones were or the seats will not fit properly.  Though some are punched for the bolt holes, they may not be exactly in the right locations.  The new pans can be laid over the old ones in the car and key points marked and measured for more precise placements. 

I hope that helps you.  Keep me posted on all and let me know what you find.  Please feel free to write at any time if you have any questions.

Art Isaacs
« Last Edit: February 19, 2009, 07:59:27 PM by amgba »
Art Isaacs
AMGBA Tech Staff


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