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MG MGB Technical - carb install high idle

I have a 73B with HIF4 carbs. Carbs were flooding, so I removed the carbs and sent them away to be rebuilt. I installed the rebuilt carbs, and was ready to tune them. Car started easily, but with idle adjusting screws backed off completely, the car idles at about 1500 rpm. I have made sure that there is nothing holding either carb open. Checked for vacume leaks and cant find anything. Using a piece of fuel line to listen for air flow at the mouth of the carb, I can here air flowing into both carbs - more in the front carb, but detectable in both, even though the throttles are completely closed.

I removed the carbs again, and throttle discs apear to be closing and seating properly. I can get a .0015 feeler in around the disk in a couple of spots, but its tight, and most of the way around the disk, the feeler wont go in at all, so I think the carbs are ok?

These carbs have the small anti run on valve in the throttle discs. They appear to be operating properly, but I really dont know how much force the springs on these are supposed to supply to keep them closed.

The carb rebuilder suggested that if the timing is advanced too far, it will cause a high idle even with the throttles closed. This does not make sense to me. If the the trottles are working properly, how can the car run without air/fuel? And I can hear air entering the carbs using a piece of fuel line.

I am at a loss as to how to proceed. The only thing I can think to do is to solder the anti run on valves closed and reinstall the carbs to see what happens. I really dont like doing this, but dont know what else to try.

Any help appreciated.
DJ Shaw

DJ - Check that the throttle disks are properly centered. If they are slightly off center, they will hold not close all the way. Cheers - Dave
David DuBois

I agree with Dave. Where are you in PA?

Greg Van Hook

I had the same problem and it was the anti run-on valves. The spring seemed nice & tight but one carb was pulling it open. Soldered closed & problem solved.

P M Gregory

David, can you tell me the procedure for centering the throttle discs? I think they are centered, but if I understand the procedure, I will check.

Greg, untill last year, I lived in Mechanicsburg - on the west shore of the Harrisburg area. I moved last year to Minneapolis MN. Should have changed my profile. My son goes to school in Philly, so I still get down that way quite it bit.
DJ Shaw


With the air filters off and the car running try covering the carb air intake openings. If the car continues to run when they are completely covered then you have a rebuild problem. If he car cuts off as it should then experiment with the butterfly centering and sealing off the valves. You could even seal the valves temporarily with some electrical tape (good and secure so it can't get sucked in).

BH Davis

I would have thought that when the carbs were rebuilt, the re-builder would automatically replace the spring loaded discs with the standard solid discs. This may be something that you can do your self. Make sure that the linkage that goes between the 2 carbs isn't holding the carbs above idle. There should be a little slack in the throttle cable at idle. This is very important. The throttle discs can be centered by loosening the holding screws and centering the discs. Then you re-tighten the holding screws. RAY
rjm RAY

DJ - "can you tell me the procedure for centering the throttle discs?"

This works best if the carbs are removed from the ca so you can look through the throat to see any light getting past the throttle disks. Loosen the two screws holding the disks in place and work the disks around for the best possible fit in the carburetor throat (least amount of light seen around the edges). This ma require pulling and pushing on the throttle shaft. Once you have an absolutely perfect fit, snug the screws down (a drop of green LocTite helps keep things in place. Cheers - Dave
David DuBois

And, yes, advancing the timing will increase the idle speed. It makes for more efficient combustion, but is not recommended for adjusting the idle since it will upset the timing of the engine above the idle range. RAY
rjm RAY

Vacuum leaks won't cause a high idle that can't be backed out by the idle screws, with one exception and that is if the bushes and spindles are so worn that mixture can get round the fully shut throttle discs.

The rebuilder should *certainly* have got the discs centred! However they may well not substitute solid for vented unless asked to do so. Personally I've just soldered mine shut, which can be done simply by removal and not dismantling. The performance gains by not having the spring in the air-flow will be negligible to non-existent in normal road use. They are a common problem, mine used to stick open during warm-up on frosty mornings.

These valves are not anti-runon valves, but pollution-reducing by simulating a gradually closing throttle on the overrun rather than a snapped shut one. The anti-runon valve is external to the carb(s) and works when the engine is idling, by which time those valves are (or should be) shut anyway.
Paul Hunt 2010

This thread was discussed between 22/05/2010 and 25/05/2010

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