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MG MGA - idling problem

I have started my engine on my MGA before putting the body back on the chassis, however the revs are rather high when the engine is idling. I have replaced most of the parts on both carbs i.e. shafts and bushes, jets, banjo bolts, concave washers and rubber washers , jet bearing kits,springs, gaskets, vacuum pipe.
Throttle discs close properly and pistons function well.
Does anyone have any idea what could be wrong. I have double checked almost everything but it did not make any difference.
Your comments are very much appreciated
c vassallo

Check the castings that you put on the new shafts. Some times the hole drilled is in the wrong location and holds the throttle plates slightly open. I have had to file clearance on the stop pad section to allow further closing. see if when you unscrew the idles screws the shaft stops are bottoming.If it is tight then filing is needed.

The only way an engine can rev is if it getting air. You can do as you like with the fuel supply but it will make no difference to the idle speed if there is no air getting through

With that in mind there are only 2 ways air can get into an engine

Either throught the carbs (throttle disks slightly open)


A leak at the manifold to head or manifold to carb joint.

The high idle speed is caused by one of these issues.

I rather fancy it will be through the carbs and that the throttle disks will not be seating perfectly.
Bob Turbo Midget England

1. Make sure the throttle discs are fitted the right way round so that the chamfered edges mate perfectly with the throttle bores.
2.Leave the throttle plate screws slightly loose, snap the throttle closed and then tighten the screws with the the throttle closed. As Bob says, the throttles being at fault is the most likely cause of too much air getting in at tickover.
Lindsay Sampford

I have taken carbs off the manifold again and could not trace any particular fault. When we lifted the piston and put in a torch to check whether light could pass through around the throttle disc we could only just see a bit of light around the disc. Now we are going to start blanking one carb at a time. I have also physically checked the bodies and manifold and I could not see anything abnormal. All comments and suggestions are welcome
c vassallo

My vote is on a throttle valve not closing properly. This may be due to the carbs not being synchronized. The tight clamps on throttle shafts between the carbs can cause this.

Reassembly all without air cleaners. Loosen one clamp on the shaft between carbs so they can move (open or close) independently. Back off all three idle screws so they don't touch the stops (fully closed). Tighten the throttle clamp between the carbs to lock them together. Without touching the idle screws, press throttle pedal slightly and start and run engine (may need choke to start).

In this condition you should have no fast idle and should have to hold the throttle cable or pedal to keep it running. When released it should slow down and die. If it continues to run, fast idle or not, it is getting some intake past the throttle plates. Place your hand over the carb intakes, one at a time. If the throttle plate is closed this would have no effect. If the throttle plate is open a bit you hand will stop the air flow and reduce engine speed or kill it all together). In any case both carbs should act the same. Another possibility is a maladjusted or sticky throttle cable that will not allow the carbs to close all the way .
Barney Gaylord

I have just been to the garage where I have spent the last three days fiddling with these blessed carbs and tried Barneys method. Yes when the engine is running and I block the intake of the air filter the engine stops even when the throttle disc is completely closed. Both shafts have been disconnected , throttle cable undone and idling screws completely removed.
Just for info the throttle discs on the carbs are the only parts that I did not replace with new.
What baffles me is the fact that the throttle discs seem to bed in so well with the carb that I cannot figure how air could be sucked in when you could not notice any gap with your bare eyes. However as already mentioned I had put in a small torch in the barrel from the side of the piston and with the disc completely closed I could see a very thin light around the disc.
I will order new discs if this would be your advise but could there also be wear on the carb body.
c vassallo

Barney.. Your vote is correct but remember that he had replaced the throttle shafts. Perhaps he did not get the holes for the lock pins correctly oriented and the pad for the idle screws is hitting the stop prior to the throttle plates closing.

Thanks for your comments darnoc but the pad for the idling screws is definitely not touching the pad. Besides I have the same symptoms on both carbs. Does the disc need to be completely air tight
The engine is presently running at 1200 revs when idling with both throttles completely shut. I have carried out my tests on each carb separately.
c vassallo

You may have the throttle plates in backward. The plate is not round but elliptical, so when closed it sits at an angle in the throat. The edge of the disc is beveled all around such that looking at it straight on when closed it is cylindrical to match the throat and will close with no air gap. If you put the throttle plate in flip side over it presents the sharp side of the bevel to the throat wall. Sharp side being larger across the tips, the plate seats at a greater angle, touching at the ends only and leaving a small gap the rest of the way around.

On the other hand, if the plate is in correct orientation it may be slightly misaligned. Loosen the two screws holding it, then nudge the plate around a bit in the shaft slot until it seats firmly all around the throat, and tighten the screws to hold it there.
Barney Gaylord

I already suggested that Barney.
Lindsay Sampford

I have checked on all your suggestions and found everything been fitted correctly.
Could this be as a result of timing, moreso when we had taken off the distributor.
c vassallo

No. You should be able to close the throttle valve completely for zero air flow, in which case the engine will not run. When you put your hand over the intake and it slows or kills the engine, the throttle valve is not fully closed. Back to looking at the throttle valve and linkages to find why it does not close.

Barney Gaylord

Errr No

I refer you to the 3rd post again.

You will probably be able to slow the engine down by changing the timing however that will simply be a fiddle. You could equally put the car in gear and let the clutch out slightly that will also slow the engine down but again is not the reason but would simply be a fiddle.

Has someone suggested to you that timing is the cause?

Robert (Bob) Midget Turbo

It is just the case that I could not detect the fault after going through all the checkpoints suggested in the thread. I have just ordered a couple of throttle discs.
Do you believe this will solve the problem.
c vassallo

Maybe a picture of your throttle plates and then some detailed ones (close up and personal) of your carbs would help.

You seem to think everything at the carbs is correct, opinion does not support you.
dominic clancy

It is true that I think everything at my carbs is correct since we have done such a big effort to try to get the best result and hence I feel very disappointed. In fact that is the reason that I started this thread since there are so many who can come up with suggestions to rectify the matter. I wish to point out that I am not technical but I have a lot of technical assistance from a local mga owner. we are both very puzzled what could be the reason for this. Sorry but i dont think I could manage to post a picture in view of my limitations for the use of the computer. someone told me that closing the throttle does not completely shut off induction. Is it the case?
I wish to thank everyone for your interest.

c vassallo

It is likely a lean mixture for some reason. As Bob stated above it could be after the carbs. You could try spraying some carb cleaner on the gasket joints and see if it makes a difference in the way the engine runs. Did you try enriching the mixture on the carbs? That should slow it down at least a bit. If it doesn't you must have a decent size air leak. I had the same problem with my '73 MGB this summer. The problem ended up being the crankcase ventilation tube where it plugs on to a barbed fitting on the intake had swelled, was loose fitting, and there was an air leak at that point. I had bypassed it for a few days. I had the throttle screws backed all the way off and still it idled about 1300-1400 RPM. After changing the leaking tube it would stall from idling so slow, so I was able to speed the idle back up with the adjusting screws like it should be. MGAs don't have that tube, but it illustrates the effect that an air leak can have. Could your vacuum line to the vacuum advance be leaking, or the diaphram on the VA unit?

L Poupard

Properly set up, closing the throttle DOES shut off all air flow through the carbs. If there was a down stream air leak there would be no fuel to go with it, and the engine would not run with throttles closed. If it runs at all the fuel is introduced with air flow through the carbs, only place the fuel can originate. Go figure out why the throttle plates are not closing all the way.
Barney Gaylord

I am taking out the carbs again for closer inspection and will revert.
c vassallo

It came to me that if shaft bushes were installed, and the bushes do not come flush with the bores, then there could be fuel/air leaks around the shaft where the bushes are.

FR Millmore

Get hold of the shafts and wiggle them -there should be no slack at all. Shine a torch into the carbs when taken off. If you can see any light from the other side then the discs are not fully seating.
J H Cole

Along with my mate Charlie, I too was fiddling around to try and find out why the idle speed on his MGA was remaining abnormally fast. At one stage we were totally convinced that the cause of fast idle was an air leak somewhere, either on the carbs or the induction manifold. We had checked everything over and over again, but could not see where any leaks were coming from. We were completely mystified. Finally, we sort of gave up, and decided to seek a third opinion. Charles has a mate who is very well into classic cars and all their workings, and showed the carbs to him. And "hey presto", this guy instantly found the defect which was causing the problem. In this thread, it was FRM who hit the nail right on the head with his very last post. The cause of the problem was from the bushes not coming flush with inside of the carb bores, and this was causing air/fuel to be sucked past the throttle discs. Having both carbs completely rebuilt with all new parts, etc., at no time did it occur to both of us that the shaft bushings were not pushed in flush with the carbs venturi bores, and we did not notice this error. Anyway, my mate Charles is now over the moon, and I'm pretty certain that he will come up again on this thread to thank everyone for his contribution to try and solve the problem. Thank you all and congratulations to FR Millmore.

F. Camilleri

Check your carb flanges for flatness. It is easy to distort them by over-tightening. If they are not flat, get out a big flat file and correct it.
Art Pearse

It seems to me that there should be a shoulder at the carburator throat that prevents the new bush from extending that far and maintains a smooth bore in the throat. Was this drilled out, so that the two sides could be align bored for the new shaft? Barneys site shows a better way to do this.
John DeWolf

I've often seen and said that most installations of bushes are far worse than the slight leakage that is generally present if the shafts are renewed in the old bushes. There are degrees of incompetence. If bushes are not installed in stopped counterbores, then the bush must go in to the throttle bore, and be finished to match the bore.

Note that the practice of reaming the bushes and fitting .010 OS shafts requires turning down the shaft to std size, outboard of the carb body. Otherwise, the various bits that fit on the shaft won't fit. If you ream these too, then you have made the arms etc. not usable with standard shafts.

Regarding making the flanges flat: the same distortion that makes the flange not flat also distorts the bore so that it is not round. Result is that the throttle plates don't fit and will not close correctly. This can be fixed by careful work with a scraper; very careful reboring of the throat could fix it, as well as the bush problem, but the final bore cannot be ANY bigger than it was originally, or else the plates won't be able to close off the bore.

FR Millmore

Just arrived home from my garage after fitting the carbs back on the engine. As already explained by Frans Camilleri, the person whose help and knowledge in this field has motivated me to almost complete this restoration, the problem was air intake from the new bushes which were slightly short from the bore. My engineer has machined a tool to push the bushes further in and then we filed them to shape. We started the engine and pronto!! problem was solved.
I wish to take this opportunity to thank everyone who has contributed to help solve this problem, and although I am still a novice , I have found these threads so helpful and encouraging
c vassallo

This thread was discussed between 02/11/2011 and 04/11/2011

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