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MG MGB Technical - High Idle? No Leaks Though

I am finally at the end of my restoration on my 72 midget. Its been two years and I am yet to drive it. I have finally resolved a few timing issues and finally the car runs clean but idles uncontrolably high. I have removed the carbs, which are brand new and have checked for air leaks and correct adjustment. Everything is fine as far as I can see. Now the car was fitted with emissions control but has been removed long ago. The only thing that is left is the run-on control valve but wiring has been disconnected, with this it should be closed. I have left it in place because it plugs the hole on my intake, could this be the problem? Or am I missing something? The vacuum advace is working correctly and all gaskets are new and tight Any help would be great.
Neil

It sounds like your throttles are open too far. This could be from the throttle stop adjustment or from the cable being clamped down too short. Disconnect the throttle cable and set the idle to the proper speed, then reconnect the throttle cable. If this doesn't solve your problem, you probably still have timing issues, provided there are no vacuum leaks. Do your throttle plates have a valve installed or do they have a small hole drilled in them? How far open are they at idle? Are the carbs topped off with oil? What is your static timing set at? How many degrees advance is your vacuum canister pulling at your current idle? Did you set your timing with the vacuum disconnected? You need to step-by-step check every aspect of the ignition and carburetion systems. If this doesn't come up with anything, you need to review your cam installation. Was the cam degreed? Is it the original, aftermarket, or performance? What other modifications did you make to the engine? If you catch my drift, there are many variables to consider, given what littl info we have to go on.
Good luck. Report back with more info?

Jeff Schlemmer

Well, I have tried again to get this thing to idle right am I cannot. 72 MG Midget 1275cc. To answer your questions,

The cam is stock, the timing chain has been replaced a week ago and everything seems is fine. The static timing was set at TDC and could not get it to fire. I retarded it 8 or 9 degrees and seemed to have found the sweet spot. I did this by ear and by driving around the lot. The timing ewas set with the vacuum advance on and I checked it while running for opreration. When I rev the engine, it sounds perfect but the idle will not fall unless I advance it, but then it runs like crap. I gave the carbs a go over to check for any leaks and everything is perfect. The oil in the dampers is 20W. The piston rises with resistance and falls softly, over 4-5 seconds. I also went over the manifold and checked for leaks.

Everything has been replaced on the ignition side, points, condenser, breaker plate, cap, wires, coil, rotor, plugs, LT & HT leads and timing chain. The coil is a MSD Blaster 2 w/ballast resistor. The fuel side replacements are new HS2 carbs, fuel filter and pump, all new gaskets from the head forward, throttle and choke cables.

I have disconnected the throttle linkage to see if that was the problem, but it is not. I have never adjusted or know if I can, the valve on the throttle plate. I also thought that this may be the problem but could never find any info that says how or where to set it. Why when I retard the timing, the car idles higher? I am lost. Please help!
Neil

Neil,

Until you pull the carbs off the car and verify the throttle plates are closing 100%, your chasing your tail. Hold the carb up to a bright light. If you can see light, air is passing. Loosen/remove split screws and snap the throttle shaft "smartly home". It takes a little fiddling but eventually they will find their way home. Be sure the bevels on the throttle plates are in the correct direction. It is possible to have them installed backwards.
Paul Hanley

Hi Paul,
I did pull the carbs yesterday and the plates looked as if they were in place exactly. Although I did not use the flashlight trick so maybe you are on to something. However when the carbs are installed and the car is idling, as I back off the idle set screw the rpm falls. And when the screw is completely backed out, the engine dies and will not start. So if it will not run then technically it should be set correctly. But if it is off even a little bit then i suppose it would be like turning the screw in farther. I'll check tomorrow.
Thanks.
Neil

I just thought of one more thing while sitting here.....My engine is equipped with a pump in the timing cover to send fumes to the carbs to be burnt. I wonder if this is forcing air at a higher rate to my carbs? I have never tried running the car with the hoses off. I will try tomorrow.
Neil

I am now a member!!!!
g Campbell

out of all that ramble I didn't even state how high it was idling. It is no less than 2000 rpm with everything backed off
Neil Campbell

If the butterfly plates are correctly seating then no fuel should be able to get past stalling the engine - regardless of any vacuum leaks or crankcase breathing. Causes of a high idle can be a butterfly not correctly seating (can be caused by being the wrong way round in the spindle), faulty poppet valves (if fitted), interconnecting spindles and clamps incorrectly adjusted so that although one butterfly is closed it is holding the other open, throttle cable without the appropriate slack at rest, maladjusted fast-idle screws.

But you say when you back off the idle screws the revs fall, and eventually the engine stalls and will not restart. That is what you *should* be happening!

The pistons should drop *smartly*, not over 4-5 seconds!

Normally with carb vacuum the idle speed *increases* as the timing advances, which makes me think you have manifold. With manifold vacuum the timing must be set with it disconnected and plugged (which reduces the idle speed), and the idle speed adjusted to compensate while doing so.
Paul Hunt

Hello everyone,

Yesterday I took the carbs and intake off, again. I worked my way back checking for any leaks or cracks and everything was fine. I glued the poppet valves shut on the throttle plates, assembled and left the car alone over night to let everything set. I woke up deadly sick this morning, but still managed to sneek down and give it a try. A couple turns of the screw driver and the car was idling at 1200. I wasn't going to get into tuning it today so I left it alone. None the less my problem is solved. Thank you very much!

After gluing the valves shut I went and dug out another set of SU's that I had to check and see how strong the valve springs were in the original set of throttle plates. They were so much thicker and therefore stronger. The new ones that are installed now, are very thin and flimsy. With vacuum created from the engine these were opening just enough to let the air through. Something else that caught my attention was that only one of the original throttle plates had the sring valve on it, where as both new ones ony the new carbs have them on each. It also explains why before carb swap I had to always run it a bit rich. Well, Thanks again for all of the help everyone!

Neil
Neil Campbell

This thread was discussed between 16/04/2004 and 18/04/2004

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