| Author | Message | ||
     Dale V Mattson Yet to post message Username: wookieecantina Post Number: 1 Registered: 5-2009 |
Hi All! We have a 19767 Silver Shadow, California smogged vehicle and are experiencing an odd problem. After warm up, choke functions correctly, the engine idle increases and thusly varies a great deal. I synchronized the carb's airflow with correct pot mount micrometer gauges, but the problem still exists. I also replaced both EGR valves which are now working wonderfully. I also adjusted the linkage which is functioning very smoothly. A local British mechanic from an MG club said it sounds as though the SU butterfly shaft is worn and is pulling in air. After wiggling the right carb, I noticed the idle can change or vary slightly. Has anyone out there experienced this sort of problem? I was told a graphite bushing can be installed which will eliminate this issue. Any help would be tremendously appreciated. Thank you! | ||
| Peter Colwell Frequent User Username: peter_colwell Post Number: 72 Registered: 3-2005 |
In the absence of advice from someone familiar with this particular model, I was able to cure a similar problem on a car with twin SUs. The linkage between the two carbs was 'loaded'. Ie. one throttle was open slightly more than the other at idle. The cure was to loosen the linkage right off, so that both carb throttles were free to close right up. Then tighten them up again, prior to the usual tuning for twin SUs. | ||
| Dale New User Username: wookieecantina Post Number: 2 Registered: 5-2009 |
Hi Peter, Thank you so much for the response, I'll check that linkage tonight and see if that is possible. That would certainly be the easiest fix. When I looked at the ajoining linkage between the two earlier in the week, the small piece that joined the two appeared to be solid, but there may be an adjustment I failed to observe. Thank you again Peter, have a great day! | ||
| Dale New User Username: wookieecantina Post Number: 3 Registered: 5-2009 |
Hi Peter, OK, it just clicked! Last week when I re-set the butterflies I adjusted the left unit first, then the right. I'll try that in reverse, as this may have been my mistake as I believe the right carb is linked directly to the first throttle rod atop the manifold and the left carb is controlled via from the right carb with a fixed linkage. Thanks again Thanks again! | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 327 Registered: 6-2006 |
The adjustment is just clamps onto the spindles. You need to back off the idle speed adjusting screw completely and make sure the fast idle screw is away from the cam before synchronising the throttles. Shut them both and clamp them up. Not sure that it would give you varying idle speeds though. Also have a look at the butterflies and see if the spring loaded valves are seating correctly and firmly. Perhaps check the hot air blend flap and vacuum advance for leaks. And that the weakener tubes and valves are OK. Blank off the speed control vacuum pipe. Also check for inlet manifold leaks. Rubber mats or Carpet holding the accelerator pedal down? | ||
| Dale New User Username: wookieecantina Post Number: 4 Registered: 5-2009 |
Hi Paul, Thank you very much! This will help a great deal. I've checked the hot air blend valve and the original hose was badly deteriorated, so I replaced the hose with a nice after-market piece. I've not checked the weakeners however, so I'll check right away. Probably have to break out the manuals for that one  I checked for leaks using the old reliable starting fluid spray test and nothing really happened which was very noticeable, but the idle did move a little when I spray the right carb linkage. Not much though. I think what perplexes me is the number of mileage is quite low, only 48600 miles. Didn't quite make sense that a shaft bushing would wear out with such low miles, but its possible. Thank you again for your help! | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 328 Registered: 6-2006 |
Maybe it's just too weak then , and hunting? | ||
| Dale New User Username: wookieecantina Post Number: 5 Registered: 5-2009 |
Hi Paul, You could be right, as the slighest touch changes the idle, even when opening and closing the throttle, sometimes it will go back to the standard idle, approx. 600RPM, and other times will return to 650 to 750. I feel as though I'm just looking the problem in the eye and just not quite recognizing it The MG mechanic stated that shaft leaks can cause this issue. so you would be right, as vacuum leak is changing based upon the random closing of the butterflies. Thank you very, very much. | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 329 Registered: 6-2006 |
Dale, so when it returns to 750 , will it just stay at 750 until you do something to it, or will it drop back to 600 by-itself after a while? If it's idling at 600, will it go to 650 or 750 without you touching anything? | ||
| Dale New User Username: wookieecantina Post Number: 6 Registered: 5-2009 |
Hi Paul, Thank you again for your assistance! Well, it will start and idle nicely, choke turns off, and the idle stays constant near 600 RPM. Once the engine heat up to thermostat opening temperature to wear you hear the radiators thermal fan clutch engage and start pulling more air, then the idle picks up. If you rev it up, sometimes it will drop to 600 to 650, sometimes it will stay near 700 to 750. What is strange, is that if you shut the car off and then very quickly restart it, the idle will be back down to 600 until the engine is rev'd up. I checked the linkage last night and everything appears satisfactory and the butterfly pinch bolts do not actually allow for much adjustment due to the groved pinch-bolt slot on the shaft. Thank you again for your assistance. | ||
| Bill Coburn Moderator Username: bill_coburn Post Number: 1121 Registered: 4-2003 |
Dale, Talking to one of our automotive sages down here the consensis is that you have weak or sticking over run valves in the butterflys. I note that Paul suggested this some time ago. If the car has been sitting around for quite a while they could well be clagged up and periodically sticking open. Best clean them and ensure there is free movement. | ||
| Dale Mattson New User Username: wookieecantina Post Number: 10 Registered: 5-2009 |
Thank you so much! I'll do just that and run some cleaner through the butterflies. After following everyone's advice, which I'm incredibly grateful for, I did find that the linkage on the right carb. was binding a little bit, the piece which connects to the actuator located on the left side. Being a Calif. smogged vehicle, the linkage seems to be a bit different from most pictured in the manual. This did appears to prevent the butterflies from closing properly which would explain why the idle screw would not seat properly or least offer limited use. I'll clean the carbs tonight, correct that linkage issue, and see how she runs. Thanks again! |
