burden@cheers.DEC (Dave Burden --- 381-2559) (08/10/84)
From: NOVA::GAVIN "BILL GAVIN" 10-AUG-1984 10:30 To: CHEERS::BURDEN Subj: RE: maybe you can help this poor soul I've run into similar symptoms with both my V8 Monza and my Austin Mini-Cooper (997cc). In both cases the problem was excessively rich mixture. The problem with the Monza was the poison-hands 'mechanic' the previous owner took it to. I pulled the carb, reset everything to specification, and found a tremendous improvement. No more running on, no pinging, and 20mpg instead of 12mpg. I'm still fooling with the auto choke and idle mixture settings, but it works adequately for an engine with 125k on it. The problem with the Mini is a little different; a thoroughly modified engine that was pretty competitive in SCCA D-Sedan racing in the late 60's. I put smaller carbs on, altered the ignition advance curve, and did a few more things to make the car streetable. I worked my way through all your symptoms. It seems that the mixture started out so rich the carbs wouldn't respond to mixture adjustments, but the idle was fairly smooth. The 20mpg mileage was the clue here, and I arbitrarily started to lean the carbs down until the engine barely ran. After some driving and further fiddling, the carbs started responding to adjustments. Once the mileage got over 30mpg, I figured the mixture was ok in idle and midrange, so I started swapping needles to dial in the whole range. Since you've done so much, I'd skip all the normal stuff and look deeper. Carefully inspect the needle for scuffing; if the needle is worn, it will give a rich idle mixture. If you adjust for this, it will give a lean mixture in mid and top range. Top tends not to be so noticeable. Stromberg needles, unlike SUs, are not symmetrical, and have to be fitted in a certain way. Check a good manual and make sure you've got it right. Also, do you have the special tools for holding the piston from rotating while you adjust the mixture ? Double check the rubber diaphragm for tiny cracks, and make sure it is seated properly in the piston and in the carb body. Check for cracks, missing seals and plugs, and worn parts (throttle shafts, etc). Strombergs have temperature compensators on them. I haven't messed with them for a long time, but they can lose their setting and cause temperature-dependant mixture problems. Check the manual, and try to figure it out before you tear into it. These can be disabled or removed; the EPA won't approve, but ........ After you're sure the basic mechanical specification is ok, and if the problem persists, try leaning down the mixture a good amount; if the problem gets better, go some more. If it gets worse, richen the mixture. Fuel pump pressure can also affect things. Make sure you have a good fuel filter in the line and that it isn't plugged. You might switch to a single SU, but the brake booster will prevent you from fitting twin SUs, which would be the best solution short of a Weber. It sounds like your problem is rich mixture, but finding the cause can be a real challenge. - Bill Gavin