wildbour Posted January 21, 2014 Report Share Posted January 21, 2014 Any idea why SU carbs would have resistance right as it opens? It's fine once you're driving but starting from a stop there's pressure on the pedal and throttle valve to a point where it is very difficult to accelerate smoothly from a stop. It requires more pressure on the throttle than it should to get the engine to open up, but only from idle. So it causes the rpm to jump up to almost 2.5k when im trying to accelerate smooth. And it only happens when the car is running. There isn't any throttle resistance when the car is off. Its not the pedal itself sticking or the throttle cable. Quote Link to comment
LenRobertson Posted January 21, 2014 Report Share Posted January 21, 2014 With the engine turned off, remove the air cleaner, reach in each carb throat, and raise the piston in there with a finger tip. The pistons should lift up smoothly without sticking and drop back somewhat slowly when released. Do yours both do that okay? Len Quote Link to comment
wildbour Posted January 21, 2014 Author Report Share Posted January 21, 2014 Yes. Both dashpot cylinders seem to be working correctly. I also just changed the oil in there to ATF; it's supposed to be 20W. Doesn't seem like the needles or jets are binding. Quote Link to comment
LenRobertson Posted January 21, 2014 Report Share Posted January 21, 2014 Okay, that is good news on the dashpots. How well does the engine idle? I'm wondering about a vacuum leak somewhere, although I don't know if that causes any different problems with SUs than normal carbs. Keep in mind I'm no SU expert. I'm just throwing out some ideas, waiting for the real SU gurus to join in. With the engine running, If you open the throttle by hand at the carbs (not with the gas pedal) you can see all the linkage moving smoothly? Nothing hanging up or binding? Did this problem come on after the car had been sitting for a period of time? Or gradually develop over time as you were running it? Also keep in mind the Ratsun saying "If you think it is a carb problem, it is probably ignition" and vice versa. Ignition is something to consider if nothing seems to be wrong with the carbs. Maybe a bad coil could cause the symptoms you are having, but that is a real guess. Len Quote Link to comment
wildbour Posted January 21, 2014 Author Report Share Posted January 21, 2014 Opening the throttle by hand produces the same sticking feeling, and I am not seeing anything binding. It gets driven once or twice a month and it seems to have developed over time. Thanks for the input, I'm just stumped. Quote Link to comment
wildbour Posted January 21, 2014 Author Report Share Posted January 21, 2014 Also it idles solid and the response is all there, but I will try and look around for leakes. Quote Link to comment
LenRobertson Posted January 21, 2014 Report Share Posted January 21, 2014 More random thoughts - If you can rig up a light and inspection mirror, make sure the screws are in place on the throttle butterflys and there is no burr or ridge in the carb bore that could hang the butterfly up. I can't remember if any SUs have a seal or O-ring where the throttle shaft goes into the side of the carb body - something that might come apart and cause a hangup on the shaft. Maybe spraying carb cleaner where the shaft goes into the body might loosen something up. Is it easy to disconnect the throttle cable so you can feel the throttle shaft action by itself? I don't know the symptoms of the fuel nozzle adjustment on the bottom of the carb being too high. It seems like if it is too low, the carb floods over. Maybe if just a hair high there would be a hesitation before the fuel was sucked into the carb throat. You may need to re-post your question in the Engine category if you don't get any better ideas than mine. More people will see it there. Hopefully someone with good SU experience will see something obvious that I'm missing. Len Quote Link to comment
exit64 Posted January 21, 2014 Report Share Posted January 21, 2014 Are you running the stock return springs? Don't use just some random spring, get the correct ones from Ztherapy or some other vendor. And make sure both sides are attached. Sounds silly, but springs that are too heavy will cause exactly this problem. I had the same thing going on with my 70 1600 when I first got it and put it back on the road. About drove me crazy trying to figure it out. Also, post this question over on www.311s.org where all the Roadster guys hang out and you will have the right answer in a jiffy. Quote Link to comment
drieseck Posted February 3, 2014 Report Share Posted February 3, 2014 Sounds like you have checked this, but it is familiar. Since you changed to ATF might be worth another look to be sure the level is correct. Quote Link to comment
bobd Posted February 4, 2014 Report Share Posted February 4, 2014 If I am reading this correctly, the throttle pedal is hard to push and the linkage is hard to move by hand. The slides are not connected to the linkage, they move with vacuum. Nothing to do with vacuum leaks, needles, jet adjustment, damping oil, etc. You have a problem with the linkage or the butterfly valve binding. Remove the dogbones to find out which carb is binding. Quote Link to comment
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