ArdWrknTrk Posted May 23, 2023 Share Posted May 23, 2023 So, I will be the first to admit that I am still learning regarding the intricacies of carburetion. When the idle is set a bit higher than it should be, the hesitation is minimal or even gone. I don't like having the truck idle at 1200 RPM in drive though. That being said, it seems that functionally the higher idle causes higher vacuum, or perhaps maybe even takes the idle circuit out of the equation. If there's no transition from the idle circuit to the main circuit, there's no hesitation. I'm reasonably confident the carb is clean and overall functioning properly. But, I think the high idle "good behavior" is a good hint. What else besides the transfer slots may be involved. Does the level of vacuum control the power valve? Maybe I have a power valve or something else that is "too firm". What can get take out of the equation with a higher (too high) idle? If I am making someone chuckle with this train of though or using the wrong nomenclature when thinking through this, I apologize. I leave this alone for a week or two then it starts bugging me again... It sounds to me like your accelerator pump needs adjustment/tuning. Link to comment Share on other sites More sharing options...
ckuske Posted May 23, 2023 Author Share Posted May 23, 2023 It sounds to me like your accelerator pump needs adjustment/tuning. Even with my limited knowledge, I’d agree that makes sense. It has been replaced twice now in the last 18 months. I have also tried changing the throw of the pump and no difference. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted May 23, 2023 Share Posted May 23, 2023 Even with my limited knowledge, I’d agree that makes sense. It has been replaced twice now in the last 18 months. I have also tried changing the throw of the pump and no difference. Are you sure the check ball is properly in place. Do you see a squirt of gas if you cycle the throttle lever by hand? Link to comment Share on other sites More sharing options...
mat in tn Posted May 23, 2023 Share Posted May 23, 2023 So, I will be the first to admit that I am still learning regarding the intricacies of carburetion. When the idle is set a bit higher than it should be, the hesitation is minimal or even gone. I don't like having the truck idle at 1200 RPM in drive though. That being said, it seems that functionally the higher idle causes higher vacuum, or perhaps maybe even takes the idle circuit out of the equation. If there's no transition from the idle circuit to the main circuit, there's no hesitation. I'm reasonably confident the carb is clean and overall functioning properly. But, I think the high idle "good behavior" is a good hint. What else besides the transfer slots may be involved. Does the level of vacuum control the power valve? Maybe I have a power valve or something else that is "too firm". What can get take out of the equation with a higher (too high) idle? If I am making someone chuckle with this train of though or using the wrong nomenclature when thinking through this, I apologize. I leave this alone for a week or two then it starts bugging me again... certainly, if there is not transition from idle circuit to venturi flow there is no stumble. at 1200 rpm you are already on the venturi flow (or you have a vacuum leak and a rich idle) and just adding throttle should be very smooth. being that it seems to come and go, how thoroughly have you checked for vacuum leaks? one point that gets dismissed a good bit is the brake booster. notice that it has the largest vacuum line of all going to it. this is because it is the largest vacuum reservoir on the truck/car. and if that diaphragm leaks it can do exactly this. the most common sign is noise or noise changes by merely pressing on the brake pedal with the engine running. usually, a hissing sound that comes from the area around where the actuating rod connects to the brake pedal. the only true test is to cap the intake port disconnecting the booster and run the engine. possibly even tune the engine that way before reconnecting the booster. if it runs differently than it is a problem even if it's not the whole problem. Link to comment Share on other sites More sharing options...
ckuske Posted May 25, 2023 Author Share Posted May 25, 2023 Are you sure the check ball is properly in place. Do you see a squirt of gas if you cycle the throttle lever by hand? Hi Jim, I will confirm, but last time I checked it was working, yes. The carb shop guy installed it, but never hurts to check again, I shouldn't take nothing for granted! Link to comment Share on other sites More sharing options...
ckuske Posted May 25, 2023 Author Share Posted May 25, 2023 certainly, if there is not transition from idle circuit to venturi flow there is no stumble. at 1200 rpm you are already on the venturi flow (or you have a vacuum leak and a rich idle) and just adding throttle should be very smooth. being that it seems to come and go, how thoroughly have you checked for vacuum leaks? one point that gets dismissed a good bit is the brake booster. notice that it has the largest vacuum line of all going to it. this is because it is the largest vacuum reservoir on the truck/car. and if that diaphragm leaks it can do exactly this. the most common sign is noise or noise changes by merely pressing on the brake pedal with the engine running. usually, a hissing sound that comes from the area around where the actuating rod connects to the brake pedal. the only true test is to cap the intake port disconnecting the booster and run the engine. possibly even tune the engine that way before reconnecting the booster. if it runs differently than it is a problem even if it's not the whole problem. Thanks Mat, I'll plug the brake booster and see if it helps. Instead of driving it around, I can duplicate it in the garage now. I just have to put it in gear, (set the parking brake) and also have one foot on the main brake. I then give just a little gas, and the RPM stays flat when it should increase. Eventually it does catch up after 3 or 4 seconds... Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted May 25, 2023 Share Posted May 25, 2023 Hi Jim, I will confirm, but last time I checked it was working, yes. The carb shop guy installed it, but never hurts to check again, I shouldn't take nothing for granted! I'm not too well versed in Cali spec feedback carbs. I certainly don't want to doubt "the carb shop guy" It is his trade after all. There should be a spec for gap if you have a set of feeler gauges. Link to comment Share on other sites More sharing options...
ckuske Posted May 25, 2023 Author Share Posted May 25, 2023 I'm not too well versed in Cali spec feedback carbs. I certainly don't want to doubt "the carb shop guy" It is his trade after all. There should be a spec for gap if you have a set of feeler gauges. Consider yourself lucky I am probably missing something, but gap of what? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted May 25, 2023 Share Posted May 25, 2023 Consider yourself lucky I am probably missing something, but gap of what? Actuator arm to diaphragm plunger. Let me see if I can find a diagram outlining this (though it may not be specific to your California emissions carb. Link to comment Share on other sites More sharing options...
ckuske Posted May 25, 2023 Author Share Posted May 25, 2023 Actuator arm to diaphragm plunger. Let me see if I can find a diagram outlining this (though it may not be specific to your California emissions carb. Oh nevermind, I can visualize it now. I am working and posting here in the background - a bad combo! Link to comment Share on other sites More sharing options...
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