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1970 Chev C10 chugging after warmed up

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  • #662693
    JosieJosie
    Participant

      Hi all;

      The project ride has hit the road and now I am trying to work out the bugs. As a little background, the truck is a Pre-EGR 350 V8 with a T350 transmission.

      When I bought it it didn’t run so well and would just about quit when I pulled up to stop lights. Its been a lot of work later, and that issue even got resolved for a bit, but now I am seeing something similar again, making me think I’ve overlooked something. So to list I.

      1) put on a Professionally rebuilt Carb (bought new to me Rochester Quadrajet like the one it had)
      2) new base gasket
      3) new Vac advance line to Carb
      4) new fuel filter in the line

      (plug, wires, Distributor cap and rotor were done last year just before buying)

      This is where its sat for the winter till I got at it again this spring. When I started it I noticed the idle was pretty high so released the idle screw some. This helped a bit but it was still running a bit high. I lubricated the linkage and replaced the throttle spring and it seemed a lot better after that.

      Then I took it for a good 30 min drive or so to see how it would do on the highway. I got it up to about 50 miles an hour for about 10 minutes and then took it around the city drive for another 20 minutes. She wasn’t running the greatest that last 15 min. Like I wasn’t getting much power. It almost seemed to be missing maybe. It was lightly raining too. I notice the miss when I let off on the throttle not so much when accelerate

      Well after doing so research, I decided since it was 25 dollars, I replaced the fuel pump today as the one on it looked pretty old. I find the throttle response is really good now. No miss or hesitation there at all when reving (from under the hood)

      So I took it for a run around the block to see how things would do and wow, the miss was really bad. Its warm out here (like 10C /50F) so no choke needed. Truck never quit on me but it felt like it could have, I got it home and it was sputtering and missing.

      I looked under the hood ad the choke was half shut trying to run. When I cut the engine I even saw some gas leaking out from the side of the carb. That explained it a bit so pulled the choke wide open manually, I tinkered for 5 minutes looking at anything else it could be and then fired it up again

      This time the thing started right up. Idles fine, no miss not sputters, no smoke at the back end. works and revs great. THEN after about 2 minutes of idle, the truck slows idle suddenly almost down to a chug. I hit the gas to keep it going. The choke is forced wide open. And I’m like WHAT THE HECK?

      I grab some carb cleaner and start spraying the base of the carb, the carb itself, all my linkages, all my air hoses, the hose to the brake cable, nothing, the idle never changes. Then this once I spray the air hose where it attaches at the brake booster and it seems to speed right up. I think I find it! But then it goes back again. I spray some more, nothing, I soak the thing then, nothing. I shut the truck off again. Let sit 5 minutes, Fire it up and idles fine for a couple minutes then chugs down again, I spray the booster hose again and this time it does nothing. I rev the engine and get it back into high idle again. I move that bake booster hose all over the place and it does;t seem to cause a thing with the idle. I leave engine alone and a minute later it goes back down to chugging again.

      Anyway, the gas tank is behind the seat with no rust in it, there is a new fuel pump and filter in the carb, and a fresh rebuild on the carb from last year.

      Then main thing I had bee trying to avoid too was I kept needing to pour gas down the carb to get it to start every morning

      (note that since the rebuild I have only had to do this like 10 times. 3 over the winter and the last 7 times I drove it) If I start it once during the day, the rest of the day it generally fires right up. Trying to fix this issue I have now have an idle issue.

      Any thoughts as to what the heck this could be? I called it a night. I’m head scratching now.

      Here is the pickup in question. Thanks again for all you help

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    • #662702
      AbdulAbdul
      Participant

        Two things to help narrow down your issue. Vacuum guage and fuel pressure guage. You should have good vacuum at idle. The needle should be steady between 18 and 20 ish only a healthy motor with stock cam. See if it changes when it starts to act up. Next is fuel. Let’s see if your pump is keeping up with your fuel demand. I don’t know the spec on your carb but generally around 5 psi is in the ballpark. Is it mechanical or electric??

        I was going to say fuel boiling in the lines but if the hood is open that may not be the issue. Especially if u have an electronic pump. I hope this helps or gives u a new idea.

        #662728
        Ryan WoffordRyan Wofford
        Participant

          did you properly adjust the carburetor after installation, It sounds like your low speed set screw is set a bit rich, but your high speed screw sounds fine. I would adjust the low until it idles fine and also revs fine. do this while the car is warm, don’t do it cold, it will make your adjustments rich.

          #662740
          JosieJosie
          Participant

            The new Fuel pump is the same as the old one, mechanical. Bolts on to the side of the block. Hose back to the tank (tank is in the cab behind the seat), hose up to the crab (new fuel filter from last year). Pretty simple. No electrical parts or wires at all. The way it was described to me was there was a lobe on the cam that pushes a rod down to lever in the pump to make it work. When I replaced it I made sure the push pin was in the up position and held there for when I put the pump on. There was no priming instructions for the pump at all.

            This does sound Carb / vacuum related though. Its like as soon as a certain temperature is reached a hose softens and then starts letting air in.

            Or there could be a mis adjusted idle screw I don’t know about. I have played with the one I do know a bit trying to fine tune. Its an external screw that just pushes the throttle linkage in (brings idle up) and then out (brings idle down). The only other screws are mixture screws. Never touched them at all, even when the truck didn’t have the issue. Was told by the re builder that they rarely need tweaking. And I admit I am a novice here so not sure about fine tuning stuff.

            It funny that with a warm or cold engine, engine works fine for 2 or 3 min after start. and then wants to conk out on it own. I’ll post some pics today with the carb and pump and such so you can all have a better look. I know she needs some gas so I’m going to get some first. 🙂

            #662775
            Gary BrownGary
            Participant

              I feel that this is carb related, I too have a 1970 quadrajet MC(married choke) in my truck. If the carb was professionally rebuilt, it will still need to be fine tuned to your vehicle. What is your warm idle speed?

              Tuning a carb requires ignition to be in tune first. Then, set your curb idle via the idle speed screw near the throttle linkage usually 600-700 for a manual and 700-750 for an automatic. From there, you can either tune by ear or connect a vacuum gauge. If by vacuum gauge, turn the idle mixture screws incrementally one at a time(the two needle valve screws in the front of the carb) and aim for highest vacuum. If Idle goes up, readjust the idle speed screw back down to your curb idle. Any idle speed 500 or below may be rough and if the truck has A/C, will not be sufficient. If tuning by ear, same procedure, just listen for a change in idle speed and turn the screws to get the leanest idle possible without starting to shudder. This is called lean drop. When you reach that point where it starts to studder and shake, move change screw direction until it smoothens back out.

              EDIT: I forgot to mention, the Quadrajet is very versatile and was used from the small 305 to massive 502s. The small primary bores and large secondaries allow this as well as an excellent and flexible overall design. Because of this versatility, fine tuning MUST be done. It is not a plug and play carb.

              #662813
              JosieJosie
              Participant

                Thanks Chevy Man

                Spoke to my Dad today and he seems to think the same thing. The only thing I have touched on the carb was the main idle screw (under the throttle linkage). I never found a problem with idle till now so I never thought the mixtures would need adjusting. That said the drive I took recently was longer than I ever had with the new carb on. Truck sat with the new carb on since last September. I would start it up on warm days just to make sure it ran OK but never for that long.

                I guess I try those adjustments now and see how that goes. I did some reading up on them and I’ll just do it by ear. Tune to highest idle for one. Then do the other. I don’t have a vacuum tester but I figure i should be OK to at least get it to run better than now. I’ll post tomorrow with my results.

                #662838
                Gary BrownGary
                Participant

                  [quote=”gsr-chick” post=135610]Thanks Chevy Man

                  Spoke to my Dad today and he seems to think the same thing. The only thing I have touched on the carb was the main idle screw (under the throttle linkage). I never found a problem with idle till now so I never thought the mixtures would need adjusting. That said the drive I took recently was longer than I ever had with the new carb on. Truck sat with the new carb on since last September. I would start it up on warm days just to make sure it ran OK but never for that long.

                  I guess I try those adjustments now and see how that goes. I did some reading up on them and I’ll just do it by ear. Tune to highest idle for one. Then do the other. I don’t have a vacuum tester but I figure i should be OK to at least get it to run better than now. I’ll post tomorrow with my results.[/quote] No problem, and what an awesome truck. Make sure that when you do the adjustment, to go easy with the mixture screws. Go about an 1/8-1/4 of a turn each time. Tuning by ear is a lost art form, if you can tune by ear it will serve you well in years to come. That’s how I learned, no tach, no gauges, no analyzers just my ear. I can get pretty spot on without my vacuum gauge but I always will double check it with the gauge for the sake of perfection.

                  #662872
                  JosieJosie
                  Participant

                    Well I am worse than before and extremely frustrated. I let the truck idle for 15 min getting it warmed up and ready to adjust mixtures. I spent some time just tweaking the screws but they made no difference at all in idle speed. I made sure the choke was wide open, made sure that the truck would idle well, Decided time for a test drive. I didn’t even make the block before it tried to die on itself.

                    Thankfully I got it home chugging and about to die. Giving it pedal did nothing for it at all. Only made it chug faster. I looked at the carb and its flooding itself. There is fuel all over the place. I tried to adjust the timing by turning the distr cap a small turn. IT made if barely run at all. Well I broke it now. GRRRR :pinch:

                    I guess I’ll have to get it towed to the mechanics. I guess I am just so mad that all these new parts to correct the problem did nothing. I can’t figure out what is causing this. Fighting to replace a part is one thing, doing it and it making no change is just so disappointing.

                    I know I am about to get taken for ride by the mechanics over this. I love doing car maintenance but this timing/ tweaking stuff your right is a lost science. I should have taken it to them in the first place before the extra 50bucks for the tow. Worst part is I never learned anything. I just for frig sakes wish I knew whats was wrong 🙁

                    OK. Im done for the day. Maybe I just need to walk away for bit. Sorry to take this all out on you.

                    Here are some pics of the setup if that helps at all. If anyone can tell me which way I should turn the dist cap to make it run leaner that would be great! It might save me the cash at least to get it towed to someone who knows what they are doing 🙁

                    #662879
                    Gary BrownGary
                    Participant

                      Ya, you and me have the same carb, mines a olds rocket 455 quadrajet but it is tuned the same way. First things first, I would not have touched the distributor. I would have simply thrown a timing light on it to check timing. I assume you are running a points style distributor?
                      Rotating the distributor does not make the mixture leaner or richer. It advances or retards ignition timing” (at what point before TDC the plugs fire). Also, when you rotate the distributor, you do it slowly and slighty. It does not take much to advance the timin.

                      When you were adjusting the carb did you turn down the idle speed screw first? If the transition slots were fully exposed past the idle circuit, the idle mixture screws will do nothing. You need to back the idle speed screw(the one near the throttle linkage) off and adjust the idle speed screws after. When you are doing this the vacuum advance hose from the distributor should be disconnected and plugged.

                      It is a lost art, but with a little patience you can learn it. I could have that problem fixed in less than 10 mins if I was there.

                      #663039
                      JosieJosie
                      Participant

                        Thanks Again Chevy man

                        Well good news is I actually DIDN’T touch the the timing. I started into a youtube video about it and indeed it has been years since I have WATCHED it get done. What i did was turn the CAP a little bit. I didn’t turn the distributor at all as I never loosened the bolt that holds it.

                        Went out yesterday to start fresh and the truck started up and idled Ok but it still has a nasty chug giving it gas and it speeds up and down on its own at idle. I started over with both Mixture screws as well. As per what I read online, I turned the screws all the way in (clockwise) softly till they were in the whole way, then I turned both of them out 2 and 3/4 turns. Truck still would idle but not take throttle. Its like it bogs right down and dies when I do. I replaced a vacuum hose on the advance to see if that had a leak but it made no difference.

                        I just read the part about blocking off the vacuum advance when adjusting the mixture but I can’t even get a smooth idle plugged or not. I am taking it to the garage tonight with my fingers crossed. I wish I did have someone with knowledge about these things close by. I’d definitely have welcomed the help!. Sadly the “big city” doesn’t have as many handy folks as a girl would hope. 😉

                        I am getting it towed today to the garage before I goof something up more trying to limp it there. Its less than a mile so no big fee for that. I did think it over a bit more and the good news is the mechanic I am taking it too loves older vehicles so i should be on good hands. I plan on taking him out to show him all the parts I replaced and fixed I personally so that he knows I am not just some dummy blonde with a bottomless bank account. I’ll be making it clear i want to see any parts that come out (if needed) and to consult with me before replacing them.

                        Will keep you all posted of what this turns out to be. Something tells me it could be more than the carb adjustment. But good news is the truck should run mint once this is fixed. Buying parts for my Acura hurt a lot more than this in the bank account area. 😉

                        #663043
                        Gary BrownGary
                        Participant

                          [quote=”gsr-chick” post=135834]Thanks Again Chevy man

                          Well good news is I actually DIDN’T touch the the timing. I started into a youtube video about it and indeed it has been years since I have WATCHED it get done. What i did was turn the CAP a little bit. I didn’t turn the distributor at all as I never loosened the bolt that holds it.

                          Went out yesterday to start fresh and the truck started up and idled Ok but it still has a nasty chug giving it gas and it speeds up and down on its own at idle. I started over with both Mixture screws as well. As per what I read online, I turned the screws all the way in (clockwise) softly till they were in the whole way, then I turned both of them out 2 and 3/4 turns. Truck still would idle but not take throttle. Its like it bogs right down and dies when I do. I replaced a vacuum hose on the advance to see if that had a leak but it made no difference.

                          I just read the part about blocking off the vacuum advance when adjusting the mixture but I can’t even get a smooth idle plugged or not. I am taking it to the garage tonight with my fingers crossed. I wish I did have someone with knowledge about these things close by. I’d definitely have welcomed the help!. Sadly the “big city” doesn’t have as many handy folks as a girl would hope. 😉

                          I am getting it towed today to the garage before I goof something up more trying to limp it there. Its less than a mile so no big fee for that. I did think it over a bit more and the good news is the mechanic I am taking it too loves older vehicles so i should be on good hands. I plan on taking him out to show him all the parts I replaced and fixed I personally so that he knows I am not just some dummy blonde with a bottomless bank account. I’ll be making it clear i want to see any parts that come out (if needed) and to consult with me before replacing them.

                          Will keep you all posted of what this turns out to be. Something tells me it could be more than the carb adjustment. But good news is the truck should run mint once this is fixed. Buying parts for my Acura hurt a lot more than this in the bank account area. ;)[/quote] Your welcome, I see, I agree, when was the last time the points(in the distributor) were changed? Dwell adjusted? I’d test the vacuum advance canister for operation as well. When you made your carb adjustments did you pull the idle speed screw off the throttle stop and then turn it back so it only lightly touches at a good idle speed? You don’t want that screw keeping the throttle open during idle, thats what the idle circuit is for. Another thought is that the throttle shaft is worn into the casting, this is common and requires bushing replacement. If the carb was professionally rebuilt though, this was already done, but I would check throttle shaft play regardless(also hopefully they used ethanol resistant seals and parts, ethanol will make quick work of an accelerator pump cup). In any case I look forward to hearing how you make out at the garage, hopefully it’s an easy fix.

                          #663051
                          JosieJosie
                          Participant

                            With me, the Distributor has never been looked at. The cap was replaced last year (just before buying) and so was the plugs and wires. I see my pics don;t show but the rotor is not under the Dist cap its on its own in a little black canister.

                            I also noticed that there is actually NO Charcoal canister at all in the truck.It was probably clogged and ripped out. This is a pre EGR type as well so there is no catalytic converter. The main hose coming out (just below the blue small line on the front in the pictures) just goes to the PCV on the drivers valve cover. If I recall looking the original setup up of these engines, there used to be a “T” off that hose going to the canister.

                            And yes, will totally keep you posted. I am hoping the fixes are easy for this too.

                            #663053
                            Gary BrownGary
                            Participant

                              [quote=”gsr-chick” post=135846]With me, the Distributor has never been looked at. The cap was replaced last year (just before buying) and so was the plugs and wires. I see my pics don;t show but the rotor is not under the Dist cap its on its own in a little black canister.

                              I also noticed that there is actually NO Charcoal canister at all in the truck.It was probably clogged and ripped out. This is a pre EGR type as well so there is no catalytic converter. The main hose coming out (just below the blue small line on the front in the pictures) just goes to the PCV on the drivers valve cover. If I recall looking the original setup up of these engines, there used to be a “T” off that hose going to the canister.

                              And yes, will totally keep you posted. I am hoping the fixes are easy for this too.[/quote] Ah, I would check out the distributor for sure. Are you sure you are not talking about the coil? The coil is externally mounted unlike HEI where it is in the cap. A points distributor requires periodic maintenance the points may be burned or oxidized. The rotor should be under the cap, it doesn’t make sense that it’s not. The advance weights may also be seized or rusty. The charcoal canister being missing doesn’t matter, they are unnecessary for pre 1975 vehicles. As long as the hoses don’t leak, all is good. Have you changed the PCV valve? A malfunctioning or incorrect PCV valve will cause issues.

                              #663055
                              JosieJosie
                              Participant

                                Yes i am probably confused hahaha. Your logic is correct. I am indeed talking about the coil. I never looked at the points as I am not sure what I am looking at. You are right that they could need checking. And yes, now that you say it, a router would need to be under the cap.

                                I have heard about the weights before and they actually failed ob my first car (also a small black chev) The car idled good then, it just hated moving 😉

                                PCV valvle “rattles” well when I pull it out. Is there another test I would need?

                                Got a feeling the mechanic is going to spend 10 minutes fixing this. Like you say, not much to really check. I hope its not a failed Distributor. That could be a pricey (if not hard to find) part. Fingers crossed!

                                #663056
                                Donnie RothDonnie
                                Participant

                                  I actually had to replace my distributor in my ’83 GM last week. Symptoms were multiple/random misfires, and very rough idle because of it that would seem to smooth out under moderate acceleration, but would bog down under heavy acceleration on occasion. Distributor cost about $215 for me, and book time says it is about an hour job.

                                  Now, my carburator is running super rich, so that’s actually going to be my next project to work on.

                                  Hope your issue can be resolved quickly!

                                  #663057
                                  JosieJosie
                                  Participant

                                    Gosh it will cost me twice that here for a new distributor (I’m in Canada). Fingers crossed that mine is not having an issue 🙁 Can these be rebuilt with a kit? Can I upgrade to haveing the coil in the cap or do I need one with external coil?

                                    Gosh, if they want 500 for one in Canada, I will be towing the truck back to my house and hitting e-bay to save me 300 :-P. I feel comfortable putting one in. Its just timing it right after it done.

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