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Shakercars's 1972 Trans Am

2019 August
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steveeoaktree

Still trying to get the ol' 76 GP 400 to idle/accelerate right...

Question

Well fellas, I will finally have some more time Saturday to get to tweaking the issues with the '76 GP's idle/accel issues. Started with running terrible and popping back through the carb. After a new fuel pump (smelled fuel in the oil), wix inline filter and new lines, and 2 Rochester carb rebuilds (soaked the parts overnight in the carb solvent the 2nd time as well as installing a new brass float), re gapped the new plugs to .045 that were gapped at .080 by the seller, new vaccum lines, and I also have the new pcv valve and lines (there wasn't one) to install Sat. It now accelerates MUCH better, but idles like it wants to die at stoplights (and does die sometimes) but starts right back up. I thought maybe to tweak the vaccum lines to see how they work best b/c some diagrams show a "t" going to the vac advance with one going to the manifold, and the other going to the carb... and some diagrams just show a single line to the carb. I also don't know it the missing pcv would cause any minor idling or accelerating issues, but I do know that it should be there. the timing is solid. I am going to drain the tank again and refill it with 89 octane fuel to also see if that helps. I am curious though, as it seems to only try to die after it's warmed up. I know there is a choke internal in the intake but it's tough to find a diagram specific enough to list all of the line connections for a 1976 400 2bbl... (most show them for the 4 bbl) If any of y'all have any tips, I'll surely give them a try either tomorrow after work, or Saturday when I do the rest... Have interior pieces to dye inbetween as well so it should be a great Poncho day -- just would feel much better if I could get this baby to run smooth! She's sooo close...


 


Thank y'all  for your time, and all of the guidance that you've given.


 


Stevee


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What RPM is it idiling @?   May be justa bump to the idle screw if its idiling too slow.  800 to 900RPM is the norm.


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What RPM is it idiling @?   May be justa bump to the idle screw if its idiling too slow.  800 to 900RPM is the norm.I

about 800... its when im trying to drive it .. when accelerating at stoplights, especially after its warm.

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Well JUSTA. I went out today, and plugged off every vacuum line that went to the air cleaner (or the ones that I thought did) and pulled the "t" off the vac advance, and ran straight ported manifold vacuum to it. I wanted to block off the fast idle and choke vacuum lines as well, but didn't. She fired right up and drove MUCH better. No more popping back thru the carb, etc... Still a tad of hesitation, but I think that I can fine tune that out. I found a "hole" in the intake between the carb and therm housing that has what looks like a straight pcv valve stuck in it with a hose going up to a 90 degree fitting in front of the carb that also goes into the intake...?? Their connected but both are in the intake? It doesn't look right to me. I will scour the 'net some more for a better, more thorough vacuum line diagram as I am sure that when it gets cold, I'll need them hooked up. At least now it'll run ok, so I can focus on the sunroof and interior issues before winter!


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Glad your making headway buddy.  If there are any shows you can hit N check out "like" engine setups, this helped me a bunch. Take pic's!


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Glad your making headway buddy.  If there are any shows you can hit N check out "like" engine setups, this helped me a bunch. Take pic's!

Thanks! My problem is, that although I am making headway, I need to focus on getting the car sealed and the hinges rebuilt... ha ha. It just bugged the sh** outta me that it wouldn't run right, and I knew the engine was tight. The hole in the intake, I believe should have a threaded "f" type vacuum fitting... and the one in it don't seal real well so that'd help too.  Thanks for the post!! If you get a chance, check out my interior log, as I am making headway there as well.

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Well fellas, I drove the car to work Sunday, and about 1/2 way there (it's about 15 miles) it started sputtering, popping, and died. About 15 times. It took a LOT of cranking to get it fired back up after each stall. On the way home, I took the back roads and kept it under 30 mph and it only died once, again about 1/2 way home. I first thought that maybe the hei coil was failing after the car is warmed up (or plug wires) but I now am leaning towards the float being set a bit too low for highway driving. I also considered an issue with the power valve (it's a Rochester 2GC). Another thing that I considered was the fuel tank cap not venting properly, or the fuel or filter itself...? Got time to troubleshoot the next two days so any direction is much appreciated! It was nice to get her out though. Thanks.


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Just wanted to update y'all. I think that I have nailed down the issue. The ICM (forgot these HEI units had them) and coil are being replaced this weekend and not going cheap this time. Along with a better set of plug wires and platinum AC Delco plugs -- should be good after that. I believe it was the ICM causing the issues, but better just upgrade the rest of the ignition while I'm in there. Thanks.


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Been reading your posts today - how are things coming?


 


Bear


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Much better after I finally broke down and pulled and completely rebuilt the distributor, and everything in it... ha ha. From the pickup coil to the plugs, all electrical ignition has been replaced. The MSD advance kit had great weights and springs inside it, and I chose to go with the medium springs. Now, I'm working on the choke thermostat,pulloff,etc.... The walker unit that I ordered through O'Reilly was backwards (in the way the spring cup was facing) so I have to send it back. I am still unsure of what that exhaust efe flap does, and whether or not it'd cause issues (or whether it's open or closed if I leave it disconnected from the vacuum line) The plugs were fine except for the outer 4 (1,2,7,8) which were all blackened with way too much fuel. Not sure why that'd be...? I am hoping that since those Delco's were in the engine throughout all of the carb and fuel pump and filter work that maybe the new set will burn cleaner. I do have a bent idle mixture screw that I am having a hard time finding. I found the original ASC sunroof but the guy wants like a grand for it. (pic attached) Will keep working on her after the rain stops, then she'll have to get into storage for awhile while the overtime stretch at work goes on until next year...


 


Thanks for asking Bear!


 


 


Stevee


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