Everything I've said about fixing a 4gc is doubtful at this point - ClassicOldsmobile.com


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Old November 8th, 2017, 05:24 PM   #1
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Everything I've said about fixing a 4gc is doubtful at this point

Well the carb on my 55 took a dump yesterday, same problem as before and before that and before again. The accelerator pump cup has split at the bottom and separated itself and the BIG bog is back coming off the line. I have a NOS leather cup and pump coming, gotta be 30 years old if it's leather and it's going in next week.I a'm looking for a different 4gc carb just to back myself up.... Frustrating.... Tedd


More to follow if you have read this link/ post before what ever I have thought I had fixed, well I haven't, so most of this is bogas information until I figure this out.... Tedd

https://classicoldsmobile.com/forums...p-problem.html
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Old November 8th, 2017, 08:33 PM   #2
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Not sure if I posted this before but they have the finest kits available. Totally ethanol resistant and I use them with no issues. The pumps come pre assembled with new springs and the nylon is clamped to the stem so it can't slip. They know their stuff,just ask them. If you can get Ron on the phone,he's the man. Call early and you might get him.




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Old November 9th, 2017, 03:13 AM   #3
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Originally Posted by Tedd Thompson View Post
Well the carb on my 55 took a dump yesterday, same problem as before and before that and before again. The accelerator pump cup has split at the bottom and separated itself and the BIG bog is back coming off the line. I have a NOS leather cup and pump coming, gotta be 30 years old if it's leather and it's going in next week.I a'm looking for a different 4gc carb just to back myself up.... Frustrating.... Tedd


More to follow if you have read this link/ post before what ever I have thought I had fixed, well I haven't, so most of this is bogas information until I figure this out.... Tedd

https://classicoldsmobile.com/forums...p-problem.html
It sounds like the custom made pump didn't work as hoped. I haven't heard that your experience was a commonplace problem with the 4GC, hence I have to believe that there is a unique situation with yours causing the repeated failure. To find a solution, first the failure mode should be accurately defined. I'm assuming that the 21/64" dimension shown in the previous thread is in error, as it is nowhere near 3/4". 21/32" would more likely be correct. The piston's cup should contact the wall of the well "all the way around" without being tight. Check to see that the lip of the cup doesn't "bottom" in the well. Such could cause it to tear. Also look for any casting irregularity in the wall of the well which could cause the cup to split. If anything is suspect with the float bowl assembly casting, and you have a stock of spare parts, you may want to try another. If the mode of failure can be defined accurately, a cure should soon follow.
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Old November 9th, 2017, 10:17 AM   #4
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Thanks Ozzie, and you are correct about the 21/64 should be 21/32, senior moment. The piston cup looked to fit correctly but then again on a couple of attempts I know I used 3/4 cups and I though they looked fine also and they failed quickly. The last cup was 21/32 but might have had to long a stem from mixing and matching parts and possibly it bottomed. The well is as smooth as a babies bottom and has been inspected with a loop twice. Steve, my carburetor guy who is a lot more knowledgeable than I is the one who found the NOS leather piston and spring back east somewhere and he wants to try that. Thirty year old leather isn't my style but but maybe it's the length along with the non alcohol resistance material I have been getting on my earlier builds.... Frustrating but I feel it's something simple that I'm overlooking time and time again..... Tedd

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Old November 10th, 2017, 08:16 AM   #5
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Is there casting flash maybe in the bore? I know on Qjets there is a slot running vertically that can catch at the bottom of the stroke. Not familiar with a 4GC so just throwing that out there.
Personally I always run a Dremel tool with a wire wheel on it up and down in my accelerator pump bore. It makes a nice surface and I've never had a problem.
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Old November 10th, 2017, 10:10 AM   #6
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Boy Trip I have looked down that bore a few times using a 10x loop and it sure looks smooth to me. It is tapered but I understand all of them are of this era 4gc.

The car is now at my buddy's shop who knows much more about carbs than I'll ever know. He does this on a regular basis and now he is now pulling his hair out. I'm convinced it's something, simple just over looked. I personally think it has something to do with pump rod length and correct cup combination but he doesn't want to bend the linkage rod to shorten the stroke and also wants to use a new old leather cup. We will see I have all winter to get it right... Tedd
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