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Pontiac of the Month

360Rocket's 1970 GTO

2019 March
of the Month

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Hi I’m new here, I’ve been sitting parking on a 1974 Pontiac GTO for about a year. I got it running for the first time in about six months yesterday after I rebuilt my carbs. I have two Holley 600 on a tunnel ram intake. I also replaced my point distributor with an HEI. The engine I have in the car is a 1975 firebird 400. The issue I’m running into is the car idles great but when I give it about half throttle it bogs down and stumbles like it’s gonna die but when I take my foot off the gas it reeves back up to the proper RPM and idles fine. Any advise? I assume it’s something amiss with my ignition system or my carbs. 

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:welcomeFP:

Unfortunately, I do not have any suggestions but I'm sure someone else will be able to help.

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Hey Jcro, welcome. My 1st thought is you may have to much carb unless you've got it dialed way back. Could also be a timing issue. I had a similar issue after I built my 428. Mine was great at idle but would fall flat when I hit the gas. If I feathered it at take off it would go, after 20 feet or so it would take off and pin you to the seat. I had set the timing by ear just like I always did and finally put a timing light on it and started playing with it. I set it at  COMPs recommended starting point and went up from there. Runs great now. One other thought is to make sure that all your vacuum hoses are hooked up and in good condition, paying attention to the ends for small cracks and brakes. Again I may be wrong but id try a single carb set up if you can just to eliminate that being part of the problem. Good luck and please keep us posted. Also pic of your car/engine would be awesome 😁   

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Yes Welcome!

I would agree! Either timing or a vacuum issue from the carbs. I.E. secondarys coming on to some, leak at the base gasket, etc. I don’t think it’s to much carb because of how you say it acts when you take your foot of the gas. I would check timing first, and not just the initial, but how it advances when you rev it. You may be getting a bad advance curve.

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I set the timing to 16 degrees and then hooked up the vaccum advance. Would that be the correct thing to do? The carbs have stock sizes for the accelerator pump/ power valve however the jets are 55 instead of 60. On my rear carb I ran a vacuum line to a PCV valve on my passenger valve cover, could that cause an issue? Also could it be a spak plug gap issue? 

325ED51C-1ED0-4A4E-833F-323178895491.jpeg

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52 minutes ago, Jcro said:

I set the timing to 16 degrees and then hooked up the vaccum advance. Would that be the correct thing to do? The carbs have stock sizes for the accelerator pump/ power valve however the jets are 55 instead of 60. On my rear carb I ran a vacuum line to a PCV valve on my passenger valve cover, could that cause an issue? Also could it be a spak plug gap issue? 

325ED51C-1ED0-4A4E-833F-323178895491.jpeg

Well a couple things. First you need to know the total vacuum your pulling. For a Pontiac 400, 36 degrees is about it. So whatever the total is with the vacuum you deduct the initial from that. The carb should be port vacuum not manifold. PCV is manifold vacuum. So that may be your biggest problem, your dropping vacuum just when you need it!

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I agree totally Indian. I don't think 16 degrees is enough. My cam is just a bit bigger then stock, nothing major. Small enough to run the stock converter and COMP told me to start with 12-14 degree advanced and see what the engine liked. I just walked out and put a light on mine to double check before speaking but with the engine cold at idle I'm at 23.5 degree @ 850 RPMs, remember mines a 428 though so I don't know how much this helps. Also my PCV is connected to a port in the manifold NOT to the carb so that might be an issue. Not that my gap size will help but I'm gapped at 0.35

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I don’t have a manifold vacuum port so should I ditch the pcv valve and  just put a breather in it’s place?  I don’t understand what deduct the initial means? Sorry this is my first classic never felt with carbs before.

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Yes you can run a breather instead of a PCV in the valve cover.. Might have to zip tie a rag around it so you don't wind up with oily film all over everything under the hood...

Just out of curiosity what are the cam specs..does it have a lot of overlap..?? Would suggest opening the spark plug gap to .045 with the HEI...

Have cut this short ......Wife just got home....Will elaborate more on timing in a couple hours.....

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Posted (edited)
1 hour ago, Jcro said:

I don’t have a manifold vacuum port so should I ditch the pcv valve and  just put a breather in it’s place?  I don’t understand what deduct the initial means? Sorry this is my first classic never felt with carbs before.

I think there’s some confusion here! The PCV isn’t the problem, or at least it should’nt be. I can’t see your engine, but the hose to the PCV should be coming from a fitting on the manifold or  large nipple coming out of one of the throttle plates. This means when the engine is running, just a idle you have high vacuum. By connecting the vacuum advance secondarys to that vacuum the secondary’s are opening at idle. When you step on the gas they close. That’s backwards! Port vacuum is taken from above the ventures, which means as you increase throttle, vacuum increases and starts to open the secondary’s. 

Timing, with the vacuum connected put a timing light on the harmonic. Increase the rpm’s until the timing stops increasing. This is the total timing of the engine. If it’s more than 36 degrees it’s to much. Check initial timing, remove and plug the vac line. Check the timing. So for instance say you have 40 total timing and 16 initial you need to set the initial at 12, not 16.

Edited by Last Indian

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Okay thank you both I’ll check it all out in the daylight tomorrow. If you think of anything else I should check just let me know 👍

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so I had the vacuum advance on manifold vac not ported and had a leak at the ported vacuume port on my rear carb so I switched the vac advance to ported and plugged the vac leak on the rear carb. Once again it runs awesome while it’s in park it reveals up really nice and idles well but when I put it into gear it starts running real rugh and idles about 200 rpm lower. Since I’m new to carborators I’m guessing it’s an issue with some kind of adjustment? What should I mess with and why does it run great in park and neutral but try to stall in gear

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Posted (edited)
16 hours ago, Jcro said:

so I had the vacuum advance on manifold vac not ported and had a leak at the ported vacuume port on my rear carb so I switched the vac advance to ported and plugged the vac leak on the rear carb. Once again it runs awesome while it’s in park it reveals up really nice and idles well but when I put it into gear it starts running real rugh and idles about 200 rpm lower. Since I’m new to carborators I’m guessing it’s an issue with some kind of adjustment? What should I mess with and why does it run great in park and neutral but try to stall in gear

Ringo’s going to move this any time now! 

That said, you don’t mention changing the vacuum secondary's of the carbs to port vacuum! Did you? If those are connected to manifold vacuum that’s a problem. Take a picture of the carb setup and post it over on the maintenance garage part of this forum.

Edited by Last Indian

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