# 1967 GTO Sputtering Issues Under Load



## TinIndian67 (Feb 11, 2017)

The GTO is up and running again. I have had this issue that is definitely heat-related, as it has been worsened by the 95-degree days we have had. I've taken it out to try and get some drive-time on her - especially in the hot weather - and she is being very temperamental.

Problem: Minor to major sputtering - could last a second or a couple seconds before she catches herself. RPM/speed does not appear to be a factor in the engine sputtering taking place. I could be accelerating from a stop sign and ease on the throttle, and she may or may not sputter. I could accelerate from the same stop-sign and fry the tires, and she may or may not sputter. I could be cruising at 50mph and go to quickly accelerate, and she may take off flawlessly, or sputter. I do feel like it runs a little better when I am running at high RPM's, but the problem is not eliminated completely. It is not a violent sputter either like she is completely out of fuel - more like a miss or something. Will not sputter at all if not under a load, even when I hold it at higher RPM's while in park.

Couple side items to note: The engine runs at 180-185 degrees even in this heat, which is great. I stalled a few times right after I replaced my eccentric during test drive when the temperature was in the 80's, and it was definitely vapor-locking. I read a bunch of helpful posts on this forum, and went with a return-line fuel filter, and installed a return line to my tank - pictures below. At this point, my car will not stall, but has the sputtering issues listed above. Even on the super hot days, I would run the engine hard and she did not overheat, stall, and would start right back up after shutting her off. One think I noticed after installing my return line is that my fuel pressure gauge will jump very quickly from 2.5'ish to 5'ish - it will fluctuate at the same speed as the mechanical fuel pump. When I ran my return line to a bucket, it does get a decent amount of fuel that returns, and I am wondering if there is too much fuel returning? Should I try and restrict it? I just purchased some heat-shielding wrap that I am going to stick on my fuel lines before and after the pump to see if it helps. I swapped distributors, and it did the same thing with both. I have tried three different carbs just to make sure, and the engine likes the Holleys more than the Edelbrock, but still has the same issue with each one. I don't think it is electrical. Plugs and wires are new, and I do have colder plugs in there. The engine compartment does get very hot, and the fan blows a ton of hot air right back at the block and all the components. Timing is good, and when she wants to run, she runs awesome.

Main questions are: should I try and restrict the return line a bit, or should it be fine? Do you think it is fuel/heat issues as I do? Anything else to check for?

Thanks for your time and thoughts in advance.


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## TinIndian67 (Feb 11, 2017)

Not sure how to edit a post, but I also wanted to mention that I do have the carb spacer on the intake manifold, and I did block the exhaust port crossover with manifold gaskets that did not have the cutout.


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## Lemans guy (Oct 14, 2014)

Well of course it could be many things. But everything you describe when the sputter is there is during acceleration, from a stop, from speed. Yes sometimes it does not,..but from your description each time it does you are accelerating.

Acceleration hesitatation that happens only from a stop and not cruise is often the transfer slot in the carb. It comes on as you open the throttle from idle. But yours also happens at speed when the throttle is past the transfer slot. That is often an indication of a weak accelerator pump, the wrong accelerator pump cam and or the wrong size accelerator pump squirter.

These are alleasy to change with a screwdriver and not expensive cam kits from holley and squirter as well. See which ones you have you may have to change the accelerator pump cam and set the gap right. Also up the squirter.

Holley has videos on their web to do all this. A likely spot for your problem.

That said a weak coil can also fail under load which is a demand for more power. Too much timing, which many cars have,can also cause a sputtering and hitching effect.

So you have some investigating to do. I would star,t with the fuel and accelerator pump and see if you get any improvement.

Let us know how you do!


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## Lemans guy (Oct 14, 2014)

I know you changed carbs but your engine may have a big need for a big shot of fuel!

Any misfires can cause sputtering as well, make sure you go back and recheck the firing order of your plug wires, easy to get those out of sync.


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## Lemans guy (Oct 14, 2014)

Oh your fuel filter already meters the return line through about an .060 orifice. If you took off that hose and looked at it you would see how small it is. So that is OK and yes you will get a lot of heat on that filter and those lines. I wrap all of my lines there. It does look as chic, but runs better. My filter is in almost the same spot and it is a fuel lab filter, but I completely cover it with DEI heat shielding.

with the low volatility of temperatures of today’s gas we have to think a hair different. We block the exhaust crossover that keeps fuel cool under the throttle plate, where we really need vaporization and we heat up exposed lines on the engine where we need colder fuel, relatively of course.

so yes shielding helps. And so does your return line and spacer. I wrap my return line above the engine as well, why send hot fuel back to the tank constantly, eventually that will raise the temp as well.

if you are going to run it over the engine I would consider a steel line and wrapping it. Originally on those fuel filters they button hooked them down in front of the engine, and they did not run over the engine for long.


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## TinIndian67 (Feb 11, 2017)

Thanks for all the recommendations, Lemans guy. I did purchase 12' of DEI wrap that I am going to wrap the supply and return line all the way to the rear of the headers, which should be sufficient. I did change the coil, and there was no difference between my Pertronix and Edelbrock coils. 

I will test drive it once I wrap those fuel lines, and if that doesn't fix it, you're right - tuning the carb would be a good step. The carb only has a few hundred miles on it, but it has been in a box since 2016. It is a 650CFM with 2X larger jets on it, but I never touched the accelerator pump or squirter. Good suggestion - I will let you know what I find. 

With regard to the carb size - I would imagine a properly tuned 650CFM carb should be sufficient for a 461. I remember reading an article in HPP years ago about over-sizing carbs for engines. My 461 has not been dyno'd yet as I am working out some things, and trying to finish breaking it in. It would be awesome to be in the 450HP range, though the torque is really what I feel as I don't live at 5K RPM. There is a chassis dyno and tune shop about 10 minutes from me - I may give them a call because I am not too savvy with carbs.


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## Lemans guy (Oct 14, 2014)

You are on the right track, and a 650 CFM carb will have a smaller accelerator pump squirter and the cam is likely not right for a 461. A smaller carb can give you a very snappy throttle response and when you drive in the low RPM’s it is good there.

The reason is the gasoline is at higher velocity going through the smaller Venturi, it is tight and fast and very responsive. So you can run it, it would suffer at higher RPM compared to a bigger carb.

I have a 461 as well and use a Quick Fuel technology 780 CFM carb with Vacumn secondaries. That carb had a small squirter for the accelerator pump, I had to up to get enough fuel for acceleration. I think you will need to as well. Up the squirter and change the cam and set the cam gap correctly.

But hey you are on it now, and you will get it!


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## armyadarkness (Dec 7, 2020)

His problem is very similar to mine, and for the record, if his avatar is accurate, I also have an ivory 67 with black cordova top ,so it's very suspicious!!!

As for carb sizing, Pontiacs like big gas, and while I agree that over carbing is worse, IMO, a 650 is no where near enough for that engine. My factory 400 wouldn't run with a 650 and a 750 added 30hp on the dyno! Now I run an 800, and it's tuned for my 400.

I'm concerned about his mentioning the heat. My car runs like a clock, but on those hot New Jersey days that we had, if I put my foot in it, it stumbled a lot too. Only on the hot days!

I'm still sorting through my issues. Making sure that timing and jetting are right, before moving on to other issues, such as return lines and shielding. Not much point in correcting the nuances, if the timing and carb are wrong.


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