Cutting out under load
 

So the car has developed a problem. It has terrible acceleration and if you kick it then it flat or stumbles. Idles great, revs perfectly as long as it’s not under load.. where do I begin

Fuel delivery system. What carb are you running? how much H/P is it making?

Dave - what size fuel line(s) and pump?

I agree with Chuckie.

I'd put a T in the fuel line right before it enters the carb. Then run a line from the T back into the passenger's compartment and mount a temporary fuel pressure gauge on or under the dash. If the pressure drops below spec you've identified the problem.

What do you mean by using the term as you did “ developed “, was the car all good until just recently?
This detail will make a huge difference in how members once again put in there time, experience and knowledge to guide you out of this.

Check your distributor advance. An engine will accelerate just fine with no load even in the absence of full distributor advance. Under load, it will want to maintain a steady state and not accelerate.

Carburetion issues are typically a temporary bog or lean out followed by normal acceleration. If too rich, the plugs foul and the car runs lousy all the time.

I called Holley tech today because like I said it just started doing it. The tech told me to back the idle mixture screws out another 1/4 turn and tell him what my vacuum said. I did and it went from fluttering to 11/12 steady. Got in it, drove it, seems fine. There is a man just down the road from me that has a 70 GT37 that runs like a raped ape all up and down the highway near my house. I am gonna go pay him a visit cause I have seen him slow down several times and look at my car. His car runs good so maybe we can tinker with mine.

Ok, once again in your first post you had said that it idled fine, when actually it did not.

It’s good that you have the idle better now with a 3 minute adjustment, but the idle mixture screws will not cure a lean stumble or a bog as you have posted about.

Note that there’s a HOLLEY carb tips thread posted up near the start of this street section, it might be a good time to start Thumbing thru it.

What does your fuel system consist of? Line sizes and fuel pump. What carb?
A weak coil can also cause the same symptoms.

Well I dont know what you considered idled fine but this thing is pretty rowdy so its hard to tell whats smooth and whats not. It smoothed out a little I guess but it didnt turn into a Caddy. It has ALWAYS free revved fine, even when the plugs were black. I put 65 miles on it today. Seems to be better. If it does it again I will explore further. After driving I pulled a plug and they seem to be fine too although I am no expert at reading plugs they arent black like the ones I took out. Man you gotta cut me some slack, EVERYTHING on this car is new. Not a damn part but the seats and steering wheel and speedometer and gas gauge was reused and they were recovered,recast, and rebuilt. I do not have it sorted out totally and I wont for a long time. It may leave me on the side of the road tonight but right now the Holley tech said adjust the idle mixture screws, I did, it runs. I do have a spare coil I will carry with me in case thats what it is. I just took my wife to the parts store with me to return some stuff from the flywheel swap and scared the hell out of her at how fast it is and its still under break in, I gave it like 2/3 throttle.

1/4 turn wouldn't really do anything. That's your mixture at idle with the throttle plates closed. It really has little to no effect once your throttle plates are into the transfer slots off idle.

So what do you think it could be?

Don't know, but it'll be back.

Agree that the idle mixture circuit is out of the equation as soon as the throttle is cracked open. It would have nothing to do with crapping out under load. It could be a fuel issue or an ignition issue.

I'd start with fuel because ignition wouldn't care (much) if under load or not.

I've found that not to be true always. Low voltage will support no load/light load and bang like crazy under high load.

Fuel pressure is too high and is blowing the needle and seat I think. The problem came back, changed black plugs, went away.

Yeah, now that you mentioned it, my Dad had that issue once. I was young, but remember it being confounding for him by the vocabulary lesson... ;)