Engine sputtering at about 65

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Dave Haertel

Dave Haertel

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I've got a 71 Dart swinger with a small block 340 using as my driver. Took a lot to get it rolling and shifting correctly and it does fine up until about 65, as soon as I ask for any more out of the engine, she starts to sputter.

Carb is a 650, wondering if that might be a bit too small or if it could be the fuel pump. Not sure of the best way to start troubleshooting this.

Unsure of the heads or the cam as I purchased it as it sits and the previous owner is clueless about all of it.

Ideas?
 
Plymouth 65 said:
Sounds like the fuel pump or filter. 65'
Click to expand...

It idles really well and pulls consistently up until that point, so I was thinking fuel pump but someone mentioned that the carb seemed pretty small for a 340 with any kind of buildup done to it. Filter is fairly new but the easiest to replace, and the fuel pump is fairly cheap so I'll probably grab both of those and start there.
 
Per your description it kind of sounds like fuel. If you have eliminated ignition, I would pull the carb and clean it and check the
secondary operation and jets. Could be plugged passages.
 
Try increasing engine speed slowly in neutral, and if it starts to miss at the same RPM as 65 mph it may be too much vacuum advance. I had that issue with a slight surge on the highway with mine.
 
DesertRat said:
Per your description it kind of sounds like fuel. If you have eliminated ignition, I would pull the carb and clean it and check the
secondary operation and jets. Could be plugged passages.
Click to expand...
The carb was rebuilt a couple of months ago and a new filter in place. Not to say that something could not have fouled it up of course.
 
qkcuda said:
Try increasing engine speed slowly in neutral, and if it starts to miss at the same RPM as 65 mph it may be too much vacuum advance. I had that issue with a slight surge on the highway with mine.
Click to expand...

I did the timing by vacuum and it’s normally just right at 20 psi. I actually don’t have a tach in the car yet, definitely on my list but just haven’t done it yet, primarily because of all the electrical still to be done. I won’t be able to check the RPM that way is what I mean
 
the sock filter inside fuel tank on sending unit could be stopped up you can use compress air and blow it clean for a while but you might knock float off sending unit like I did.
 
Interestingly, I haven't had a problem with stalls in the 6 months I own it but it stalled twice on me today, the first time it was still cold, probably left a bit too early I thought but then 20 minutes later, at full operating temp she stalled again just driving down the road.
 
So, is this when you are just cruising or just trying to slowly accelerate, or when you are at 65 MPH and then punch it? If this is happening under cruise conditions then it won't be the secondaries; the engine will be running on the primaries at the point if cruising on flat land. And this is not the accelerator pump if this is at steady speeds.

(BTW, if you have a vacuum gauge in the car, you can get a pretty good idea of what phase of carb operation is going on with vacuum readings.)

The suggestion of the plugged fuel system is most likely. Be ready to pull the tank and clean or replace it. At this car's age, the tank and lines are suspect. It can get into the fuel pump's check valves and muck that up, as well as into the carb. If you don't know if this has been done by a PO, it is time for a full fuel system cleaning. THIS HAPPENS ALL THE TIME TO THIS VINTAGE OF CAR.

BTW, with a 20" vacuum at idle, the cam is not big. So a 650 is fine. I ran a 351C to 6500 RPM all day long for years and years on an old Holley 1850 600 cfm.
 
I had a '71 Satellite with a 318 with a worn distributor. It would sputter while accelerating, and got worse with more electrical loads (headlamps, heater fan, wipers, etc...). Swapped for a new distributor, all was good.
 
Dave Haertel said:
The carb was rebuilt a couple of months ago and a new filter in place. Not to say that something could not have fouled it up of course.
Click to expand...
Looking for what's changed since you owned it, a carb rebuild. Carb may be fine but sounds like something (see previous suggestions) is starving it for fuel.
Best of luck with it!
 
nm9stheham said:
So, is this when you are just cruising or just trying to slowly accelerate, or when you are at 65 MPH and then punch it? If this is happening under cruise conditions then it won't be the secondaries; the engine will be running on the primaries at the point if cruising on flat land. And this is not the accelerator pump if this is at steady speeds.

(BTW, if you have a vacuum gauge in the car, you can get a pretty good idea of what phase of carb operation is going on with vacuum readings.)

The suggestion of the plugged fuel system is most likely. Be ready to pull the tank and clean or replace it. At this car's age, the tank and lines are suspect. It can get into the fuel pump's check valves and muck that up, as well as into the carb. If you don't know if this has been done by a PO, it is time for a full fuel system cleaning. THIS HAPPENS ALL THE TIME TO THIS VINTAGE OF CAR.

BTW, with a 20" vacuum at idle, the cam is not big. So a 650 is fine. I ran a 351C to 6500 RPM all day long for years and years on an old Holley 1850 600 cfm.
Click to expand...

Thanks for the info on the cam, by the way, I didn't know that you could pick up on the cam by the vacuum. I don't have a gauge in the car, I just hooked up an external one when I was checking timing. I will probably end up doing a rebuild on this at some point, so knowing that there's not much of a cam in there already is good info to have.

This happens with a moderate acceleration, if I slowly try and increase it, there's pretty much no response, I have to ask for it moderately to get a response and the response is the sputtering.

The sending unit and the fuel gauge has never worked, but neither has any other gauges on the dash so I was hoping that was an electrical issue. Perhaps dropping the tank is going to have to happen. I'm not looking forward to this if I end up having to replumb all the fuel lines.

There's such a laundry list of things to do with this car, but as far as the motor goes, I had decent performance with it right from the beginning, so this is bugging me and a little bit strange.
 
nm9stheham said:
So, is this when you are just cruising or just trying to slowly accelerate, or when you are at 65 MPH and then punch it? If this is happening under cruise conditions then it won't be the secondaries; the engine will be running on the primaries at the point if cruising on flat land. And this is not the accelerator pump if this is at steady speeds.

(BTW, if you have a vacuum gauge in the car, you can get a pretty good idea of what phase of carb operation is going on with vacuum readings.)

The suggestion of the plugged fuel system is most likely. Be ready to pull the tank and clean or replace it. At this car's age, the tank and lines are suspect. It can get into the fuel pump's check valves and muck that up, as well as into the carb. If you don't know if this has been done by a PO, it is time for a full fuel system cleaning. THIS HAPPENS ALL THE TIME TO THIS VINTAGE OF CAR.

BTW, with a 20" vacuum at idle, the cam is not big. So a 650 is fine. I ran a 351C to 6500 RPM all day long for years and years on an old Holley 1850 600 cfm.
Click to expand...
HaH I got you beat,I ran a 340 to valve float on a Carter 2bbl.
 
Dave Haertel said:
Thanks for the info on the cam, by the way, I didn't know that you could pick up on the cam by the vacuum. I don't have a gauge in the car, I just hooked up an external one when I was checking timing. I will probably end up doing a rebuild on this at some point, so knowing that there's not much of a cam in there already is good info to have.

This happens with a moderate acceleration, if I slowly try and increase it, there's pretty much no response, I have to ask for it moderately to get a response and the response is the sputtering.

The sending unit and the fuel gauge has never worked, but neither has any other gauges on the dash so I was hoping that was an electrical issue. Perhaps dropping the tank is going to have to happen. I'm not looking forward to this if I end up having to replumb all the fuel lines.

There's such a laundry list of things to do with this car, but as far as the motor goes, I had decent performance with it right from the beginning, so this is bugging me and a little bit strange.
Click to expand...
This happens with a moderate acceleration, if I slowly try and increase it, there's pretty much no response, I have to ask for it moderately to get a response and the response is the sputtering.
late-to-the-party powervalve



Before you start all that, just do a fuel-pump volume test ; 1 qt in 1 minute at 500 rpm
 
Last edited:
AJ/FormS said:
This happens with a moderate acceleration, if I slowly try and increase it, there's pretty much no response, I have to ask for it moderately to get a response and the response is the sputtering.
late-to-the-party powervalve



Before you start all that, just do a fuel-pump volume test ; 1 qt in 1 minute at 500 rpm
Click to expand...
This was hard to test because the idle is set higher than 500, but it did seem a little weak so I’m replacing the fuel pump this morning. Luckily it’s fairly easy to get to. Will report back what I find.
 
Dave Haertel said:
This was hard to test because the idle is set higher than 500, but it did seem a little weak so I’m replacing the fuel pump this morning. Luckily it’s fairly easy to get to. Will report back what I find.
Click to expand...
Oh jeeprs, that's the minimum, there is nothing to stop you from testing at a higher rpm,lol. I guess I should said that. I'm not big on throwing parts at a problem. But if it's a Holley,I would check the manifold vacuum, at the point of the problem, and throw a new PV in it...........of course I have a tester for that............
For me, the testing is the fun part.
 
Dave Haertel said:
This was hard to test because the idle is set higher than 500, but it did seem a little weak so I’m replacing the fuel pump this morning. Luckily it’s fairly easy to get to. Will report back what I find.
Click to expand...
I had same problem it was accelerator pump. For me anyway
 
I would check throttle movement for full actuation and vaacum secondary settings.
I would bet you are not getting full secondary opening.
Or you have a fuel line kink/blockage or weak fuel pump.
 
AJ/FormS said:
Oh jeeprs, that's the minimum, there is nothing to stop you from testing at a higher rpm,lol. I guess I should said that. I'm not big on throwing parts at a problem. But if it's a Holley,I would check the manifold vacuum, at the point of the problem, and throw a new PV in it...........of course I have a tester for that............
For me, the testing is the fun part.
Click to expand...


It is a Holley, a 650 with a manual choke not sure of the exact model. So, I've had quite the day of this, the first pump I bought went in but did absolutely nothing, so I went back to the parts store to swap it out. Grabbed some additional fuel line because the line bridging from the steel line coming up from the tank was dry and cracked on me twice just putting it back together. Come to find out, the line to the filter and from the filter to the carb were 3/8, not 5/16 as is proper, so I'm sure that was part of the problem. I got her running but when I got back from the test drive and the headers were good and hot, the line running to the inlet of the fuel pump had started leaking because I didn't tighten clamp down enough on it. Now to wait for the headers to cool a bit and I'll go and tighten it down again. It ran like crap so now I'm wondering if the timing is all out of whack now.
 
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