Feels like dragging an anchor

[jcskokos]

1967 coronet 4 door 383/727.
Car drives great most of the time. Every now and then I'll be cruising along then no thrust no matter the accelerator input. I can let off and it feels like I am dragging something until I hit about 10mph and feels like something kicks loose then will drive great again, Almost like a downshift.
I had just resealed the trans and adjusted the bands while I was in there. Shifting is smooth up and down. No signs of a motor stall or misfire. Clean oil in motor and trans. Brakes not dragging. Muffler is damaged a bit, old coworker said it looked like it had a backfire that lit inside the muffler. I am concerned that the muffler is giving excessive back pressure but I doubt it would be enough to give this issue.

 

[WP29440SE]

is the kickdown linkage adjusted correctly? sounds like it could be 2-3 shift overlap. what did you adjust the bands to. how did the fluid look.

 

[steve340]

The muffler could be causing the problem. I saw one once that a part of the internal baffle that had dropped and choked the exhaust.
The symptoms were similar to yours. Initially it would run Ok but get it on the open road and it would choke up.

 

[jcskokos]

is the kickdown linkage adjusted correctly? sounds like it could be 2-3 shift overlap. what did you adjust the bands to. how did the fluid look.

It's very possible it's not adjusted correctly. I wasnt sure on proper adjustment and I think I set it to bottom out with WOT. Fluid looks clean still. Has less than 15 miles on it since it was changed. Old fluid was pretty goopy thanks to some blue devil additive that was put in some time ago.

 

[WP29440SE]

I've seen screwed up fuel systems and carbs that would kill power. not to mention wacky timing.
a little history here would help. when did this start happening.
if you had the throttle wide open and the TP arm on the trans adjusted all the way back you are very close to having it right.

 

[myk r sanchez]

Yes, is this a recent problem? Was the car running perfectly and then it developed these symptoms? Do you know where timing is set at?

 

[jcskokos]

Car always had some hiccups when accelerating and hard starting. Replaced the timing chain and rebuilt carb. Now starts right up even after 3 days sitting. Trans was apart due to a few leaks that needed attention. Resealed and adjusted bands while I was in there.

Like I said, it shifts good, starts and runs good. Normally picks up pretty good but once in a while I can be cruising at 40mph then suddenly feels like I just released a chute that drops out below 10mph then drives normally once again.

 

[Shorthorse]

Lots of possibilities here but you mentioned "rebuilt carb". Did you rebuild it or was it an over the counter rebuilt carb? I don't have much faith in those. As long as you're looking at stuff, check the float settings on that carb. Could be starving for fuel and once you slow down the bowl has a chance to catch up.

 

[jcskokos]

I rebuilt it myself. Carter 2bbl (the square one, not the round one). The only change I made to that was running the dist off manifold vacuum instead of ported at the recommendations of a few engine builders. All linkage and float adjustments were done to the installation instructions that came with the rebuild kit. I had to move my accelerator pump pin to compensate for cutting out from a stop.
Before all the work done I do remember having really bad pinging when going up even the smallest hills. Seems to be better currently but I'm waiting till I get my new tires on before extended driving.
I didnt change any jetting on it and I have never had a problem with maintaining constant throttle, say merging onto a freeway.

 

[oldbee]

Don't ever forget about sticking emer. brake cables either.

 

[Don Frelier]

I would suggest fuel pump pushrod or sending unit pickup sock.

 

[Outsider]

Fuel pump or vapor lock?

 

[Curiousyellow71]

It may be the filter sock on the pickup in the fuel tank...IF it has one yet..they can start plugging and will give some fuel but starve it...it will likely get worse if that's what it is. If you had a 4 barrel it would likely be way worse.

 

[70chall440]

Brakes dragging??

 

[jcskokos]

Brakes dragging??

My original thought was maybe the brakes locked up on me but the 2nd time it happened I had done nothing with the brakes. It slowed for about 100 feet then popped loose. Very much felt like something physically releasing but like I said, I gave no input to the brakes before, during, or after the event

 

[Wietse]

Every now and then I'll be cruising along then no thrust no matter the accelerator input.

Does the engine rpm increase when you apply more throttle, but the car just does not increase speed?
I think that if the engine does respond on the throttle the issue will not be engine related but more likely in the transmission.

 

[Rockland71GTX]

The muffler could be causing the problem. I saw one once that a part of the internal baffle that had dropped and choked the exhaust.
The symptoms were similar to yours. Initially it would run Ok but get it on the open road and it would choke up.

I agree with Steve. You mentioned the muffler so it sounds like you want to replace it anyway. They're cheap enough that you should just change it now to eliminate that as a possibility.

 

[BB BELLA]

water in fuel?

 

[T2R9]

I would suggest fuel pump pushrod or sending unit pickup sock.

I agree. I had similar symptoms in my 71 Road Runner and finally one day it got so bad I could only get up to 20MPH, limped home and pulled the fuel pump and found this.

 

[Wietse]

I also had problems with a worn fuel pump push rod, look at the picture.
It supposed to be 3.220" long, with this i was still driving but engine died under high load/WOT.
Coming off the throttle and let it coast maybe 20ft and it would be driving normal again until you stomp the throttle again.
As the OP describes he really needs to come down to like 10mph.
That's why i asked, if the engine is still reacting on throttle, it will not be engine related.