No hard acceleration - please advise

thethee

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Could really use some help here. I'm having acceleration problems with my '75 Imperial (440 & 727). I don't know how long it has been going on but I've noticed it this last week. Part throttle seems to be fine, but under hard acceleration the engine is breaking up. It's difficult to describe but it's almost like it goes in steps, so accelerate-pause-accelerate-pause-accelerate. Today it seemed to be all over the place. Like I said, normal city driving (~30mph) is fine, but hard throttle to try and make the traffic light and it will refuse to rev.

Recently gone through the entire ignition system and because it will rev in park or neutral all day long I don't feel it's ignition related, but I don't know. Bench tested my electric fuel pump and it's okay. Also replaced all lines under the hood as well as fuel filter. Unhooked fuel line from carb to check afterwards and there is good fuel flow. Thermoquad was rebuilt not too long ago and I'd rather not open it up again but will do if I have to. It produces a solid stream of fuel from the accelerator pump. Timing chain was replaced this winter which I advanced for more low down torque as it's a heavy car.

I don't really know how to troubleshoot this issue since it only happens under load and part throttle seems fine. Kind of dreading if it's transmission related. I'm really hoping this isn't anything major as I don't have the funds for that right now.

Any and ll advice will be much appreciated.
 
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Just some general info. If it's missing under load it could be spark plugs, spark plug wires or distributor cap as they will tend to fail under load first before going completely bad. Also could be fuel pump or fuel filter since under load you require more fuel. I don't know the failure modes of the ignition box, but maybe?? Looks like you already checked several of these things.
 

413

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It could be something very simple and basic. If you advance the cam you lose higher rpm power. Something to keep in mind.

You say things are good, but not the test results. So tell us more about the fuel pump output. How much in a can in 30 seconds? What is the fuel pressure at idle an WOT?

what ignition system and coil do you have?
 

Davea Lux

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Also, check the ground on your electronic ignition control module. If the mount screws are rusty, that will lead to erratic performance.

Dave
 

thethee

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It could be something very simple and basic. If you advance the cam you lose higher rpm power. Something to keep in mind.

You say things are good, but not the test results. So tell us more about the fuel pump output. How much in a can in 30 seconds? What is the fuel pressure at idle an WOT?

what ignition system and coil do you have?
Yes I know about the loss of power at higher rpm cause of cam advance but I would expect it to at least run smooth through the rpm's.

Running electrical ignition with standard motor products UC12 coil and standard motor products LX101 ECU, both are less than a month old. Distributor was also replaced at that time. Ballast resistor is also new.

Don't have real figures of the fuel pump output but I took it apart and cleaned it and it is working like the first day it was on there, before these problems started, which is why I thought it to be good. But will test the output in a clear can to be sure. Don't have a fuel pressure sensor though.

Next I'll look at some simple stuff like plugs, wires, and cap and maybe swap a coil. I feel like it should be something simple so hopefully I'll find something with this.
 

Davea Lux

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Also, check the ground on your electronic ignition control module. If the mount screws are rusty, that will lead to erratic performance.

Dave

Also check your ECM to see if it is one of the cheap ChiCom knockoffs, those are a POS.

Dave
 

pioneer

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Could be your fuel pump pushrod wearing out. Not an uncommon thing to happen to these old big blocks. I've had to replace lots of them. When that wears out the car will start fine and drive around town all day, but will not accelerate or drive fast
 

thethee

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Plugs #1-3-5-7
20210814_164031.jpg


Plugs #2-4-6-8
20210814_164021.jpg


Cap
20210814_164506.jpg


Rotor
20210814_164530.jpg


These all look okay to me
 

thethee

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For tomorrow I'm planning to unhook the secondaries and see if that changes anything. Quick and dirty way to check if it's fuel related and if I need to open up the carb.

Don't know if it matters but noticed my kickdown is out of adjustment so I'll sort that out as well.

Also, check the ground on your electronic ignition control module. If the mount screws are rusty, that will lead to erratic performance.

Dave

Bolts are cleaned and there are star washers between ecu and firewall
 

413

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Always put the star washers under The bolt heads.

Remember those factory bolts with the barbs on the washer for grounding.
 

Imperial dude

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This is just a shot in the dark, but I see you're in Holland
Does it have a catalytic converter on the exhaust?
What you describe sounds just like a bad converter
Can drive around all day, cruise 70 on the highway, but kick it to merge on the on ramp and it'll stumble and surge until you back off a bit
If you don't have a CC check or restricted exhaust
Like I say, it's a long shot, but sometimes it's the simplest things
 

Davea Lux

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Also, if this engine has an EGR valve, it might be sticking open due to carbon fouling of the valve seat. This is a common problem in areas where blended fuels containing methanol or ethanol are used. Easiest fix is to block off the EGR port with a steel plate. Several after marked EGR eliminator plates are available and there is one that bolts on under the EGR valve to maintain a stock look.

Dave
 

thethee

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Well it's fixed!

Adjusted the kickdown lever so the end of the slot was touching the throttle pin again. Put back the plugs, wires, cap, and rotor. Swapped out the coil for the spare in the trunk. Topped off the engine oil.

One of those must have done it and I was thinking coil so I put the first coil back and that seemed to work also which had me stumped. But I now distrust it a little so I'm putting in my spare coil and keeping the first one in the trunk. Could also be something as simple as a loose spark plug wire, loose coil to distributor wire, or wrong firing order. Who knows, but still, I'm just happy she's running good again!
 

thethee

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This is just a shot in the dark, but I see you're in Holland
Does it have a catalytic converter on the exhaust?
What you describe sounds just like a bad converter
Can drive around all day, cruise 70 on the highway, but kick it to merge on the on ramp and it'll stumble and surge until you back off a bit
If you don't have a CC check or restricted exhaust
Like I say, it's a long shot, but sometimes it's the simplest things
It doesn't have a cat, not required for these old cars. Emission requirements are pretty loose if they're around 50 years old. But thanks for the idea!
Also, if this engine has an EGR valve, it might be sticking open due to carbon fouling of the valve seat. This is a common problem in areas where blended fuels containing methanol or ethanol are used. Easiest fix is to block off the EGR port with a steel plate. Several after marked EGR eliminator plates are available and there is one that bolts on under the EGR valve to maintain a stock look.

Dave
EGR has been deleted long time ago. I didn't really want to but the valve itself had a vacuum leak so it was easiest to just remove completely. When I replaced valley pan there was indeed big carbon buildup!
 

thethee

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I spoke too soon.

So I drove it around town for a while and enjoying she was running like a champ. Stopped at a gas station for a couple liters of fuel and returned home. Car sat for about an hour and a half before I took it out again. This time the stumble on hard acceleration was back... :(

I don't know what changed BUT at least I know it shouldn't be anything major and it should be fixable. Having had a taste of how she can run I'm more determined than ever to find the cause.

It feels fuel related so I will continue that way. Start at the back and work my way up. Perhaps run the electric fuel pump off of a make shift secondary fuel tank to rule out junk clogging up the fuel sending unit? Double check pump wiring again. Then I'll probably open up the carb to check for blocked passages.
 

1970cat

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I spoke too soon.

So I drove it around town for a while and enjoying she was running like a champ. Stopped at a gas station for a couple liters of fuel and returned home. Car sat for about an hour and a half before I took it out again. This time the stumble on hard acceleration was back... :(

I don't know what changed BUT at least I know it shouldn't be anything major and it should be fixable. Having had a taste of how she can run I'm more determined than ever to find the cause.

It feels fuel related so I will continue that way. Start at the back and work my way up. Perhaps run the electric fuel pump off of a make shift secondary fuel tank to rule out junk clogging up the fuel sending unit? Double check pump wiring again. Then I'll probably open up the carb to check for blocked passages.
sounds like you stirred up some junk in the tank when you refueled.
 

thethee

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I'm out for the season...

upload_2021-8-17_13-23-36.png


Talk about one step forward two steps back. I was ready to confirm that my acceleration problem was because of junk in the tank by rigging up a second temporary fuel tank but when I tried to start it no dice. Heard the starter spinning freely so I thought the solenoid could be bad. Pulled the starter to bench test it only to find it was working perfectly. Crawled back under the car to find it was missing a bunch of teeth on the engine side.

Up till now I've managed to do all the work myself in the parking lot behind my building but this'll be above my skill level I fear. I'm guessing I need to pull the trans and I doubt I'm able to do that without a lift or something.. Don't even know what this part is called or if I need a whole new torque converter..
 

Big_John

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In theory, you can change the ring gear on the converter. I think the FSM may even show it being done.

In practice though, you are better off replacing the converter. I doubt if you can easily find the ring gear, then fitting and welding it on right is going to be above most guy's heads.
 
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