low power when hot?

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Diaddigo

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Howdy!
Recently got my 72 Duster back on the road, and its been doing well for short jaunts around the block. Today, I tested it out for a slightly longer drive to a local car show (my first one and I was the only Mopar there). On the way back, towards the end of my drive, It was really low on power and not super responsive. I live on a decent size hill so I usually have to park it at the bottom and wait for it to be dead cold before I take it back up (which it does fine just off cold)

Idle is fine and it never cuts off unless its hard braking (I have just been left foot braking with the slightest bit of gas until I stop then it recovers fine)

my theory is its leaning out when the mixture gets warm/hot. but I want to gets some more input. Is this as simple as richening the mixture + a little higher idle? (currently 1.5-2ish turns out from seat)
 
My theory is that the valve lash needs adjusting.
Mine works really well at .013int/ .023 exhaust, hot lash.

But be careful, the valve stems like to wear ruts in the rockers, so you cannot use the regular-width feelers, which will span the ruts and give excess lash.

You can prove if I'm right by doing a hot-compression test before the lashing. And another after resetting the valves.

If no change, then either I'm wrong, or it's too late, one or more valves is burned. In and case,the test will point you in the right direction.
 
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is your heat riser stuck closed? That would create a very hot carb stove. Id check that first, EZ enough just look and see where the heat rider is sitting when warm. If its closed (activated, redirecting exhaust up into carb stove) see if a little blip of carb forces it open. Not sure if its supposed to be open when warm or so little pressure on it that it swings open when the slightest exhaust is going through it off idle. Seen it explained both ways.
 
My 340 ( nearly new) used to bounce the flapper around at idle making a characteristic rattle that I thought all 340s had. Back in the day,if you didn't hear it, it was an instant tip off that the car probably didn't have logs, so play it cool.
 
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I'll second the heat riser. Freed mine up a few years ago. This summer mileage was down a little and didn't seem as responsive. Found it stuck part way this time. Freed it up and running good again.
 
I did set the valve lash cold when I was going through everything (per @pishta 's recommendations on a previous thread) so even if its off, it shouldnt been that far out of whack but I'll go back and set it hot in the near future. Also - given that this is a 2ish rebuild engine and I'm skeptical about the integrity of the last one, a burnt valve isn't out of the question. However, how would that not show itself at every temp or every RPM? The good news is I have another head that seems to be in good shape so worst case I can swap it.

I've checked the heat riser before but I'll do some more intensive testing with the heat cycles. Worst case, Would it be the end of the world to get it stuck in its "warm" position or remove it? cold starts aren't much of "cold" starts in Georgia and even then its in the garage.
 
I've always done the valve lash hot and running. What clearances did you set them to cold? Have you rebuilt the carb?
 
Howdy!
Recently got my 72 Duster back on the road, and its been doing well for short jaunts around the block. Today, I tested it out for a slightly longer drive to a local car show (my first one and I was the only Mopar there). On the way back, towards the end of my drive, It was really low on power and not super responsive. I live on a decent size hill so I usually have to park it at the bottom and wait for it to be dead cold before I take it back up (which it does fine just off cold)

Idle is fine and it never cuts off unless its hard braking (I have just been left foot braking with the slightest bit of gas until I stop then it recovers fine)

my theory is its leaning out when the mixture gets warm/hot. but I want to gets some more input. Is this as simple as richening the mixture + a little higher idle? (currently 1.5-2ish turns out from seat)
Need more info on engine, general stuff.
If it stalls out upon braking or heavy turning it is too rich and choking out.
I know that does not sound right, trust me, read the spark plugs they will tell the story.
 
My theory is that the valve lash needs adjusting.
Mine works really well at .013int/ .023 exhaust, hot lash.

But be careful, the valve stems like to wear ruts in the rockers, so you cannot use the regular-width feelers, which will span the ruts and give excess lash.

You can prove if I'm right by doing a hot-compression test before the lashing. And another after resetting the valves.

If no change, then either I'm wrong, or it's too late, one or more valves is burned. In and case,the test will point you in the right direction.
I wonder if his valve springs are weak.
I had a motor that ran great for the first 10-15 min...then got flat up top.. turned out to be light spring pressure.
 
If the jetting is stock it could be slightly lean on today’s gas. Not sure what gas you’re able to get there but here in so cal my falcon 6 responded well to 2 jet sizes up on our crappy ethanol gas. Also you don’t mention timing but if it’s set to stock spec (0-5 deg btdc) there is some improvement to be had there as well. But you’ll have to limit total timing.
 
Yeah, wire it open for a few days see if it changed anything. Cold lash to hot lash is <.002 on the worst of days. Guys run them +.002 all day here so I doubt your cold lash is the culprit. Where is your fuel filter in proximity to your exhaust manifold? You want to check out @slantsixdan vertical filter recommendation to fight vapor lock.
 
Here's the not so short story:
I got this car from my great grandad when I was in high school (so i'm the 2nd owner which is fun) and I loved cars but was clueless on engines and I got completely taken for a ride on a "rebuild" from a mechanic and if I didn't see it in pieces, I would never believe that anything was done besides some cobalt blue spray paint. So the thing shortly thereafter bit the dust and I parked it, went to college and on the side I was teaching myself about cars by doing basics on my daily driver as well as watching and reading everything I could get my hands on about old mopars. I graduated in May and came back with a vengeance to get my car back on the road. turns out the bolts on the rocker shaft had backed themselves out (one of them completely out) so I checked over all the rods, got the rocker put back on and torqued, and checked the timing gears, rotor, cap, points, put new plugs in (although now that they've lived through a lot of test starts and tuning, I might should grab another set) set the valve lash cold (Intake .011/ Exhaust .021) and "bench set" the carb to get it running, then made some idle adjustments from there to get it ballparked. So since then its been sounding good, starts instantly hot or cold, and I've taken it for a quick spin a few times and when its cold, it actually feels decently peppy but i wouldn't be surprised if there were some serious underlying issues that come up and opted for the roadside service and tow on my insurance. I haven't looked at timing since I don't have a timing light (nor did I see a mark on the balancer then I re-installed it) but I probably should figure that one out. Also, I was told at some point it was bored 30 thou over but I have no way of verifying that.

The carb is a holley 1920 reman unit, but I do have the original and a rebuild kit (sans float since those seem to be hard to get)

Side note: what do you recommend for gas because I've seen some conflicting answers. right now it has a fresh tank of regular 87 in it.

I know that's a lot to sift through, and my car has a long way to go but thanks for all the input so far.
 
fuel filter is vertical between the fan and the manifold where the hardline comes up towards the carb
 
Here's the not so short story:
I got this car from my great grandad when I was in high school (so i'm the 2nd owner which is fun) and I loved cars but was clueless on engines and I got completely taken for a ride on a "rebuild" from a mechanic and if I didn't see it in pieces, I would never believe that anything was done besides some cobalt blue spray paint. So the thing shortly thereafter bit the dust and I parked it, went to college and on the side I was teaching myself about cars by doing basics on my daily driver as well as watching and reading everything I could get my hands on about old mopars. I graduated in May and came back with a vengeance to get my car back on the road. turns out the bolts on the rocker shaft had backed themselves out (one of them completely out) so I checked over all the rods, got the rocker put back on and torqued, and checked the timing gears, rotor, cap, points, put new plugs in (although now that they've lived through a lot of test starts and tuning, I might should grab another set) set the valve lash cold (Intake .011/ Exhaust .021) and "bench set" the carb to get it running, then made some idle adjustments from there to get it ballparked. So since then its been sounding good, starts instantly hot or cold, and I've taken it for a quick spin a few times and when its cold, it actually feels decently peppy but i wouldn't be surprised if there were some serious underlying issues that come up and opted for the roadside service and tow on my insurance. I haven't looked at timing since I don't have a timing light (nor did I see a mark on the balancer then I re-installed it) but I probably should figure that one out. Also, I was told at some point it was bored 30 thou over but I have no way of verifying that.

The carb is a holley 1920 reman unit, but I do have the original and a rebuild kit (sans float since those seem to be hard to get)

Side note: what do you recommend for gas because I've seen some conflicting answers. right now it has a fresh tank of regular 87 in it.

I know that's a lot to sift through, and my car has a long way to go but thanks for all the input so far.
Post some close up pic's of the old plugs when you change them.
Still dont know much about your engine?
Depending on the compression ratio 87 octane may be OK, but NO ethanol.
 
I will post some plug pics when I swap them.

is there something specific you would like to know about the engine that might help you? as far as I know its stock aside from the bore.

So of 87 is just OK, what should I be putting in it? Also - I'm slightly out in the country so there's not really a broad selection of gas
 
I will post some plug pics when I swap them.

is there something specific you would like to know about the engine that might help you? as far as I know its stock aside from the bore.

So of 87 is just OK, what should I be putting in it? Also - I'm slightly out in the country so there's not really a broad selection of gas
Well the size of the engine is a good place to start, /6, 318 or 340?
Factory stock carb, ignition, exhaust? Automatic or shifter car?
 
Sorry I thought I posted this in the slant six engine section, If I didn't, I apologize. its a 72 slant six, remanned holley 1920 (same as stock), stock ignition, stock exhaust. Automatic (904a) on the column. the only option it had factory was AC which is not functional so I took the lines off and don't have the belt on it to minimize parasitic loss.
 
Sorry I thought I posted this in the slant six engine section, If I didn't, I apologize. its a 72 slant six, remanned holley 1920 (same as stock), stock ignition, stock exhaust. Automatic (904a) on the column. the only option it had factory was AC which is not functional so I took the lines off and don't have the belt on it to minimize parasitic loss.
Sounds like a nice daily driver.
First thing I would go after is the plugs, they will tell you where you need to go.
 
fuel filter is vertical between the fan and the manifold where the hardline comes up towards the carb

It's not just whether the fuel filter is vertical or not. It's the routing of the fuel line in the stock location absorbing heat as it hugs the block.
What you want to do is remove the existing hard line. Then make a new one using new rubber hose rated for fuel use. Today's gas tends to "boil" at a lower temp than the stuff that was available when these cars were new. The recommended fix per SlantSixDan is referenced multiple times in this section. (easy to search) I did this using a couple Holley 90 degree tubes purchased from Summit. Basically, one 90 degree fitting is screwed directly into the fuel pump outlet so it points straight up toward the valve cover. The other is screwed into the carb inlet and points directly toward the valve cover. Then new 5/16" fuel hose connects them with the fuel filter now located on the side of the engine kinda behind the alternator. This should solve or prevent any fuel perforation issues.

As already mentioned, verify the heat riser operation.

Also you might try swapping another coil. Sometimes they go bad when hot.

Get a timing light or rent one for free from a large auto parts chain. Do not set to factory spec but set it between 5 and 10 degrees BTDC (Advanced) with the vacuum advance hose disconnected and plugged. (Golf tee)

From what I gather, it is hard to find a good 1920. I'd be inclined to rebuild your original. Check with Daytona Carb to get a good kit AND a new float or even a good used one. I bought a new float for my Carter from them.

Exactly what happens when you try to drive up your hill when hot? Have you tried putting the trans in low and crawling up the hill at 5-10 MPH just to get it home?

There is no reason your car cannot run like it did in 1972, or better, assuming the guy who rebuilt the engine knew what he was doing. Unless you have paperwork showing the work done, you won't know if it is .030 over or not unless you pull the head. The bore diameter can be measured but oversized pistons should be stamped .030, .040 etc. on the top.
 
Alright, Had some time to work in the garage today and changed the plugs, and a quick oil change now that I've put a few road miles on it. Here are the old plugs, cylinder #1 is on the bottom of the picture. The heat riser seems to be in operation however, when I was checking it, there was some pretty hot air puffing at my hand (and its a tad loud but I have never heard another slant run in person and my jeep has a few holes in the exhaust so I really don't have a good reference) so I suspect that there's a leak down there that's heating the carb even when the riser is shut. Is there a slightly easier way to tighten things up without taking the intake off and stuff?

IMG_4720.jpg
 
Alright, Had some time to work in the garage today and changed the plugs, and a quick oil change now that I've put a few road miles on it. Here are the old plugs, cylinder #1 is on the bottom of the picture. The heat riser seems to be in operation however, when I was checking it, there was some pretty hot air puffing at my hand (and its a tad loud but I have never heard another slant run in person and my jeep has a few holes in the exhaust so I really don't have a good reference) so I suspect that there's a leak down there that's heating the carb even when the riser is shut. Is there a slightly easier way to tighten things up without taking the intake off and stuff?

View attachment 1715617070
If you would please a couple of pic's closer up looking in the end of the plug.
How many miles on these plugs?
From the little I can see my first impression is they look maybe lean.
 
My guess would be somewhere around 30 miles? but they've lived through a lot of starts and stops while I was troubleshooting the engine, and then trying to get the tune right so I wanted to change them now that the car is drivable and down to finer adjustments on the tune so I could get a more honest reading next time
 
Also, the plugs got oily from the workbench, they were dry when I pulled them
 
Those plugs look pig rich. @yellow rose will be able to tell you more. But I would start looking at fueling problems. Like stuck needle/seat, float level way too high, things like that. Does it drip fuel out of the boosters while it’s idleing? Take the air cleaner off and look straight down the carb while it’s running.
 
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