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Low Top End Power
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Powerflite
Posted 2019-11-04 10:32 AM (#589689)
Subject: Low Top End Power



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My '58 Coronet with 325 poly has lost a lot of it's top end power. It still idles OK and has good low end torque, but once I open the throttles further, it doesn't do much. I've tried adjusting & changing the carb and adjusting the timing, to no avail. I know that the motor has some blowby, but I would expect a serious compression issue to cause poor idle quality too, that would improve with rpm, not get worse. Any suggestions before I do a compression test? Is it possible that the single exhaust has become clogged? How would I test that?
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BigBlockMopar
Posted 2019-11-04 11:50 AM (#589696 - in reply to #589689)
Subject: Re: Low Top End Power



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An AFR-gauge would show quickly what's happening to the mixture.
Could very well be leaning out.

Check the (mechanical) ignition timing at 2000 and 3000rpm.

You could also install a light pressure-gauge in the exhaust and see if/how much pressure it builds, or just disconnect it as a test and drive again.

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57chizler
Posted 2019-11-04 12:37 PM (#589700 - in reply to #589689)
Subject: RE: Low Top End Power



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Powerflite - 2019-11-04 7:32 AM

Is it possible that the single exhaust has become clogged? How would I test that?


Seen it before and it produces your symptoms. Disconnect the head pipe.
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Powerflite
Posted 2019-11-04 10:11 PM (#589721 - in reply to #589689)
Subject: Re: Low Top End Power



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I don' think it's the mixture at this point. Three different carbs with a range of rich to lean settings and no real change. A wideband sensor would be nice, but I don't think that's the problem. Mechanical timing issues are a possibility. I'll check it out as well as the exhaust. Thanks for the suggestions.
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1960fury
Posted 2019-11-05 5:52 AM (#589734 - in reply to #589696)
Subject: Re: Low Top End Power



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BigBlockMopar - 2019-11-04 11:50 AM

An AFR-gauge would show quickly what's happening to the mixture.
Could very well be leaning out.




Actually this would lead probably to overheating and very likely pinging. Nathan didn't mention that.
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1960fury
Posted 2019-11-05 5:56 AM (#589735 - in reply to #589689)
Subject: RE: Low Top End Power



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Powerflite - 2019-11-04 10:32 AM

My '58 Coronet with 325 poly has lost a lot of it's top end power. It still idles OK and has good low end torque, but once I open the throttles further, it doesn't do much. I've tried adjusting & changing the carb and adjusting the timing, to no avail. I know that the motor has some blowby, but I would expect a serious compression issue to cause poor idle quality too, that would improve with rpm, not get worse. Any suggestions before I do a compression test? Is it possible that the single exhaust has become clogged? How would I test that?


Blowby, as bad as it is, will not affect engine performance as much at high piston speeds. While it certainly harms performance, it will not reduce high rpm performance noticeable.


Edited by 1960fury 2019-11-05 5:58 AM
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BigBlockMopar
Posted 2019-11-05 5:50 PM (#589773 - in reply to #589689)
Subject: Re: Low Top End Power



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You'll need to remove the intake for this but it can happen that the intake valves have a lot of carbon buildup and crud baked on them, especially if the valve seals aren't sealing properly.
This could limit the breathing capability of the heads and engine severly if bad enough.

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Powerflite
Posted 2019-11-07 12:19 PM (#589859 - in reply to #589689)
Subject: RE: Low Top End Power



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I may have found the problem. The heat riser is stuck solid. Is anyone familiar with these enough to figure out if this is stuck open or closed, and which way it needs to rotate? I plan to start whacking it with a punch & hammer but want to hit it in the right direction.



(58Coronet Heat Riser.jpg)



Attachments
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Attachments 58Coronet Heat Riser.jpg (116KB - 9 downloads)
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wizard
Posted 2019-11-07 2:52 PM (#589866 - in reply to #589689)
Subject: Re: Low Top End Power



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Spray it with some good rust solvent and heat it up with a heat gun or by running the engine. Spray some more and tap the shaft with a brass rod. try to wiggle it loose carefully.
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jboymechanic
Posted 2019-11-07 5:34 PM (#589872 - in reply to #589689)
Subject: Re: Low Top End Power



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I always remove these from my engines as this condition seems to be the eventual outcome. I remove the assembly, drill and tap the holes and then install screws into the holes in the manifold to seal them up. I've never had cold starting or driveability issues with this arrangement either.
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Powerflite
Posted 2019-11-07 5:52 PM (#589873 - in reply to #589689)
Subject: Re: Low Top End Power



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That's what I would do too, but I don't really want to take it all apart right now. When I convert to dual exhaust, I will do it then. Right now, I just want to be able to continue to drive it. I'm hoping I can jamb it in the open position and it will stay there.
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wizard
Posted 2019-11-08 2:07 AM (#589883 - in reply to #589689)
Subject: Re: Low Top End Power



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The butterfly needs just some minor maintenance for to work properly. Every carburetor engine will need a working heat riser for to give the best cold start conditions and economy.

A race engine will of course not need a heat riser.

The car manufactorers practically never mounts a part not needed
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Powerflite
Posted 2019-11-08 10:01 AM (#589898 - in reply to #589689)
Subject: Re: Low Top End Power



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Needed for winter use, but we have no winter here so it really isn't useful to me.
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Powerflite
Posted 2019-11-08 12:40 PM (#589910 - in reply to #589689)
Subject: RE: Low Top End Power



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The spring naturally pushes in the counter-clockwise position so I assume that is the closed position. Is that correct? I was able to get it to move up against that little weight on it. Does this look like the open position?



(58Coronet Heat Riser Original.jpg)



(58Coronet Heat Riser Moved.jpg)



Attachments
----------------
Attachments 58Coronet Heat Riser Original.jpg (120KB - 12 downloads)
Attachments 58Coronet Heat Riser Moved.jpg (149KB - 13 downloads)
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1960fury
Posted 2019-11-09 8:01 AM (#589937 - in reply to #589883)
Subject: Re: Low Top End Power



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wizard - 2019-11-08 2:07 AM

The butterfly needs just some minor maintenance for to work properly. Every carburetor engine will need a working heat riser for to give the best cold start conditions and economy.

A race engine will of course not need a heat riser.

The car manufactorers practically never mounts a part not needed ;)



That is undoubtably true. Some kind of intake warming device is a M-U-S-T for E-V-E-R-Y carbureted engine.

"Heat riser" was my first thought, but knowing the OP is from Nathan I didn't expect that to happen (and didn't dare to suggest)
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58coronet
Posted 2019-11-09 9:22 AM (#589943 - in reply to #589689)
Subject: Re: Low Top End Power


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Your photos look like it is in the closed position. The weight causes it to drop when the spring heats up and relaxes, at which point the weight would be near the bottom. I have a 58 Dodge with the 325 and installed a NOS heat riser when I rebuilt the engine back in the 1980's, as my original was stuck as well. As a side note I have found with other cars I have owned, that once the heat riser starts sticking, you can free it up with penetrant but it will just stick again shortly. I think it's because the shaft wears and carbon builds up between the shaft and its bore.
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Powerflite
Posted 2019-11-09 9:54 AM (#589945 - in reply to #589689)
Subject: Re: Low Top End Power



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Which photo is in the closed position? You can see from the photo that the spring pushes in the counter-clockwise direction. The last photo, I rotated it in the clockwise direction, thinking that's how it should be when it is open. Do you agree with that or should it go even further? I have no way of determining if it is correct. Can you post a photo of yours in the open position?
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58coronet
Posted 2019-11-09 2:45 PM (#589962 - in reply to #589689)
Subject: Re: Low Top End Power


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I will try to take a photo and post it in the open position, and as I recall the weight is in about the 5 o'clock position when the valve is open. Always possible that baffles have broken loose within the muffler and are internally blocking the flow. The 325 typically makes good power and mine did even when extremely worn out prior to the rebuild.
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jboymechanic
Posted 2019-11-09 9:21 PM (#589966 - in reply to #589937)
Subject: Re: Low Top End Power



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1960fury - 2019-11-09 7:01 AM

wizard - 2019-11-08 2:07 AM

The butterfly needs just some minor maintenance for to work properly. Every carburetor engine will need a working heat riser for to give the best cold start conditions and economy.

A race engine will of course not need a heat riser.

The car manufactorers practically never mounts a part not needed ;)



That is undoubtably true. Some kind of intake warming device is a M-U-S-T for E-V-E-R-Y carbureted engine.

"Heat riser" was my first thought, but knowing the OP is from Nathan I didn't expect that to happen (and didn't dare to suggest) ;)


I agree about warming the intake, but that function will still exist as the exhaust crossover is still there. There is a chance the car could become more cold-blooded if you remove the butterfly valve, but I have not found that to be the case with either of my cars.
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Powerflite
Posted 2019-11-10 2:56 AM (#589975 - in reply to #589689)
Subject: Re: Low Top End Power



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This issue motivated me to purchase a dual exhaust system for the car. I used to get a stainless 2.5" system from Pypes but they dropped the 59 Impala setup that I used and they raised the price a lot to around $400 for anything else that might be close. Ugh. That's a lot of money for something that I still have to modify and doesn't even include the mufflers. So I found a '55-57 Chevy set of tailpipes on ebay for $130 and will give them a try. Hopefully they won't require too much modification to make them work well and the price is more in line with what it should be.

https://www.ebay.com/itm/Stainless-2-5-Prebent-Exhaust-Tailpipe-Syst...

Edited by Powerflite 2019-11-10 2:58 AM
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1960fury
Posted 2019-11-10 8:10 AM (#589980 - in reply to #589966)
Subject: Re: Low Top End Power



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jboymechanic - 2019-11-09 9:21 PM

1960fury - 2019-11-09 7:01 AM

wizard - 2019-11-08 2:07 AM

The butterfly needs just some minor maintenance for to work properly. Every carburetor engine will need a working heat riser for to give the best cold start conditions and economy.

A race engine will of course not need a heat riser.

The car manufactorers practically never mounts a part not needed ;)



That is undoubtably true. Some kind of intake warming device is a M-U-S-T for E-V-E-R-Y carbureted engine.

"Heat riser" was my first thought, but knowing the OP is from Nathan I didn't expect that to happen (and didn't dare to suggest) ;)


I agree about warming the intake, but that function will still exist as the exhaust crossover is still there. There is a chance the car could become more cold-blooded if you remove the butterfly valve, but I have not found that to be the case with either of my cars.


I wanted to disagree but thought about it and to be honest I have only experience with dry-intake engines (big block) and you could be right if you got a working thermostat. On a big block on the other hand, in cold weather, there is no way to run it efficiently and troublefree without a heat riser, as the fast idle will stay on forever.
One thing for sure is, even on a wet intake car the lack of a heat riser will delay warming up and theoretically increases engine wear.
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jboymechanic
Posted 2019-11-10 9:58 PM (#589995 - in reply to #589689)
Subject: Re: Low Top End Power



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You're right about the big blocks Sid. My dad just built his 440 and blocked the exhaust crossover and he regrets it. The carb and intake are full of condensation when it runs and it never gets over 160 degrees F. In the spring, he's going to correct his mistake.

In my 318 in my Plymouth, it does take a little longer for the carb to come off high idle. I have a 185 thermostat in there now and will probably change to a 195 next year.
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Powerflite
Posted 2019-11-11 1:50 AM (#589999 - in reply to #589995)
Subject: Re: Low Top End Power



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Does your father's motor have an aluminum intake? Because they transfer heat much better and many of them come blocked. The aluminum hemi heads also have no crossover passage built into them. But they are aluminum, and if used with an aluminum intake, I am assuming that it will work fine; but I haven't tried it in cold climates yet.

Edited by Powerflite 2019-11-11 1:51 AM
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1960fury
Posted 2019-11-11 5:57 AM (#590004 - in reply to #589999)
Subject: Re: Low Top End Power



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Powerflite - 2019-11-11 1:50 AM

Does your father's motor have an aluminum intake? Because they transfer heat much better and many of them come blocked.


My 383 has and in cold weather it still needs a heat riser, if only for the reason that I do not want to press a drive button at full fast idle. The idle drops down pretty fast with a heat riser as it has an open choke well with just a thin sheet metal housing for the choke spring.

Yes, many come without a heat crossover (aftermarket, never OE) and headers sell without a heat riser too, as the aftermarket manufacturers do not pay your gas bill, do not pay for your engine rebuild and don't clean your spark plugs either. Some, or most old cars today, are just used for racing and/or warm weather weekend car show trips, degraded to toys. Every REAL car, that is meant for daily use, will need a heat riser with a dry intake.

Edited by 1960fury 2019-11-11 7:08 AM
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jboymechanic
Posted 2019-11-11 12:03 PM (#590020 - in reply to #589999)
Subject: Re: Low Top End Power



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Powerflite - 2019-11-11 12:50 AM

Does your father's motor have an aluminum intake? Because they transfer heat much better and many of them come blocked. The aluminum hemi heads also have no crossover passage built into them. But they are aluminum, and if used with an aluminum intake, I am assuming that it will work fine; but I haven't tried it in cold climates yet.


My dad's 440 is running the stock cast iron intake, he used a valley pan/intake gasket that blocked off the exhaust crossover. His whole engine build was an experiment really, he thought this block off would provide better exhaust flow performance.
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Powerflite
Posted 2019-11-11 2:08 PM (#590023 - in reply to #589689)
Subject: Re: Low Top End Power



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My experience is the dry cast iron manifold really MUST have the crossover. It doesn't transfer enough heat to make it work without it and the evaporation of the flowing fuel cools it down quite a bit. I drove my Coronet with the crossover blocked and under aggressive driving, I could actually ice up the carb in the middle of summer. But I've never had the same experience with the aluminum intake manifolds on my hemi motors. They don't have crossovers built into them, but they seem to work fine without them.
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1960fury
Posted 2019-11-11 2:28 PM (#590024 - in reply to #590023)
Subject: Re: Low Top End Power



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Powerflite - 2019-11-11 2:08 PM

My experience is the dry cast iron manifold really MUST have the crossover. It doesn't transfer enough heat to make it work without it and the evaporation of the flowing fuel cools it down quite a bit. I drove my Coronet with the crossover blocked and under aggressive driving, I could actually ice up the carb in the middle of summer. But I've never had the same experience with the aluminum intake manifolds on my hemi motors. They don't have crossovers built into them, but they seem to work fine without them.


Did you ever cold start/drive in sub 10°F temperatures?
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Powerflite
Posted 2019-11-11 4:53 PM (#590031 - in reply to #589689)
Subject: Re: Low Top End Power



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No, nor do I intend to. But if you do, then you should take that into consideration.
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