273 issue on 65 Valiant

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LAP Valiant

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So, I am desperate at this point and need anyone's input. My 65 Plymouth Valiant Signet convertible has been at a mechanic's shop for over 4 years now. It is getting many improvements including a mild cam, shorty headers, electronic ignition, larger 2 bbl Holley carb, new plugs and wires, new smaller starter (headers), borgeson power steering, and A/C. They have it put together and running after 4 sets of rods. Compression all 100 or over and timing is spot on. It idles great and runs good but seems a little underpowered considering the improvements and loses power when it gets to 70 MPH. The mechanic, who is very good at troubleshooting, is baffled. Any input would be greatly appreciated - I just want my car back.
 

The cam came with rods (and valves) that were the wrong size? then he reordered twice and they didn't work as well. He finally had some custom made that he thinks are right now.
 
That's a whole lot of changes at once to be able to pinpoint a single cause...
Exactly what cam was installed? Hydraulic or solid lifter? When you say the "The cam came with rods (and valves)", I assume you meant lifters and valve springs? Or not?
And I find that whole "larger 2 bbl Holley carb" statement to be somewhat intriguing, can you elaborate on that? What carb, what manifold (or adapter), what linkage, etc.
If this was a 350 or 500 cfm Holley 2bbl. on the stock manifold using an adapter, I'm guessing you have some linkage issues going on, both throttle and kickdown. If it's an aftermarket manifold, which one? '64/'65 273s had a different bolt arrangement that most currently available aftermarket manifolds do not work with unless they're modified to do so, which could lead to significant vacuum leaks (also an issue with carb adapters).
We need some blanks filled in here.
 
Last edited:
valve issues? valve geometry issues?
tow it home to your shop now!
these guys may be "milkin' ya, or don't get it
but if you hired them, and YOU are having to troubleshot this
just take over the project. after 4 years, bring it home
we'll help you figure it out before those guys break something costly

good luck!
 
if its a 65 273 you had a mechanical cam and pushrods from the factory. Why would new valves come with a cam and lifter set? Sounds weird.
1783566506369.png
if you have a 65 engine in the car you should have already had a mechanical cam and lifters. Did your mechanic change it over to hydraulics? Sounds weird to us. A 273 should make good power even at 70MPH
 
this is from the mechanic -It is hydraulic camshaft w/new lifters, springs and custom made push rods, Holley 500cfm and a Mr. Gasket Adapter plate.
Here is what I found in my receipts. Hope that helps.
-Comp cam 20-225-4-Comp Cams Extreme energy camshaft
-CCA-667-16 Lifters, hydraulic flat tappet AMC, Mopar
-Holley 435 - HLY-0-44125
-Summit Racing Billet Electronic Mopar Distributor, SUM-851003-1
 
What are the actual compression readings? 110 all cylinders.....not too great. 150......pretty good and should run good. 175.....excellent and should run that way. This is assuming the timing (both cam and ignition) are optimal, and the fuel system is A1 too.
 
A terrible choice for a camshaft on what we are to assume is a stock compression, stock heads, stock intake 2bl 273. It's never going to be anything resembling fast in today's world. As far as losing power, is it stalling out or just not pulling hard? Fuel tank vents intact and clean?
 
I went with what the experts at Summit Racing suggested - what would you suggest for a cam? I have forwarded that question but he has said it loses power but does not stall.
 
jeff alder said:
if its a 65 273 you had a mechanical cam and pushrods from the factory. Why would new valves come with a cam and lifter set? Sounds weird.
View attachment 1716563993if you have a 65 engine in the car you should have already had a mechanical cam and lifters. Did your mechanic change it over to hydraulics? Sounds weird to us. A 273 should make good power even at 70MPH
Click to expand...
No new valves - my mistake
 
Scampin said:
A terrible choice for a camshaft on what we are to assume is a stock compression, stock heads, stock intake 2bl 273. It's never going to be anything resembling fast in today's world. As far as losing power, is it stalling out or just not pulling hard? Fuel tank vents intact and clean?
Click to expand...
I went with what the "experts" at Summit Racing suggested - what would you suggest for a cam? I have forwarded that question but he has said it loses power but does not stall. Or rather, has trouble maintaining power at 70.
 
Depending on how the Holley 500's linkage was adapted, you quite easily could not be getting complete throttle opening, resulting in it running out of steam at a certain point- in your case at ~70mph. And I guarantee if the throttle ain't right, the transmission kickdown rod ain't right either.
 
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