Slant Six 225


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Mar 8, 2016
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Hi my name is Brandon and I am having so problems with my 1969 Plymouth Valiant 100 slant six 225. The car runs fine in Park when you put in Drive or Reverse the car starts to shake real bad as soon as you put a load on it at a stop but as soon as you drive and get going it smooths out with no problems. I have replace the plugs, plug wires, carburetor, and put on the pertronix electronic ignition to get rid of the points and still has this problem does anyone have an idea to this problem or have had this problem with there slant six?
Welcome to FABO...

I would look for a vacuum leak. Try the carb base, and the head where the intake goes to the head. Also trace any vacuum lines and look.

Try spraying wd-40 or something to see if it idles up or down on the suspected leak areas...
maybe a bad plug wire or not pushed in all the way, carb running too lean, cracked distributor cap, check pcv connection...
Brad13, welcome to the world of Slant Sickness.The 67 to 72 Valiants are my favorites.A service manual for 1969 will be your friend in the long run.As many of us will be willing and try to help the best we can,without standing there and hearing it first hand,we are doing our best to help you. Kinda tough over the internet.Do ONE thing at a time.krazykuda has a good point as the Slant engines tend to actually wear out the intake gasket.The combo intake/exhaust,and the attachment of the manifold to the head,you will see that it has special washers and arrangement.The manifolds, due to heat cycles move everytime you run the engine and over time the gasket wears away.When I did my 63 Valiant there was hardly any gasket after 50 years.You are lucky being in Az so rusty bolts won't be much of a problem.But that is just ONE area to check. As stated above check the carb attachment bolts and the carb airhorn to base screws too.Everyone above made valid points that need to be looked at.Welcome aboard and keep asking questions.Remember there are no dumb questions.
All of the above and one more... check the timing
Lots of good info on this site. Most important is do one step at a time and stick with it... you'll figure it out and have great satisfaction knowing you solved the problem... keep us posted on your progress
I would set the valve lash before any carburetor tuning. stock slant is .10 intake valve and .20 exhaust . I feel like slants like a snug .12 and .22. Also check ur timing advance. And as stated above check for VACUUM LEAKS around intake/exhaust gasket and carb mounting gasket. Good luck ! And keep on keeping on!!
Brandon. Yes, common problem in slants w/ the Holley 1920 carburetor. I dealt with exactly that for decades in my 1969 Dart. Mechanics did valve jobs, swapped intakes (vacuum leak?), finally a whole new long block gave the same issue. A 4th Holley 1920 made it idle like a dream. Rebuilt ones are usually no good, though my last one was. Maybe try a later Holley 1945 or even a fuel injection system like a few slant guys have (and I hope to).
if the car has vacium accest brakes.the buster could be the problem.