RocketJock

I've been lucky enough to have a fully restored 1956 with a 2x4 setup and powerglide handed down to me, but I don't have a clue about 1950's tech. My issue is that the car idles fine in park once warm, but once in drive, it will increase speed and run right at 50 mph and stay there with no input from the throttle. I have to shift the car into neutral so I can give the brakes a fighting chance to bring it to a stop. In neutral, the engine is idling very high. Blipping the throttle does nothing when it is in drive, but does occasionally bring it down to its normal idle when in neutral. This car only has 100 miles on it since a ground-up, restoration and obviously needs some tweaking- any recommendations on what I can do to correct this?

rich5962

If the carburetor linkages and associated fuel system mechanicals(i.e. the kickdown linkage) have been checked out and not the problem(which I would verify first if it has not been done yet)..............

....then I would suspect that you have a bad modulator valve diaphragm, which is allowing transmission fluid vapors and some fluid itself to be drawn into the intake and be the "secondary" fuel system causing your problem. Try disconnecting the vacuum line to the modulator at the source, and plug the port at the carburetor and see if the problem disappears on a test drive.

Rich

56Heap

No modulator valve if it's a cast iron PG. Disconnect the throttle valve kick down linkage and see what the idle does then

Vette Daddy

Turn the carb scew to the right.

Just kidding!

1955 copper

56-57 has no modulator valve that started in 58

rich5962

Hmmmm, ok didn't know that, but may it has a later PG???

I recall someone else had this kind of problem on the TDB Roy, and that's where I learned about the valve causing rpm's to rise. Oh well, I tried to help.



Rich

1955 copper

Dont feel bad we all learn something each day

sub006

Toyota wants to subpoena your car for a "runaway acceleration" defense: "They all do that!"

67L36Driver

Broken driver's side motor mount? At least that was the excuse back in the day.

Bowtyeguy

Check the return spring on the carb, its either hooked up wrong, to weak or sticking.

Kerrmudgeon

Hey guys, don't ya just love newborns? They're just so innocent and cute.
Greg, early auto "technology" is not that complex. No rebooting, downloading drivers,blah, blah. You have a simple mechanical fuel delivery system here. It's getting too much fuel. Causes.....Probably a throntle return spring not returning the gas flow to idle level, or a stuck choke that must be unstuck. Biggest advise I would give you, is don't mess around with 'settings' that used to work great, and concentrate on stuff that time (storage and not being run) changes and deteriorates. And buy a shop manual!!!!!!!!!
And they grow up so fast.........

62Jeff

Also, if your car doesn't already have one, add an extra return spring to the one that is already there.

RocketJock

Thanks for all of the input- I'll have to double check the return spring tomorrow.

I do own an assembly manual and a '56 passenger car shop manual that I've been studying hard for the past few days. One thing that may have some impact that I failed to mention is that this car has a '62 powerglide in it. After looking at the shop manual, I've come to realize that there is linkage from the trans to the carb (at least on a passenger car) that dictates some behaviors with how the carb functions. The shift pattern on the '62 trans is the opposite from a '56 (Park is forward instead of to the rear), could this cause some issues with the carb's operation if this linkage is set up for a '56 trans instead of a '62?

And yes, most of this is not that complicated, but plugging a hand-held programmer into a port for modifying engine/trans management is a hell of a lot simpler to me!!

Kerrmudgeon

Greg, that extra linkage operates the 'passing gear' of the transmission. When you step on the gas to the floor, that linkage kick's the trans into low gear. Should have nothing to do with carb operation. There may be a vacum line for the automatic, and that and any other vacum line should be checked for leaks. Leaks can make your idle go way up. Good luck.

Subfixer

Sounds like it could be a broken motor mount on the driver's side.

Kerrmudgeon

Wow that's thinking outside the envelope, but could be quite true. If the motor is out of kilter, the linkage could be binding somewhere.

67L36Driver

That's why the General came up with interlocking motor mounts in the mid to late '60s. A separated driver's side mount would let the engine 'self feed' the throttle linkage. Also why all the mfgrs went to cable throttles.

One of the first safety recalls by the auto industry. Followed by the Ford automatic shifter falling into drive from neutral.

Subfixer

Happened to me with a 65 Impala 327 w/powerglide. As soon as I put it in Drive, the engine lifted on the left side, pulling the throttle open due to the "hard mounted" throttle linkage of that time period. Not uncommon at the time.

65 Pro Vette

56 does not have same motor mount as a 63 and up very unlikly to be that. I would also check the aligment of the carb's gaskets, all repo parts are junk and I know in the 64 AFB some gaskets are to small and the plates get caught on the gasket. Did you say it had 2-4's? Check all.

RocketJock

I finally got around to messing with it today and the first thing I noticed was that the return spring on the rear carb was a "full" spring and the one on the front was a "partial". This is exactly backwards with what is shown in the assembly manual. I tried to switch them, but the distance between the retainer bracket and the lever is not the same. But, the full spring stretched and fit tight on the rear carb, so I disconnected the linkage to the front carb and gave it a try. The car ran fine with the full spring on the rear carb so clearly the partial spring didn't have enough pull. I'll order a correct set of front and rear springs and hopefully that'll do it. Thanks again for the advice- I would have over analyzed it for sure if this had not been recommended.

Now I just have to get the rear end down to normal height and figure out how to get the secondary gauges working and I'll be in good shape!