1956 Engine rpm increases on slope | Ford Thunderbird forum club group 1955-2005 models

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1956 Engine rpm increases on slope

D

DaveRo

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Thunderbird Year
1956
I have a 1956 Tbird with 292 cuin engine with 3 speed standard. On the flat part of the driveway, it idles at about 7-800rpm. When I roll down the sloped driveway, backwards, the rpm increases to about 2000. This is in neutral, no clutch. The engine doesn't have the original carb. It does have a new original fuel/windshield washer mechanical pump. The only vacuum lines are to the distributor advance and the windshield wipers. The wipers operate fine. There are no vacuum leaks. When I drive the car back to the flat area, the idle returns to 7-800. The only thing I can think of is the float bowl in the carb. But that shouldn't change with the angle of the car. Any suggestions?

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Dave, a very intriguing problem., I'm also thinking about the float bowl, too.
You say there are no vacuum leaks... have you checked for vacuum leaks when the car is on the slope?
Have you tried with the front end of the car pointing down the slope? Then do a 180 and have the car with the front end pointing up?
Does the RPM still change to the 2000 rpm in each direction?
 
DrDISK, I didn't check facing down the driveway, I will. The wipers work fine on flat and on the slope, indicating no change in vacuum. Thanks for the reply.
 
I would plug the vacuum advance distributor line (you might have to bump up the idle while testing) and try backing up.
Next I would wire the accelerator linkage temporarily at the idle setting and try it again.
Not a Ford carb guy but could the 'fast idle' cam be engaging when backing up.?
 
cokefirst said:
You state that you do not have the original carburetor. The original carburetor was designed to work with the distributor as a unit. It is a Load-0-matic, engineered by Holley. If you have changed the carburetor, it is likely that you need to change the distributor to one that is compatible with the carburetor.
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Hence my test above to block off the vacuum, however vacuum shouldn't change with the slope of travel, carb float bowls contents or moving throttle linkage (fast idle cam, return spring not closing off throttle plates fully) could - hence my other test recommendation... Its a weird problem and the only thing I recall close is early GM 4 bbl carbs stalling on sharp left turns...
 
Try letting the car down the drive to where the engine speed goes up .Emergency brake it and wheel block it so you can get under the hood. Now you can check throttle position, disconnect vacuum lines and so on till you discover the problem.
 
Bruce Hardy said:
Try letting the car down the drive to where the engine speed goes up .Emergency brake it and wheel block it so you can get under the hood. Now you can check throttle position, disconnect vacuum lines and so on till you discover the problem.
Click to expand...
Yeah, be daggone careful NOT to blip the throttle if you try this - very dangerous....even with chocks and e-brake set.
 
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