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

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1956 Engine rpm increases on slope

  • Thread starter Thread starter DaveRo
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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.
Click to expand...
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.
 
Does it do it wether you're going forward or backward on the incline? Check your carb float settings, also check the power valve. If the power valve diaphram is damaged the valve can leak fuel into the carb. Depending on the fuel bowl level the incline may make a difference. I had a leaky power valve which caused a high (about 900-1000 rpm) idle I didn't immediately notice but I did notice my mpg dropped. A quick Shady tree garage test I found was to let the car idle and turn the fuel idle adjustment screws down and watch for the idle to drop (count the number of turns you take on the screws so you know where to set them after the test). You should be able to turn it down to where the engine stalls. In my case I was able to get it down a couple hundred RPMs then futher adjusting didn't do anything. At that point I pulled the power valve, vacuum checked and it found it was bad and replaced it. Made a big difference in performance and mileage. There are a lot of you tube videos on replacing one.
 
My thought in braking and blocking the car so you can get under the hood, the car ofcourse would be in neutral.Blip the throttle all you want to find the answer.Checking motor mounts is also a good idea.
 
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