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  #1  
Old 01-31-2021, 06:31 PM
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Throttle resistance after Weber carb conversion

Hi all! I own a 1970 Mercedes 250c. I have searched this forum as well as the rest of the net for info on my particular problem but have not had any luck.

I have just had my rear main seal and crankshaft/flywheel replaced by a local mechanic. I also purchased the Redline Weber 38/38 DGES carb kit and he installed those as well.

It feels as if there is something preventing the throttle plates from opening slowly and easily. I'm guessing vacuum pressure (?). I have to use much force on the gas pedal to break through this resistance, causing the engine to rev. It's either off or on, making driving very uncomfortable.

I have gone through all the linkages and they do not bind. The accelerator return spring at the firewall has been removed. In fact, this only happens when the engine is running! Pressing the pedal is nice and easy when the engine is off.

The mechanic did not utilize the heat shields, so I am left with an extra spacer for each carb. Will that extra 1/4" of space between the throttle plates and the manifold make any difference?

What I can say is that opening the throttle plates by the turning the idle speed screw in makes a difference (decreases the resistance), but at this point the idle RPMs is too high.

Any ideas? Thanks much in advance.

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Old 01-31-2021, 07:03 PM
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Join Date: Feb 2011
Location: Modesto CA
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Quote:
Originally Posted by freshtogeof View Post
Hi all! I own a 1970 Mercedes 250c.
I have to use much force on the gas pedal to break through this resistance, causing the engine to rev. It's either off or on, making driving very uncomfortable.

I have gone through all the linkages and they do not bind. The accelerator return spring at the firewall has been removed. In fact, this only happens when the engine is running! Pressing the pedal is nice and easy when the engine is off.

Try disconnecting the control pressure link to the transmission. There should not be enough force through that link to cause the described problem, but check.

Quote:
The mechanic did not utilize the heat shields, so I am left with an extra spacer for each carb. Will that extra 1/4" of space between the throttle plates and the manifold make any difference?
Space the carbs up from the manifolds such that the throttle shafts are at their original height.
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Old 02-22-2021, 11:31 AM
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Quote:
Originally Posted by Frank Reiner View Post

Try disconnecting the control pressure link to the transmission. There should not be enough force through that link to cause the described problem, but check.

Thanks for your response, Frank. Sorry for responding just now. I finally had time to do as you suggested and test things out. Is the transmission control pressure link the other rod connected to the bottom of the factory bell crank up front? I wasn’t sure, but I actually disconnected the rod between that bell crank and the front carb (essentially disconnecting communication from the gas pedal). I still had the same problem of breaking through a “wall” when opening the throttle by hand.

Space the carbs up from the manifolds such that the throttle shafts are at their original height.
I have installed the heat shields and extra spacers. No difference. Is there such a thing as too much vacuum coming from the manifolds? I’m no mechanic, but I always thought that meant good compression. Could the mechanic that worked on it cause these symptoms by replacing the rear main seal, crankshaft, flywheel, and bearings?
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Old 02-22-2021, 04:40 PM
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Join Date: Feb 2011
Location: Modesto CA
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Quote:
Originally Posted by Frank Reiner View Post

Try disconnecting the control pressure link to the transmission. There should not be enough force through that link to cause the described problem, but check.
Let's try this again. Did you disconnect the linkage rod from the angled bellcrank to the transmission? Yes/No. And did you try the throttles with the engine running and the control pressure link disconnected? Yes/No.
The point of doing the above is to either confirm or rule out the possibility that the unusual force required to move the throttle is a transmission problem.


Quote:
Originally Posted by freshtogeof View Post
I have installed the heat shields and extra spacers. No difference. Is there such a thing as too much vacuum coming from the manifolds? I’m no mechanic, but I always thought that meant good compression. Could the mechanic that worked on it cause these symptoms by replacing the rear main seal, crankshaft, flywheel, and bearings?
No, there is not "too much" vacuum. And the lower end work is not a cause.

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