‘73 620 | High RPM at idle, must tap accelerator to settle RPM

[bisonjk69]

Hi there! 
 

I just purchased a 1973 620 that generally runs, but it idles quite high until you briefly jab the accelerator. Most of the time this helps, but sometimes it increases the RPM at idle and other times RPM then drops so low the car shakes and it feels like it may stall (though it hasn’t yet). I can’t give exact RPM as I don’t have the gauge. For background, previous owner recently installed a new Weber carburetor.

 

I’ve seen mentions of the accelerator tapping to drop RPM at idle in other threads but can’t seem to find a full discussion - apologies if this is user error!  

[Tucson620]

The Weber 32/36 throttle shaft is sensitive to over tightening - causes the throttle to hang up like this, not drop all the way back down. Since you mentioned PO install, I'd look at that. Not uncommon to replace the throttle levers during install, thus have the nut off/on - for Datsun cable throttle lever, etc.

 

Quote

CAUTION: Do not over tighten throttle shaft nut.

Proper tightness can be achieved by installing nut just slightly more than finger-tight (finger tight then one more flat of the nut) and bend lock tab

http://www.redlineweber.com/html/Tech/weber_carburetor_bench_assembly.htm

 

And then maybe PO didn't adjust idle properly for fully closed, cranked open idle jet. So too low at 'real' idle. Read up on Weber setup - super easy if you follow the rules for best lean idle.

Edited May 16, 2023 by Tucson620

[Stoffregen Motorsports]

Yes, many times the washer on the throttle shaft or the linkage piece hang up against the carb body. It is crucial to have the correct spacer between the carb body and the linkage/washer. If you need more space, add a shim, or smooth down the edge of the carb body.

 

Other than that, it could be a twisted throttle shaft causing the high idle.

 

Is your throttle screw all the way out? If it is and you still have a high idle, try pulling the linkage with your hand to see if the idle drops. If it does, then binding it likely the issue. If it does not help lower the idle, you probably have a twisted shaft or as mentioned, a throttle plate that wasn't installed correctly.

[Stoffregen Motorsports]

Note - when I'm rebuilding a carb, I will hold the carb body up to light so I can view the thin sliver of light that comes in around the throttle plate. It should be even on the two ends of the plate and the two barrels should have the same amount of light around the plate.