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914World.com _ 914World Garage _ 1974 1.8 Light Load Stumble

Posted by: andrewvolsen Jan 24 2017, 10:17 AM

My '74 1.8 L-Jet 914 has developed a light load hesitation/stumble. It is the most pronounced when cruising on flat ground with light accelerator application, maybe 10% opening of the butterfly valve. The stumble is not noticeable when accelerating.

The car has:
- New vacuum lines
- New plugs, wires, cap, rotor, and coil (still running points)
- Cleaned throttle body and checked operation of TPS
- New accelerator pedal and throttle cable
- Rebuilt MAF
- Recent valve adjustment

Does anyone have any suggestions for how to proceed with diagnosing the cause of my light load stumble

Posted by: scott_in_nh Jan 24 2017, 12:08 PM

By chance does it happen right around 2k rpm?
If so you may need a slight adjustment to the TPS.

If the ECU thinks the throttle is closed above 2k rpm, it kills the fuel to the engine.
When/if the engine hits 2k rpm, it adds fuel again.

When the TPS is misadjusted, you can be trying to do a part throttle cruise, but the ECU cuts in and out with the fuel.
If it is happening above 2k rpm, you may have a lean condition requiring a slight tweak of the MPS.

Posted by: porschetub Jan 24 2017, 12:11 PM

When was the last time the distributor was cleaned ?,is the advance plate moving freely under vacuum,pull the hose and suck on it ,it shouldn't take much effort.
So many of these distributors are gummed up after all these years.
Also check your timing to see you are getting the correct maximum advance ,and your timing is correct @idle.
Good luck.