Mechanical Fuel Pump Pressure vs Accel Jets

[RIQUE]

Hi list, my car hesitates when I accelerate from standstill and then picks up speed after slight coffing. I have carefully regulated and restored the carbs but I can't free my car of this problem. I installed new pump seals for the accelerator jets and tested volume correctly. My question is could this problem be due to low pressure from the fuel pump...? Thanks, Enrique

 

[Malc]

Hi Rique,
From what I remember the mechanical fuel pump is very suseptable to air leaks, especially on the suction side from the tank.
In addition:
Check that the diaphram is ok
Some pumps have a built in microfilter which can get blocked.
Some pumps the built in valves can wear badly.
The above might be causing your fuel delivery problems.
HTH
Malc

 

[Honolulu]

Don't think it's the pump

obviously your carb fuel bowls have enough fuel to run, so they ought to have enough fuel to supply the accelerator pump.

I would look into ignition timing first, followed by raising or lowering mixture richness.

This requires, of course that your plugs are good and point gap and dwell (which inter-relate) are properly set.

 

[RIQUE]

Hono, I hate to admit, my plugs are new, timing according to the book,
idle syncked, mixture checked with innverness meter. ordered a new pump but I just don't know what else to do... :cry:

 

[Malc]

Rique,
Have you checked all your fuel lines for kinks, flat spots, other damage, and holes?
Have you checked the pickup in the tank? There is often rust and "crud".
Malc

 

[Sven]

Two other things to consider:

1. The ignition coil. My CS (with Zeniths) would cut-out/cough between 3-4k rpms when accelerating. Replacing the coil fixed this.

2. If running with Zeniths, they had a tendency to warp at the bottom of the carb where they mate to the insulating gasket. Flat sanding this usually helped.

 

[blumax]

Hesitation at take-off--another possibility

pull the airbox--engine off with warm motor so flaps in carbs are fully open--press throttle linkage--do you see the little injectors--one in each carb spraying nice uniform delivery of fuel into carb throats--these little guys are key to a smooth launch and if clogged or not correctly aimed into the primary throats they can cause the symptom you describe. If not done--and most kits do not include same--I would replace them with new ones

If you plan to remove them to clean, adjust or replace--BE VERY CAREFUL--DO NOT DROP SAME INTO CARB THROATS--they are brass and can drop all the way into intake manifold and magnet won't help in retrieving same.

BTW--similar vintage early 1970's MBZ 250 and 280's were carbed with same Zenith carbs--MBZ dealers did have spares available at much friendlier prices than BMW.

HTH