Solex 4a1 - backfires under heavy load

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patrykrebisz

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1973 w114 280, m110 engine

Joined Oct 16, 2016

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518 Posts

Discussion starter · #1 · Aug 27, 2018

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I recently installed a rebuilt Solex 4a1 carb on my engine. I set the secondaries with the recommended weight but i sometimes get pops from carburetor while driving uphills and pushing the gas pedal to the floor.

As far as understand it, the pop is from a lean condition (also indicating that when i open idle screws more my RPM increases). I presume the idle screws only operate at idle so my up-the-hill pop isn't really affected by lean idle - yes? So my question, do my secondaries (which i understand are there for just this sort of occasions) open too soon thus creating a lean condition? Should i tighten the secondaries spring a bit more?

 

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patrykrebisz

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1973 w114 280, m110 engine

Joined Oct 16, 2016

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518 Posts

Discussion starter · #2 · Aug 29, 2018

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I enrichened the mixture and now the backfire is gone BUT the car still has very little power when going up hill. Should i tighten the secondaries spring or loosen it a bit?????

 

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patrykrebisz

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1973 w114 280, m110 engine

Joined Oct 16, 2016

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518 Posts

Discussion starter · #3 · Aug 31, 2018

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To answer my own question, i lessened the tension of the spring (turning the spring knob clockwise) and the car felt flat on its face - no power and backfire through the carb while pressing gas to the metal while going uphill. This means that the flaps open TOO SOON and there is not enough pressure to suck fuel for the secondaries (thus leaning out mixture).

So i turned the knob the other direction and the car run perfect. A good way to judge the "perfection" is your temp gauge. Driving on flat road with constant RPM you will find out your average engine temperature. Then going up hill pressing the gas to the metal if the temperature raises - that means that the flaps open slightly too soon leaning out the mixture (lean mixture burns hotter).

Remember that besides the spring, there are other two factors that affect your secondaries: float level and airbypass screws. If the float is too low the spring should be tightened more and vice versa. If the air bypass is set too tight, it can act like a choke plate and cause a rich condition. Too loose and, as stated above, you'll get a lean bog.