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Old 04-01-2003, 08:35 PM
Registered User
Join Date: May 2002
Posts: 40
Lightbulb Solex 4a1 fun..secondaries or accelerator pump

I have replaced all the electrics. The car starts and idles like a dream. When I hit the gas it bogs down but never quite dies then backfires and drives off but just does not run right and lacks power , it will even properly down shift when I stomp it but then goes slower.... I have ordered the re-build kit for the Solex 4A1 and it has been suggested on this board by those who have decades more experience than myself that my carb may be warped causing sticking on the secondaries but after much study I now think this bogginess may be related to the acelerator pump...and that the carb is not warped afterall and only needs a cleaning and a re-build with new diaphrams etc ... Could someone please pontificate on how a faulty accelerator pump may manifest itself in the running of the car and if this manifestation could be perceived as a secondaries sticking issue by those to whom I have poorly described the running of the car.. I have to theorize about it uintil I figure it out as I cannot afford to hand it off to a mechanic to throw parts at the problem.. I want to keep it original, no hollys or webbers.


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Old 04-01-2003, 09:47 PM
Registered User
Join Date: May 2002
Posts: 40
Unhappy seems fine

When I manually actuate the pump it does squirt a nice big stream into the carb and it seems to do it consistently when idleing in park. And the car does have days where it runs really well but more often than not it backfires and gets bogged down and lacks pedal power

Also, the cars does have a bit of white smoke when running and seems to be flooding out when I do hit the gas

.. maybe I need to adjust the pump to squirt less and drive it.. the previous owner ( dad) might have cranked up the pump ? I hadn't even thought of it.. thanks for the direction

Thanks Many...

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Old 04-01-2003, 10:17 PM
Registered User
Join Date: Feb 2003
Location: Ontario, Canada
Posts: 3,160
I am not familiar with the carb. you're describing, but if you suspect the accelerator pump. you could look down the throat of the carb. while pumping the linkage or throttle cable ( WITH THE ENGINE OFF ).
You should see a nice big gulp of fuel.
2007 C 230 Sport.
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Old 04-02-2003, 10:26 AM
G-Benz's Avatar
Razorback Soccer Dad
Join Date: Apr 2001
Location: Dallas/Fort-Worth
Posts: 5,711
Having a VW, I am familiar with the "touchy" Solex carbs!

Check the carb float. I've had SEVERE performance problems that were remedied with a float adjustment.

The accelerator diaphram may be only slightly ruptured, just enough to manifest itself under hard acceleration...your symptoms sound like that.

Also check the carb jets for obstruction, and the housing for wear. Even if the case is not warped, the butterfly linkages could be worn beyond spec.

Also look for loose or missing parts. I lost a piece to a linkage that would cause sluggish performance (on a dual carb setup).

Just some things to consider...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car
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Old 05-26-2003, 04:05 AM
Registered User
Join Date: May 2001
Location: Manila, Philippines
Posts: 66
Sorry for the late post... I hope its still useful.

Bogs with the Solex 4A1 can be caused by several things. Here's what I've seen with my own Solex'ed 250T (now gone).

Accelerator Pump:
There are two ways a working accelerator pump can cause the bog you described. Either the pump shot is late or the duration of the pump shot is too short.

My observations with the accerator pump on the Solex 4A1 is that the accelerator pump lever should *just* touch the accelerator pump's piston when the engine is at idle and the engine running. Otherwise, the shot would be late and you experience a bog then it goes.

Then there's a adjustable screw mounted on the accelerator pump lever itself that limits the travel of the piston -- my observation is that controls the duration (and quantity) of the pump shot.

A late pump shot would go: bog... vroom.
A short pump shot would go: quick vroom... bog... vroom

dirt in the idle jets and main jets can manifest as bogging down when accelerating from a stand-still.

sticky secondaries
Sticky secondaries (meaning the secondary plate doesn't open) doesn't show up as a bog... but rather a loss of power/grunt at 4000 rpm up.

Secondaries that open without giving more fuel will bog down the engine. A loose air-flap can do this, or blocked secondary fuel circuits, or an empty secondary fuel-bowl.

Oh yeah, before you start tearing up your Solex... have you checked your ignition timing? dwell? etc?

Hope that helps,
Johan Limcangco
Manila, Philippines
79 350SE (W116)
86 230E (W124)
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Old 05-26-2003, 10:07 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Float level was a major factor in those carbs. The level of the fuel should be 5mm below the parting surface of the top cover. To measure, disable igition, remove bowl cover, hold float pivot retaining sping down with ones finger (not imparing float rotation), sop up enough gas that it is significantly below the level, and have someone crank the car till the bowl fills. Do not measure the miniscus of the fuel go by the surface level.

Be sure your main jets measure the 100 that they should be. Take them to 105 if you wish.

And to fix the problem one must deal with the secondary air valve. If one looks it can be seen that the secondaries open whenever the throttle goes beyond a certain point. There is a point at the secondary throttle shaft that one can wire the secondaries closed (there is a small lever and a startegically place whole for the wire) so that they never open. This can occur as the linkage from the primaries is through a spring and it just stretches when the secondaries don't open. I would advise that for diagnostic purposes you wire the secondaries shut and take it for a drive. You will probably be amazed at how well it works on 2 barrels. I have wired a number shut where the secondary airvalve was beyond repair.

Once, sure that this gets rid of the bog, one can attempt a repair. The concept behind the secondary air valve is that the secondaries are so big that on openning the venturi velocity is so small that no fuel flows and that is where the bog comes from: two big holes adding air with no fuel. The airvalve acts like a choke and allows a certain negative pressure to be developed under the plate (which then opens the plate). This pressure replaces the venturi pressure differential to move fuel untill wide open and of sufficient velocity. The act of openning the secondary airvalve also lifts the secondary metering rods. Even with the rods all the way up, if the valve goes wide open there is no pressure difference to move fuel and the thing just goes slower.

MB made a set of weights and a fixture to place them on the airvalve trying to get quantitative with their adjustment; won't happen. The balance is dynamic to other variables and has to be set by feel.

The first thing is to make sure the choke is off and place one finger on the airvalve. does it spring open and closed against the finger? Does it bind? The warping that affects this problem happens in the top and causes the three pivot points of the airvalve shaft to be non colinear. This causes the most serious binding. I have reamed the holes and sanded the shafts strategically to make a smooth rotation. It doesn't need to be smooth for more than 90 degrees of rotation. Slightly bending the shaft can help (when done at precisely the right point.)

Once the shaft moves freely the spring must be adjusted. At one end of the airvalve shaft there is a small allen set screw that tightens on a small shaft that has a small screwdriver slot in the end. Turning this shaft winds up the spring. *Before loosening the set screw be very sure that you are in the screw slot positively* as it will unwind in a hurry when the set screw is loosened.

Loosen the set screw and tighten the spring to taste (bouncing the plate with ones finger for flavor). The real test is on the road. One should start loose and drive. There will be a bog as the secondaries are engaged with the throttle (by the way remove the wire holding the secondaries closed for this part). Tighten some more. Theoretically when one passes the optimum spring tention there will be a small puff of black smoke from too rich a mixture and it may feel similar to the original bog. It will bog from too little or too much fuel.

Mostly you won't get it too tight; for one nasty reason. As you reach too tight the spring will come off the little adjuster we have been fooling with. This will require removing the top to reset the spring, so be carefull not to go that far.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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