Tuning Solex P11 40-4 carbs after rebuild

[JohnLiles]

Hi all , above carbs are fitted to a 912 motor in the back of my pre A 356 !

Will be collecting them from Fenn Lane Motorsport ( UK ) next week after a rebuild .
Car is in Cheshire UK , will refit the carbs myself ( I took them off ) but then what ?
I have never messed around with tuning and only possess basic mechanical tools.
If there is a basic procedure to get the car running , not necessarily perfect , I will have a go ,

Otherwise -

Does anyone know of a chap that will come and set them up for me ? I made a note of the obvious adjustment screw settings before taking them off for a rebuild but imagine there is more to it than that . Float levels ? Balancing air flow ?


Cheers and TIA

[David Jones]

John, you will need a carb synchronizer of some kind. This is the one I would recommend. https://www.amazon.com/Carburetor-Synch ... B00SLH03R4
Set the float level first. This can be done on the bench before installation I have successfully used a length of fuel tubing and a funnel with the funnel set about 2 metres above the carb and adjust the fuel level until fuel just seeps out of the plug on the side of the carb. Set the idle screws about 1 turn in from when they contact the stop and the mixture screws about 2 turns out. Get the car running and use the throttle to get it warmed up enough to keep running then it is a case of adjusting to best idle while adjusting for even flow in all 4 throats. The factory manual has a good chapter on setting up Solex carbs.

[Doug McDonnell]

https://porsche356registry.org/article/47? From the articles section.

[Chuck Allard]

If you can set the float levels on the bench before you install the carbs in the car, actuate the throttles multiple times until you have fuel squirting out of the accelerator pump nozzles. Refill the float chamber until fuel just sits at the bottom lip of the plug in the rear of the carbs. Install on the car and the engine should start immediately since fuel will not have to be pumped up into the float bowls or prime the accelerator pump.
Enjoy.

[JohnLiles]

OK guys , thanks for all the advice , here is an update !!

After I got the carbs back from being rebuilt , I was told that they had set the float levels using isopropylalcohol (IPA) to just below the level checking screw , and had also set idle speed screws a half turn in against throttle lever , and the idle mixture screws one and a half turns out from fully in.

Before fitting to car , I decided to check the accelerator pump delivery , after pumping out the IPA and making sure it was all out , filling up with petrol till weeping from the ( removed) level checking screw , then giving 2 quick full length pumps of the throttle lever . I adjusted till each carb delivering the same , and decided to sit with 0.3 ml being delivered . This is a bit short of 0.45ml 'summer' recommendation but I am using the highest octane fuel (98 euro spec ) , can always adjust it up if not enough , at least it is balanced.

I fitted the carbs and found the idle speed too low , just about 500rpm and would not keep running when cold. I managed to keep it running to get thing warm and steady but then checked the airflow using an SK synchrometer , RH carb pulling 7kg/h and LH carb pulling 3kg/h . On double checking the RH carb idle speed had been turned in 1.5 turns rather than 0.5 . I then went wrong by deciding to make the LH carb pull more air by turning in the idle speed another turn to make it 1.5 like I had found the RH to be ( stupid to assume it was starting from 0.5 turn in , I know ! ) . Engine would not run properly , plenty of popping and banging , switched it off , went to have a think.

Came back and decided to check spark plugs by pulling them out and having a look . This started a whole saga of dropping one ( left hand front ) in the tinwork / cylinder head space. I got some helpful advice from Registry members on how to deal with this . Eventually removed all plugs , gaps were good , but all were sooty with loose carbon deposits , I had a set of new shiny ones to hand so I fitted 4 new plugs . To do all this I had removed the carburettors again so while they were out I double checked all the settings and used a micrometer gauge to set the same slightly open throttle butterfly position on both using the idle speed screw.

OK , so after refitting carbs , with reset throttle butterfly opening and default 1.5 turns out on all 4 idle mixture screws , tried again . Got it running OK, settled into a fast idle about 1100- 1200 , checked airflow , now 6kg/h each side for both barrels . Hoorah !
Noticed leak of petrol on LH fuel connection banjo so shut down , tightened it up , but leak persisted so replaced red fibre washer . Restart , no further fuel leak , but now settled into persistent 'cough' on idle.

I checked airflows , 3 of the 4 barrels had the previous 6kg/h rate , BUT the RH back carb barrel had erratic flow , dipping down from 6 to ? something small when the cough occurred .

Peered down RH back barrel , saw red light (flame?) when cough came back on that barrel . Switched off !!

Car is fitted with electronic ignition so should be nothing to adjust there , was serviced a year and only 600 miles ago , don't know if valves were adjusted then , but seemed to be running OK before carbs rebuilt .

I would be grateful for advice on what to do next !!

[David Jones]

Coughing back is indicative of a lean mixture when at idle. Possible reasons are, idle screw too far in which is most likely, given your symptoms, turn it out half turn at a time until it quits coughing back. Air leak because of warped carb bottom or bad gasket or idle jet blocked.
By the way if you work on the carbs on the bench use paint thinner or mineral spirits. Safer than isopropyl alcohol much less flammable but same SG as gasoline.
98 octane is way higher than needed unless you are running real high CR. Even the SC and S90 only required 95 octane.
I hope you have a factory workshop manual if you are doing your own work as it has good instructions on carbs.

[JohnLiles]

A bit if trial and error was needed after David Jones suggested that the misfire back through one of the carburettor barrels was probably caused by too lean a mixture - it was ! - but you soon unbalance another barrel as you move through the process.

I read back on the maintenance history over last 12 months , have only had the car 14 months , and it turned out that the valves had been adjusted , the spark plugs replaced , and the distributor points swapped out for electronic ignition. Just as well I bothered to check as had already swapped out the plugs for new ones this month , so the 'old ones' had only done about 500 miles !

OK , I discovered that if you set idle speed with aircleaners removed , when you refit you lose a couple of hundred RPM , so in my case was dropping from700-800 to 500-600 , which seemed to be awkward for depressing the accelerator and getting a smooth pick up of revs.

So I decided to clean out the aircleaners using petrol , it took a few rinses before the grit and brown colouration from loose rust subsided . I then squirted WD40 on inside and outside surfaces of the mesh , generous helping , and set them aside to dry off .

I started up and it took a few cranks and then put a few more revs on to a target of 1000 RPM , forgot to detach one of the carb link drop down rods , so had to repeat the exercise , but this was pretty straightforward using the SK air flow meter.
BTW I had to cut down the rubber collar so it fitted snug without the injector / diffusertubes getting in the way. Really pleased I bought it , would not want to attempt balancing without it .

The tricky bit was adjusting the 'hidden' idle mixture screws at the 'front' of the carbs when you are leaning in over the 'back' of the engine .....there is a slotted screw ( jet holder? ) just above the slotted idle mixture screw so it is easy working blind to try to ajust the wrong slotted screw. If you have superpower fingers that can feel and grip the correct knurled finish screw and rotate it against the ( new and strong ) spring then you will do a better job than me . Eventually after a bit of slipping out of the slot partway through adjustments and losing track of how much adjustment had actually been achieved up to the point of slipping , I had a reasonably running engine running at a fast idle around 1000 -1100 RPM , no aircleaners attached , balanced flows on all 4 barrels , with a slight cough every couple of minutes , could not pin down which barrel was misbehaving , so quit while ahead.

Following day I made up 2 tools to deal with idle mixture screw setting . I then refitted aircleaners , started up and engine settled to an idle around 800-900 RPM , no coughing , maybe that bit less RPM was more in balance with mix screw settings

The first tool was a flat blade screwdriver bit with 1/4 inch hex drive , I put this inside a 1/4 inch hex socket to give me something stubby enough to work as a screwdriver in the restricted pre A engine bay , a padded cell with vinyl padding !

The second tool was a modified plastic ballpoint pen cap , cut about 12mm off the part that caps the pen , cut off most of the pocket clip , you now have something that can be pushed down over the 8mm diameter knurled brass mixture screw , with an opening that tapers from just over 8mm dia to just under. This can be push fit over one of the hidden 'front' screws and left fitted while adjustments made , the sawn off pocket clip stub acts as a pointer that gives a good indication by feel of the screw position.

Here is a picture showing the tools I used/made , the background kitchen tiles are 100mm square to give an idea of scale

[David Jones]

John you found a solution but if you want a better tool try these. Amazon UK
https://www.amazon.co.uk/Mini-Screwdriv ... crewdriver
https://www.amazon.co.uk/d/4bo/innovati ... crewdriver

[JohnLiles]

John you found a solution but if you want a better tool try these. Amazon UK
https://www.amazon.co.uk/Mini-Screwdriv ... crewdriver
https://www.amazon.co.uk/d/4bo/innovati ... crewdriver

That second one looks good , but I really like the plastic 'throw-away' plastic pen top ! Thanks for your help on this David !

[WilliamVaughan]

For split shaft solex 40 P II idle mixture adjust screws I used to use a penny. Now I use a nickel. It fits the slot better. Easy to count turns.