Solex Carbs

[ix0ifan]

Have just bought a 901/01 with Solex carbs and 911S cams and installed it into a SWB car.

It has a terrible flat spot at around 4500rpm and is quite difficult to start as it floods quite easily.

The carbs look in quite good condition and are certainly clean and presentable.

The mechanical recirculating pumps have been replaced by a pair of Facet Solid State Electric Pumps and the main pump has a pressure regulator set at 2.5psi.

I haved a Porsche manual which gives some information about these carbs but I wonder if there is a Solex 'shop' manaul available so I can try to figure out the problem.

[Schläfer]

The search function is your friend!

The flat spot seems quite high up the rev range for it to be the classic solex circuit change over flat spot. The cams you're using may be something to do with this. What's the full spec of the engine?

[BURN-BROS]

Does the flatspot happen regardless of engine temp? How about the hard start?

I would have Phenolic spacers made to try and keep heat transfer to a minimum. This modification would be #1 on my list before I tried to chase any heat related issues.

[ix0ifan]

The engine has had a reeasonable number if mods but is basically built to an FIA comaptible spec.

It has standard valves, pistons are forged and compression is about 9.6:1.

Oil pump is from an SC, the crank has been knife edged, cases are boat tailed and barrels are half-mooned and it has piston squirters.

Exhaust is a set of 1.5" SSI Heat Exchangers with a two in two out straight through muffler.

We haven't driven it hard enough to find the flat spot when cold but is is always there when warm. The engine has a very low milage still.

Starting is a problem hot or cold but when cold it just wets the plugs and is a real pain.

It trundles around on a light throttle without too much trouble but any hard acceleration doen't really work at all.

I am just about to open up the carbs and see what venturies jets and emulsion tubes are fitted.

[Bobs 67S]

The solex conundrum has been fixed by Kevin.

When we drove our 1965's to Florida for Rennsport III he experimented with mulitple settings and jetting to find and correct the flatspot and hesitation and idle circuit richness on his solexes.
His first setting were rich and continued to lean - until he had both even power, mileage and absolutely no flat spots or starting issues.

When at Rennsport III in conversation with Hubert Kinsler (the mechanic who the factory sent to Brumos in 1965 to train all the USA mechanics - and possibly the most experienced factory Solex person) Kevin identified the corrections to get his carbs performing - Hubert was amazed we had made the trip down without "hole-ing a piston". The 4K round trip was without incident.

Kevin would be willing to consult on Solex. Talk to him at birdfixer at charter.net.

[ix0ifan]

Bob, Sounds interesting and I will send Kevin a message, thanks.

The carbs are generally quite dirty - at least to the standards that I would expect to see and the jetting doesn't seem to make sense.

Venturi 35mm

The venturis have clearly been machined to a size, which I believe is too big for a motor with a potential 160-165 BHP.

The jetting is as follows:

Main Jet 120

This jet is marked on the side of the jet and does not have an X

My Workshop Manual states the following: the shape of the mian jet bore is machined to be compatible with the direction of fuel flow. To facilitate identification , the calibration number stamped on the face of the jet has a prefix X. When replacing of installing the Main Jet make certain that only those jets bearing the prefix X are used.

Does anyone know the difference this is likely to make.

It also states in the case of complaints pertaining to poor power transition in the case of engines equipped with carburetor covers of the older design

I imagine 'cover' means top cover and does anyone know how to tell the older design from the new design and how to modify?

Emulsion Tube 8

Idle Metering Jet 55

Air Correcteion Jet 180

I will start a cleaning process suggested in 'Solex Black Magic' aand then start to make the other measurements such as float height and injector volume for the pumps.

I think that I possibly need to make some smaller venturies maybe 32mm and look for different jets.

[edmayo]

I had also tried using the Solexes with 906 cams and although it had better running characteristics than yours it was still not responsive enough in the lower ranges. Although I had a large selection of jets, the lack of different styles of emulsion tubes to try was, I felt, the problem with trying to use Solexes. I wanted to use them for the better flow characteristics due to the straighter end manifolds. I had worked with Solexes for years on stock engines and was always successful. I blamed the problem on too much cam for the emulsion tube set-up. As Bob has said, Kevin is very good on these and I can also give you a few tips as related to stock set -ups. The X on the jets was to distinguish them from the identical looking, but reverse flowing, jets used on the 356 Solexes. The floats tend to get fuel soaked, get too heavy, and cause the level to be too high. Weigh the float, it should only be 7 grams. You can make a simple manometer to check float levels. This is all covered in the shop manual. See what Kevin says, my own feeling is that the secret is in the emulsion tubes.

[ix0ifan]

Thanks for all the helpful comments.

I guess that the Phenolic Spacers sit between the carb body and the inlet manifold?

I am in the process of weighing the floats and trying to measure the parts to see if it is all basically correct.

[edmayo]

Phenolic spacers, when used, go between the manifold and the head.

[ix0ifan]

I guess I should have realised that

I have drawn them up and plan to make them from 'Whalebone' Tufnol and will have them Water jet cut.

I imagine 6mm (0.25") should be thick enough?

I have sourced emulsion tubes and a range of jets (120, 125, 130 and 135).

32mm venturis are being machined.

The only thing I haven't understood as yet is the difference between early and late top covers.