Solex Carb Rejetting Experiments

[Cliff Murray]

What results would you like to share about your experiences rejetting Solex carbs after modifying your engine? All advise should come with assurances that the engine was healthy and all other adjustments were spot on. Throttle shafts were also good. If you are at high altitude, let us know.

Here is an opinion to start the conversation. My work was done on a stock S90 with a NPR big bore kit. If you have the needle/seat units supplied with the kits, you will most likely find that the adjustments you make will not make clear changes. Unfortunately the car will not show real obviously that the needle/seat is the problem. As soon as you change to the units with the spring loaded ball, the car will respond nicely to changes. Also, I have found that engines with 30 degree pistons and 22 degree heads have poor combustion and respond vaguely to jetting changes. Cutting the heads to 30 degrees will net a great performance improvement.

To make the Solexes work well around town without loading up I have had good results going a size or two smaller main air correction jets instead of going to larger mains which cause over richness in the midrange. Air correction jets have more effect at higher RPMs. To deal with the stumble when accelerating from 2500 to 3500 RPM a 60 idle jet helps.

There are many ways to skin this cat(no offense to cat lovers) so it will be interesting to hear how you made your car drive as though it were fuel injected.

[Norm Miller]

Cliff,

At 5280' and above on a stock engine one step larger on the air correction has worked best for the last 50 years Zenith or Solex.
Dellorto's are happy as set for sea level and Webers are a nightmare.
If the cam is changed it seems to be a fiddle 'til you find the right combo.
Higher compression usually needs a bit larger main, although todays gas doesn't work well with 10.5 or 11 CR.

Just my 20pfg worth .......................norm

Post generated using Mail2Forum via email.

[Cliff Murray]

I was asked in an email if I was talking about GrossJets. My reply...

The GrossJet is a float valve that uses two balls instead of one needle(cone
shaped) to stop flow when the float bowl is full. Some people love them and some
people hate them. I have never used GrossJets.

The spring loaded valve that I refer to is used where the needles make contact
with the float. Solex used them but the cheap ones in the kits have no spring
loaded ball. The spring must take up the engine vibration and the fuel sloshing
so that fuel does not dribble past and cause intermittent richness. Until the
float level is constant, the carbs are erratic and jetting changes don,t help
much. Hope that helps.

[Cliff Murray]

Funny how one problem masks another. My 1720cc 30 degree chamber, otherwise stock S90 seemed to run better at 36 degree timing but did not seem right or respond well to carb tuning. The car always had mid range stumbling to one degree or another no matter what carb changes were made. The cure came when I bought high quality float valves from Harry Bieker. The carbs then responded well to small carb changes but still did not run perfectly. It took awhile to realize that the timing I was using was too much for my engine. When I changed the timing to 32 degrees the car was transformed and now became smooth and torquey! When you up compression and get combustion right, you need less timing and often less fuel. I now use stock jetting and less than stock accel pump outputs to get fantastic city driving charicteristics. The real/original problem was that the weepy float valves made the car a bit rich making it want more timing. When the valves were replaced, the wrong timing wanted more fuel but still ran poorly in the mid range thus the tail chasing round and round! Hope this helps others.