There is not much I can write about proper Dellorto jetting that has not already been written, so I will just share my experience with it.

My Dellorto carbs came fitted as follows:

-  Size 45mm
-  Choke 37mm
-  Main Jet 135
-  Main Jet Emulsion Tube 7772.8
-  Main Jet Air Correction 150
-  Idle Jet 55
-  Idle Jet Holder 7850.9

When I initially started the engine, following conversion, tuned it and drove it, the engine stumbled and popped back through the carbs during the progression phase, 1500-3000 rpm. Above that, the car had great pickup as the main jets kicked in. However upon deceleration from higher rpm and when coasting downhill, I experienced pop backs through the carburetors. All indicators were that the progression phase mixture (fed by the idle jets) was too lean.

To deal with the lean progression phase mixture (fed by the idle jets), I installed 7850.7 idle jet holders. These holders create a richer idle mixture by decreasing the amount of air used to emulsify the fuel drawn through the idle jet. Don’t let the small numerical change in the idle jet holder number fool you. The holder numbering system is not numerically progressive. I actually went from a lean-side holder to a rich-side holder.

This change made a marked difference in drivability and I was well on my way. Not perfect yet, but a vast improvement. The stumble during progression all but vanished and the engine did not pop back during deceleration any more.

Since there was a little stumble during low-rpm acceleration still, I ordered a one-size-larger set of idle jets, 58s. They entirely eliminated the stumbles.

By the way, my JH passed the Delaware emissions test with no problems with the new Dellortos installed, even with idle jets that were a little bit off.

Well, what’s next? Even with the engine performing well, the larger carburetors and the large choke (37mm) are more set up for racing than for street use. The power is a little far out on the rpm range for my liking and the low-end torque suffers for it. I may opt for changing to smaller chokes and rejetting the carbs to get me into a power range more suited for suburban driving.

If it’s any help, here is an EXCEL spreadsheet that lists jetting setups on a number of drivable automobiles as well as recommendations from some supply shops. If you have a successful combination, or a correction to the ones on the spreadsheet, send it to me and I will post it along with the others. Most of the Dellortos seem to come set up for modified lotus engines, which mine is not.

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