View single post by Esprit2
 Posted: 11-12-2016 11:00 pm
PM Quote Reply Full Topic


Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
DonBurns wrote:
My engine is 2.2, 104 / 107 combination, 9.6 compression, Big Valves (West Coast Cylinder Head). The 45 Dellotos are as supplied by the club store. Perhaps Greg could publish all of the carb specs for 2.0 and 2.2 jetting on these? Maybe spec 5 and spec 9 respectively?I don't know how the Dellortos supplied by the JHPS store are set-up. I'm a Lotus guy and not a JHPS member (I crash the gate on the forum), so I can't get into the website far enough to explore those details. But scroll up a few messages. I posted the jetting set-up for Specs 5 (2.0) & 9 (2.2) in message #8.

Both Lotus Spec 5 & 9 set-ups are influenced by emissions standards to increasing degrees (9 more so than 5), and not as rich as the dyno-tuner might like. Especially the Spec 9 Idle Air Correctors. Instead, use the 7850-7 Idle Air Correctors from Spec 5, then tweak the Idle Jets to suit your engine (see below). The rest of Spec 9 should be about right for a street driven 2.2 that's built-up as you describe. For peak track-day performance (like the dyno-tuner was probably trying to achieve) slightly richer jets might be required; but then you'll be trading emissions and fuel economy for a few more horsepower. How bad do you want it for the way you'll be driving the car?

Nobody can give you a standard, 'canned' jetting combination that will always work. Engines are all unique even if you try to build them the same, and carbs are all unique without the precise repeatability of ECU controlled fuel injection. They're all different... accept that. Start with a carb set-up that's close, like Spec 9, then tune your engine/ carbs/ ignition to be the best it can be as an individual. It's more like a woman than a computer.

DonBurns wrote:
I have so far not opened the carbs. As delivered the timing was off spec (way advanced) so I corrected that. I have a Carbtune Pro, and ColorTune, and have been playing with those and have the engine running pretty well, but doesn't feel perfect. Also running hot in spite of new 3-row, largest available oil cooler which could suggest running lean? It seems like air flow, flame color and RPM are not reacting as much to mixture screw adjustments as the Des Hammill book suggests they should.How much advance are you running. My 2.2 converted 907 with two 104 cams likes 14-16 BTDC for autocross, and 12 BTDC for the street. But then, my centrifugal advance curve is stock 'Federal' Lotus, and pretty tame.

Don't put too much stock in the ColorTune. I have one, tried it, and it's been untouched in the box for years. The instructions are simplisticly short on details, and focus too hard on achieving stoichiometric, blue flame combustion. That's fine for fuel injection controlled by O2 sensors and feedback loops, but most carbs can't support that. With a blue flame, you're toeing the line of burned pistons, and carbs don't have the fine resolution to safely dance that line consistently. Put a little yellow in the flame. Better yet, park the ColorTune, listen to the engine, and tune to where it runs well and makes good power.

DonBurns wrote:
When the dyno tuner delivered the car, he made a comment that he had to "open up" jets to get it running right. That would be OK I suppose if it made it run right, but the engine as delivered ran like cr%p. So my question is does someone have jet specs for what hole sizes each jet should have? I would like examine the jets and check with pin gauges to see what these yahoos did to me, and correct if necessary. I can't help you with dyno-tuners near you. But start with Spec 9 plus 7850-7 Idle Air Correctors. The big valves do require slightly bigger jets, but I don't have specific recommendations for you.

Warm the engine up, and set the idle speed as slow as it will go consistent with smooth running... no stumbling or spitting. Then open the throttle slowly enough that the accelerator pump isn't much of a factor. If it hesitates just off idle, then the Idle Air Correctorr is too lean. If your carbs have the 7850-9 correctors, the engine probably will stumble... switch to 7850.7.

Or, to do it right, if it stumbles, go a step or two richer with the Idle Air Correctors until the stumble just disappears, but no richer. If the engine doesn't stumble, then go a step or two leaner until a hesitation just shows up, then go back a step richer until it just disappears.

Then go for a drive. Idling in the driveway is one thing, but driving in traffic is something else. Just rich enough to kill the hesitation might still be a step or two too lean for the real world.

With the Idle Air Corrector sorted, find a stretch of open road. Accelerate at full throttle in a mid-gear. Not first, since you'll rip through it too quickly. Not fourth/ fifth, since you'll get a ticket. But select a gear, start slow, and accelerate at full throttle to near 4000 rpm. The Idle Circuit hands off to the Main Circuit at 3200 rpm. If the engine stumbles at or a little before 3200 rpm, then the Idle Jet is too lean, and the Idle Circuit is running out of capacity before the mains take over.

Go a step or two richer on the Idle Jet until the stumble just goes away, no more. If the engine doesn't stumble, then go a step of two leaner until a stumble just occurs, then back a step richer until it just disappears again. No richer.

The Idle Jet and Idle Air Corrector affect each other. So once you get one adjusted, you may find the other one is now off. It's an itterative process, and you need to repeat it a few times until the last round results in no further change. Yes, it's putzy work. And yes, a full selection of jets would be very helpful.

Adjusting the main jets and air correctors is best done on a dyno, unless you like getting tickets. But if the engine falls on it's face before reaching redline, the main jets are too small. But Spec 9 is pretty close on the main circuit, so focus your efforts on the idle circuit. Save the mains for the Dyno.

Tim Engel

Last edited on 01-09-2017 12:19 am by Esprit2