Lotus Elan

Weber Jets and Chokes for 1720 engine

PostPost by: Davidb » Sat Mar 18, 2017 5:23 pm

I have tried doing a search on this but could not find the info I need.

What chokes, jets, etc should a 1720 Twin Cam need-for a starting point? My engine has 30/70/70/30 cams at .385 lift and about 10.5:1 compression with Sprint size valves.

Any guidance on this appreciated.
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PostPost by: nmauduit » Sat Mar 18, 2017 5:44 pm

you may start with what I have on a 1700 sprint like : choke 33 or 34, main 130 or 135, Idle 40, emulsion F16 air 180 (bottom 50, pump 35)...
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PostPost by: Davidb » Sun Mar 19, 2017 5:58 pm

Many thanks nmauduit.

Anybody else have suggestions to add?
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PostPost by: mbell » Sun Mar 19, 2017 6:53 pm

That suggestion sounds a good starting point to me. You of course need to fine tune it once running.

The 40 idle jet surprises me a little, using a 50 equivalent in my stand big valve engine, as std 45 was to lean (transition flat spots) for modern fuels here in US. Would have expected the larger engine to need something larger 45/50/55 but I have no experience.
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PostPost by: 1owner69Elan » Sun Mar 19, 2017 9:20 pm

Another reference point on a 1700 stroked engine:

But, with larger valves (1.625 in, 1.375 ex), high lift cam (.44), and street-ported SAS head, which may make the below less applicable to your setup?

Weber 40 151's with 32 mm chokes.

Pump gas (91 octane), US
CR: 10.25
33 Deg adv.
F-16 E-tube
40 pump
45 F-9 idle
165 Air
140 Fuel
2" air horns, stock

Dyno: 181 hp @ 6900 and 143 torque @ 5100
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PostPost by: gjz30075 » Sun Mar 19, 2017 11:11 pm

1owner...., do you have the transition flat spot dialed out?
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PostPost by: 1owner69Elan » Mon Mar 20, 2017 12:42 am

Jury is still out on full range response as I don't have the car on the road yet.

Just dyno time with measurements starting at 4000rpm.

We'll see how it does in a month or two on the road with the possible dreaded flat spot.
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PostPost by: rgh0 » Mon Mar 20, 2017 9:22 am

The dreaded flat spot is solvable in any twin cam of any state of tune just needs time and patience to tune it to suit your engine , its state of development, your local fuel and the carbs your using.

The alternative is convert to fuel injection and that costs you a lot more :shock:

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PostPost by: nmauduit » Tue Mar 21, 2017 1:30 pm

mbell wrote:The 40 idle jet surprises me a little, using a 50 equivalent in my stand big valve engine, as std 45 was to lean (transition flat spots) for modern fuels here in US. Would have expected the larger engine to need something larger 45/50/55 but I have no experience.


Yes, me too now that you mention it : I had just glanced at the notes, then digged a bit further to remember these 40DCOE are not 18, but more likely 34/35 (no back vent), so the (3) transition holes are probably different from the standard used by Lotus... It took me a while to realize the venting issue, during which I had tried a wide range to "idle" jets (up yo 60). After finding out I modified the caps then I did more tuning using an AFR sensor and decreased the idle jets so that richness was "reasonably rich" in the transition region (12.5-13 while it was sputtering black sooth before), then eventually proceeded to tune above 4000 rpm under load (air corrector), which requires long straights in the countryside or a dyno (in project), not so easy to find nearby where I live.

std disclaimer: the process is not completely finished (is it ever?)
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