Lotus Elan

L14/Q420 cam idle speed

PostPost by: Pfreen » Sat Aug 08, 2020 3:40 pm

I just fired up my tc stromberg engine after installing q420 cams.

What idle speed do people use with this cam? With my sprint cam, 850 rpm was good. With the new cam, at about 1150 RPM, it seems to idle well. The mixture is adjusted correctly because I have an exhaust oxygen sensor.

The cam timing mop is set at 106/106.
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PostPost by: TBG » Sat Aug 08, 2020 4:14 pm

Mine is a Weber engine with 420s and about 800 is fine.
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PostPost by: nmauduit » Sat Aug 08, 2020 7:35 pm

changing the cam profiles may require you to redo the idle tuning (not the same vacuum level at low revs etc), so I would not be worried at this point. I note that you opted for a timing that is different from the one recommended by the manufacturer (100/106), so that may also contribute to a change in settings.

All that aside, I prefer to set the idle on my S4se with Q420 camshafts to 1000rpm (even though it would be smooth at 800), as I believe it reduces the thumps on the bearings: this costs a little in gas mileage, but I don't drive it that much and usually not in comuter traffic jams.
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PostPost by: Pfreen » Sun Aug 09, 2020 1:14 am

I have the Kent cams L14 cam and they recommend the 106/106 mop timing. I did a lot of engine modeling on the subject and found this to be the best compromise of low speed torque and peak horsepower.

I am going to play with the idle spark advance (I have a distributorless programmable ignition) and the mixture to smooth out the idle. My car also has ac, so the idle is important to be strong so it doesn't stall. On the sprint cam, if I set the idle at 1100, it would drop to 850 with the ac on. With this cam and the idle set at 1100, the ac pulls it down too much and it barely runs.
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PostPost by: rgh0 » Sun Aug 09, 2020 1:53 am

What sort of balance pipe arrangement do you have as absence of the pipe will affect the idle smoothness ?
The fact that it is harder to crank with ignition connected suggests to much advance.
I would also check the oil in the dampers in the Strombergs.

You may want to fit an idle speed increase solenoid that kicks the idle speed up up when the air con compressor clutch engages. A number of cars especially small engined ones had them.

cheers
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