Intermittent rev drop on TC

[dunclentdr]

Still in the process of getting my early Plus 2 working well after previous owner used it little over 5 years. Back from suspension and brake rebuild today.

Previously I have had new Webers fitted as originals in poor condition and running very poorly. These new ones were fitted by Aldon and rolling road showed about 100bhp.

Now able to road test properly and not running as I hoped. Under acceleration (but also normal running) there seems to be an intermittent stumble where the revs drop rapidly, the car hesitates and then normal power resumed. The rev counter drops rapidly (as through it has lost power) and jumps back up.

My working hypothesis is that this is not a carb/fuel problem but maybe an electrical, maybe faulty earth problem.

Can anyone advise please?

Thanks

David

[mbell]

What is the history on the ignition system?

Have you changed dizzy cap, rotor arm, leads, plugs, coil? If so what parts were used. There been some quality issues with some replacement parts, especially rotor arms/dizzy caps.

Does it happen at a consistent rev range? If so what rev range? I presume the carbs were jetted roughly inline with Lotus recommendations?

[dunclentdr]

Doesn't occur in the same rev range - car has electronic ignition which was checked over by Aldon and considered to be working OK.

I'm trying to work out whether to get the car back to them to be sorted or whether it might be another problem.

[Hawksfield]

Hi Dunc

Sounds definitely an electrical problem of an intermittent nature, check all connections as the tacho is seeing loss of power.

[rgh0]

Do you still have the original current sensing tacho and is the aldon unit wired from the coil positive or from an independent power supply?

[dunclentdr]

Not sure! Could you advise on how to find the answer to the questions please...

[mbell]

Look for the positive (red) and earth wiring coming from the electronic ignition unit inside the dizzy. The earth wire should be connected to the coil. What is the red wire connected to?

It should be connected to a 12V source. Often people connect it to the 12V side of the coil as it is easily accessible and tidy wiring wise. However if the tacho hasn't been modified from RVI (current sense) to RVC (voltage sense), the tacho can pick up current fluctuation from the supply side of the electronic ignition that it registers as a current fluctuation cause by a cylinder firing. This causes the tacho to read incorrectly.

Often this can be avoid if the 12v supply to the electronic ignition is on a separate power feed and not the positive side of the coil. The true fix is to get the tacho converted from RVI to RVC but this normally means sending it away.

Some good background at:
http://www.lotuselan.net/wiki/Bouncing_Tach

All this will only deal with an inaccurate tacho it won't help with any real hesitations from the engine. Did the rest of the ignition system receive a refresh at the time of the elec ignition?

[dunclentdr]

Thanks - will check it out.

The tacho doesn't bounce around - rather it drops rapidly as the engine hesitates and then comes straight back.

Electronic ignition was done by PO but assume was checked when carbs were done and rolling road test. May be best to take it back to them...