Hitting a (power) wall *Tach Issue*

[1969Plus2]

After bringing back the plus 2 back from a winter hibernation / the birth of my first child, I went out for test drive and seem to be battling a bit of a strange problem...

While the twin cam fires up fine, idles smooth and seems to run well, it feels like power really hits a wall around 5k RPM. It pulls fine but seems to sort of hit a wall right there. It felt this way before I put it away for the winter / parenthood but now that I am getting some time back in the garage I am back on a mission to figure out what is going on. This is my first lotus and so I don't have much to benchmark it against but this doesn't quite seem right.

Timing seems OK (petronix distributor), new fuel filter, plugs seem like the right color. I think the tach is reading right but that is next on my to-do list.

Any thoughts from the community on where to start troubleshooting next would be very much appreciated!

[rgh0]

In second gear the engine should get very quickly to 6500 rpm with a steady continuous rev climb it certainly should not stop at 5000 rpm.

yes you need to check the tacho is accurate as with electronic systems this is often not the case

The next place I would check is the ignition timing at advance curve to ensure your are getting enough advance. The manual advance curves are somewhat confusing. Is your car Weber or Stromberg ?

If the ignition timing looks Ok and its a Stromberg engine I would then start looking at the pistons and diaphragms and springs to ensure they are fully opening and no holes

cheers
Rohan

[mikealdren]

I don't know the petronix distributor but the original had a centrifugal cut out on the rotor arm and if this failed, it could limit the revs.

[alanr]

A simple first check..
Check that when the accelerator pedal is on the floor you are actually getting full throttle at the carburettors?...adjust cable accordingly.

Alan

[1969Plus2]

OK... Feeling a little silly because I don't think that there is a "power problem" but I do think I have a tach problem...

I walked into the garage with a fresh frame of mind and went right back to basic. Fuel, air, spark. After checking mixture, timing and the throttle linkage again and all seemed right I installed a temporary, modern, tach. I wanted to be sure that the stock tach was reading right. Sure enough, it wasn't...

After going for a drive it looks like the stock tach is a tad inaccurate more generally its also jumpy and doesn't seem to want to keep up with the car. Rolling into the throttle the stock tach climbed quickly to 3k rpm, outpacing my newly installed modern one, before the needle slowed at a near standstill around 4.5k rpm. The new modern tach swept smoothy across the dial demonstrating that the engine was indeed pulling smoothly and with vigor to 6k, before I downshifted. The net of all this is that I think the old tach is just plain inaccurate and I am not really hitting a power wall.

Like I said, I feel a little silly since I thought I was still accelerating before but the tach wasn't showing as much. I generally try to trust my instruments!

So, onto the next problem. Why doesn't my tach work?

Upon closer inspection it appears that the wiring doesn't make that much sense.

Attached to the negative side of the coil is the distributor.

Attached to the positive side of the coil is 1) the distributor, 2) a 12 volt power source (green and black) and 3) a red and green wire. My Assumption is that the tach is reading off the red and green wire but I didn't think that a tach would even work if you hooked it to the positive side of a coil. Am I missing something here?

This car has been a bit of a electrical basket case since I bought it and I am slowly working through a number of issues but this one is just plain confusing.

[mbell]

If wired as original the tach sits on the line to the coil and measures the current pulses.

A petronix unit can create extra pulses and cause strange behavior if powered from the coil.

Or more likely in this case the capacitors in tach are failing due to age. Replacing them might sort it out or could go the whole house and fit a spyida rvc board to raise the original.

[1969Plus2]

Many thanks for the feedback.

Any suggestions for where I might source a new techno / replacement resistors?