I have always had a problem with an erratic rpm reading on my car since it was built on carbs and its still there on efi too, it's especially a problem when over 4000rpm. The RPM reading will start jumping around all over the place (usually jumping higher and off the scale).

I thought it might be something to do with my rev limiter, so disconnected that and no difference.

I get the same readings with megasquirt and on my dash (both are connected to the coil for input). I also got the same readings on my old acewell dash before it was changed to the AIM one.

I have fitted a supressor to the coil which has made no difference.

Its beginning to bug me, and is making getting a nice smooth map difficult too.

Any ideas?

David

Is there anywhere else you could take the rev reading from?

Could be that its not a clean feed.

I know nothing about the systems you are using just an idea and worth a try.

I have similar prob with my acewell.
I managed to improve it by fitting shielded wire through the engine bay for the signal wire.
cheers

I have sheilded wire to my megasquirt. Could try fitting shielded wire to my dash too...

Does the engine sound okay or does it sound like the rpm is changing???

Talking split second rpm jumps. Engine sounds ok as far as I can tell.

David

What parts are common to the engine running carbs and EFI? Might pay to check them?

The only parts of the ignition which are common are the dizzy and coil. Everything else has been changed at some point including the wiring to the dash. Dizzy and coil were both new from H&H ignitions, electronic ignition and dry coil (cossie style).

It does seem a bit better after removing the rev limiter, most probably as the spikes were playing havoc with that....

Possible fault with distributor or coil perhaps? May be the ignition module?. Can you use the Megasquirt to run an Edis unit and coil pack?

I had a similar problem a while back with an ETB Tacho and it was the earth to the Tacho not properly fixed, could be something similar?

Are you running resistor spark plugs?
Non resistive ones can cause large voltage spikes

No I am not, I am running NGK AP7FS plugs. That could be part of the problem then...

I could try getting some resistor ones.

Cheers,
David

Right I have at least partly solved the problem. Turned out the rotor arm in the dizzy wasnt a very good fit on the key and was wobbling around!

Only trouble is the new one I have just fitted isnt much better! Seems the key in the rotor arm is just too small for the cut out in the dizzy shaft which seems very odd...

David

can you not pack it out so it is a secure fit?

my first thoughts were a dodgy coil that is playing up at higher revs but think you would be able to tell from other engine problems.....

sheilded cable to tacho could be wirth a try although mine isnt sheilded in the slightest and i dont get a problem....

I am using resistor type plugs with my pinto if that makes any difference.........

I have been in touch with H&H who supplied the dizzy and they informed me that many of the non OE rotor arms are a poor fit and to try and source a bosch one or a Facet one.

So will see what I can find. Would much rather it be right than bodged!

David