Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
?fuel starvation on 26VA
Just read an interesting old PWA7C back number with a tech piece by Mark Sharpe on Coils, saying that most are NOT earthed now but early ones need to be... and how to check if they are or aren't. Is there a straight yes or no for Lucas Austin 7 coils, or is it good practice to earth 'em anyway?
(I was thinking of creating two modern press on terminal on the earth and the LT connector so I can have a second condenser set up, and it makes sense to run a bulkhead wire back to earth block if it doesnt show as a good earth already.)
Hi JonE

The Lucas A7 coils are of the commonplace type where the "cold" end of the HT winding is internally connected to one end of the LT winding, so the return current for the spark plugs flows through them to earth.  The metal casing isn't internally connected to anything, but is normally earthed anyway by virtue of the bulkhead fixing screws.

Cheers,  John Cornforth
(20-04-2019, 06:33 PM)Slack Alice Wrote: Re: condenser

I use a digital meter on the highest, in my case, 20 megohm resistance setting.

Make sure the condenser is not in circuit with anything else.

Apply the test leads to the condenser, usually to the case and the one lead or terminal that it has. 

Don't have a finger on each of the leads or you will introduce your own resistance into the circuit.

You should get either no reading, or, more usually a "kick" of a reading - say 5 megohm - which steadily decreases to zero.

Reverse the connections.   You should get a bigger "kick" which again reduces to zero.

Any reading above zero means the condenser is scrap.
Simon - this was useful. On a cheap Chinese multimeter, I can follow exactly but the reading comes down to '1' suddenly (its reads 1 when the the probes are unconnected) and stays there... on two original condensers. On a presumed new 25D4 modern, it didnt work!
On a Fluke where I can't select the maxi setting, it comes down to zero and then immediately starts hunting again, before coming back to zero.
Can I assume that both of these sound "good" despite the differences in machines?

Haven't got anything which can test the microfarads yet.

Thanks John - at least I can not worry about the coil!
I would say they are useable.

I would set the engine itself up as a tester - have no condenser connected inside the dizzy - wire/crock clip/Sellotape a lead from a condenser to the contact on the outside of the dizzy, and fix the case or the other wire to the cylinder head.   Usually by trapping a lug or a wire under a head nut.

I buy odd condensers from my local car spares shop, preferably with two wire leads, and they are handy as spares, or known good ones to test against when fixed as described above.

Note an engine will often start and run on a faulty condenser, but wont pick up because the condenser breaks down under load.   Feels like fuel starvation, but isn't.

OK, well I identified three unusable condensers.
Created a secondary condenser on the coil which I can use in event of failure.
Ordered a chinese faradmeter.
Checked fuel flow and thought that a pipe might be kinking, so redid all the hose and replaced large filter. The old one was very packed with rust, and on starting flow tests, I realised the on/off fuel button was partially blocked. Redid all that and realised that.... there was hardly ANY fuel in the tank.
So presumed it MAY have been fuel starvation!

Retested running after all that sorted and problem with pickup still. Decided to replace DK4 distributor with accuspark again. Same problem. So must be the carb.
Took off emusion block and replaced with the one I'd cleaned, refaced and left in coca cola. Bolted back up and all sorted.

Not the right thread, but the difference in the accuspark made it possible for the low speed trimming screw (on top) to actually do something for the first time. Very noticeable.
And the first run out was back to fabulous running. Felt like having a V8 - I could pootle along in third (top) round bends at low speeds.

So I'm back to fettling the DK4 to get it running as good as the accuspark!

Original problem - not quite sure why the carb started playing up at that time. Presumably was connected to user-induced low flow conditions (i.e. me being a pillock) but muck couldnt get through the filter, so could air have created some sort of lock in the emulsion perhaps?
(21-04-2019, 09:16 PM)Howard Wright Wrote: Hi Jon

I use one of these to check capacitance

AIMO M320 Digital Multimeter DMM Frequency Capacitance Meter Auto Range B1C9



my new meter turned up, but doesn't seem to work!
Do I use a 2ū or a 200n setting for the capacitance scale please? (as I'm meant to be expecting 1.4 or something)
and does it make a difference whether you put red wire to the body or the terminal??
Hi JonE

The capacitance value you would expect will be between 0.25 uF (microfarads) and 0.4 uF.  This is the same as 250 nF (nanofarads) to 400 nF.  Most meters change range automatically, if not set the maximum scale to something like 1 or 2 uF.

The polarity of the test leads will not matter.

You will only get a valid test if the coil is disconnected and the points are open, so that you are measuring the condenser alone and not other things hanging off it.

Having said all this,  a measurement with this sort of meter (which works at a fairly low signal level) may show the capacitance value to be OK but the capacitor can still be faulty.  For example it may have high/intermittent internal series resistance  or leakage or breakdown under high voltage conditions.  So it's a start - but not a conclusive test.

Cheers,  John Cornforth
Those modern clear plastic in line filters, not the ones with chrome end fittings, are designed for a pressure feed and restrict the flow on a gravity system where in your case JonE you have very little head.
I found & fitted a little  (probably old Motorbike) filter in the down pipe. It is quite early (40/50's) with a small glass bowl & a pin That screws in to shut off the flow

Forum Jump:

Users browsing this thread: 1 Guest(s)