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Compression test?

You MUST have a condener across the points -and, because this component is so important, I never have an engine without a newish spare ready to fit.

Usually this is any condenser my local spares shop has in stock with a lead on it - the lead will go to the dizzy, and the screw tag nips under a head stud washer.
Hi simon

I have electric timing that is triggered by a sensor , my understanding of a condensers purpose was to stop false firing at the gap in points by regulating or blocking fluctuations in the voltage ?

So thus the condenser is mute in my configuration?

Happy to be corrected if this fixes my issues!
Oh, right, OK - I don't know anything about this new-fangled stuff Rolleyes
Are you sure that it is timed correctly? Check that when No1 cylinder is on compression the rotor arm is pointing to the correct plug lead. Also ensure the firing order is correct.
With electronic ignition there should be no condenser.
Two things come to mind. 

This first is that, whilst you say you have a spark at each plug at the right time, the spark may be degrading after a few revolutions. However, I imagine this is not the case since you have a new fangled electronic system. All the same it might be worth fitting your ignition to a friend's Seven to prove it is good.

The second is sticking valves. You mentioned you've fitted new springs but that the engine has sat for some time. When I had low compressions, similar to what you have on all four cylinders, on a couple of mine the cause turned out to be sticky valves. My engine had double valve springs but had not run properly since I got the car.

Good luck!
Thanks Colin,

Other than hand turning and oiling the valve train is there anything you did to release and stop the sticking valves ?

Hi Steven
I removed the valves and cleaned the valve guides with a special wire brush that fits in an electric drill. They come as a set of different sizes and comprise about ten pieces of wire held in an arc surrounding and in line with the central spindle.
I can't remember now what they're called or where I got them from, but Nick Turley put me onto them.
I would describe the effect they have on the guide bores as "burnishing".
If Nick sees this he may enlighten you. I shall be back in the garage tomorrow and will post a photo.
I think the fit you're looking for is to be able to shift the lightly oiled valve up and down with your little finger. No slop but no tightness anywhere.
Newfangled electronic systems don’t just get worse as the voltage decreases, they suddenly stop- as I’ve found out more than once on my Ulster with a total loss system. It might be that the voltage drop with the stater is enough that the electronics don’t work. Try a push start.
Alan Fairless
Searching eBay for "Valve guide brush" brought up this, which is what I have got, but where to get them from new I don't know I'm afraid.

Found 'em!
(13-01-2020, 07:32 PM)stevenm Wrote: Hi Chris, 

Hope you dont mind im having some trouble getting a 1928 austin 750 engine to start. 

What would be the reading for a cold compression test ? 

Im showing 50 50 48 50 1 through 4. Im failing to get more than a puff puff and some exhaust gasses so somethings happening but i'm failing to get her running.

Ive checked spark ( good converted to 12 v) so strong spark all plugs.

I have a brass updraft carb ( zenith 22 ) and i'm getting suck at the air inlet. No idea if the carb is good or how the mix is controlled but theres some fume smell in the chambers but not a lot. i worked my way from the chamber through manifolds and sealed up all leaks.

Ive adjusted timing right round and appears quite retarded is best setting currently but again nothing more than puffs smoke and as i cant run her 10 minutes to check hot compression wondered what would be expected, seems very very low to myself.

The standard Austin 7 Compression Ratios are-

1923-1933 4.8 (or 4.9) to 1

1933-1936 5.2 ? to 1

1936- 1939 6.0 (or 6.2?) to 1

Compression Pressures are between 17 and 20 times the Compression Ratio in a good engine giving :

4.8 to 1 = 80 to 96 psi

5.2 to 1 = 88 to 104 psi

6.0 to 1 = 102 to 120 psi   

Hand cranking with a 'dry' engine and throttle open you should get at least 80 psi from a standard early head (single bolt water manifold) and 100 psi from the 1936 head (two stud water manifold) if rings and valves are good.

All cylinders should be within 5 psi.


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