Pre-war timing question

[Brucegirvan]

I have read a few of the twin timing threads, and I am aware of the sensitivity (given last weekend Tappet woes with Eddie) so I thought I’d check VO (1930 Long 2) engine whilst the car is still in bits. I found that inlet valve is opening before the flywheel indicates TDC. I realise as I type this that I do need to confirm flywheel TDC is TDC, but let’s assume it is. Then given the images below do I need to reset the cam timing? And if so, what’s the advice on how to do so? The timing dots on the cam and crank gears line up fairly well…I think.

Eager to learn from you masters.

EB0D0E2E-1B02-4CD9-976A-85E107DF01D1.jpeg

0EEA7649-B4B6-4F7A-B446-6207CC02420D.jpeg

[Keith Clements]

It is normal for inlet to open before TDC so as to be ready for suction about to be created when piston descends. The degree of overlap would depend on design rev limit goal for maximum power . Javelin is set at 13 thou lift at 12 deg before TDC.
The exhaust valve also closes after the inlet valve opens to increase the suction using the momentum of the expelling exhaust charge. A bit like the suction you get in the Underground when a train leaves the station. This also makes sure all the spent exhaust is replaced by the incoming new charge which pushes it out. The design of the combustion chamber is critical in creating the swirl to improve this replacement.

[k. rogers]

It seems to vary from engine to engine, but I'm quite sure that on the 7s the inlet should be just opening at TDC, certainly that's the case on pre war so can't imagine any different for vintage. I believe it shows this in the handbooks.

[k. rogers]

20220405_123726.jpg

[k. rogers]

The above was taken from the 1930 handbook. Please note the tappet clearances for setting up.

[Keith Clements]

Good one Ken. So the inlet is open by 6 thou at TDC when properly set up. Not as much as Javelin which is 10 thou at 12 deg before TDC since tappet clearance is 3 thou. The 13 thou setting is using a dial indicator with zero tappet clearance and measuring on push rod.

[Brucegirvan]

And the tech manual indicates running clearance of 0.004 for a 1930. However, is the timing still set on 0.002 as per 1929 or also 4 thou?

247F9A6F-BB2C-41BA-AEE0-84E19494B788.png

[Keith Clements]

Any instruction written in the 1920s about engine set up has to be viewed with consideration of the many things which might have changed. Knowledge of combustion chamber fluid dynamics has advanced, parts such as the camshaft may have worn or indeed been modified, petrol has changed considerably in volatility, and road conditions have changed. I doubt if many vintage Jowetts have been through a tuning session on a dynamometer to see what changes could improve performance.

[k. rogers]

Strangely, they've missed that out on that table, however, if you look at the diagram I posted it states 10 thou for setting up then reduce to 4 thou for running.

[k. rogers]

Before doing anything drastic I would set the clearances up at the required 10 thou and ascertain that the inlet valves are indeed just opening. You may well find that it is all tickety boo!

[Tony Fearn]

Hello Bruce and Ken.
I agree with Ken before drastic action is taken.
I've always thought that the timing of the 7hp pre-war engine belongs in 'The Dark Arts' cupboard,
especially as there are two positions for the key on the front end of the crankshaft where the sprocket fits, and
three choices on the camshaft sprocket, which taken together alter things by a degree or two.
Nevertheless, I remember some time ago that this was discussed here on JowettTalk.
Ben Hicks penned a missive which KC posted, and if you want some bed-time reading, here it is again.

viewtopic.php?p=21408&hilit=timing#p21408

Tony.

[Brucegirvan]

Thank you, Ken. I thought I’d ask. I like your valve timing chart. I’ll try 0.010” and check as I don’t want to change what’s “good”. When you tried this did you find much difference between inlet valves on cylinder 1 & 2? Ie if set ok for cylinder 1 is 2 also spot on or is markedly different? Ie are the camshafts pretty good?

Keith raises an interesting point - I wonder how many 7/17s have been worked on a dyno? Would be fun to try. From my experience, the advance and retard makes some difference but is quite subtle….more ear and feel than raw power.

Really appreciate your information. I’m in Coventry this week, so will take a few days before I can test this out.

[Brucegirvan]

The Ben Hicks description is really useful.

[Keith Clements]

The Search Server finds the article in the 1983-05 Jowetteer as its 7th result using search word 'camshaft'.
Pick up the original copy in viewtopic.php?p=24944#p24944

You can search 60 years of Jowett knowledge using the search server. There is only 20 years of Jowett knowledge using the JowettTalk search.

[Tony Fearn]

The Ben Hicks description is really useful.

Hello again.
A number of the crankshaft pulley keys that I have seen are as below in the sketch.
I suppose that when turned through 180 degrees the timing might alter ever so slightly.
They fit into the hole on the front end of the crankshaft, but the top-half is only half a circle, and this fits into the keyway of the sprocket.

Crankshaft key.jpg

Does this muddy the waters?
Tony.