[jackal]

Bottom main bearing of my Tecumseh lawn mower engine developed excessive wear (ran out of oil!). Have stripped engine and made up a new bush.  Unfortunately cam shaft fell out prior to re-assembly.  Camshaft has a punch mark on timing gear but no equivalent on crankshaft.  I've tried at both possible top dead centre position but won't run - seems to be firing too early.

Any info out there on how to re-time the valves relative to top dead centre please?

Alternatively, any sources for a spare engine (marked 0277 LAVR35-1A/C114M   8703714)?
Jackal

[Gwiz]

there is normally a mark of some discription on these.
on some it's a punch mark, others have a line scored on it, and if memory serves ( it does every now and again ), the drive ring on the crank has one of the teeth chamfered on it's edge.
Hope that helps!

[Gwiz]

Just as an after thought......
The timing ring that fits on the crankshaft is often of a type that slides off, but is keyed on.
If you've taken it off when doing the main bearing, have you put it on upside down, so you can't now see the timing mark?

Just a thought.
done it myself......

[Gwiz]

cor, it's gone very quiet in here........

[jackal]

Thanks for your advice.  There is a timing mark on the camshaft gear but absolutely nothing I can detect on the crankshaft gear - and I didn't take it off and turn it over!  I eventually got it going from first principles i.e. I decided at what point in the cycle the inlet valve should be just opening and the exhaust shutting and inserted the camshaft to suit. The timing mark is lagging top dead centre by about(?) six teeth (should counted for future reference but didn't).  Don't understand it but it runs!!!!!

Thanks again,
Jackal

[Larkshall]

From memory of motor-cycle engines (many years ago) there some variables involved in timing. Generally speaking valves need a slight overlap between exhaust closing and inlet opening. This should occur at top dead centre on the non-firing TDC. Ignition timing should be several degrees before the firing top dead centre (this will vary according to the length of stroke in relation to the bore). A "Square" engine (stroke and bore the same) will require less advance than a "Long Stroke" engine (where the stroke is longer than the bore). This is to allow for the delay in the gas mixture burning. No! petrol mixture does not explode, it burns rapidly. Diesel oil does explode, as it reaches ignition temperature by compression and then all of it fires at the same time, that's where the diesel "Knock" comes from.

Of course, we are talking about 4 stroke engines, as 2 stroke engines do not have "valves" but have "ports" in the cylinder walls which do the same job and therefore are fixed but ignition timing is critical, although most industrial type engines have fixed ignition timing.

[jackal]

Thanks for the advice based on motorcycle engines.  So far the machine has continued to run well.  Must have got it right after all!
Jackal