Fourth Sorrowful Mystery

[dgently]

Trying to sort a recent intermittent problem with rough running. Sometimes smooth, sometimes very rough.

Whipped out my disty
Confirmed firing order 1-3-4-2 (anticlockwise on cap)
Engine to TDC (timing mark on crank, #4 cam lobe under oil filler pointing in, #1 piston visible just below plughole)
Reinsert disty with rotor @ 10 o'clock
ignition on, twist disty and fix where #1 plug fires
Start engine
Rotate disty to where it idles best (this is quite a way clockwise)
Let it warm up
Timing light on #1 lead, engine revved to 2000, crank mark steady @ due north, 12 o'clock, must be over 30 degrees btdc!

How could that be where my engine runs best? It has always been in or around 10 BTDC. Has my timing chain jumped a tooth?

It is running quite smooth, but was before too and then went very rough, esp under load.

Any and all advice welcome!

[SADLOTUS]

To me, intermittent sometimes rough sometimes smooth means something breaking down - maybe a crack in the dissy cap, a dodgey plug, leads shorting. Any of those things give up the ghost quickly under load if they're feeling tired out.

If the rough running is at a specific rpm I'd put it down to a more mechanical problem eg points timing, valve timing, or an obvious failing item eg plug/cap.

This forum can be a help or it can state the obvious, sometimes the original poster just needs a spark of an idea to send him(her) off in the right direction. Clean everything, wipe everything. Good luck.

If i think of anything else...
Paul

Just reread your post -
'Start engine
Rotate disty to where it idles best (this is quite a way clockwise)
Let it warm up '

should you do that? don't rotate the distributer, let it warm up. if the timing has always been 10btdc leave it there.

[dgently]

Just reread your post -
'Start engine
Rotate disty to where it idles best (this is quite a way clockwise)
Let it warm up '

should you do that? don't rotate the distributer, let it warm up. if the timing has always been 10btdc leave it there.

THanks Paul. Cap, rotor and plugs are new. Leads are the last suspect.
When rotating the distributor, there's quite a narrow band where it will start/run at all, and my practice is to find it and err on the "retarded" side of it.

What is flummoxing me is that that "sweet" spot is (according to the timing mark on the crank pulley) 30 degrees plus btdc. I know that can't be right, but I can't understand why that's where is engine is happiest now.

[paddy]

Is the timing advancing correctly with engine speed?

Also, I seem to remember reading somewhere about crank pulleys from some other engine variant on which the timing mark was in a different place (maybe out by 10 degrees). Are you sure the timing mark is accurate?

Paddy

[Hawksfield]

Are you saying it will not run at 10-12 degrees btdc, if this is the case then something is not right mechanically.

Check every thing, TDC mark against piston at TDC, Camshaft timing marks at TDC.

[stuartgb100]

I would start with that timing mark on the pulley.

You really need a dial guage set over no.1 piston and bring it up to tdc on the guage.
Then check the alignment of marks on pulley and cover.

Regards,
Stuart.