'funny' ignition timing

[DevonAid]

Finally connected a timing gun to our T25 in a bid to sort the very big flat spot we get when pulling away (always makes pulling away from roundabouts a bit of fun).

TIming was close but tweaked the distributor to get it spot on. However, even the most tiny increase in revs seemed to make the timing leap back (greater advance) with no amount of progressive change notice - just a big step. Not sure what this equates to in degrees but it's about 25mm on the pulley wheel.

Checked the mechanical advance springs by twisting the rotor arm - plenty of spring and spring return there. Checked the vacuum advance with suction on the tube to the distributor - plenty of resistance the and distributor plate moves.

I'm not sure if this is giving the flat spot problem or is normal. All the other possibilties (full service, check/clean/replace HT ignition parts, clean out carburettor, check/confirm all vacumm lines are servicealbe etc) have been checked out as OK.

The only other possibilities are:

1: Is 5 deg ATDC with vacuum advance about right for a DF engine (although I only have a single line on the pulley)
2: small amount of exhasust leak, adding to 'popping' from exhaust on overrun.

Any thoughts greatly appreciated - cheers!

[kevtherev]

By the sound of it you have an inlet side vacuum leak.
As flat spots and backfiring on the overun are classic symptoms
If the timing is correct I would be checking for leaks/renewing vacuum hoses, including gaskets and the brake servo hose which is a common culprit.

other reasons for lack of power are
fuel pump fault
incorrect timing
fuel filter choked
At worst back firing is from burnt valves

[DevonAid]

Thanks for the response.
Well I think I've check the vacuum hose side of things carefully including changing the servo hose, carried out a carburettor base gasket test (spraying WD40 at the base with engine running, no change in engine note), fuel filters are new and fuel pump reasonably new.

I don't like the sound of burnt valves, although as far as I know the 120,000 miles is on the original engine - perhaps a compression test is in order, but another check of vacuum hoses might be a good idea as clearly that is an easier fix if I do find a problem.

Just a thought, has anyone tried different timing on a DF - I've never had a vehicle with timing 5 degrees ATDC - seems rather 'late' ?.

[kevtherev]

For your information

5°±1° ATDC (with crankcase breather disconnected) with vacuum.
Ignition advance checks ..
0°-2° @ 1500RPM without vacuum and basic timing
6°-10° @ 2400RPM without vacuum and basic timing
20-24 @3800RPM without vacuum and basic timing

vacuum advance range 14°-18° engine degrees

compression pressure 10-13 bar (150 psi-195psi)

[DevonAid]

Very useful numbers, more detail than Mr Haynes. Gives me something to use on the next check over to get to the bottom of this.

Many thanks

[Miffanwee]

I found a T-bar connector with a missing connection in the middle of my vacuum advance cable, so it was sucking air in and not advancing properly. Some GIT from history had advanced the dizzy +15 degree to get the damn thing to run.....
Obviously fuel consumption was a problem. When I found it....I plugged the hole and retarded the dizzy to correct position and she drives like a dream.
Still don't know what the vacuum advance t-bar connection was supposed to go to!

[California Dreamin]

Worth having a close look at the accelerator pump (It should be giving a clean diffused spray and not dribbling)

Martin