Triumph Repair: 1975 Spitfire Timing Confusion..., static vacuum, vacuum advance

Question
QUESTION: I'm trying to arrive at the correct timing specifications for my '75 Spitfire.

In the Bentley manual, the Driver's Handbook indicates 2 degrees ATDC @700-900rpm (dynamic) and 10 degrees BTDC (static).

Also in the Bentley manual, the Workshop Manual indicates 10 degrees BTDC (static), and instructs the reader to perform static timing only.

On the triumphspitfire.com, under tuning, it specifies 10 degrees BTDC (static) and 2 degrees ATDC (dynamic). The site also specifies 14 degrees BTDC @ 1100rpm (dynamic), and recommends against static timing. It's also not clear whether the vacuum advance should be disconnected and plugged.

I've tried setting the timing at 2 degrees ATDC (dynamic), but the car runs very rough. I set it at 10 degrees (dynamic) BTDC and it's running better, but not optimally.

There is no timing data on the label under the bonnet. There's a Crane XR7000 electronic ignition installed. All other ingnition components are new.

My question is, dynamic or static, vacuum advance disconnected, and what angle TDC?

Thanks.

ANSWER: Jeff,

Keep in mind that the ignition settings are based on EVERYTHING else being within spec.  Carb condition (dashpot, needle, diaphram), carb settings (butterfly, needle), plugs, cam timing, timing chain and tensioner condition, etc.

Also keep in mind that although the factory published numbers that look like absolutes, in reality there are tolerances in all the engine components that might mean a car will run better a few degrees +/- from the "official" number.

Having said that, with the Crane installed it'll be tough to do a static timing with any accuracy.  Vacuum advance disconnected and plugged at the manifold end and 10 degrees BTDC.

Does your car have the Lucas distributor body or an earlier Delco unit?

Cheers,

Jim


---------- FOLLOW-UP ----------

QUESTION: Thanks Jim. The distributor body is Lucas. The carbs are off a '67 Spit. I guess that makes it even tougher.

Okay. So timing is not an exact science, and aftermarket modifications would cause it to drift from factory specs.

My question is this: is there a way to determine the optimal timing for a given setup? Or as long as it's running, should I move onto setting the carbs? (I've already done the valve clearances.)

Thanks again.

Answer
Jeff,

It's really a bit of an iterative process... you adjust the timing then tweak the carbs then maybe adjust the timing a bit more.

One tuning method is to put in a tank of low octane and advance the timing just to the point that it starts to ping, then back it off a few degrees.  

Do pay attention to the spark plugs as they're a good indicator for timing/mixture issues.

Cheers,

Jim