TR2/3/3A Carburetor Rebuild

[TR3driver]

Vacuum retard was only on later cars, not a 57 TR3.

TR2-4 used 4 BTDC (not ATDC) as the factory (static) ignition timing, and then had a very aggressive centrifugal advance so the timing was advanced much more than usual as the engine came off idle. The purpose, I believe, was to make sure the timing was late enough to avoid kickback when hand cranking. With the spark occurring significantly before TDC, all it takes is for the cylinder to be full of combustible mixture and the crankshaft to be moving very slowly, to cause the cylinder to fire and force the crankshaft to turn backwards.

You'd have to check with Jeff for details, but I believe he installs a slower advance curve, such that the total advance is about the same at normal operating rpm, even starting with 16 BTDC static. (Or maybe just a pinch more, the factory total advance is fairly conservative.)

Just for grins, here is a comparison of the centrifugal curves for a couple of TR2-4 distributors, and a TR6 for comparison.

 

[SASSAMON]

Thank you Randall, with the warning about hand cranking the engine with the aggressive 16 degree BTDC timing.

 

[CJD]

Interesting. I never thought about hand cranking being the reason for the TR3's odd "knee" on the timing curve, but that makes perfect sense. It essentially acts as a switch, so timing comes in very fast after the engine is started, and then follows a more traditional curve...or rather line.

 

[ckeithjordan]

Last week in this thread I offered Randall a digital copy of the second version of the Triumph Instruction Book (the TR2 book updated with TR3 info), but my PM to him wouldn't allow an attachment. Given that the scanned PDF is over 13mb, I can't post it here either, so I've put on my website as a hyperlink and making it available to all. The link to the page is:

https://losangelesdivision.com/LA_Division/Triumph_Stuff.html

There are two versions, one that is 13+mb, another reduced-size version at 5mb. The latter's scanned pages' photos aren't as crisp, however. Your choice.

 

[TR3driver]

Thanks, Keith!

 

[carpecursusII]

I think there may be some confusion here regarding timing. Triumph called for 4 degrees BTDC static, Jeff at AD calls for anywhere from 10-14 BTDC AT IDLE* with the vacuum disconnected. It's two very distinct ways of setting the timing. I would question how well the case would run if you set 16 degrees static timing.

 

[sp53]

I agree, how it could. The timing would fall off and run out of advancement before the engine could reach its maximum RPM/road speed and the engine probably bog down.