Triumph Repair: 1976 Triumph Spitfire 1500, Stromberg, Spitfire

Question
Hi Jim,
First off, thanks for all of your answers, you have a wealth of information on
these pages (and in your head!).

My Spit had been running rich when we got it (lots of black smoke and
sooting), and had a hole in the exhaust system. It had a flat spot between 3-
4000 rpm.

I replaced the exhaust pipe and muffler and checked for leaks. I replaced the
stock air cleaner with a longflo "free flow" air cleaner. I believe the car began
to run very lean at that point because i started experiencing a
backfire/spitback through the carb. I adjusted the carb (stromberg 150
single) mixture about a half rotation clockwise and reset the idle screws which
helped but didn't eliminate it. I then checked the timing which was way off
and reset it to 10 degrees BTDC (I believe, but am not sure, that I have a lucas
breakerless distributor, so I left the vacuum advance hooked up as per the
haynes manual). This helped a lot, however this morning (after a cold night)
when I started it up, if I blipped the throttle it backfired through the carb and
shudders when I apply throttle with no choke. I set the choke at half way and
took it out for a drive, and the car ran beautifully, but when I came back and
checked again with the choke off, more backfiring.

Any clue if this is symptomatic of a still lean mixture? Is this longflo free flow
air filter thing not providing enough resistance?  I sprayed the intake manifold
with carb cleaner to check vacuum but heard no change in rpms.

Any and all help would be greatly appreciated, thanks so much again!

-Hugo

Answer
Hugo,

Yes, it sounds like the mixture it still off.  Have you made sure the carb dashpot is full?  Just needs to be within about 1" from the top of the opening.

Have you tried pulling a spark plug (or two) to see how they look?   Bright white insulator is a sign of a too lean mixture.  

To see how the car is running on the road one thing you can try is a plug cut.   Find a nice stretch of road with little traffic.  Run the car at a constant speed for minute or two then just shut off the ignition and coast to the side.   Let the engine cool for a couple minutes then carefully pull a spark plug.   That'll show how the running mixture is.

Cheers,

Jim