Triumph Repair: 1976 Triumph Spitfire 1500, triumph spitfire 1500, partial vacuum

Question
Hi again Jim!
Thanks for the pointers on my last question about running very lean
(definitely related). I replaced the spark plugs and tried your suggestion of
pulling a spark plug while out to see the running condition. I am definitely still
running very lean, my only way of getting the engine to respond relatively well
(ie no hesitation between 3-4k rpm) is by pulling the choke out about halfway.  
I triple checked the timing  and set it to 10 deg. BTDC @ 800 RPM. The carb.
needle has been set as far rich as I can get it, it will not turn any farther.
My thoughts are that this is an issue with fuel delivery or the choke/throttle
plate in the carb. I had a look at the fuel filter and fuel pump which look to be
in good condition, however I didn't open either up because they look like they
have been serviced or replaced pretty recently. Could this problem be the
float bowl? I just added some fuel line cleaner to my gas tank to hopefully
show me something. Any guidance you can offer would be greatly
appreciated.

Also, these issues all began once I replaced the stock air box/filter with a
longflo foam air filter. The recirculation hose on the original box was removed
before we ever got the car. Could this have something to do with it?

Thanks so much and take care!

Answer
Hugo,

The stock paper air filter adds a bit of restriction to the airflow in to the carb, causing partial vacuum that helps lift gas into the airflow into the engine.  A low restriction air filter will cause the car to run lean.

It's also possible that the float is sticking in the float bowl, not allowing the bowl to fill completely.  Or there may be sediment blocking the hole in the needle valve, etc etc etc.

You need to make sure that any holes used in the exhaust recirculation and fuel system fume recovery are blocked, otherwise you're sucking in air that will also cause the car to run lean.

If you get through all that and it's still too lean you can either look for a richer needle, or carefully thin down your needle with fine emery paper.


Cheers,

Jim