Triumph Repair: Spitfire 1500 speed, stromberg carburetor, vacuum gauge

Question
QUESTION: Hi, Just wondering if there's a guide anywhere that goes through things to check to ensure a standard 1500 spitfire is achieving its potential (have searched online without success). My 1976 UK 1500 Spitfire will do 65mph and no more. Speed isnt everything i know but i am interested in why it wont go faster!
Tappets checked today and set to 10thou;
Compression test completed today - 160,165,165, and 175 - so seems ok to me;
Battery is new;
Points and timing were checked 12months ago...
Fuel - usually supermarket unleaded with Castrol octane booster.
Air Filters new;
Plugs new;

Any thoughts would be appreciated.
Many thanks
Andy

ANSWER: Hi Andy,
65 MPH is too slow even for the US version and the UK version should run much faster. We had one Stromberg carburetor and I understood that the UK version had two SU carbs. Is that correct?

The compression test is ok but you can't count on a year old timing so set the points and check the timing.

Another test you need to run is a vacuum test while at 65 MPH. Put a vacuum gauge under a wiper arm so you can read it at speed and run a long hose under the hood (bonnet) and connect it to straight manifold vacuum and if the vacuum is less than 5 in of vacuum full throttle full speed you have a problem. My first thought would be a partially stopped up exhaust system. Next if that is not the case I would check the lift of each cam lobe and then check cam timing.

What RPM are you turning at 65 MPH? You don't have an unusual low gearing do you?
Something is wrong as it should run faster than that.
Howard

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

QUESTION: Thanks for your reply Howard; 2 SUs is correct. I will get the points set and check the timing. I will have a think about the vacuum test - i haven't got the necessary kit and am very much an amateur in all this but have the manual (for example i am not sure whether you mean that checking the vacuum is related to the partially stopped up exhaust (sorry i'm just not mechanically minded enough!).
How easy is it to check the cam lobe lift and timing??

I'll have a look at the rev counter next time i'm upto 65mph.

Dont think the gearing is anything other than standard.

Your advice is appreciated!

Andy

Answer
Yes Andy, a manifold vacuum test is a preliminary test of a stopped up exhaust system but must be done at the speed that the car levels off at and will not go any faster. All street engines when pushed to max throttle under max load (your 65 MPH) will maintain higher vacuum than 5 in. ALL engines that have a partial stopped up exhaust will at that time go to or close to -0- vacuum.

Even though some other things can cause the -0- reading like cam lobes worn down or incorrect cam timing, a stopped up exhaust is the more common.

A vacuum gauge is not expensive and available at any auto parts store. The same with extra hose.
It is a tool that can be used on any gasoline engine.

There is a 100% test for a stopped up exhaust and that is to open the exhaust pipe before any muffler or CAT. On some cars that is not easy so the next method is to take a short piece of brake line (about two inches long) and drill a hole just a little smaller than the diameter of the brake line piece and drive it into the pipe and put a hose on that pipe and run it up to a low pressure gauge and put that gauge under a wiper arm and drive the car and watch the gauge. (most vacuum gauges have a section for low pressure testing for fuel pressure so one gauge serves two jobs.) At full throttle, full load the pressure must not exceed 1.5 PSI. A partially stopped exhaust will quickly go to above 5 to 7 PSI. This is more work but is a 100% test.

You can guess at what is wrong but I always found that testing is the ONLY way to correct problems.
Howard