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Triumph Repair/Weber Carb- 79 Spitfire

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Question
While rebuilding a Weber 32/36 DGV 5A carb, I found conflicting info on locations for secondary & primary parts so I guessed & used the most popular info. I assembled as follows:  Idle Jet (55-Primary, 50-Secondary).  Main Jet (135-Primary, 140-Secondary).  Air Jet (160-Prim, 165-Sec).  Emulsion Tubes (F50-Prim, F6-Sec).

Car starts & runs pretty good except minor flat spot on wide open throttle. Does it sound like any of thoses parts are in the incorrect location?

Thanks y'all & Merry Christmas!

Answer
Hi Steve,

The listing I have for a Weber 32/36 DGV 5a is for a 1599 cc Capri GT. Which does seem to be usable on a 1500 Spitfire. All the listed jets you quote are correct for that carburetor. (providing the engine is not modified)

If you have raised the compression ratio or put in a cam or even opened up the exhaust you may need to rejet. I found from building and tuning race engines, that you should start with large main jet and come down toward lean to a correct jet. Then move down to the lower speed jetting and do the same (start at too rich and work toward correct jetting)

The reason you start with the main jet is that the restriction of fuel in the carburetor at lower throttle settings is not the main jet. The main jet is normally only the metering or restriction at 3/4 to full throttle. On most carburetors you can remove the main jet and put it in your pocket and run the engine at lower RPM ranges and never note a problem.  

Weber jets are expensive. If you have the intake manifold and carb as a kit to go on a Spitfire, the jets are probably correct.

Ever since removal of lead form gasoline it is more difficult to take plug readings for mixture adjustments at high throttle settings, but it can be still done. It is better to have CO equipment but not many have that.

So what can be done is to find a long straight road with no side roads or homes and make a full throttle run (You can use third gear) and turn off the ignition at the same time you back off the throttle. (Be sure not to go into the lock position and lock the steering) Stop as quickly as possible (remember that you may also loose braking without the engine if your car has a brake booster) Be sure to operate the engine enough to get operating temp on the oil as well as coolant and be sure to use the correct heat range spark plug. And start the test with clean plugs or new plugs.

Pull several plugs and note the color of the center insulator of the plug. It should be only slightly colored. Not clean and white and not flat black coated. the rim of the plug should be colored flat black. A clean insulator and a clean rim is way too lean. A clean insulator and slightly colored rim is just too lean. A flat black insulator and black rim is too rich.

Let me know,

Howard  

Triumph Repair

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Howard M. Fitzcharles III

Expertise

Triumph TR-4 up & Spitfire, and Engine theory

Experience

Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Organizations
Associate member SAE EAA member

Publications
Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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