• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

HS SU carb problem

Share this page

A

Alex_McMillin

Senior Member
Country flag
Offline
I am having trouble tuning a pair of rebuit SU carbs I have recently installed on my TR4. When I start the car the rpms are very high (2500) and stays that way. I can't figure out why. I have done what I think are all the basics; centeralizing jet, set float level, etc., but I can't get the idle to come down. I have tried adjusting the the jets but it doesn't seem to make a really big difference. The exhaust smoke seems alright. The forward carb has lots of suctions, more than the rear. When I put my fingers over the air inlet the engine speed drops but the exhaust smoke increases A LOT. I would appreciate any words of wisdom or hints anyone might have.

........Cheers, Alex
 
Hello Alex,
this sounds like the throttle spindles are not closing or the throttle discs have not been set correctly. For the first, look at the linkage, including the choke high idle screw, which should not touch the throttle when the choke is fully home (i.e. the jet fully up.) For the latter you will need to remove the carburettor and look carefully through the choke with the throttle fully closed. You should see virtually no light. If you do slacken the two clamp screws and adjust the disc in the spindle to get the best fit.

Alec
 
Alex,
Sounds like a vacuum leak. Since these carbs were recently re-built, make sure the throttle plate on the carbs fully seats when closed. Check any vacuum hoses, or lines, for leaks. You might also check for a manifold leak, where the manifold attaches to the head and where the carbs attach to the manifold. With the engine running, you can try squirting carb cleaner around the manifold and also around each bushing on the carbs. If there's a leak, it should alter the engine note.
 
Alex: I agree with Alec as to the most likely culprit. If the discs are fully seated then there must be a vacuum leak somewhere.
What is your PCV setup? There may be a leak there. Are the HS6 the exact type used on some TR4s or are they originally from some other application, such as a Volvo, that do not use a Smiths PCV, but that connect the crankcase breather directly to a port on the back side of the carb body? If these are present but not used (or plugged) they could be a source of leakage.
 
Back
Top