Revs slowly return to idle

[richardb]

Hi All,
I have a 1968 Sporto with the original Webers and ignition distributor (but with the vacuum unit disconnected).

I have been tuning the carbs and am pretty confident they are correctly setup as I have a very stable, smooth idle now.

However, when I rev the car to 2,000-3,000 rpm the revs drop quite slowly back to idle. It takes 3-5 seconds for the revs to return to idle.

Is this normal or is there something else I need to check?

Thanks in advance, Richard

[1QuickS]

Two comments:

• I believe vacuum dizzy on 1968 cars was a retard feature which would slow engine speed when throttles are closed
• carbs may be running smoothly but this is not the real test, you need to set idle mixtures to "Lean Best" adjustment with STE (Type SK) Synchrometer readings of about 5. If idle mixtures are rich then you could open throttles enough to expose first progression hole which would make mixture even richer rquiring more air flow to support combustion. The rich mixture will result in a STE reading fo possibly 7 or more. A rich mixture will cause return to idle to be slow.

[richardb]

Paul,
Thanks for your reply, this is a great help.

I was using a SK type synchrometer to balance the carbs and have a consistent reading of around 8 at idle. But you're right I had to screw the two main throttle stop screws in to get the car to idle consistently.

So if I understand your message correctly then I need to adjust each of the idle screws to lean the mixture at idle more. Which screws to I need to adjust and should I turn them clockwise or anti-clockwise?

Best, Richard

[1QuickS]

Have a look at my Tech1 web page and review both "Periodic Maintenance" and "Standard Procedures". Look especially at "Lean Best Idle Mixture Tuning - Traditional Method" which is on the "Standard Procedures" page.

[richardb]

Thanks Paul, but I think I'll have to send the car to someone. The tuning procedure looks way too complicated for my skills!

[1QuickS]

OK but be sure that whoever you use is honestly capable with air cooled 911s. I had a customer who sent his car with my refurbished carbs to a "venerable" dealer in San Francisco that had been there from the 1950s. They knew everything about Porsches. He called to complain the carbs were junk but was willing to try tuning them himself. He followed the information on my website and called me two weeks later saying "I have two things to say to you: 1) The carbs are great & I am happy with them and 2) Nobody except I will ever touch my Webers again."

I have other stories that are similar, three of them from this year alone.

Actually the process is not hard, just a series of step by step procedures that when followed will empower and enable you in tuning your own engine. Every 6000 miles the tuneup interval includes changing oil, setting valves, adjusting timing, checking plugs and tweeking your Webers. Hopefully you achieve the 6000 mile interval annually!