what does an erratic vacuum gauge reading mean?

[prof_rik]

I apologise for the naive question. My '95 S3 was running very poorly - symptoms: irregular idle, delay in increase in rpm (and consequently power) when opening throttle - I thought it was a carburettor problem so cleaned them as much as I know how (which is quite limited - check jets/needle) but it doesn't seem to have changed anything. I used a vacuum gauge and the needle's are extremely erratic (flicks rapidly between 0 and 25 in Hg) at idle (~1000rpm) (for all 3 cylinders). Only when I open the throttle does the needle hold steady at 5in. What's up with that? A search on the net suggests the valve guides may be worn. Is this correct? I was hoping that I simply have to tweak something in the carburettor (sync?) and all would be good. Any feedback would be much appreciated.

rik

[Martin_R]

You have the restrictor pieces (smaller hose pieces with only a needlepoint sized opening) on the vacuum gauge hoses, right?

At least on the CarbTune without the restrictors the reading will be very erratic. Also I have found the bar type vacuum gauges much easier to read than the needle gauges.

[John21]

Believe this or not, you are more liley to have a coil problem than a carb problem. Mine did the near same thing with eratic idle and poor low speed performance. I replaced with Nology coils and cured everything (after spending $350 in the wrong areas first).........John

[prof_rik]

Thank you very much for the feedback - I thought I must have been doing something simple wrong - I think Martin has the solution - there were some extra assorting tubing/fittings that I didn't use when attaching the gauges - I'll bet that's where I went wrong and underlies the erratic needles. And to John - I've heard of the dodgy coils - particularly on the '95 model so once I've figured out the vacuum gauge I might swap coils around and see if my rough idling is due to a particular coil - I'd be surprised if they all went bad at the same time but it sounds like if I'm going to replace one I'm better off replacing them all. Once again - thanks for the helpful feedback. rik

[Major Kong]

prof,
You may want to check the condition of the carb rubbers (airbox to carb and carb to head). If there is a leak there, it could also cause a poor idle. You should also check the vacuum line running from the carb slide vacuum and the inter carb rubber for cracks. This could really set off the opening of the slides.

Is this a recent problem? Are the carbs stock? Has someone installed aftermarket needles or jets? Did you check float hight? If the floats are set wrong, it will run poorly.

You also should have a small restrictor adjustment in the line for the gauges to help damp out intake occilations and provide less needle hopping.

Godspeed,
Kong