Vacuum leak testing question

Scott R · Scott R's Garage

I went to my mechanic yesterday to have the mixture set on my 80sc and we could not set it, the HC's were almost off the meter, and when the mix was set it would just pop and miss. I went home to start checking for vacuum leaks and I think I have one, however I thought the idle speed should increase when I spray carb cleaner?!? I can actually start to drag the engine down while I am spraying around my new airbox, is this normal?

This is after a complete CIS rebuild BTW, air box and all associated parts and seals.

Grady Clay · 03-25-2006, 12:48 PM

Scott,

I will speculate the mixture is already very rich even with the vacuum leak so replacing air with carb cleaner hydrocarbons simply makes it richer yet. Usually a small vacuum leak causes a lean condition (popping & such) and the carb cleaner brings the mixture closer to proper and the idle speeds up.

Why is it so rich? Did you adjust the mixture? Why was it apart? Was it adjusted prior to the disassembly to compensate for some condition?

If the idle changes at all – there is a vacuum leak.

I would systematically look at every connection and check every fastener. WD-40 is slightly less explosive to test with and may tend to seal a small leak for a few minutes. At the extreme, pull it all back apart.

I hope you are doing this outside on this beautiful Colorado day and have a helper standing by with a BIG fire extinguisher. We don’t want to see you, your nice SC or your great garage up in flames!

Best,
Grady

Scott R · Scott R's Garage

Quote:

Originally posted by Grady Clay
Scott,

I will speculate the mixture is already very rich even with the vacuum leak so replacing air with carb cleaner hydrocarbons simply makes it richer yet. Usually a small vacuum leak causes a lean condition (popping & such) and the carb cleaner brings the mixture closer to proper and the idle speeds up.

Why is it so rich? Did you adjust the mixture? Why was it apart? Was it adjusted prior to the disassembly to compensate for some condition?

If the idle changes at all – there is a vacuum leak.

I would systematically look at every connection and check every fastener. WD-40 is slightly less explosive to test with and may tend to seal a small leak for a few minutes. At the extreme, pull it all back apart.

I hope you are doing this outside on this beautiful Colorado day and have a helper standing by with a BIG fire extinguisher. We don’t want to see you, your nice SC or your great garage up in flames!

Best,
Grady

The initial issue was a cracked air box that the PO had repaired with epoxy, well over the years the epoxy became brittle and the box blew out again.

So after I rebuilt and replaced the system I set the mixture just enough to get me down to Eurosport for an adjust, that's when we found the HC's at around 18,000.

Scott R · Scott R's Garage

Ok, well I spent 5 or so hours on this today without success, I seem to have a leak all around the throttlebody, however I have removed and reseated it twice now. I had a leak at the break booster connection so I took the metal fitting out and used some plumbers tape on it, that one seems to be gone. The thing still won't run right, not that it ever has for that matter.

Might be time to throw in the towell, I guess it was a bad idea to assume that I could do any better than my mechanic on this, he was never able to get it wokring either.