A CIS Air Box Cautionary Tale

HarryD · 03:23 PM

A few weeks ago, I went to start my CIS car and there was a loud boom followed by my being unable to start the car. Knowing I had a flapper inthe air box, I was sure I did not blow up my air box but something was not right.

I opened the engine room and saw that one of my 30+ year old rubber straps holding the air cleaner cover to the box had broken and was askew. I removed the cover and saw the flapper vavle lying next to the hole it formerly occupied.

I pushed the valve back into the hole and used some 3M Blue Tape to hold hte air cleaner in place and started the car. It started without a hitch.

When I got home, I cleaned up the old epoxy, cleaned the area with carb cleaner, and used JB Weld to glue the valve back into place. I ordered two replacement straps and installed them as well.

After waiting 2 days, I started the car for the first time. I noticed it started easier than it has ever and the idle was smoother.

I think the improvements came from the fact that, over time, the valve had developed a slight crack at the glue joint that allowed a small bypass of air.

Anyone who has one of these things should periodically check for failure of the glue and reglue as needed to ensure a leak free install.

Mikey Hrv · 04-04-2009, 03:21 PM

great to know thanks ..

ossiblue · 04-04-2009, 05:43 PM

+1 to HarryD's post.

My 73.5, which was running smooth as silk, suddenly began to have difficulty starting (which gradually got worse) and a rough idle. After checking the usual suspects, it was discovered the pop-off valve had developed cracks and air leaks around the base, just like Harry's. Hard to see unless you looked and tested for a leak. Cleaned it up and reglued and all is well. This should be a periodic check, IMO, for all us CIS owners.

cab83_750 · 04-04-2009, 11:18 PM

My first pop-off valve broke after 2 months. It was replaced under warranty.

don gilbert · don gilbert's Garage

i have put in a lot of pop valves, and have found that black urathane(not silicon) will not ever leak and so far, one has never popped out. i have had several jb weld ones come loose, sometimes a month later, sometimes years, and when it comes out you are stuck most of the time.