Guide Me - Backfiring Through Airbox

pryord · pryord's Garage

My SC is running poorly, so help me with a plan of action or point me to a previous post.

It's an '80 SC with 90k miles. Top end rebuild and rings done about 10k ago. Recently I adjusted the valves and put on a Euro premuffler.

After the valve adjustment, the car ran poorly. I checked the valves 3 times, so I'm pretty sure they are OK. If anything, they might be a tad tight. I could get the feeler gauge in and move it, but the fit was snug. However, I found I had dropped a plug and ruined the gap. Regapped it and things ran much better.

Since then, the car is progressively running worse. It frequently backfires through the airbox (pop-off installed) and I get many pops through muffler on deceleration. It cranks right up, but the idle is a little rough. When idling, if I rev it quickly, I almost always get a pop through the airbox.

Plug wires, cap and rotor are new. Next I'll check for spark. If that proves OK, then what?

I'd love any help!

klaucke · 05-27-2005, 11:26 AM

You may need to adjust the mixture screw which is located next to the fuel distributor. This can account for the system being slightly out of adjustment. It takes a 3mm allen wrench. Do a search there are many threads on adjusting this.

If that doesn't work, it could me many things, however, you will probably want to buy the CIS pressure testing gauges, and first check to see if your WUR is in spec. You can make an adjustable WUR if it is out of spec. It may just need cleaning. The best thing to do is do a search involving CIS problems and WUR problems.

Also, check all your hoses for a vacuum leak. Doing some searches will yield a lot of information and threads where users have fixed their problems.

gerard vaglio · 05-27-2005, 05:34 PM

You say after the valve adjustment the car runs poorly. Does that mean the car ran fine before the valve adjustment?

If so you might look at the valve adjustment. You could have done everything correctly with the feeler gauge but missinterpreted the marks on the pully, it's actually easy to do.

One trick that helps is to paint the marks with nail polish for clarity as you rotate the engine over and over again. One color for TDC and another color for the other two marks.

Try to pick colors that say "spring".