Big backfire!

[Ron.G]

I've had a problem starting the car when cold. It starts right up and then dies. I do this about four times then it runs at a very low idle and then comes up to normal idol after about thirty seconds.

Yesterday when I went to start it (cold) It did the usual but this time with a very large back fire.
When I got it started the RPMS were up around 2K ,much higher than ever before.

So I lower the idol back to 900 and went for a ride. It drove fine but when I let off on the gas the RMPS dropped off and the car stalled twice. when I got home I adjusted the idol so the car would stay running again Back to 900.

Whats going on? I did a search on cold starts but did not find a answer. just a link to the cold start circuit.
I have a pop off valve and it seems to be fine.

I also noticed the small screws in the air box were backed out so I tried to screw them in with no success.

Could I have blown my air box even with a pop off?
would the RPM's come up if the box is leaking?
I checked the fuel presure when cold and it looked good.
It's a 1980SC.
Thanks in advance!

[Paulporsche]

Did you check the cold control pressure? It might just be your WUR, especially since your symptoms changed as the engine warmed up. Or possibly just your mixture setting.

However, It is possible to blow the airbox even with a popoff valve, although not too likely. I'm thinking your airbox may be OK because the engine usually does not want to run at all if the box is blown.

You may have other air leaks. Check AAR, AAV, DEcel Valve, intake runners.

[sammyg2]

If your screws on the air box were backed out, you prolly blew the air box.

[Ron.G]

But would it run if the box was blown?
Otherwise it runs great. It was the two very small screws on the inside bottom to the right of the pop off valve.

[Gunter]

I'll take a look at my airbox. It seems to me there was something about these little screws being spring-loaded. I tried to find some detailed drawings here on PP without luck. Can someone find a schematic showing the airbox details?
Except for the initial one-time high idle, it sounds like a vacuum leak somewhere. I set my idle more like 950-1000. Search this board for how to trace a vacuum leak and a closer look at AAV, AAR, etc.
http://members.rennlist.com/jimwms/CIS/CIShome.html

[kach22i]

Did you check to see if the rotor slid up on you?

Here is my backfire story:
Pertronix install - big backfire - 1977 2.7L

[ruf-porsche]

You don't need to have a GAPING hole for an air box to be blown. Probably a crack in the airbox is letting in additional air and leaning out the mixture. When you reset the IDLE it was okay for a cold start condition, but once the engine had warmup, the IDLE was to low.

[Ron.G]

Thanks. I'll have to check the air box out. might be a good time to get a state inspection if it is lean. It's a bear to start when cold. It takes a least four tries than it cacthes and runs great.

[Paulporsche]

I noticed that you checked your fuel pressure, but did you check your cold control pressure? Is your mixture set rich enough?

[bagpiper47]

I had a few screws missing and a crack in the airbox on my 1981 and drove to Denver to get it replaced (127 miles), drove a lot better after I lost weight in my wallet!! Check the front screw on the left side. Mine was almost out. It still ran OK though, no stalls etc. Good Luck.

[Ron.G]

We checked the presure when it was cold right after the engine rebuild. I'm pretty sure it was fifty PSI and thirty something when warm.
What should it be for a 1980?

I might need to carry a extra air box in the trunk till I figure this one out.

[ruf-porsche]

Replacing an airbox is not a job you want to tackle on the side of the road.

If you are having problem with the airbox, replace it while you have access to a garage.

[Gunter]

Before you get into major $$, do a general check for vacuum leaks. A propane torch, or starter fluid, does not work very well because it does not do a pin-point spray. I found that an acetylene torch with a small tip works best. If you don't have one, go to a shop and ask them to do it; it's a lot cheaper than a new airbox. Besides, if the leak is somewhere else, you still have a problem after you install a new box. If you go with a new airbox, install the pop-off valve.

[Ron.G]

I have a acetylene torch. I'll give that a shot. I do have a pop off valve but the backfire was a pretty good one.

I'll let you know what I find. the intake boots are in great shape and are all tight, but I'll snoop around with my torch and see what going on.
Would'nt I get some poping when going down hill and using the trany to slow the car. Or on regular decell if I had a vacume leak?

I'm almost positive it's running lean because it burns very clean and you can"t really smell gas at the tail pipe. I think it was set that way after the engine rebuild.
Is there a inexpensive exhaust analyzer that can be had?

[Gunter]

Check with the torch nozzle around the gaskets where the intake runners meet the engine case and all the vacuum lines for brakes, etc. Also, tighten all the clamps on the lines coming from the oil filler stud; I had to put an extra clamp right where it meets the OFStud. If there are any leaks, the RPM will pick up a little right away.
I don't use an exhaust analyzer; I do it by ear, adjusting with the famous 3mm long Allen. Just a little turn on the mix adjust screw; Clockwise is richer. Don't leave the Allen in there when reving up!Tweek the idle adjustment screw to set it around 950. You may have to play with both these adjustments before the engine is happy.

[Paulporsche]

Try richening the mixture in 1/16 or 1/8 turn increments and see if that helps/. If the idle starts to fluctuate, then go just slightly leaner.

Another poster recently noticed his sensor plate was gummed up and not operating freely. Make sure that it's OK.

[mperkins]

My airbox blew about 3 weeks ago. $1,300 later - i'm back on the road. It is a costly affair.

[Ron.G]

I just thought of something. When I take my oil cap off the RPM's come up. Would that mean that my air box may still be good?

I'm gonna rich up the mixture a little and see if that helps. When I take it for inspection I'll lean it back out.

Thanks for all the help on this. I'll let you guy's know how it goes.

[kach22i]

Originally posted by mperkins
My airbox blew about 3 weeks ago. $1,300 later - i'm back on the road. It is a costly affair.

I thought it was a 2 or 3 hundred dollar box, is that $1,000 labor?

[mperkins]

$450 for a new air box. Then new gaskets and boots and pop-off valve., plus other misc items. About $600 labor. Its a DIY job if you have a garage.