Air filter blown off

[Ellie]

Hi, what would cause the hose connecting the air filter to blow off?. Happily driving along then the engine started running rough. Fortunately was close to home so limped into the drive. Then while I was parked listening to the engine, the main pipe connecting the air filter blew off as if there was a large amount of pressure pushing back. Engine of course then stopped running. I reconnected the pipe and started the car again. This time the engine was still running rough and pressure tried to escape through all the smaller various pipes in the system. Something blocked somewhere maybe but any clues or suggestions where to start would be most appreciated

[Yadkin]

Sounds like you had a backfire in one or more cylinders. From wiki:

Backfires may also occur before the combustion chamber. One possible cause of this is timing. If the timing is too early, the spark plug fires before the intake valves close, causing the combustion to propagate into the intake manifold, further igniting the air-fuel mixture there; the resulting explosion then travels out of the carburetor and air filter. On many small marine engines, a screen is placed over the intake of the carburetor as a flame arrestor, to prevent these flames from escaping the intake and potentially igniting fuel or fuel vapors in the enclosed sump or bilge of the boat, causing a fire or explosion. Alternatively, the ignition timing may be late, in which case the combustion is not completed by the time the exhaust valves open, allowing the combustion to propagate into the exhaust system.

Additionally, improperly adjusted carburetors that create a lean condition during acceleration can cause the air–fuel mixture to burn so slowly that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire. In this situation it is conceivable that there is a backfire occurring in the intake manifold and exhaust manifold simultaneously.

Time for a basic tune-up.

[Ellie]

Well that is something I certainly hadn’t considered, thanks so much for that advice and makes sense considering I had my timing belt replaced last year and have had some occasional coughing starts since then.

[Yadkin]

Well that is something I certainly hadn’t considered, thanks so much for that advice and makes sense considering I had my timing belt replaced last year and have had some occasional coughing starts since then.

Interesting. You could be a tooth off on one of the cams.

[RRoller123]

Timing belt being off a few teeth, by either slipping or mis-aligned at installation, can give a backfire and blow the hose off the AFM and/or the throttle body.

[Ellie]

Hey thanks for the help so far. She wont start at all now. I've checked all air pipes and hoses are on tight, when turning over the flap in the AFM opens and closes like I think it should, I can see the timing belt turning everything looks ok, but obviously is not. Then there is a slight Pffffft sound and briefly stops cranking before continuing again. Does this still sound like a timing belt issue? should I be able to see the COGS on the timing wheels?

[aj81spider]

Turn the crank by hand until the timing marks are lined up. This will tell you if you've slipped a few teeth. If you have then you need to adjust the belt. If you have slipped and you adjust the belt and it still won't start then do a compression check. If it slips enough you can bend a valve (ask me how I know).

[azruss]

Also check to make sure your dizzy is bolted down tight.

[Ellie]

Sound advice, much appreciated. Um, what's a dizzy?

[Ellie]

Ok, so I'll think before I type, a bit of that might get my spider back on the road quicker....I did pull the distributor cap off and checked the insides but didn't pay much attention to the rest of that system so will go over it all and make sure everything is secure. Thanks again for the tip

[Ellie]

Hi, so I`ve finally found the time to check the timing. I was hoping that with a slight adjustment I`d be back on the road but it all it looks good. On to testing compression. Can you tell me what this may indicate or if there are any other suggestions that may be causing my misfire and non-start?

[aj81spider]

If your compression is good across all 4 cylinders (roughly the same value and a reasonable level say bigger than 100) then you can move on to other debugging. One cylinder with very low (or no) compression can mean a bent valve that's stuck open (that's what I had) or perhaps a bad head gasket. Two adjacent cylinders with low compression means the head gasket is blown between them.

[Ellie]

Still on the hunt, but in the meantime... how do you know if a valve is bent? and, A.J. what happened to yours?

[aj81spider]

My timing belt slipped a few teeth. When I did a compression check I had no compression on one cylinder. Pulled the head and the piston had bent the valve so it wouldn't close. Replaced the valve and put it all back together and it worked fine.

If a valve is bent your compression on that cylinder will be 0.

[Ellie]

Thanks A.J. Well it looks like my next task is to take the head off, unfortunately compression readings were 70, 80, 140 and 150. Problem is in my number one and two cylinders (gasket or valves, dont know yet) as I presume those 70 & 80 PSI are a tiny bit too low to start the engine. And it is such awesome weather here at the moment so am missing out on some great driving, but getting to know my car is good too.