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MG MGB GT V8 Factory Originals Technical - Sound changed ... how do I tell if all cyls. firing???
|How can I tell if all 8 cylinders are firing? My V8's noise has changed much for the worse over the last couple of days. It sounds like a miss or something -- there's an unevenness that seems to vary directly with the rpms -- and the growl sounds a little sickly. Car's in super shape, but perhaps I've hooked up something wrong or bonked something. Could the timing simply be off?|
|Disconnect and reconnect each sparkplug wire as it idles to determine which has no effect on how it runs.|
|You could even have a plug wires swapped. I did that on my 4cyl truck, she ran but ran terrible, and that was with 2 cyl wrong, so a 8cyl would be harder to track down..|
|Good advice, once the car is warmed up, you may also want to take a water bottle and squirt some water on each branch of the exhaust manifolds to see if the stuff doesn't go away very fast.|
The absulute best way is to scope the engine, but not necessary. If you have recently messed with ignition of course check timing, firing order, etc. First, make sure you have no vacuum leaks and that the choke isn't accidentally engaged. Get a squirt bottle of water and spray down all of the spark plug wires and dist cap. If you can see any voltage shorting to ground replace wire or cap, and check resistance of all other wires (roughly 7000 ohms/foot for regular wires) and check dist cap for tracks,cracks, dampness. If no results do as George says-
1) Note engine RPM.
2) one at a time, pull a spark plug wire from its spark plug for 5 seconds (rest the wire close to a ground on body or engine so that it still sparks to avoid overstressing components)
3) note drop in RPM
4) reconnect wire and wait 10 seconds before you move on to the next wire and repeat until done
All cylinders should have roughly equal drop in rpm
NOTE: if you have a RISE in engine rpm when you disconnect a wire this normally points to valve problems.
check compression, spark plugs, wire of offending cylinder.
if ignition seems to be in order check cooling system for possible bad head gasket, or cracked heads.
check carburetor balance and mixture.
Note that accelerating up hill as fast as possible so as to maximize engine load will usually make ignition problems reveal themselves as the engine will miss badly, possibly backfiring.
Fuel problems with a carburetor will usually make engine run equally rough and not sound like a single "miss".
Timing problems will lead to loss of performance(possible overheating), but not a steady "missing" sound.
Good luck and hope it's something simple.
|Well, I borrowed a timing light and it turns out I must have really bonked the diss at some point having neglected to tighten the hold-down bolt enough ... the reason for the change in sound was simply that I had about 20 degrees too much advance. Put it back to the normal position (~10 degrees before TDC @ 1000 rpm, vac disconnected and it sounds like it always did! Thanks for the heads-ups!|
This thread was discussed between 24/05/2001 and 26/05/2001
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