I have a 77 xj6 which after being "tuned", by an apparently knowledgeble friend of mine has developed a nasty case of over-running when switching off. I can't help thinking that it can't be good for it apart from anything else.
Any ideas? Timing seems spot on and retarding doesn't seem to help.........aaaarrrrrggghhhhhhhh?
Help!!!!
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It's caused by particles in the combustion chamber glowing and continuing to ignite the fuel/air mixture after the ignition has been switched off.
An anti-run-on valve could be fitted which opens the inlet manifold up to atmospheric pressure after the engine is switched off, and was commonly fitted on Ford OHVs to cure a known problem.
But the cure could be as simple as slowing down the engine idle speed if it's idling too fast, and/or correcting the fuel/air mixture setting.
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ok, you've confirmed my suspicion as to the mechanics of the thing. Idle speed is quite low when in gear (auto box), around 500 to 600rpm so i'm not sure if it's that and the fule/air mixture hasn't been touched. I should mention that it seems to be accompanied by low power on moving off. So - with your particle point in mind could this mean that it's coked up? Sorry if i'm stating the obvious here but it's been driving me nuts!
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Does it only do it once its warmed up or hot??
Has he changed the heat range (or length) of the plugs.
Running lean and overheating valves?
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Only does it when warmed / hot.
Plugs are NGK as was fitted (book says they should be Champion)
I did richen one carb but plug colour looked spot on after a long run (before fiddling)
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oops!
sorry i meant hat i leaned one carb out. THe front 3 plugs were sooty on inspection so i leaned the carb a bit, plugs were "biscuit", colour after.
Thanks
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I'm guessing this has lived on a diet of leaded 4 star for most of its life, so undoubtedly has considerable deposits in the combustion chambers. These both raise compression and provide hot spots which promote running on - a common problem which isn't seen so much these days as unleaded burns more cleanly. I'd be absolutely sure it's not running weak and/or over advanced, both of which promote this problem. If it really isn't, then I'm sure a good old-fashioned decoke would solve the problem.
JS
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Its caused by excessive build of carbon on cylinder head/piston crown..What happens is the carbon builds up heat to the point it fires the incoming gases. After ignition is switched off..Usualy causing a pinking at the same time(meaning premature ignition). Decoke.ie cylinder head/s off then clean all deposits off both heads and pistons. I wouldnt use an additive to sort it as neighbours may complain
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Steve
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As mentioned above the problem is probably due to a build up of carbon, but there are several things I would try before resorting to stripping the heads.
1. Italian tune up - a sustained high speed (high revs) run to blow the carbon out.
2. Try using a higher octane fuel with more detergents in, such as Shell Optimax, BP ultimate or Texaco super unleaded.
3. Try adding some redex to the fuel tank each time you fill up.
4. Put Redex down the plug holes and leave to soak overnight to decoke (as mentioned above, lots of smoke on initial start up next morning)
If these don't work then it's got to be a strip down.
This is assuming that everything is set up correctly. With no disrespect to your knowledgable friend, there is a possibilty though that the tune up may not have been done correctly if, as you said, the problem only occurred after the tune up.
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I had the same problem with a 1953 Rover 75 I owned as a student. Perisistnet running on.
I soaked the bores in Redex for 2 days : as described above.
When I started up, very little smoke.. great I thought. Well I drove down the road and as the exhaust warmed up I laid a cloud of white smoke some 20 feet high for the next mile. Fortunately it was a country road. It was a still cold winter morning and very impressive. No cops around .. but very dangerous and the smell!
After all that it still ran on.....
(I had obviously used too much, unburnt redex had been pushed into the exhaust .. etc.
If you do that with a catalyst you may destroy it!
I'd go the Optimax/Italian tune/new plugs route: far safer imo...
madf
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as Kith said with no disrepect to my friend he is most correct! I couldn't see what he did wrong (timing looked good while tuning etc), but it didn't do it before! Shall i take bets with him as to what it turns our to be?
I've now decided to have it proffessionally attended to as it's cold outside and there are a couple of other things that need attending to before winter sets in and i'm left stranded by the roadside in the dark somewhere.
Thanks for all the replies, i'll keep you all posted as to what the man with the spanners says post fiddle which hopefully will be this week.
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