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Spike2
Joined: 11 Jul 2011 Posts: 16
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Posted: Thu Nov 28, 2013 8:14 pm Post subject: Morris 12/4 misfire |
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Hi Guys.On starting my series2 12/4 side valve from cold she runs on 3 cylinders for a few seconds and then kicks in onto 4, no smoke from the exhaust and runs quite nicely.Anyone any ideas, Thanks in anticipation. |
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Bitumen Boy
Joined: 26 Jan 2012 Posts: 1740 Location: Above the snow line in old Monmouthshire
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Posted: Thu Nov 28, 2013 8:49 pm Post subject: |
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If it's just for the first few seconds after a cold start, but actually starts without drama and runs fine on all 4 the rest of the time, then I'd suggest there likely isn't a problem at all. Modern cars with computer controlled this and electronic that can and will run perfectly with cold engines, but oldies with distributors, carburettors and manual chokes tend to have a little more character - that's why we love 'em |
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Farmer John
Joined: 18 Feb 2010 Posts: 181 Location: Manawatu NZ
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Posted: Thu Nov 28, 2013 9:45 pm Post subject: |
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Hello Spike. Simplest quickest things first. Find which cylinder is faulty before the engine starts firing on four. If you think you will have time to start up and then remove each plug lead in turn and short it to the head to see which cylinder is unaffected by lead removal then do that. The type of terminal you have can make this test very simple or a bit awkward just stop the current getting to the plug even if by using a screwdriver to short it to earth.
Otherwise, remove a lead, start the engine briefly to assess, then turn off. Do each cylinder in turn.
There is a chance that if it runs for long enough on three and you turn it off the instant the fourth kicks, you will be able to feel a temperature difference either in the base of a spark plug or a branch of the exhaust manifold adjacent to the port. If two cylinders share a port do not bother with that one.
You should, by using one or all of these methods, be able to say that it is number "x"
Now, this is your car so your choice, but to confirm your result, leave it for twenty four hours then start up with the offending cylinder lead disconnected to confirm.
Dealing now with the lead for the offending cylinder you need to test the spark that it will create. The end needs to be bare or have something like a split pin inserted. A gap of 4mm from the cylinder head is what it needs, but expect up to 6mm. If that is OK swap the offending plug with it's neighbour. Try the engine again tomarrow and test to find the faulty cylinder. This is to find out whether the problem moves with the plug. If it does it is absolute confirmation that it needs to be thrown away.
While this looks drawn out it is critical to get the diagnosis on a sound basis, the last thing we need is two, or five, variables.
Please report your findings so that we can perform the miracle cure.
John |
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peter scott
Joined: 18 Dec 2007 Posts: 7126 Location: Edinburgh
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Posted: Sun Dec 01, 2013 12:18 am Post subject: |
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A sticking valve?
Peter _________________ http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon |
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ka
Joined: 03 Dec 2007 Posts: 600 Location: Orkney.
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Posted: Sun Dec 01, 2013 10:36 am Post subject: |
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Have you carried out a compression test? if you do it whilst cold, and then when it goes on to four, this will identify any mechanical issue (sticking valves, gremlins etc).
Also the more basic type of timing light using the zenon tube, and where the sensor clips around the plug lead, shows a possible mis-fire indicating electrical issues.
A bit more on this, like most of all of you, I decided that running points, I would benefit from a more technical system to check them. I looked on the 'well known web site' and purchased a Crypton machine, and arm and a leg in their day, now around £100 as they are not required for modern vehicles. Mine came with the handbook, and using the oscilloscope it identified play in the distributor shaft, points wear etc, and in the comfort of the garage, and not paying professional rates, it is surprising how much can be achieved. Whilst tinkering, I also found the cheapo timing lights do not, or intermittent flash if there is no spark at the plug.
I now run the Maplins ignition assist system, that is worth more than its weight in gold, but that is a different thread! _________________ KA
Better three than four. |
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Spike2
Joined: 11 Jul 2011 Posts: 16
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Posted: Sun Dec 01, 2013 11:14 am Post subject: 12/4 misfire |
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Many thanks everyone for your inputs. as soon as the decorating is complete I will try out the ideas. This probably sounds a daft question but how does a valve stick when I assume the spring is always trying to re-seat the valve?. Thanks again everyone. |
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peter scott
Joined: 18 Dec 2007 Posts: 7126 Location: Edinburgh
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Posted: Tue Dec 03, 2013 10:00 am Post subject: Re: 12/4 misfire |
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Spike2 wrote: | but how does a valve stick when I assume the spring is always trying to re-seat the valve?. Thanks again everyone. |
Hi Spike,
The guides/valve stems can get gummed up with burst residues. If you take the tappet cover off and turn your engine over on the starting handle (when cold) then you'll probably be able to see any valves that have excessively big valve clearances.
Peter _________________ http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon |
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Spike2
Joined: 11 Jul 2011 Posts: 16
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Posted: Tue Dec 17, 2013 10:23 am Post subject: 12/4 misfire |
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Hi everyone.
Carried out compression test when cold, No1 = 3.1 bar. No2 =5.6 bar. No3 = 7.1 bar No4 = 6.7 bar..
Then ran engine for no more than 30 secs and re did test.
No1 7.8 bar. No2= 7.2 bar No3 = 7.1 Bar. No4 = 7.0Bar.
Runs quite nicely on the road but perhaps lacks a bit of oomph. though that might be me expecting too much.
Thought I would check out valve clearances and timing etc. which leads me to ask how do I time this engine, I cannot see any marks on the bottom pulley or timing chain cover.
Also my SU carb has no damper fitted to the dashpot,( correctly I believe) but the oil does not stay in the chamber very long.( oil used was 3in1). Would you expect to top up frequently with this carb.
Every day brings a new question. Thanks in anticipation. |
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NONORT
Joined: 26 Apr 2012 Posts: 55 Location: Southampton
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Posted: Tue Dec 17, 2013 7:21 pm Post subject: |
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Hi Think no damper means no oil what would it do. |
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Phil - Nottingham
Joined: 01 Jan 2008 Posts: 1252 Location: Nottingham
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Posted: Tue Dec 17, 2013 7:26 pm Post subject: |
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Is it a down draft SU with horizontal dash pot? If so no damper as it relies on weight. Oil hole is just to provide general lube inside to stop sticking _________________ Rover P2
Rover P4
Rover P5 & P5B
Land Rover S2 & S3
Morris Mini Traveller Mk2 |
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Spike2
Joined: 11 Jul 2011 Posts: 16
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Posted: Tue Dec 17, 2013 7:42 pm Post subject: 12/4 misfire |
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Thanks for your input.I think it should have oil as the brass cap has" use cycle oil" stamped on it, but as you say what damping effect can there be. The carb has a vertical dashpot. |
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Roger-hatchy
Joined: 07 Dec 2007 Posts: 2135 Location: Tiptree, Essex
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Posted: Tue Dec 17, 2013 10:59 pm Post subject: |
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My twin Su MC2's do not have dampers, the oil is to lube the piston and to assist in the vacuum.
As stated the damping is due to the piston weight,
and also due to spring tension, Springs can be changed to alter the piston rate, they are colour coded but I have yet to see any with a colour on them. |
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NONORT
Joined: 26 Apr 2012 Posts: 55 Location: Southampton
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Posted: Wed Dec 18, 2013 6:01 pm Post subject: |
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The dashpot on my Moggy has a centeral section that is filled with oil. The outer area is kept dry. The addition of oil to this outer area is more likely to gumm up the works not help the situation. There are labrynth rings turned on the outside of the piston which are the only seal. I dont know quite how they work but they do, if the inside off the dashpot get mucky the piston will stick giving a rich mixture. The brass lid would probably have had a damper attached to it? A rod with a doffer on the end. Very early SUs have cast iron pistons and no dampers which work under there own weight and a light spring, again should not be oiled as the parts shouln't actually touch. A bit of history SU stands for Skinners Union the Skinner part of this the same as the Lily and Skinner of leather shoe fame their early carbs had leather diaphragms. Much the same as Stromberg carbs. ps a picture would realy help. |
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Spike2
Joined: 11 Jul 2011 Posts: 16
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Posted: Thu Dec 19, 2013 4:38 pm Post subject: 12/4 misfire |
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thanks nonort. Have located a exploded drawing for my carb. No damper, no spring. Must be just the weight of the damper. Interestingly after i put some cycle oil in the dashpot the engine ran very very rich. more so than before. Should I then remove the dashpot and wipe clean. |
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alanb
Joined: 10 Sep 2012 Posts: 516 Location: Berkshire.
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Posted: Thu Dec 19, 2013 5:16 pm Post subject: |
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If after filling the dash pot with suitable oil such as 3in1 cycle/light machine oil the engine now runs rich it would suggest the dash pot was sticking or sluggish, and you now need to adjust the mixture by adjusting the hexagon nut under the carburator body. _________________ old tourer
Morris 8 two seater |
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