Hi,
I have a misfire / pop / bang on my 3.5L V8 (SU carbs) and I am rather stuck as to where to go from here. I know that a misfire can be many things, and I also know I should look to change the distributer cap, HT leads, spark plugs, ignition coil, maybe camshaft, etc. etc. however this misfire seems not to relate to these.
The misfire occurs almost exclusively at low ish revs (1000-1400 rpm) when I accelerate with a partial - medium throttle. I expect it would do it at full throttle in the same gear at the same revs too, but I have not tried in case I blow the engine up. If I drop a gear to get the revs up, then accelerate it does not do it hardly at all. Also, you can predict when it is going to do it as the engine feels very slugish and has a 'fluttering' sound, a sound similar to one you would get if there was a hole in a manifold.
I have recently fitted a stainless steel exhaust and checked that everything is tight and no leaks I can see, and I have tried making the air / fuel mixture both weak and rich. Setting it rich seems to reduce the misfire, but not reove it fully, and most recently I have actually set the mixture fairly weak, and this too seems to now reduce it so I have no idea where to go next.
I have used a Colortune to look at the flame in the cylinder, and while the flame at various revs is a reasonable blue colour, at idle it is a very yellow flame which cannot easily be changed to blue, and therefore my spark plugs soot up when idling for any long periods.
To add to this, once I have accelerated very gently, or very hard to avoid this misfire area, the engine runs fine. Will pull reasonably well and will do easily motorway speeds on level ground (up hills are still a little lacking in power but this is not a new problem).
I can only imagine I am going to have to learn how to rebuild a carb and try to get a service kit, but before I do, are there any suggestions to help?
Thanks.
I have a misfire / pop / bang on my 3.5L V8 (SU carbs) and I am rather stuck as to where to go from here. I know that a misfire can be many things, and I also know I should look to change the distributer cap, HT leads, spark plugs, ignition coil, maybe camshaft, etc. etc. however this misfire seems not to relate to these.
The misfire occurs almost exclusively at low ish revs (1000-1400 rpm) when I accelerate with a partial - medium throttle. I expect it would do it at full throttle in the same gear at the same revs too, but I have not tried in case I blow the engine up. If I drop a gear to get the revs up, then accelerate it does not do it hardly at all. Also, you can predict when it is going to do it as the engine feels very slugish and has a 'fluttering' sound, a sound similar to one you would get if there was a hole in a manifold.
I have recently fitted a stainless steel exhaust and checked that everything is tight and no leaks I can see, and I have tried making the air / fuel mixture both weak and rich. Setting it rich seems to reduce the misfire, but not reove it fully, and most recently I have actually set the mixture fairly weak, and this too seems to now reduce it so I have no idea where to go next.
I have used a Colortune to look at the flame in the cylinder, and while the flame at various revs is a reasonable blue colour, at idle it is a very yellow flame which cannot easily be changed to blue, and therefore my spark plugs soot up when idling for any long periods.
To add to this, once I have accelerated very gently, or very hard to avoid this misfire area, the engine runs fine. Will pull reasonably well and will do easily motorway speeds on level ground (up hills are still a little lacking in power but this is not a new problem).
I can only imagine I am going to have to learn how to rebuild a carb and try to get a service kit, but before I do, are there any suggestions to help?
Thanks.