Misfiring when permanent/constant revs above 2500 rpm

[Peter B.]

Hello,
today I do need a little advice regarding an effect I never came across so far. This weekend I did something I do not do normaly, I used the Autobahn to get there more quickly. Due to traffic, speedlimit and fuel consumption I intended to ride with a constant 3000 or max 3500 rpm. After 5 to at least 10 minutes I got misfiring, especially when just carefully increasing the revs. Putting the pedal to the metal and anything is fine, no misfiring then, nice revving up etc. But driving more steadily with revs above 2500 rpm and misfiring will come back, with short notice of some slight "resistance" in driving. Tried to reproduce that on minor roads, but due to my auto tranny I?m too fast with 2500 rpm and have to use 2.gear, but with a more open throttle, and there nothing happens, no misfire.
Coil is fairly new, as are the ignition wires together with the spark plugs, and as already said, on normal roads use (between 2000 and 2500 rpm cruising and accelarating up to 5000 rpm occasionally) the car is just fine.
I do also suspect the carbs, mayby float level or fuel starving, but I do have a Pierburg since 4 years without any problems as replacement for my faulty SU, and I?m able to get the car up to a top speed in excess of 200 km/h.
Does anybody have any idea how this can be explained and then cured? I?m left quite clueless in the moment.
Many thanks for your help!
Peter B.
p.s. or should I just forget about it and just don?t drive with constant revs above 2500 rpm?!
p.p.s What "damage" has to be expected by occasional misfiring?

[Heuer]

Peter

Sounds like a fuelling problem to me. Have you checked all the fuel filters - sump, bowl and SU gauzes? After that you need to check the SU floats and the other carb settings. Misfires need to be attended to as they can be a sign of impending failure and can cause damage.

To give you an example of such consequences the accelerator pedal return spring broke on my car and whilst it was still drivable it was not a comfortable experience. Snapping off the pedal would drop the revs to idle but after gentle pressure taking the foot off would leave the engine idling at 2,500 rpm - not fun when approaching a corner or stop! I eventually pulled over and switched off and there was an almighty bang; the engine would not restart. I checked under the bonnet and found one of the manifold core plugs had been blown out. Switching off at high revs had left lots of un-burnt fuel in the manifold which ignited blowing out the plug. I pressed it back in, used a rubber bunge rope as a return spring and was on my way. Had I not found that small silver disc I would have been stranded - so yes misfires are bad news and need to be fixed.

[Peter B.]

David,

thank you for your reply. Posted as well in ther german Jaguar forum and like you, everybody suspects the carbs/fuel side.
I will have to tune the carbs and see if it will go away, some tend to "too lean fuel mixture at part load" (hopefully translated well enough).
This will have to be done by an expert here, I do not feel capable of "fooling around" with the 3 SU?s....
Thanks
Peter B.