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I've got this problem that I can't find the cause of. When starting the engine, which goes very easily, it idles smoothly and stable, rising the rpm a bit still gives no problems. But when driving at high rpm speeds eg. on the highway, and i declutch rpm falls back to 300-400rpm. It's raised again to idle after a second or four. Sometimes i have to rev it up myself or it'll stall. There's no loss in power, pinging or whatever. I've checked the following:

  • Air leaks, there seem to be none.
  • Mixture, it's set to around 4% CO or lambda 0.93 which is fine for this car ('76 Triumph TR7, SU carbs)
  • Compression
  • Ignition advance
  • Carbs are synced and the dampers go up and down fine
  • Just put new Duckhams Q 20W50 in it
  • Drove with and without my PCV system connected to the carbs

The engine has been revised 6 months ago so everything is still clean and in good condition.(ie. nothing clogged or coked or whatever) I've got a Facet performance fuel pump, and the fuelsystem also works fine. It's equipped with stock needles and jets. As far as I know, i've checked everything. A friend of mine told me that although you can't see that, one or both dampers may not go down smooth enough as they should, causing this problem. I doubt that though. As a side note: after the revisation it has as somewhat higher compression, it tends to run on a bit after shutdown. I don't suppose this has something to do with the problem. Suggestions or ideas anyone? Anything is much appreciated. Cheers.

  • What is meant by "revisation"? – Pᴀᴜʟsᴛᴇʀ2 Nov 13 '16 at 15:41
  • @Pᴀᴜʟsᴛᴇʀ2 Sorry poor translation, the word I meant is revision or overhaul. – Bart Nov 13 '16 at 18:58
  • I don't know nearly as much about carb vehicles compared to FI, but does it have an idle control valve, or is it just a hole in the throttle plate? – Hari Ganti Apr 11 '17 at 1:01
  • @HariGanti It doesn't have an idle valve, but there's a overpressure valve in the throttlevalve itself, which opens under heavy manifold vacuum. Older carbs don't have those though. – Bart Apr 11 '17 at 12:18
  • I'd check that valve. It functions as a mechanical idle control valve, so if the spring is weak or you have a vacuum leak, it might not be opening. I'd suggest having a friend rev up the engine (or just pull the cable yourself), then suddenly release the throttle. See if that valve opens or not. – Hari Ganti Apr 11 '17 at 17:25

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