Fast running

I have just bought a 1984 mK1 cabrio, and i have noticed that once it warms up a bit it starts running a bit fast, 1500rpm, can anyone advise me as to why this is, I have just replaced the battery but it was like this with the old battery too.

Thanks in advance  

To slow it down you need to tune the engine by adjusting the screw in the throttle housing- see the 'Fuel Injection' link on the left. Make sure the timing is good first and the air cleaner is clean. These adjustments should be made when the engine is hot. Things are probably just set-up incorrectly. However…

When the engine is cold, there are a couple of things that that happen to help the engine run at a reasonable speed. It could be that these are not working properly. They should operate only temporarily during warm-up of the engine, it could be that they are continuing to work even when the engine gets warm, or it could be that they aren't working at all - and to compensate, the tuning of the engine has been fiddled to allow the engine to run even when cold.

If the timing, throttle screw etc. are set-up correctly, you might want to check the throttle bypass valve, which should be open when the engine is cold, and should gradually close as the engine runs. The control pressure regulator could also be faulty?

good luck

oops, I should have called the 'throttle bypass valve' the 'auxilliary air valve', as in the technical article.

Thanks for the advice, BUT…. I have just given  the car its first proper run, (more than ten mins to the shop) and it seems to have settled at about 800 - 900 rpm, (better then 1900 that it was running)

So It has been sat for a while and maybe just needed a decent run.

This might sound like a daft resolution, but I had a similar problem with the cabby I've just sold to my brother. I tuned it, changed various parts, checked the F/A mixture, checked all vacum lines and all was OK or was put right yet the problem persisted!. After much head scratching it was my brother that actually found the problem (I think the second time he drove it) turned out that the accelerator pedal wasn't fully returning intermitantly (was kind of gunked up a little on the hinges) and as such was reving higher  :redfaced:  please note that you can not feel or see a difference in the pedal location as it was very minor. The next time it does this put your foot under the pedal and lift it up and see whether the revs drop, if it does, clean/lubricate the pedal hinges and job done. I fixed mine with some GT40 spray

Might not be this but definately worth a check!

I will check that, that didn't even occur to me.

can happen on the throttle too, with the butterflies sticking. same thing - wd40 and some wiggling.