simon67 Posted April 29, 2005 Share Posted April 29, 2005 My VX HPC on carbs has started to misfire at 5k revs. It runs beautifully until you hit 5k then massive misfire, as if it has hit the rev limiter. I have just set the carbs up but can't see this causing it. Any ideas gratefully received Cheers Simon '91 VX HPC making lots of noise in Kent Link to comment Share on other sites More sharing options...
EFA Posted April 29, 2005 Share Posted April 29, 2005 Venturi screw loose hence rotating at resonant frequency. Link to comment Share on other sites More sharing options...
simon67 Posted April 29, 2005 Author Share Posted April 29, 2005 Thanks - will take a look first thing in the morning. Hopefully a simple solution, but one that I would never have found myself! Simon '91 VX HPC making lots of noise in Kent Link to comment Share on other sites More sharing options...
pete g Posted April 29, 2005 Share Posted April 29, 2005 check your leads not breaking down under load. normally at spark plug end Link to comment Share on other sites More sharing options...
stevefoster Posted April 29, 2005 Share Posted April 29, 2005 If the crank sensor lead is running with the plug leads then any plug lead break down can interfere with it. Best to route it separately. Just a long shot. Hants (North) and Berkshire Area club site here My racing info site here Link to comment Share on other sites More sharing options...
simon67 Posted April 30, 2005 Author Share Posted April 30, 2005 Just checked venturi screw but all is ok there. Crank sensor lead is routed away from plug leads. Do I bite the bullet and buy new plug leads or are there any other posibilities? Cheers Simon '91 VX HPC making lots of noise in Kent Link to comment Share on other sites More sharing options...
pete g Posted April 30, 2005 Share Posted April 30, 2005 borrow some Link to comment Share on other sites More sharing options...
simon67 Posted April 30, 2005 Author Share Posted April 30, 2005 Good idea - anyone near West Malling willing to let me do a quick test ? '91 VX HPC making lots of noise in Kent Link to comment Share on other sites More sharing options...
EFA Posted April 30, 2005 Share Posted April 30, 2005 Remove and clean the crank sensor. if you can, borrow a known good and test as well! Link to comment Share on other sites More sharing options...
StewartG Posted April 30, 2005 Share Posted April 30, 2005 When you say you checked the venturi screw did you check all four of them? Link to comment Share on other sites More sharing options...
simon67 Posted April 30, 2005 Author Share Posted April 30, 2005 Thanks guys. Been trying to track the problem down all afternoon. Thought I'd cracked it then big misfire at 5200rpm. Then Had almighty rattle under the bonet - I thought that the day was going from bad to worse but it turned out to be the spark plug cover that had come loose ! I now think that the misfire is intermitent and that it is acompanied by a loss of power. When I came back from the Quaife factory tour on Thursday night she was absolutely flying, with no misfire. Today she just felt less explosive and the misfire kicks in. Ill clean the crank sensor tomorrow. Yes checked all 4 venturi screws and they are wired too. Checked rotor arm and dizy cap & all is well. Leads are Beru and look in top condition. If the ignition leads are breaking down would they only show the misfire at high revs? It feels like a much more serious misfire than 1 plug lead breaking down 7 going on to 3. I'd guess that all 4 sparks are suffering. I hate this sort of problem & just want to get to the bottom of it. Cheers Simon '91 VX HPC making lots of noise in Kent Link to comment Share on other sites More sharing options...
Marius Posted April 30, 2005 Share Posted April 30, 2005 Check the alternator plugs. Mine had come loose once and when it hit ground (chassis) it started to misfire, felt like rev-limiter. But that was under cornering. Can´t see why it should touch ground at just 5k revs. Link to comment Share on other sites More sharing options...
pete g Posted April 30, 2005 Share Posted April 30, 2005 had one go on mine ,some years back. took me ages to find it. lead resitances were good but it was burnt out at the spark plug lead end. will get worse if this is the problem. Link to comment Share on other sites More sharing options...
stevefoster Posted April 30, 2005 Share Posted April 30, 2005 I had a misfire below 3500 rpm that persisted through all cures/tracing until I changed the fuel. I reckon there was water in the fuel or it was the wrong sort (whatever that could mean)?! Long shot: Did you fill up just before popping home and then have problems next blat? Hants (North) and Berkshire Area club site here My racing info site here Link to comment Share on other sites More sharing options...
simon67 Posted May 2, 2005 Author Share Posted May 2, 2005 Don't think it's fuel - had 2 X tanks in since misfire first started. There is another thread with an identical problem on an XF. The suggestion there is that it might be the coil breaking down. This seems plausable - any thoughts ? Cheers Simon '91 VX HPC making lots of noise in Kent Link to comment Share on other sites More sharing options...
simon67 Posted May 2, 2005 Author Share Posted May 2, 2005 All fixed now. The connector block that joins the crank sensor lead to the loom was full of engine oil!! Must have been 10ml in there - it just poured out when I disconnected it to take a look. The connector is located underneath the dry sump oil filler so must have been covered when the last owner changed the oil. Dried it out and now everything is back to normal. What a relief when you fix a niggling electrical problem at last, & it was so simple! So If you have a VX with Caterham DS take care when topping up the oil! Thanks for all of your help guys Simon '91 VX HPC making lots of noise in Kent Link to comment Share on other sites More sharing options...
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