Susan S. Posted May 1, 2003 Share Posted May 1, 2003 I am continuing the saga of tuning the carbs on my Super Sprint. It will not idle smoothly and backfires with quite a bit. I have a Gunsons colourtune and I cannot get it to be yellow on tickover - it just stays at bunsen blue whatever I do. It is sugegsted that theter might be a leak on the inlet manifold, a crankcase ventilation problem or a blocked idle jet. When I take it for a run (really bad performance) and come back it idles at 2000rpm, having been set to about 1000rpm at tickover when warm and standing. Any suggestions as this is becoming frustrating! Cheers Chris Link to comment Share on other sites More sharing options...
Big Brother Posted May 1, 2003 Share Posted May 1, 2003 Chris, I would start by looking for an air leak. Sometimes just spraying WD40 onto the manifold to carb join will highlight a leak. Listen for the engine speed to rise. It may be worth checking the thackery washers are intact and not broken. These are the spring type washers used to allow the carbs to move a little. (This is to stop fuel frothing caused by engine vibration) They have a habit of breaking in half and this can/will lead to an air leak. Redline can supply a rubber bobbin type replacement that are a lot more reliable. When you say the colour stays blue when using the colortune - is this the same on every cylinder? If so, a blocked idle jet is unlikely - unless all 4 are blocked. Thinking about it....the idle jets only come into play up to around 2000rpm. After this the mains take over. If you are getting a poor idle - bad performance and lots of backfires (= lean running), I would start by looking for an air leak or possibly a fuel supply problem. Hope this helps Steve Se7en-Up! Less is more! Remember - an Amateur built the Ark - It took Professionals to build the Titanic! Link to comment Share on other sites More sharing options...
Joe 90 Posted May 2, 2003 Share Posted May 2, 2003 Air leak has to be #1 suspect, but low levels in the float chambers are a possibility. This can be a problem with the needle valves or the fuel pump. 100,016 *biggrin*miles so far Link to comment Share on other sites More sharing options...
rjc Posted May 2, 2003 Share Posted May 2, 2003 I had exactly the same problem. In my case it turned out that the man who sold me the carbs had given me a set up that represented an amalgam of the bits he had rolling around at the back of Webber bits draw and were not correct at all for my previous 1700 supersprint. If you look down the throat of your carbs whilst the filters are off the restriction with the circle is a separate part to the body - the choke. It should have the diameter scribed on it somewhere. Note the number Remove the wingnut cap on the top of one of the carbs and there are two sets of brass jets, the small two are the idles and the large two are the main jets. Remove one of each, they unscrew and pull out. The idle jet has a cap and a one-piece jet. It will have a number something like 55 F6 on it. Note the number. The main jet has a cap and a jet in three parts, each part will have a number. The top part is the air correction jet, it will have a number like 160. The midlle part will be longer with grooved holes in it - the emulsion tube. It will have a number like F16. The bottom part is the main jet and it will have a number like 145 on it. The Webber Company Tuning Manual -available from Webcon UK ltd explains how they all work. You'll need a degree in mechanics ! It lists base calibration figures for these parts for 40 DCOE's as Chokes 30mm, Idle Jet 45 F9, Air Correction Jet 115, Emulsion Tube F11, Main Jet 115. See what you've got. If you can't get the idle rich, your idle jets simply aren't big enough. As for chokes you can run different sizes, bigger sizes maximise power but reduce mid throttle response. Smaller sizes restrict inlet flow and full power but provides good part throttle response. The main jets should'nt be too far off what the calibration figures are. Rodger King or James Whiting should be able to send you any replacement jets / chokes (or get them from Webcon) and also advise on the most appropriate sizes. Check all the jets and blow them through with compressed air whilst you're at it. It won't hurt. Bear in mind that you are opening up petrol, so no naked flames and plenty of ventilation. If you want more details get back to me directly with a fax no. Regards Rowland Link to comment Share on other sites More sharing options...
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