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Car Forum / MINI / February 2005Erratic idle
Can anyone help? I recently fitted a new main Waxstat jet to the SU carb on my '89 Mini 1000 as the old one was worn and running very rich. So much so that the jet was screwed almost to the top and still idling quite fast. The new jet is a great improvement but still not quite right. Even after adjusting the mixture to the specfications shown in the SU kit and Haynes manual the engine still idles too high even with the idle screw right out with no more adjustment. Also, when I blip the throttle and the revs increase and decrease, the engine nearly stalls before idling again. Is this normal? I filled the dashpot with engine oil and the throttle and choke cables appear to be set correctly. Someone said to me that some Minis just idle high. Is this true or are there any other adjustments/checks I can make? Should the engine still idle with the idle screw right out? And Elvis was sitting next to ln1gaw in the spaceship, which I thought was kinda weird, but then they turned to me and said: > Can anyone help? > [quoted text clipped - 12 lines] > there any other adjustments/checks I can make? Should the engine still > idle with the idle screw right out? Firstly, check that the choke mechanism is working correctly - ie. with the choke closed the jet is actually making contact with the jet adjusting nut and not being held open by the choke mechanism. Secondly, some models of Mini suffer terrible mixture problems due to air leaks in the crankcase breather system. If your car has the crankcase breather connected up to the side of the carb, check it very carefully for air leaks. The rubber hoses perish very badly, and the smallest air leak in the system makes it impossible to set the idle mixture correctly. Thirdly, is it the correct type of jet? There are two jet types, 0.09" and 0.10" (there are also 0.125", but they aren't found in Minis) The correct jet type must be used for the needle installed. The smaller jet is for Axx series needles, the larger for Bxx. It's not unusual to find the wrong series of needle fitted to Minis that have been mucked about with. Generally, the larger jet is found in 6 series (1-3/4") and larger carbs, the smaller jet in 4 series (1-1/2) and smaller. You can identify the size of the jet by the grooves machined into the outside of the jet tube - one groove on the 0.10" jets, none on the 0.09 (two grooves indicates a 0.125" jet) The jet type should match the needle in use, not the carb. Thanks for the tip. I reckon I had the incorrect larger jet originally and now have the correct size. As for the idling though, could it be an air leak from the vacuum advance unit? There doesn't seem to be any resistance when I suck or blow down the tube suggesting a possible diaphragm leak? If so would this cause mixture problems and would it be safe just to blank the pipe/unit off, as the general engine performance seems fine? And Elvis was sitting next to ln1gaw in the spaceship, which I thought was kinda weird, but then they turned to me and said: > Thanks for the tip. I reckon I had the incorrect larger jet originally and > now have the correct size. As for the idling though, could it be an air > leak from the vacuum advance unit? There doesn't seem to be any resistance > when I suck or blow down the tube suggesting a possible diaphragm leak? If It would certainly upset the idle mixture, but probably within the range that you could 'adjust out' when you were fiddling with the jet adjuster. The problem with any induction-side air leak is that if you don't notice it, you *can* adjust the carb to compensate for it and end up with the mixture way too rich over the rest of the operating range. Minis are very prone to air leaks, either from perished crankcase breather lines, poor sealing of the combined inlet/exhaust manifold or (as in your case) torn vac advance diaphragm. A huge number of problems along the lines of "the car is running really badly and using loads of fuel" turn out to be this. > so would this cause mixture problems and would it be safe just to blank > the pipe/unit off, as the general engine performance seems fine? I would recommend getting a new vac advance unit. On most models of Mini it advances the ignition timing at idle, which tends to allow a more stable lean idle. it also contributes quite a bit to part-throttle economy and helps to reduce emissions. It doesn't affect running at wide throttle, that's entirely down to the centrifugal advance. Besides, you can take the opportunity to check the condition of points and set the gap, and also check that the centrifugal advance moves freely (a drop of oil on the pivots is a good idea). You should mark the position of the dizzy before you remove it so that you can put it back in the same position, but consider that as only a rough setting to get the engine started - you should always reset the timing with a strobe light whenever the dizzy is disturbed. Timing is adjusted with the vac advance disconnected, BTW. If you don't intend to replace the advance unit, then you should certainly block up the vacuum tube.
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