500 idle problems & light throttle misfire

[crazycats]

Hi Folks,

Wonder if you can help me with a problem with my GF's 500?

It's had a history with intermittent idle problems, which showed up again a few weeks ago, this time joined by a light throttle misfire. I did a search on here, which pointed me in the direction of the stepper motor.

Took it out and it was completly seized.

Replaced today, and seemed to be fine. Then all of a sudden, it cut out again and would not idle. Threw the tools down in a huff and sulked in the corner for a while!

After it'd cooled for a bit, I tried again, and after a few coughs and splutters, it idled perfectly for about 20 minutes. Drove out of the garage, for it to all of sudden cut out and now won't idle again.

Took it for a spin up the road and the misfire is still there.

Any ideas from the great and good Griff clan?

Thanks

edited to add..... Don't know if this is part of the problem, but whenever I switch off,the rev counter sticks. eg, just ran it up, held at 2000 RPM, killed the ign, and the needle stays put instead of dropping to zero.

I presume the counter takes it's feed from the coil?

Edited by crazycats on Saturday 3rd July 18:27

Edited by crazycats on Saturday 3rd July 20:31

[BeastMaster]

Hiya - to eliminate the stepper motor, when you get a steady idle disconnect it, think you will have to do this while running because if you turn off ignition the stepper motor will reset.

Another way would be to plug the bypass, and use the bleed valve to manually set tickover.

Andy

[crazycats]

Thanks Andy,

Will try that.

Problem is getting a steady idle though. Runs fine with a bit of throttle, and cuts out dead when you let go.

I let it cool this afternoon, fired it up and it it idled perfectly during warm up, but as soon as it got to normal running temp, started to play up.

That points me in the direction ot the coolant temp sensor. Although I checked the resistance when hot - 300 ohms, which according to the bible, is about right.

I'll disconnect the stepper during warm up in the morning, and see what happens.

Off for a beer and a ponder

Thanks again

Paul

[crazycats]

Quick update!

Run up from cold today. Disconnected the stepper motor.

Unfortunatley, the same issue remains.

Did notice that the rev counter, as well as sticking when I kill the ign, is very sluggish to react to engine speed.

Going to replace the coil I think, see if that makes any difference. clutching at straws a bit. But it looks like its been there from new, is only a few quid, and would be another thing eliminated.

[blitzracing]

You can do a bit more diagnosis with it costing anything. Firstly have a look at the plug colours (one or more on each side). If they are black you have a misfire or overfueling. If one side only is black, its a misfire on that bank only (Rotor arm, cap or leads). The coil and ignition amp can be tested by removing the distributor cap, and unplugging the king HT lead, and placing the end near the chassis to for a spark gap. Then rotate the engine slowly so one of the magnetic lobes on the distributor trigger line ups with the pickup point. Then turn on the ignition and use a flat blade screwdriver to close the gap between the lobe and pickup (jab it on / off) and the coil should fire. If it wont, a magnet attached to the screw diver shaft can help. If the coil is healthy you should be able to get the spark to jump 1- 1.5 cm to ground. Another reason for loss of idle can be an air leak around the plenum somewhere, as it causes the engine to run very lean.

[Colin RedGriff]

From your description it sounds like the problem happens once the engine gets hot. Could be the coil breaking down when it gets hot.

You say the stepper motor was seized and you replaced it. Did you clean out the plenum stepper hole before fitting the new steppper, might need the crud clearing out?