8 replies to this topic

[mikko]

I'm trying to diagnose a rough stuttery engine for a mate, it starts and idles fine maybe a tiny bit lumpy but there may be a cam in it, its undrivable with hesitation but revs freely in neutral and when i tried to check timing the strobe was all over the pulley erratically from tdc to about 20before and wont stay put. new plugs, leads, cap, rotor, points. can a duff coil cause this sort of thing? i'm out to get a new one tomoro. thanks

[jaydee]

At what revs you have the problem?
With an hot cam it wont idle smoothly at less than 1100-1200 rpm and the gun will read exactly as you describe.
I had this problem also with a faulty sports coil once and once due to a bad earth.

[sledgehammer]

Obvious question - did you remove & block the vac advance pipe ?

if you did - check the rotor for movement sideways

also a dry rotor can cause the points heel to vibrate & wear- so a bit of grease on them may help (only a bit)

check the earth cable on the plate

& even new condensers can be faulty - giving weird faults

best of luck

[tiger99]

Electrical faults including the condenser will certainly cause bad running, but they do not change the instant of points opening, which is what triggers the strobe.

Timing jitter is almost always caused by severely worn distributor bearings or timing chain. The distributor is easily checked, just take the cap off and feel for sideways movement in the shaft. More than a couple of thou is too much.

[mikko]

thanks guys, that a few things to go check/change, i did check for play in the dist. shaft and there was none

[dklawson]

Before you BUY a new coil, see if you have a friend from whom you can borrow one.

Coils do fail but their failure symptoms typically do not show up at idle. The normal behavior for a failing coil is increased misfiring with engine speed OR misfiring and failure when the engine/coil is hot. That said, I have seen a coil failure that showed up as severe fluttering of the tachometer at certain RMP.

When looking at the dizzy, make sure that the points are mounted and gapped properly. If the dizzy has not been serviced in a long time, it might not hurt to take it out and inspect the components below the breaker plate. Make sure the advance springs are intact and connected and that the bob weights are free to move. Since compared to other parts coils are expensive, double check the distributor/coil connections, check the dizzy for play, and borrow a test coil if you can. Save that coil purchase for after investigating other possible failures.

[Shifty]

Have you pulled a plug out to check that the mixture is correct? Also the dashpot has oil in it? Also vaccum leaks as well

Changing the points to electronic ignition can make a big difference, the cheap ebay accuspark ones are ok.

Personally I don't bother with a timing light on minis, I set it by ear.

[mikko]

i found an old gold lucas coil that i changed out for a new one 5 years ago when my mini was A powered, fixed. never have seen a bad coil behave like that.

thanks for the input.

[dklawson]

I'm sorry it turned out to be the coil but I am glad you had an old one to try first before buying a new one. It's always good to have some spare parts around for things like this.