Hi everyone,
I've got a mostly stock '75 GC, which has been in the family since new, and recently had an intermittent misfire/timing issue, where the car runs fine, but every few seconds, it seems to go out of timing and 'barks' or shudders before going back to normal.
I'd figured it was a potential electrical issue, so have just changed plugs, points, leads, condenser, rotor button, dizzy cap, and coil, but it still seems to have the problem, and may even be worse! I can't figure it out! Even my dad, who owned the car before me, is stumped!
Essentially, it feels like the car is running rough. Dwell angle, revs, and timing are set right, but when it seems to misfire, the timing goes out (from 5deg BTDC to about 10deg BTDC), the car shudders, and then returns to normal.
Anyone had anything like this before? Any ideas??? Definitely appreciate some help, cos I can't figure it out!
Cheers,
Luke.
Strange intermittent misfire/timing issue on a GC
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LukeAussie
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- Location: Adelaide
Re: Strange intermittent misfire/timing issue on a GC
Any chance of some more info?
eg:
Does it happen at idle only or under load?
Does it still have the problem once it gets over a certain part of the rev range?
I would assume, from your description, that it's probably unlikely an electrical problem? It's pretty hard to still be that when you're replaced everything?
When was the last time the carby was serviced?
I'm wondering if either there's a bit of crap in there?
Dave...
eg:
Does it happen at idle only or under load?
Does it still have the problem once it gets over a certain part of the rev range?
I would assume, from your description, that it's probably unlikely an electrical problem? It's pretty hard to still be that when you're replaced everything?
When was the last time the carby was serviced?
I'm wondering if either there's a bit of crap in there?
Dave...
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Re: Strange intermittent misfire/timing issue on a GC
Maybe the vacuum advance on the dizzy is playing up?
Re: Strange intermittent misfire/timing issue on a GC
If it has a mechanical counterweight advance on the dizzy I would have said lubricate that and free that up, could be after revs the counterweights stay out and only slowly return as they have not been lubricated? This would have the effect of advancing timing and returning it to normal over time.
I am unfmailiar with this dizzy though so not sure if it applies. Normally if the dizzy has mech couterwights there is a felt centre under the rotor button that you keep topped up with oil. The oil soaked felt then keeps the counterweights lubricated.
I am unfmailiar with this dizzy though so not sure if it applies. Normally if the dizzy has mech couterwights there is a felt centre under the rotor button that you keep topped up with oil. The oil soaked felt then keeps the counterweights lubricated.
Re: Strange intermittent misfire/timing issue on a GC
I have a similar problem that is doing my head in. At idle its fine, anything above idle the revs will go up and it sounds like just one cylinder misses or pings every second or so, and the higher you rev the more frequent it becomes. Same with you?
It gets better with retarded timing but it gets to a point that i cant retard it anymore. I have a very crazy cam in there however.
Carbs are brand new, plugs are brand new, it runs rich, dizzy is fully rebuilt but not re graphed for cam, but no i havent oiled the weights..
Let me know if you fix it.
It gets better with retarded timing but it gets to a point that i cant retard it anymore. I have a very crazy cam in there however.
Carbs are brand new, plugs are brand new, it runs rich, dizzy is fully rebuilt but not re graphed for cam, but no i havent oiled the weights..
Let me know if you fix it.
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Re: Strange intermittent misfire/timing issue on a GC
Shuggy, try a different set of plug leads. I had this problem ages ago (well similar anyway). The lead was intermittent and only showed up with the misfire at a certain part in the rev range. It was very frustrating. Could believe it ended up being one plug lead and that was it?
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Re: Strange intermittent misfire/timing issue on a GC
Hi Shuggy, I agree with Dave, sounds more like a lead / plug / spark issue.
Flex each lead as you use your ohmeter across the ends as this may discover open circuits not there when the lead is straight.
A cracked dizzy cap may cause the same issue and the crack may be barely visible and hard to find. The crack in a dizzy cap allows the spark to run through trapped dirt / filings to an easier earth route.
Flex each lead as you use your ohmeter across the ends as this may discover open circuits not there when the lead is straight.
A cracked dizzy cap may cause the same issue and the crack may be barely visible and hard to find. The crack in a dizzy cap allows the spark to run through trapped dirt / filings to an easier earth route.
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LukeAussie
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- Location: Adelaide
Re: Strange intermittent misfire/timing issue on a GC
Hi all,
Thanks for the replies... haven't had a chance to get back 'til now.
Some more details of the problem: the issue is most obvious at idle, and I believe it remains as revs increase, though it's hard to tell, since the frequency increases and any roughness seems to disappear. So the easiest thing to say is that the car is fine at high revs, but pretty bad at low.
The vacuum advance isn't an issue, as it's also new (got it from a guy in Queensland, Advance Diaphragm Options: decent price, quick delivery, re-furbished unit that works great!). I've also attempted fiddling with the timing without the advance connected (sealed up the vacuum line from the carb, obviously!), but no luck.
The mechanical counter-weights in the dizzy may be an issue, though the rotor button does rotate by hand and return freely (while the car is stopped). I do plan to investigate this one more closely though...
I don't think it's a carb/fuel issue, as I've taken the car out for a spin, and it drives okay, with expected pick up and acceleration, and it drives smooth even at higher speed, but just idles like rubbish.
Another point that may clarify is that when I adjust the timing by moving the dizzy, it gets close to 5deg BTDC, but is very touchy. As I tighten the nut to secure the dizzy in place, the timing changes (more than I'd usually expect), and as it continues to idle rough, the timing mark on the flywheel dances about all over the place.
Another mate suggested looking at a sticking valve, but I don't think it'd be this, though it's possibly worth checking.
Any ideas where to look? I'm leaning towards closer inspection of the dizzy... but am grateful for any responses, as I'm starting to pull my hair out :-)
Cheers!
Thanks for the replies... haven't had a chance to get back 'til now.
Some more details of the problem: the issue is most obvious at idle, and I believe it remains as revs increase, though it's hard to tell, since the frequency increases and any roughness seems to disappear. So the easiest thing to say is that the car is fine at high revs, but pretty bad at low.
The vacuum advance isn't an issue, as it's also new (got it from a guy in Queensland, Advance Diaphragm Options: decent price, quick delivery, re-furbished unit that works great!). I've also attempted fiddling with the timing without the advance connected (sealed up the vacuum line from the carb, obviously!), but no luck.
The mechanical counter-weights in the dizzy may be an issue, though the rotor button does rotate by hand and return freely (while the car is stopped). I do plan to investigate this one more closely though...
I don't think it's a carb/fuel issue, as I've taken the car out for a spin, and it drives okay, with expected pick up and acceleration, and it drives smooth even at higher speed, but just idles like rubbish.
Another point that may clarify is that when I adjust the timing by moving the dizzy, it gets close to 5deg BTDC, but is very touchy. As I tighten the nut to secure the dizzy in place, the timing changes (more than I'd usually expect), and as it continues to idle rough, the timing mark on the flywheel dances about all over the place.
Another mate suggested looking at a sticking valve, but I don't think it'd be this, though it's possibly worth checking.
Any ideas where to look? I'm leaning towards closer inspection of the dizzy... but am grateful for any responses, as I'm starting to pull my hair out :-)
Cheers!
Re: Strange intermittent misfire/timing issue on a GC
Vacuum leak maybe?
So maybe:
intake gasket?
Carby base gasket? Could also be sucking coolant through that same gasket?
So maybe:
intake gasket?
Carby base gasket? Could also be sucking coolant through that same gasket?
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Re: Strange intermittent misfire/timing issue on a GC
So the car started behaving like this before you replaced all those items listed?
The timing mark bounces around when it runs rough? I wouldn't expect that behaviour from a vacuum leak as the timing shouldn't change. (NB I am not ruling this out however)
You state the counter weights advance and release by hand okay.
This is sounding like worn distributor shaft bushes, allowing the distributor shaft side to side movement which would affect dwell, timing, etc...
Have you checked for this freeplay in the shaft?
The timing mark bounces around when it runs rough? I wouldn't expect that behaviour from a vacuum leak as the timing shouldn't change. (NB I am not ruling this out however)
You state the counter weights advance and release by hand okay.
This is sounding like worn distributor shaft bushes, allowing the distributor shaft side to side movement which would affect dwell, timing, etc...
Have you checked for this freeplay in the shaft?
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LukeAussie
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- Location: Adelaide
Re: Strange intermittent misfire/timing issue on a GC
Hi all,
I've been a bit flat out the last couple of weeks, and haven't had a chance to look at the car, but started work on it again yesterday :-)
I checked that the carbie was tight on the manifold and that it was tight to the engine, and all that was fine, which is nice.
What I did notice however was that removing the spark plug lead from cylinder three didn't seem to make any difference to the car's idle. So I'm thinking that the problem may be localised to that cylinder, and might in fact be a sticky valve (or some other, perhaps sinister) issue. Removing the other leads one-by-one obviously significantly affects the idle, but number 3 doesn't seem to have an effect. I know the lead is sparking (tested for that!), and even swapped out the lead, but no luck.
I'm going to check the actual plug tonight, in case one of the new ones I just put in was dodgey from the box (unlikely), and also do a compression test to look at the valve health. Hopefully I can work out the issue and get my Gal back on the road!
I'll keep you all posted...
Cheers,
Luke.
I've been a bit flat out the last couple of weeks, and haven't had a chance to look at the car, but started work on it again yesterday :-)
I checked that the carbie was tight on the manifold and that it was tight to the engine, and all that was fine, which is nice.
What I did notice however was that removing the spark plug lead from cylinder three didn't seem to make any difference to the car's idle. So I'm thinking that the problem may be localised to that cylinder, and might in fact be a sticky valve (or some other, perhaps sinister) issue. Removing the other leads one-by-one obviously significantly affects the idle, but number 3 doesn't seem to have an effect. I know the lead is sparking (tested for that!), and even swapped out the lead, but no luck.
I'm going to check the actual plug tonight, in case one of the new ones I just put in was dodgey from the box (unlikely), and also do a compression test to look at the valve health. Hopefully I can work out the issue and get my Gal back on the road!
I'll keep you all posted...
Cheers,
Luke.
Re: Strange intermittent misfire/timing issue on a GC
That's interesting?LukeAussie wrote:What I did notice however was that removing the spark plug lead from cylinder three didn't seem to make any difference to the car's idle. So I'm thinking that the problem may be localised to that cylinder, and might in fact be a sticky valve (or some other, perhaps sinister) issue. Removing the other leads one-by-one obviously significantly affects the idle, but number 3 doesn't seem to have an effect. I know the lead is sparking (tested for that!), and even swapped out the lead, but no luck.
Yep, exactly what I'd do with what you've said you've foundLukeAussie wrote:I'm going to check the actual plug tonight, in case one of the new ones I just put in was dodgey from the box (unlikely), and also do a compression test to look at the valve health.
Dave...
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LukeAussie
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- Location: Adelaide
Re: Strange intermittent misfire/timing issue on a GC
Hmmm, things keep getting more odd... I did a compression test on all four cylinders today, and they came out pretty much text-book perfect. Got 125psi on each cylinder (not sure how accurate my gauge is, but it's fair to say I'm getting the same compression on each cylinder!), and swapped my plugs around... still no luck. The same problem perpetuated.
I got the missus out to give a hand with some testing, and after putting the plugs back in, I got her to rev the car to 1500rpm and hold it, and I again pulled out plugs one-by-one to see what happened. As expected, plugs on 1, 2 and 4 adversely affected the car's running, and while cyl 3 didn't make a huge difference, it did drop the revs a bit. So the cylinder/plug is doing something, but just not what it should.
What I don't get with this problem is how it's actually happened or what could cause it, since it doesn't seem to be any of the electrical components (they're all new), and the vacuum all seems okay too. Cylinder 3 doesn't seem to be working properly, but has okay compression. Odd.
To answer an earlier question, before changing any of the electrical gear, the car was "mostly" running okay; it would idle smoothly, but every few mins, it would go slightly off, like a misfire, and recover fine, and it would do this more frequently as the car was hotter. Which is why I decided to change the electrical gear in the first place, in case it was an intermittent component.
However, the whole thing runs so much worse now, and there's really not that much that I did that could cause this? I wonder if it still could be a sticky valve or some other cylinder/head type problem, despite getting okay compression? Any ideas? I'm starting to pull out my hair (lucky I've got a good boof with plenty of hair to pull!).
Thanks guys!
Luke.
I got the missus out to give a hand with some testing, and after putting the plugs back in, I got her to rev the car to 1500rpm and hold it, and I again pulled out plugs one-by-one to see what happened. As expected, plugs on 1, 2 and 4 adversely affected the car's running, and while cyl 3 didn't make a huge difference, it did drop the revs a bit. So the cylinder/plug is doing something, but just not what it should.
What I don't get with this problem is how it's actually happened or what could cause it, since it doesn't seem to be any of the electrical components (they're all new), and the vacuum all seems okay too. Cylinder 3 doesn't seem to be working properly, but has okay compression. Odd.
To answer an earlier question, before changing any of the electrical gear, the car was "mostly" running okay; it would idle smoothly, but every few mins, it would go slightly off, like a misfire, and recover fine, and it would do this more frequently as the car was hotter. Which is why I decided to change the electrical gear in the first place, in case it was an intermittent component.
However, the whole thing runs so much worse now, and there's really not that much that I did that could cause this? I wonder if it still could be a sticky valve or some other cylinder/head type problem, despite getting okay compression? Any ideas? I'm starting to pull out my hair (lucky I've got a good boof with plenty of hair to pull!).
Thanks guys!
Luke.
Re: Strange intermittent misfire/timing issue on a GC
Timing?
I had the same issue with mine when I first got it running, had the timing too far advanced, turns out it likes about 25btdc, lol
I had the same issue with mine when I first got it running, had the timing too far advanced, turns out it likes about 25btdc, lol
Re: Strange intermittent misfire/timing issue on a GC
I would try several things:
1) take the plug out connect your comp gauge and start the car and watch the comp gaug for fluctuation (mech failure)
2) turn the dizzy cap over and check for arcing marks or gouges. Could be signs of out of centre shaft etc (previously mentioned)
3) run the timing light on this cylinder to see if the misfire is electrical (sparotic flash of the light)
If this all checks okay then you have to be chasing fuel air issues.
1) take the plug out connect your comp gauge and start the car and watch the comp gaug for fluctuation (mech failure)
2) turn the dizzy cap over and check for arcing marks or gouges. Could be signs of out of centre shaft etc (previously mentioned)
3) run the timing light on this cylinder to see if the misfire is electrical (sparotic flash of the light)
If this all checks okay then you have to be chasing fuel air issues.
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LukeAussie
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- Location: Adelaide
Re: Strange intermittent misfire/timing issue on a GC
Hi guys,
Thanks for the tips. I'll take another look tonight...
Just another thought came across my mind... the new plugs I installed are resistor plugs, where previously had non-resistor plugs. I'm not sure that it would make much difference, but I'm starting to clutch at straws here... I wonder if the combination of resistive leads and plugs could be causing grief? Resistors with inductive/capacitive loads (like coils, condensers, etc) do make for a time delay and/or weakening of the spark. Has anyone else had these types of issues?
I'll keep you posted with what happens...
Luke.
Thanks for the tips. I'll take another look tonight...
Just another thought came across my mind... the new plugs I installed are resistor plugs, where previously had non-resistor plugs. I'm not sure that it would make much difference, but I'm starting to clutch at straws here... I wonder if the combination of resistive leads and plugs could be causing grief? Resistors with inductive/capacitive loads (like coils, condensers, etc) do make for a time delay and/or weakening of the spark. Has anyone else had these types of issues?
I'll keep you posted with what happens...
Luke.
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LukeAussie
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- Joined: Fri May 18, 2012 9:55 am
- Location: Adelaide
Re: Strange intermittent misfire/timing issue on a GC
Well, to answer my own post, changing the plugs over didn't do much. The car's still running rough as, with an occasional puff and splutter at the tailpipe. I'm going to go down the air-leak/vacuum checking path, but think I'll save it for the weekend. It's too hard coming home from work and putting in a "second day" at the office under the hood! Ha ha! Bring on daylight savings!
I'll keep you all posted with anything I find. And fingers crossed I do find something... can't wait to get my orange baby back on the road!
Luke.
I'll keep you all posted with anything I find. And fingers crossed I do find something... can't wait to get my orange baby back on the road!
Luke.
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LukeAussie
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- Location: Adelaide
Re: Strange intermittent misfire/timing issue on a GC
Well, I think I finally got everything all sorted, and my little GC is back on the road!
It turns out that it looks like I had three problems that revealed themselves in stages. First was my intermittent misfire, addressed by changing the electrical stuff. Once sorting this out, the car ran rough-as-guts at idle, which ended up being the result of a dodgey gasket on the carbie-inlet manifold, which was likely letting in water to the inlet too. So that was nasty. Fixed it up, but still the idle wasn't quite right. We tuned her up, got everything right, and the idle would be okay, but then would rise slightly before settling down again. I had quick chat to a mechanic mate, and he suggested looking at the fuel/air mix, which was well off. A little tweak with that, and she seems sweet as once again! So that's made me pretty happy. (It may have even been a dodgey fuel/air mix since I recently changed from 91 Octane to 98, about 3-4 months ago, but never adjusted the mix or tweaked anything from then...)
So if you're in Adelaide, look out for my little orange girl cruising around each day!
Thanks for the tips and advice everyone!
It turns out that it looks like I had three problems that revealed themselves in stages. First was my intermittent misfire, addressed by changing the electrical stuff. Once sorting this out, the car ran rough-as-guts at idle, which ended up being the result of a dodgey gasket on the carbie-inlet manifold, which was likely letting in water to the inlet too. So that was nasty. Fixed it up, but still the idle wasn't quite right. We tuned her up, got everything right, and the idle would be okay, but then would rise slightly before settling down again. I had quick chat to a mechanic mate, and he suggested looking at the fuel/air mix, which was well off. A little tweak with that, and she seems sweet as once again! So that's made me pretty happy. (It may have even been a dodgey fuel/air mix since I recently changed from 91 Octane to 98, about 3-4 months ago, but never adjusted the mix or tweaked anything from then...)
So if you're in Adelaide, look out for my little orange girl cruising around each day!
Thanks for the tips and advice everyone!