Carb'd 308 with electronic ignition - idle rises & falls.

I did start a similar thread a while back but I've done a fair bit of work over the winter and I think the problem can be narrowed down.

There are lots of way the problem manifests itself but I think this is the most telling:

1. Car warmed up. Stationary.
2. Lift foot of throttle and get it to idle at 1100 rpm
3. As I walk away from the car the idle will drop to 600rpm/800rpm for a mintute or two
4. The engine revs will then pick up fairly sharply to 1200rpm/1300rpm
5. Give it a minute or two and it will slowly drop back to 600rpm/800rpm and stumble along
6. Repeat

What I have done so far:
1. New set of plug & king leads
2. Checked, cleaned and gapped plugs
3. Checked and cleaned Distributor Cap (contacts all clean).
4. Checked, cleaned and lubricated Advance/Retard mechanism (SM805A - bob weights)
5. Checked timing at flywheel - spot on
6. Checked carburettor throttles/butterflies are closing positively

I'm now thinking that one of the coils is on it's way out. And that bank is getting a variable strength spark???

All contributions welcome!

 

That's a good point BUT my counter would be that this problem first occurred last year (i.e. before I touched anything). Up until that point it had been running very sweetly. The problem I have now is the same as I had last year.
I'm very confident that following my attempts to resolve the issue, I have put everything back exactly as it came out. But re-checking how the dizzy is set is a great shout and will go on my list of things to try!

 

Thanks Sam, that all makes good sense. This problem, having persisted for some time, therefore, is unlikely to be a coil failure.

The fact that the idle increases and decreases on it's own, with the car stationary, suggests to me that it can't be anything mechanical (especially as I have checked the advance/retard mechanism).
I don't think it can be the timing. When I set the distributor, I checked that cylinder #1 was at the top, I lined up the rotor and the the distributor using the red marks, started the car and the timing light literally locked onto the 6 degree market. I didn't have to rotate the dizzy or anything, it was perfect. Interestingly, I checked the timing immediately after putting all the bits back (dizzy, leads, plugs, air cleaner etc.) and it must have been idling OK then as I thought I had resolved the issue. It's only when I started the car up subsequently that the problem reappeared. Could this be a battery/alternator type issue? Sorry to think out aloud.

 

Single Distributor.
My experience with carbs is that once set they don't go out of sync. (I've had the car almost 25 years). I rebuilt them about 5 years ago and the car has been running better than ever. I did check the balance on the rear bank and they were good, I didn't/couldn't incorparate checking against the front bank pair, as it would have involved removing the engine cover.

Would an air leak cause the idle to vary under static conditions?
I guess checking the balance side/side & front/back would kill both of these birds with one stone.

One other scenario to ponder.
Pulling away, the engine feels slightly underpowered and doesn't sound as sharp as it would normally. Pushing the throttle further and further, the power will tail off at about 4000rpm (it feels like the brakes are on) and the engine note will go a bit odd. After a few seconds or more, the engine will surge, the revs will leap and the power returns. When decelerating, the engine burbles more than normal and the exhaust will crack loudly occassionaly. Either this is unburnt fuel igniting because there was no spark or it could be a proper back fire because of the timing? I don't know.

 

Vacuum leak does sound plausible - my first search produced this:
"The normal repair person might think that the ignition system has caused the symptoms. Hiding just out of reach, the illusive vacuum leak can be detected if you know what to look for."

Which pretty much describes my efforts so far and this thread!

 

My car has always pulled well, rebuilding the carbs 5 years ago was an improvement in tractability and emissions (it was able to pass an MOT test without 'help' ! )

 

Thanks Derek. That's useful insight. I am very, very aware of that thread!

Interesting what you say regarding air leaks being less significant at higher revs. This does makes sense. I didn't mention the performance at higher RPM's earlier as it could have been information overload.

 

Well, I've fixed the idle. I removed the cold-start mechanism. It didn't seem to be moving smoothly. The idle is now steady.
I will clean up and test the cold-start mechanism because I don't think it's a bad thing to have - but there's no rush.
But this isn't an issue with the idle any more.

However...
The car is still losing power at around 4000rpm, pushing the throttle down the engine note hardens but there is no increase in power.
If I lift off there is occassionaly a very loud crack out of the exhaust, as the revs drop. If I carry on accelerating at some point the power will suddenly return and car will leap forward.

I've dismissed a vacuum leak for the now.
I had sprayed 'Easy Start' round the base of the carbs with no noticeable change in engine note or rpm. But the car is idling steadily now, which it wouldn't if there was a vacuum leak by all accounts.

I'm happy that the timing is correct (see earlier posts) at idle anyway.

I really don't want it to be electrics!

 

Thanks Derek, I appreciate your input. The problem is that the misfire or lack of power isn't constant or consistent. I accelerate up to around 4000rpm and then the engine note changes (to a thrum) and the power stops coming. It feels like the handbrake is on (it isn't!) After a few seconds, it magically clears and the car accelerates & leaps forward to match how far the gas pedal has been pushed. This doesn't happen every time. As a result, I don't think there would be a noticeable temperature difference in the exhaust. I will give it a go though.

When the car doesn't get bogged down, it's running fine and feels as lively as ever, this makes me think the timing is good at higher revs (the timing at idle is spot on).
The loud crack that I occassionaly get, sounds more as though it would be a backfire though rather than fuel burning off in the exhaust. But this WOULD indicate timing?
The advance/retard bob weights were fine, I gave them a very light oiling and they were moving freely - I am happy that they aren't sticking.