Back- and Mis- firing issues...

[jackkelleher]

Well, Minsk is now on the road again, and we've been using her a lot in the last few days in order to give the engine a good running. However, there are a few problems with general running, though nothing too serious.
When the engine is warm (about ten minutes' normal driving at average speed about 45) the car starts to misfire when in all gears when the foot is removed from the pedal. The car idles healthily, though some choke is always necessary for a while after starting or she cuts out when the engine cycles down to idle from running. She cuts out even after the time mentioned above if there is no choke and the revs are not kept fairly high when in neutral or with the clutch down. The problem gets progressively worse, and eventually the engine starts backfiring in third and fourth as the foot, again, is removed and the revs go down. Someone suggested dirt in the fuel system?..
By the way, what's the way to tell revs to an exact figure? I have no rev meter... I wouldn't have the faintest if she idles at 800 or 2000 (though i could tell them apart, obviously!)
The exhaust is completely colourless and invisible, according to the car behind (a family member acting as an escort in case of breakdown...) but the interior of the exhaust is black and sooty, as are the spark plugs when removed, and random flecks of black liquid leave the exhaust at idle (seems like petrol).
The first drive she did, she eventually cut out, though she did restart first time with no effort. I checked the plugs and they were slightly too wide in the gap (0.045) so I put the gap to .025. That hasn't stopped the misfires though.
As people following my other postings will know, I have just cleaned and replaced a lot of the ignition parts... So the problem is unlikely to lie there, though it's possible.

This is probably a really simple problem, sorry people...

[jackkelleher]

New Idea. Could a surfeit of lead replacement be causing the problem?

[Packedup]

Aside from doing the usual compression, valve clearances and igntion tests, I'd be looking at the carb. Partly because I think they look nice, but also because it's probably not set right. And if it is, then see if the choke linkage is operating correctly, and not sticking on.

[Kevin]

but the interior of the exhaust is black and sooty, as are the spark plugs when removed,

Well if the plugs are black its definatly running far to rich as packedup has said check the carb settings.

[jackkelleher]

The carb checks in the Haynes require set rpms. How do I determine these? It's most likely to be the carb, I think.

[alex_holden]

If you don't have a rev counter it's possible to use a fluorescent light and a stroboscopic disc attached to the crank pulley. See this article for more information. Remember the line frequency in the US is 60Hz.

[jackkelleher]

All right... Alternatively, is it possible to fit a rev counter easily? And obtain one?

[alex_holden]

They shouldn't be too difficult to fit I wouldn't have thought, though it'd need to have the same polarity as your vehicle. Personally I use an old mechanical tachometer I bought on eBay a few years ago.

[Packedup]

Easy to fit, easy to find... But for just diagnostics you might be better off with a hand held thingy - I've got a couple, neither cost a great deal at the car boot sale. Will let you check up to 20v (plenty for a 12v car system) dc, dwell, revs, etc.

[Willie]

A sooty exhaust indicates a rich mixture but stalling on deceleration indicates a weak mixture. You need to try the lifting pin, beneath the piston chamber. Take up the free play and then raise the pin by a 1./16"
the revs(at tickover with the engine at normal running temperature) should rise slightly and then settle back again. If the revs go up then the mixture nut needs to go UP, if the revs drop then the nut needs to go DOWN. Adjust this nut one flat at a time.

[jackkelleher]

Well, I've tested and the revs drop sharply. So I'm going to enrich the mixture... I'm still slightly uncomfortable with it, though, considering the deposits being so black...

[jackkelleher]

Well, the lower adjustment screw changed the revs noticably as I screwed it in and out, going down as I unscrewed down and up as I screwed up. Is this normal?
However much I screwed in either direction, the revs still went down when the piston was raised, so in the end, seeing as screwing down made the revs fall to a range where the engine was only just barely revving and cut out when the piston was raised, I did the opposite of what I was planning to do and screwed up. Which ostensibly has weakened the mixture. But which in fact made no difference to the rev drop and therefore, as it did in fact make the engin rev at a more comfortable speed, seemed the sensible option.
Please tell me if I got it all wrong... I don't want to compromise the car's running.
Should I change the other screw? Are the two perhaps switched (though I don't see how they could be)? Both screws have the same net effect; increasing or decreasing the throttle incrementally, it's just that the lower one (which I've altered) does so with reference to the choke.

[alex_holden]

I think you may be adjusting the choke screw rather than the mixture nut (the big hex-head brass thing on the bottom of the carb).

[jackkelleher]

Aaaaahhhh... God, I'm a dolt.

[bmcecosse]

Using the 'lift' pin requires a deft touch - you only actually lift the piston by about 1/8" each time. Just enough to test the mixture. You don't need a revmeter - just judge it by sound and feel. The slowest practical idle is about 500 rpm. You can adjust the mixture at idle until it's as smooth as possible, then adjust the timing forwards and back slightly to again get the smoothest setting, then do the mixture again. It's not difficult. Make sure the carb piston can rise easily with a light finger push and then falls back with a light clunk. Put some thin oil in the top of the carb. Also check for any leaks on the little tube that runs from the carb to the dizzy - and you need to check if the vacuum advance in the dizzy is working properly. Don't worry about exhaust pipe colour - that's just the short runs - and the little flecks are just condensed water again because of short running time.

[jackkelleher]

Thanks...
When I lift the piston it does resist a bit, and I do need to apply a bit of force. When it slides down, it does so slowly and touches down so gently that there is no sound whatsoever. Sometimes it may not even quite reach the bottom...

[alex_holden]

That doesn't sound right. Take the top off the carb and have a look at the piston and the damper mechanism. I wonder if the big spring could be broken/missing?

[jackkelleher]

Oh, it's definitely there- I had the carb completely out and apart sometime last year, and the spring was present and correct. It could have broken since, though...
By the way, does adjusting the timing mean removing the front of the car? Because if so I can't do it, as the front end is horribly misaligned and will a) be extremely hard (if not impossible without breaking something) to take apart and b) be nigh on impossible to put back on the way it is...

[bmcecosse]

No - adjusting the ignition timing just means twiddling the little knurled knob on the side of the dizzy. Carb doesn't sound right - take the damper rod out (ie unscrew the black plastic knob on the top) and try the rise/fall test. It should certainly fall back with a nice 'clunk' now. If not - remove the bell and the piston and carefully clean them both with white spirit - maybe just a touch of Brasso (metal polish liquid) - certainly nothing more abrasive. Also try to decide if the needle is straight - or has it been slightly bent. Then re-assemble and try again.

[jackkelleher]

Well, the needle is straight, but it moves considerably within its housing. With the tightening screw fully tightened, there is a play of about two millimetres in every direction. I have dismantled and rebuilt two or three different carbs (I have a whole load lying around) and none of them have had this.
I've redone that unit and the piston moves easily for about half a centimetre, and after that the resistance is so great that I can barely lift the piston. It settles with a nice clunk, though. Could it be that the oil is overfilled in the damper? How full should it be?