Points Gap and Dwell

[Ducked]

The Car (Daihatsu Skywing, Like a G10/G11 Charade) has rather suddenly stopped idling.

It'll start, and run if I keep the revs above about 1500, but it stalls out if I take my foot off the accelerator. Hasn't recently run out of petrol or experienced any other problems that seem especially likely to have introduced dirt to the carburretor.

I havn't adjusted the points since I got it, so I thought I'd better start there. On the Skywing it seems more difficult than I remember.

Firstly, the distributor faces to the rear, so you can't really see what you're doing.



I tried using a mirror but then all your actions are reversed. This would take a long time to get used to, and you might not be able to walk once you had.

Short of lowering myself head-first into the engine compartment, Mission Impossible stylee,

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my best bet might be to rig up a webcam and laptop so I can see what I'm doing. I tried this but still couldn't see very well. I'll try it again with a light tomorrow.



The Charade G10 engine manual (distributor seems identical and the points look like a Nippon-Denso design) pretty much ignores the points gap. It specifies the heel gap, and gives the same figure for the points gap as "reference information" . I'm surprised these figures are identical, since it doesn't seem to necessarily follow, but maybe they designed it that way deliberately.

Points gap and heel gap are clearly related, but I'm not sure if its enough just to set the heel gap and assume the points gap will be correct. For one thing, I've lightly filed the points which might alter the relationship a little.

They perhaps ignore the points gap because they tell you to check the dwell angle. I don't have a dwell meter here and would probably have trouble finding anyone who knew what one was, though I suppose I could get one sent from abroad or possibly find a high-end non-automotive meter that did duty cycle. I had a Gunson meter in the UK that did tach and dwell (and some other stuff) but its probably in landfill by now.

I suppose an ordinary analogue meter reading resistance would tend to average a rapidly oscillating signal like the primary ignition circuit, due to its mechanical inertia, so its indication would be proportional to dwell, but I dunno if it'd be linear or accurate enough to be useful.

Would it be worth measuring the dwell angle "statically" (i.e. with the engine cranked manually) , fitting a pointer to the distributor-arm and directly measuring the angle it moves through while the points are open/test light is off?

 

[eljefino]

I wonder if this isn't "classic" points ignition but it uses an old-school distributor to fire an HEI system, so all it needs is for the points to open and close "some of the time".

 

[BigCahuna]

Before I'd mess with the points, I'd change the fuel filter first, and see if the problem goes away. It sounds like you sucked a bunch of dirt into the filter when you ran out of gas. If your points were out of adjustment it wouldn't run much at all.,,

 

[Eddie]

Or use a clean feeler gauge. )

 

[Number_35]

I was never great with setting point gap - dwell angle was the gold standard.

I'm sure dwell meters are still available - I've still got one myself.

When I worked as a tune-up tech many years ago, we used to remove the distributors from 1st-gen Honda Civics to replace the points & condenser. The distributor sat horizontally, and it was easy to drop the mounting screws otherwise. We'd turn the harmonic balancer to TDC first, with the rotor pointing @ #1.

It was typical for Japanese cars to have a specified dwell angle of about 52 degrees, if I recall correctly.

As well, the adjustment screw was offset so that turning it varied the gap and thus dwell angle.

So we'd remove the distributor, replace the points & condenser, replace the distributor, start the engine, and check the dwell. If the dwell was off, we'd stop the engine, pop the distributor cap & rotor, and tweak the adjustment screw (easy to do with the distributor in place). Repeat as necessary.

Dwell is the number of degrees the shaft turns while the points are closed - thus, the number goes up as the number of cylinders decreases. If you didn't have specs, the rough estimate was (360 degrees/number of cylinders) x 2/3, which yields 30 degrees for V8s, 40 degrees for 6s, and 60 degrees for 4s. So, as point gap increases, dwell decreases.

Each degree change in dwell changes timing by one degree. So, check the timing after setting the point gap/dwell angle.

Good luck!

 

[SteveSRT8]

This is where you need my old distributor calibration machine!

 

[Wampahoofus]

If you have a digital multimeter that reads duty cycle, that will read dwell for you. As others have said, shoot for 55-60 degrees for a 4 cylinder if you don't know the manufacturer's spec.

Edit: Whoops, it's a 3 cylinder. Going by the 'old math', that would put you at 75-80 degrees.

 

[bmwtechguy]

A great place to find a dwell meter is eBay or garage sales....not sure if that is an option for you.

 

[Wampahoofus]

Well, I did a right awful job explaining myself, and my edit time expired

Let me try again.

If you have a digital multimeter that reads duty cycle, that will read dwell for you. As others have said, shoot for 54-60 degrees for a 4 cylinder if you don't know the manufacturer's spec. The meter will read in %. Example: if the spec is 54 degrees, the meter will read 60%. (remember, 360 degrees/# of cylinders is the maximum amount of dwell - so for a 4 cylinder maximum dwell will be 90 degrees. 54 degrees specified dwell/90 degrees maximum dwell = 60%). Likewise, if you set it to 60 degrees, the meter will read 66.7%.

Edit: Whoops, it's a 3 cylinder. Going by the old rule of thumb, 2/3 of maximum dwell, that would put you at 80 degrees, or 66.7%.

 

[Bottom_Feeder]

Originally Posted By: Ducked
The Car (Daihatsu Skywing, Like a G10/G11 Charade) has rather suddenly stopped idling.

-snip-

I havn't adjusted the points since I got it, so I thought I'd better start there.

I don't understand your troubleshooting method here.

If the problem arose suddenly, why would you start tinkering with a mechanical part that isn't known for causing immediate problems like yours? If anything, by touching the points (which have apparently worked fine up until now) you've introduced another variable for why the car won't run correctly.

 

[69GTX]

For all those years I adjusted points on my daily drivers I never once looked at dwell angle. Set the gap with a feeler gauge, advance the timing 0-15 deg BTDC based on application. A few times I changed springs to adjust total timing. The cars always seemed to run nice and smooth. For whatever reason, dwell angle wasn't something I was taught/told.

 

[HerrStig]

With an volt meter or 12 volt bulb across the points , set the point gap so that the light goes ON or the meter reads 12 volts when the points are riding on the very top of the cam lobe. That's close enough to get it running. Still, points don't wear out over night. If you can SEE them open at the top of the lobe they're likely ok.

 

[Brybo86]

poor idle with a questionable gas history

I would definitely suspect the idle/pilot circuit of the carb

take the bowl off and see what kind of junk is in there

 

[Brad_C]

Originally Posted By: 69GTX
For whatever reason, dwell angle wasn't something I was taught/told.


For most mild commuter vehicles it's not particularly important.

Too much dwell and you cook coils, too little dwell and you get a weak spark. The thing is dwell was/is kinda a hack. It was the best way available at the time to set the energy available to charge up the coil. The problem is the charging time of a coil is actually simply a factor of the coil, ballast resistor and system voltage. The dwell is a percentage of crankshaft rotation, which means as an absolute time it's maximum at idle and gets progressively smaller as the revs increase. The impact of this is you have maximum spark energy at low revs and it progressively falls off as the revs increase because the time available to re-charge the coil gets smaller and smaller as the engine speeds up.

These days it's all computer controlled and simply not a factor, but with a points system you can significantly alter an engines performance in difficult to fire conditions (lean burn, super rich burn, under boost..) by ensuring the coil is fully charged. This can mean at low speed you wind up with a hot coil and ballast resistor as any energy excess to that required to form (charge) the magnetic field in the coil is dissipated as heat.

That turned out more complex than I had intended it to be and probably confused more than it explained.. sorry about that.

 

[CBR.worm]

That was actually a pretty good refresher of something that I haven't had to think about in 30 years.

 

[Ducked]

Thanks all for the replies. Just scanned them so far.

Re the "questionable gas history" there is no questionable gas history. The car runs on petrol, and it has not been run dry.

That's what I wrote, and that's what I meant.

Of course that doesn't actually exclude dirt in the petrol, and I suppose that'd logically be the first/best guess, as pointed out above.

I started with the points because I'd been putting it off.

Got a light and did the webcam thing this morning to get the points set. Didn't fix it but didn't make it any worse. I'll look into getting a dwell meter, starting with trying to get my brother to send me my old one from the UK.

Spose I'd better look at fuel then. This may involve trying to understand all the anti-pollution plumbing hanging off the 80's carburettor. Last time I did that I got a headache and my choke stopped working.

 

[mk378]

Carburetor car that drives OK but stalls instead of idling is the classic symptom of... a vacuum leak. Excess air leans out the mixture and it dies. With the thottle open the leaking air is a negligible percent of that that is being burned and it runs OK.

Potential big leak points are the brake booster, canister purge, and EGR system, as well as of course any of the various hoses.

The other likely cause would be as someone else noted, blocked idle circuit in the carb.

Many cars had an "antidiesel solenoid" which intentionally cuts off the idle fuel when the key is turned off to prevent a knocking and rattling after-running incident. (The sound that turns everyone's heads in the parking lot and makes them say "What car is that? I need to make sure I never buy one of those.") The solenoid needs to be working and receiving power or the car won't idle.

If the throttle plate closes too far (e.g. fallapart of the idle stop / fast idle cam) engine will stall. You would be able to get a normal 800 rpm idle by very gently pressing the gas in that case. If you have to gun it to 1500 to stay running at all it's not that.

It's almost certainly fuel related not going to be the sparks.

 

[Ducked]

Originally Posted By: CBR.worm
That was actually a pretty good refresher of something that I haven't had to think about in 30 years.


Bingo. Car's 1986. I'm living history.

 

[Ducked]

Originally Posted By: mk378
Carburetor car that drives OK but stalls instead of idling is the classic symptom of... a vacuum leak. Excess air leans out the mixture and it dies. With the thottle open the leaking air is a negligible percent of that that is being burned and it runs OK.

Potential big leak points are the brake booster, canister purge, and EGR system, as well as of course any of the various hoses.


The other likely cause would be as someone else noted, blocked idle circuit in the carb.

Many cars had an "antidiesel solenoid" which intentionally cuts off the idle fuel when the key is turned off to prevent a knocking and rattling after-running incident. (The sound that turns everyone's heads in the parking lot and makes them say "What car is that? I need to make sure I never buy one of those.") The solenoid needs to be working and receiving power or the car won't idle.

If the throttle plate closes too far (e.g. fallapart of the idle stop / fast idle cam) engine will stall. You would be able to get a normal 800 rpm idle by very gently pressing the gas in that case. If you have to gun it to 1500 to stay running at all it's not that.

It's almost certainly fuel related not going to be the sparks.


Thanks. Some stuff to check there. Don't think it's got an anti-diesel solenoid (I don't have a manual for this car, but I've got a G10 which is similar) because it used to run-on before I water-cleaned the cylinders.

 

[Ducked]

The idle seems to have improved, though I dunno why.

I'd blown butane over the plumbing surrounding the carb, and over the intake manifold, but the engine note only changed when the gas was directed at the air intake, so I didn't detect any vacuum leak.

I dunno how convinced I am by this procedure though, and might try smoke. Joss sticks might work, though it might look as if I've gone native and am invoking the help of some local deities, of which there are a lot.

I cleaned the fuel filter (auto-accessories places tried didn't have a replacement)by back-washing with a hypodermic syringe and water/detergent, my carb cleaning weapons of choice. You can't actually see the filter surface so this procedure isn't ideal, and I'll try and source a see-through/take apart replacement.

Some rust washed out. I doubt this had any effect though, since I'd expect a clogged fuel filter to have a greater effect at high revs where fuel demand is higher.

I'd planned to give the fuel tank a drain-and-flush, since it has a drain plug and its probably never been done.

That didn't work out so well, since, although I cleaned around the drain plug, fuel tracked along the under-surface of the tank and got pretty thoroughly contaminated.

Since I didn't want to loose all the fuel (tank was about half-full) I aborted. Lost about a gallon (real gallon, not one of those wee toy US gallons for measuring cowboy hat sizes) but I can use it for cleaning a motorcycle chain or something once it settles. I'll let the tank get almost empty and then do it again.

I suppose its possible the partial drain of the fuel tank removed some troublesome sediment.

In the US/Europe this might be more plausible, since water/alcohol phase separation is apparently now an issue there.

I don't know if Taiwan petrol contains alcohol, but I'd have thought not, since there's no farmer grain-alcohol lobby, and the sugar industry (which used to produce lots of alcohol) was killed off ages ago by WTO membership eliminating protective tariffs.

Anyway, for whatever reason, its somewhat improved. Thanks all.