removing shrunk on gear

[Burnerman]

Its a bit difficult to tell at this stage, but the Stromberg is a pretty simple bit of kit -as is the SU. The dash pot oil is too thin, and I'd say the oil is disappearing past the dash piston and being burnt - maybe due to wear.
Is the jet adjustment the same as the SU - the large nut on the bottom?
Electrical wise, I've had all sorts of issues regarding duff points and condensers (excess sparking across the points) and using a coil that isn't compatible with a ballast resistor.
Did you use a strobe lamp to set the timing?
John

 

[jj4091]

I've emptied the dash pot of the light oil & replaced with engine oil (did not realise there only needs to be about an inch of it in the tube) but it has not made any difference, Swopped the coil for a spare one I had but it seemed to run worse with that one in. I checked the input volts to the coil & it is only 4.5v with the ignition switched on & 11.5v with the engine running. I am presuming this is due to the ballast resistor but not sure exactly what volts there should be. I am tempted to just wire 12v from the ignition to see if a little higher voltage would make any difference.

 

[Burnerman]

The ballast resistor lowers the voltage going through the ignition system in an attempt to reduce wear on the contact breaker points, so theoretically the coil gets a full 12v for starting up, and then this reduces to about half that when the engine is running. The ballast resistor may be a solid component or contained within one of the coil wires.
How effective the system is, I've no idea!
I guess the coil is connected the correct way round?
I recall in the days when I ran knackered cars with knackered SU's the dashpot oil would often vanish after a while - the difference was negligible.
John

 

[jj4091]

That seems to be the exact opposite to mine with the volts low with engine not running & high once it is. Yes the coil is connected the right way round.

 

[Burnerman]

You could temporarily bypass the existing coil by connecting a known good one direct to the battery positive, and then onto the distributor to see what happens?
John

 

[Harry Bloomfield]

The ballast resistor lowers the voltage going through the ignition system in an attempt to reduce wear on the contact breaker points, so theoretically the coil gets a full 12v for starting up, and then this reduces to about half that when the engine is running.

Idea was - when the engine was being cranked, the battery voltage would drop so less battery voltage available at the coil. To get around this, they fed a coil rated at around 9v with the full battery voltage whilst cranking, then when not cranking the resistor was in circuit. Sometimes the resistor is an actual resistor mounted on the coil bracket, sometimes they used resistance wire in the loom.

 

[Harry Bloomfield]

Yes it is a Sromberg carb, I don't think it is a spark issue as everything is relatively new. Having said that I did fit one of those electronic ignition kits & had to revert back to points as it did not perform well. Wouldn't the electrical side break down more under load though? I'm thinking it's some sort of fueling issue though because of the fact that it happens when it does.

I have in the past, traced such problems to ignition systems or inadequate spark. More air, more fuel and not enough spark to properly ignite the mix in the cylinder.

In the day, I had great success with converting systems over to electronic ignition. The fact that your didn't perform well, suggests there is an issue with the HT side - coil, leads, or plugs. These are fed with a more robust voltage by the electronic system and under more strain, something breaks down.

 

[jj4091]

I have in the past, traced such problems to ignition systems or inadequate spark. More air, more fuel and not enough spark to properly ignite the mix in the cylinder.

In the day, I had great success with converting systems over to electronic ignition. The fact that your didn't perform well, suggests there is an issue with the HT side - coil, leads, or plugs. These are fed with a more robust voltage by the electronic system and under more strain, something breaks down.

Yes I am thinking that may be the first thing to do in order to definitely rule out either a fuel or spark problem. Thanks

 

[T5 TDI]

I remember that inlet manifold hotspot collapsing burnerman mentions too! Bloody things! Make sure the distributor spindle isn't worn leading to side to side movement and varying points gap. If I remember correctly that also shows up on a strobe timing light check as the notch on the crank pulley shows as jumping around.

Some of those old Stromberg carbs have the mixture adjustment down through the top with a special tool which both holds the piston and has a long allen key down the centre. It shouldn't be far enough out to cause a problem except at idle though. An air leak is a common cause of popping back on the overrun. Make sure the inlet manifold gasket isn't leaking and be sure to check the servo pipe and that manifold hot spot if it still has it. I remember the HC Vivas had it but not sure about the Chevette.

 

[jj4091]

I've just fitted a 3ohm coil & electronic ignition & bypassed the ballast resistor. Still mis-firing. I noticed during this work that the coil to distributor lead looks a bit iffy so waiting for a new one. If that does not cure it then checking the inlet manifold is the next thing I will try, but the engine only has just over 50k miles from new so I hope it is not something like that. The plug colour tends to indicate that the mixture setting is spot on, but it does seem to blow a lot of soot out of the exhaust when first starting, I'm hoping that is just condensation in the silencer getting blown out. I did notice something else that I really cannot understand which is that if I drive it backwards in reverse gear it doesn't misfire at all over 100 yards, but going forwards over the same 100 yards it starts to mis again. I really can't see any logic in that though so am tending to ignore it as any indication of anything. Unless of course someone here knows better

 

[Harry Bloomfield]

I did notice something else that I really cannot understand which is that if I drive it backwards in reverse gear it doesn't misfire at all over 100 yards, but going forwards over the same 100 yards it starts to mis again. I really can't see any logic in that though so am tending to ignore it as any indication of anything. Unless of course someone here knows better

Engine mounts will come under opposite tension/ engine will lean opposite way, I wonder if it might be electrical in that wires are being moved by the engine to make better contact in reverse?

 

[T5 TDI]

The HT side normally is usually worse under load (although everything is possible) but you said

..that it misfires or begins to judder when the engine revs are low. Starts first time & ticks over fine, revs freely. OK when accelerating & all times the engine is loaded, it just starts when I take my foot off on flat ground & there is no load on the engine.

So I took that to be popping back in the exhaust but is that the case?

I presume you have changed the rotor arm? My old boss (c1972) used to have a fun but reliable test for those Start with the points closed and the ignition on. You then hold the unclipped cap complete with the connected HT leads in your hand. Then with the same hand, you hold the rotor arm on the end of your index finger. Centre the top of the brass conductor of the rotor arm onto the spring-loaded carbon brush (or carbon blob in the case of your fairly crude Delco distributor) so that your index finger in effect becomes the distributor spindle for this test. Choose any cylinder out of the four other carbon poles in the cap and aim the rotor at it. Then flick the points..

Try it on a couple of distributor segments just in case the cylinder you have chosen is on the exhaust stroke. The engine will either give a cough and move slightly (keep hands clear of fan blades) which means the rotor arm is fine. If the rotor arm is cracked you will get a small (ish) belt off it. Nothing serious, you won't die but it's always good for a laugh! We had an old Champion spark plug tester machine that could test them more sensibly but it wasn't as reliable and only half the fun! Note- The engine cannot start during this test, it will only give a small kick powered by one cylinder).

 

[jj4091]

Nothing serious, you won't die

Unless you jump up & impale the back of your head on the bonnet latch

 

[Mottie]

Apart from the inlet manifold problem, another thing that we had happen on a regular basis that gave rough running on the Viva's was the oil pump. That is driven by a gear onto the camshaft and then there was a type of pen with an offset slot cut into it that the offset tongue on the distributor slotted into. The smaller of the two halves used to snap off resulting in erratic ignition timing. Just whip the distributor out to check that you have two halves.