SU woes

[Austin in the Shed]

Do you have a spare mag to try to eliminate that having a fault under load.
I remember someone who had an historic single seater who changed almost everything on the engine to get rid of a misfire,except the mag.
When he finally changed it the problem  disappeared.

[Hugh Barnes]

I refer the honourable member to where I stated that I have already swapped out the mag to eliminate that possiblity - plus new plugs, twice, of diffferent types to eliminate that possibility as well...

At the risk of tempting providence, I can't believe its ignition...

[Erich]

Perhaps I misunderstand, but did you change out the dizzy/mag cap and perhaps the wires. I also like the coil suggestion. Aside from those(which it could be as faults tend to show up under load) the mixture could be the issue as previously mentioned. But without obvious air leaks, I hate to suggest this, but is compression still good? Hard to think it is the carbs as it ran well for 2000 miles.

Best,

Erich

[stuartu]

Hugh, you say the pump (which I deduce is an SU) behaves flawlessly but I do wonder if its diaphragm has been affected by modern fuel. I am trying to think of faults that could "just happen". I notice you have not yet substituted another pump.

Since you can produce the fault in your garage, could you rig a temporary fuel feed from another source to rule out fuel supply as a cause?

Regards,
Stuart

[Speedex750]

And a further check for ignition is to gap the plugs down so the spark jumps easier instead of finding an alternative path. Try 10 or 15 thou as a test?

Dave

[Hugh Barnes]

Well, I've now eliminated the inlet manifold, having swapped it this afternoon. Float levels on the carbs were checked yesterday. I had a go at eliminating the fuel pump a while back though, to be fair, not terrribly effectively. I'm going to have another go at that next, though as the engine dies instantly when the throttle is lifted, I cant believe the float chambers run short of fuel in an instant..

However unlikely things may appear, I've go to the point now where I have to check them thoroughly.. The other thing I will do in the next round is check compressions, but again, on a brand new engine that shows no sign of problemmatic running (when it does run!) I think it unlikely I will find something there..

Further out in my thinking is to convert to single carb manifold and run the engine on each of the carbs in turn and see what happen then (with suitably changed needles, of course...)

thanks for the ongoing ideas..

[Special Bits]

Well, I've now eliminated the inlet manifold, having swapped it this afternoon. Float levels on the carbs were checked yesterday. I had a go at eliminating the fuel pump a while back though, to be fair, not terrribly effectively. I'm going to have another go at that next, though as the engine dies instantly when the throttle is lifted, I cant believe the float chambers run short of fuel in an instant..

However unlikely things may appear, I've go to the point now where I have to check them thoroughly.. The other thing I will do in the next round is check compressions, but again, on a brand new engine that shows no sign of problemmatic running (when it does run!) I think it unlikely I will find something there..

Further out in my thinking is to convert to single carb manifold and run the engine on each of the carbs in turn and see what happen then (with suitably changed needles, of course...)

thanks for the ongoing ideas..

Have you got to the bottom of this yet Hugh?

[Hugh Barnes]

Sorry, I've been away for a while, only returning this weekend. The last change I made was to replace the twin carbs with a single carb set up with a replacement manifold and an SU carb scavenged from my Cabriolet. The upshot of that was no change in behaviour. So that eliminates carbs in any way.

I also did a compression test and got a reading of about 85psi from each cylinder. I didn't know what reading I would get, but the fact they were all pretty much equal was a good sign. So no issues with bores/pistons/valve timing (nor should there be as it is a newly rebuilt engine that has done 2-3000 miles).

I hate to say it, but I am coming to the conclusion it may be stale petrol! After all the work I've done, that's pretty much the only thing left. The fuel in the tank probably dates from the summer but I had not considered it as a possibilty as only a week earlier the car had run perfectly. Upon reflection, the previous time it ran was during the day on a warm'ish autumn afternoon, the time the fault appeared was a cold evening. Maybe that was enough...

I shall be trying fresh fuel this week, but if that doesn't resolve it, I really will be stumped. I will report back...

[austin]

Buy "super" grade or get ready to replace it every week, as mentioned before I've been through this...

[Tony Press]

Sorry, I've been away for a while, only returning this weekend. The last change I made was to replace the twin carbs with a single carb set up with a replacement manifold and an SU carb scavenged from my Cabriolet. The upshot of that was no change in behaviour. So that eliminates carbs in any way.

I also did a compression test and got a reading of about 85psi from each cylinder. I didn't know what reading I would get, but the fact they were all pretty much equal was a good sign. So no issues with bores/pistons/valve timing (nor should there be as it is a newly rebuilt engine that has done 2-3000 miles).

I hate to say it, but I am coming to the conclusion it may be stale petrol! After all the work I've done, that's pretty much the only thing left. The fuel in the tank probably dates from the summer but I had not considered it as a possibilty as only a week earlier the car had run perfectly. Upon reflection, the previous time it ran was during the day on a warm'ish autumn afternoon, the time the fault appeared was a cold evening. Maybe that was enough...

I shall be trying fresh fuel this week, but if that doesn't resolve it, I really will be stumped. I will report back...

Hugh,

I would think that 85 PSI is low for a recently rebuilt engine - if the CR is around 6 I would expect over 100.

Cheers, Tony.