Stall at speed when clutch depressed

[JeffW]

Hi all

I just had a rebuilt set of Stromberg carbs fitted and balanced and the the car runs perfectly.

However, at 40mph and above, if I depress the clutch and coast for a second or two, the car stalls immediately. It will restart instantly (by letting out the clutch and basically bump-starting it) and will consistently stall it at any speed above 40mph. The bizarre part is the car will not stall at below 40 whatsoever.

Any ideas? Faversham installed the carbs and are scratching their collective heads. They completely cleaned them (even though there are rebuilt and unused) and ensured the pistons move freely. If it was a sticky piston, however, it would not be limited to different speeds, hence the head scratching.

Thanks

Jeff

[Lingen]

Jeff.
This is something that my Stag did occasionally, and the only reason I could think of was that above 40 mph the ram air being blown into the air filter was weakening the mixture enough to cause the engine to stall at tick over. It may need re setting to run a little richer.
Try disconnecting the rubber gaiter between the air filter inlet, and the ducting above the radiator. Leave it connected at the filter end to deflect the ram air, and see if that cures the problem. If so, then the mixture it is. If not.....?
Mike.

[JeffW]

Jeff.
This is something that my Stag did occasionally, and the only reason I could think of was that above 40 mph the ram air being blown into the air filter was weakening the mixture enough to cause the engine to stall at tick over. It may need re setting to run a little richer.
Try disconnecting the rubber gaiter between the air filter inlet, and the ducting above the radiator. Leave it connected at the filter end to deflect the ram air, and see if that cures the problem. If so, then the mixture it is. If not.....?
Mike.

Thanks Mike - I was thinking the same as what else at speed would be affecting the engine? The engine at 2000 rpm @ 20mph and the engine at 2000rpm @40mph should be the same, the only extra being air coming through.

I'll give it a try!

Thanks again

Jeff

[DJT]

I had a similar problem recently with reconditioned Strombergs, possibly from the same supplier. In my case, one or more pistons were sticking in the raised position. The stalling happened when I depressed the clutch to when slowing down for a junction or roundabout. I found that there were traces of a talcum-like dust inside the air valve and these were jamming the piston. It took a couple of removals of the air valves and cleaning everything with carb cleaner spray to remove all traces of dust. Since then they have been fine. I did advise the supplier who apologised and admitted it did happen occasionally, which is disappointing considering he claimed to have supplied over 600 pairs of Stag carbs.............

[JeffW]

DJT Thanks! Did your issue happen at different speeds or all the time? I think irrespective of that I should clean them a couple of times as you suggest.

jeff

[DJT]

I didn’t travel far until I’d fixed the problem. The engine would tickover fine on my drive, but revving it up with the air filter and intake elbows removed, I could see that the air valves (pistons) were sticking. Carb tops off, air valve assemblies removed, and gave them all a really good clean with carb cleaner aerosol and a clean cloth. I think the dust was the remnants of the soda blasting media he uses.

[JeffW]

Thanks DJT . I will give them both a good clean and also test it with the gaiter disconnected to see if it's a mixture issue.

Jeff

[JeffW]

UPDATE - disconnected the rubber gaiter between the inlet and the air box and this has completely cured the problem (thanks Mike Lingen!) I am now going to do a compression test as Faversham feels my engine may be tired, and if compression is low in one cylinder this could cause the stalling issue at speed. Will update after I test.

Jeff

[dasadrew]

a depressed clutch and a tired engine - I can relate to that!

[Lingen]

Jeff. Getting there! I will be interested to hear about your compression test. Mine stalled with all the compressions even and ok. And I don't remember it happening when my engine was "Tired" before I rebuilt it.
I still think it is a carburettor related problem.
Mike.

[JeffW]

Lingen Thanks Mike. I am hoping the compression test does not show any alarmingly low cylinders. I also do not have any of the usual signs of a tired engine - no blue smoke, no loss of oil or water, strong oil pressure. Maybe it just needs a nap. I know I do.

Jeff

[JeffW]

a depressed clutch and a tired engine - I can relate to that!

Best quote ever Drew!

[JeffW]

Another update - did the compression test today:

1 130
2 110
3 128
4 110
5 110
6 89
7 115
8 115

It appears cylinder 6 is completely exhausted.

I did the test one cylinder at a time without removing all the plugs nor opening the throttle, but the engine was at operating temperature. I have read on here that results will be higher if I do these things. I have also not tried any oil in the top of cylinder 6 to see if it's a ring or valve problem.

Should I re-do the test with removing all the plugs and opening the throttle each time? I am puzzled why the throttle would have any effect on compression.

Thanks
Jeff

[barkerwilliams]

If the throttle is not open each cylinder can only draw in a restricted amount of air which it then compresses. Less air equates to lower compression readings.
How much lower depends on many things such as a manifold air leak which might influence just one or two cylinders adversely.

Same with plugs, one high compression cylinder will momentarily slow down the cranking and will have a detrimental effect on the next cylinder to fire or be read by your gauge. And compressing all cylinders with all plugs in will slow down the overall cranking speed, and with eight cylinders to be tested it will put an unnecessary strain on the starter motor. I always have a battery charger connected whilst doing a compression test to try and maintain a similar voltage supply to the starter motor.

And try (impossible) to crank the same number of revolutions for each cylinder tested.

So it might have a large effect, or perhaps none at all, but you have no way of knowing so removing plugs and wide open throttle just removes so many unknowns and allows you to measure consistently over all cylinders, and repeatedly over the years.

Did your remove the gauge and re-fit into six at least once to see if the low reading is repeatable, or may have been caused by an ill-fitting gauge?


Alan

[JeffW]

Thanks Alan, now I understand why you need to open the throttle!

I did cylinder 6 twice and it was only marginally different the second time (within a couple of PSI).

Will repeat tomorrow with plugs out, battery charger on, throttle open and repost.

Thanks again

Jeff