(Very) old engine power loss diagnosis...
Discussion
I wonder if anyone with old engine knowledge has any ideas as to what is causing intermittent loss of power in my 1931 Alvis 12/60. I have some old car experience but it has me stumped especially as the car has just come back from a / the Alvis specialist check over and retune, partly for the same issue - which as ever with old cars didn't happen when they road tested it. It's a vintage engine of course but fairly conventional, twin SU carb, four cylinder set-up though with magneto ignition.
The car loses most if not all power whilst churning uphill with the throttle open, particularly if it is in a high gear. Seems to happen when the car/weather is hot. Engine doesn't stall as such but if you open the throttle further it just dies; i.e. you can hear the carbs sucking but there is clearly no ignition (or fuel?). If you dip the clutch however, the engine picks up again and idles/revs. So it happens when the engine is under load. On a recent 3+ hour motorway drive at constant high (for a 90 year old!) speed but level gradient, it was fine.
I don't think it is just misfiring on one cylinder. The leads and plugs are all new. Timing has just been reset correctly and manually retarding it makes no difference. No backfire or carb pops. Car idles and runs smoothly otherwise, including at high revs. I'm wondering about a fuelling issue due to gradient and throttle opening? Or it's over-enriching and effectively choking the engine. Could be hot magneto related but that shouldn't be affected by engine load. Compression test all fine. Any thoughts would be most welcome - especially as it has to get to Le Mans on Friday! Thank you.
The car loses most if not all power whilst churning uphill with the throttle open, particularly if it is in a high gear. Seems to happen when the car/weather is hot. Engine doesn't stall as such but if you open the throttle further it just dies; i.e. you can hear the carbs sucking but there is clearly no ignition (or fuel?). If you dip the clutch however, the engine picks up again and idles/revs. So it happens when the engine is under load. On a recent 3+ hour motorway drive at constant high (for a 90 year old!) speed but level gradient, it was fine.
I don't think it is just misfiring on one cylinder. The leads and plugs are all new. Timing has just been reset correctly and manually retarding it makes no difference. No backfire or carb pops. Car idles and runs smoothly otherwise, including at high revs. I'm wondering about a fuelling issue due to gradient and throttle opening? Or it's over-enriching and effectively choking the engine. Could be hot magneto related but that shouldn't be affected by engine load. Compression test all fine. Any thoughts would be most welcome - especially as it has to get to Le Mans on Friday! Thank you.
Without knowing any more... Is it running air filters, and do they take hot air from under the bonnet? My old truck was playing up a bit under load until I fitted an air filter and it started drawing air away from the heat of the engine - the underbonnet temperatures in this weather were very high and the hot, dry air was starving the motor of fuel.
Also, I've known of magnetoes on old bikes to play up when hot, but would have thought that would show up as a misfire rather than a loss of power.
Best of luck with it.
Also, I've known of magnetoes on old bikes to play up when hot, but would have thought that would show up as a misfire rather than a loss of power.
Best of luck with it.
spoodler said:
Without knowing any more... Is it running air filters, and do they take hot air from under the bonnet? My old truck was playing up a bit under load until I fitted an air filter and it started drawing air away from the heat of the engine - the underbonnet temperatures in this weather were very high and the hot, dry air was starving the motor of fuel.
Also, I've known of magnetoes on old bikes to play up when hot, but would have thought that would show up as a misfire rather than a loss of power.
Best of luck with it.
Yes I experienced this very recently on my MG Magnette ZB, very hot weather towing a small caravan all was fine until I got stuck in traffic. As soon as I was able to make progress the problem was resolved.Also, I've known of magnetoes on old bikes to play up when hot, but would have thought that would show up as a misfire rather than a loss of power.
Best of luck with it.
Thanks all. It doesn't run air filters (not sure it could) but hot air and fuel starvation could be a culprit. It certainly gets very hot under the bonnet and there is no fan. It 'feels' more like a fuelling rather than ignition issue though mags can do mysterious things. I have a spare mag that I may try out though it's a faff. Fuel filter is clean; I think if the jet is partially blocked I'd have a more constant issue. If it is hot dry air related that suggests it is over-lean doesn't it? In which case I could enrichen the mixture from the dash control going uphill and see if it helps - though never had to before.
Sounds like classic fuel vapour locking,as you say there is no backfiring and happens when hot(weather)along with the lack of power when opening the throttle.there will be enough fuel fumes to allow idle but not enough to produce power.
might be worth rerouting the fuel line if it is mounted high up in the engine bay or running next to hot components and insulating it can help.also a cold air box housing taking in cold air will help if there is room.
driving at speed is fine it's the heat soak that builds at slow speeds or in traffic.
my classic runs efi and I had fuel boiling last week and the engine cut dead as you would expect with efi , fortunately I
was almost on the drive and had enough momentum to get off the main road.left for a few minutes and started fine again.
might be worth rerouting the fuel line if it is mounted high up in the engine bay or running next to hot components and insulating it can help.also a cold air box housing taking in cold air will help if there is room.
driving at speed is fine it's the heat soak that builds at slow speeds or in traffic.
my classic runs efi and I had fuel boiling last week and the engine cut dead as you would expect with efi , fortunately I
was almost on the drive and had enough momentum to get off the main road.left for a few minutes and started fine again.
Thanks again all, very helpful indeed. Vapour lock does sound like a probable culprit and certainly the symptoms match. As you can see, the fuel line is high in the engine bay and also runs close to the exhaust manifold; the pump is also on the bulkhead in the engine bay so will get hot too as will the carbs themselves. Thermosyphon cooling as noted so temperature under the bonnet does get quite high. There isn't an inlet manifold heater and I don't believe it is fuel level related - though I suppose it is possible that the fuel pump is weak when hot.
Less clear on how to identify where the fuel is actually vapourising and fix - especially trying to keep it all original. Maybe just something to live with...though the drive to Le Mans this weekend will test it given the forecast!
Less clear on how to identify where the fuel is actually vapourising and fix - especially trying to keep it all original. Maybe just something to live with...though the drive to Le Mans this weekend will test it given the forecast!
Loss of power under heavy load with the throttle wide open when otherwise running perfectly certainly sounds like fuel starvation. I suspect the fuel tank breather might be blocked. Does your car have a breather hole in the fuel cap? If so it might just need a clean through. Worth a try?
Definitely give the car a run in cool weather conditions. if problem disappears you have the answer.
Assume at this stage it is fuel vaporisation I would fashion a heat shield between manifold and carbs. Additionally you could wrap the fuel feed lines, tape is a aialbale that could be used.
Both of these are obviously easily removed without damage to that beautiful engine bay.
Assume at this stage it is fuel vaporisation I would fashion a heat shield between manifold and carbs. Additionally you could wrap the fuel feed lines, tape is a aialbale that could be used.
Both of these are obviously easily removed without damage to that beautiful engine bay.
Personally I would remove each fuel line union and blow through each line individually, and check the pickup in the tank, it may have a gauze mesh on the end. It sounds like classic starvation/vaporisation issues. Fine on normal running but as soon as the engine is really demanding fuel and needing it, the system can't keep up.
On my old Series Landy I had an issue a couple of years ago accelerating down a dark bypass to get up to dual carriageway speeds. It spluttered, coughed and slowed to a crawl after having my foot down. Very scary with no hazard lights and no hard shoulder avaliable! I managed to nurse it to the next exit and it sat there ticking over like a sewing machine. I revved it up and it revved cleanly and freely. I set off again, and exactly the same thing happened. Having a closer look this time the rubber fuel hose had split open at the pickup above the tank. It didn't leak due to the suction of the pump, and was obviously getting a little through along with a lot of air to stay running, but not sufficient under load. I had no tools on me at the time so had to drive the remaining 7 miles or so very, very gently at around 30mph. Smooth as silk all the way home.
You never know the pump could be weak as has been said, float needles could be blocked, stuck float, float height set incorrectly, etc
It may be worth lagging the pipes in some kind of heatproof material too and seeing if that makes any odds.
On my old Series Landy I had an issue a couple of years ago accelerating down a dark bypass to get up to dual carriageway speeds. It spluttered, coughed and slowed to a crawl after having my foot down. Very scary with no hazard lights and no hard shoulder avaliable! I managed to nurse it to the next exit and it sat there ticking over like a sewing machine. I revved it up and it revved cleanly and freely. I set off again, and exactly the same thing happened. Having a closer look this time the rubber fuel hose had split open at the pickup above the tank. It didn't leak due to the suction of the pump, and was obviously getting a little through along with a lot of air to stay running, but not sufficient under load. I had no tools on me at the time so had to drive the remaining 7 miles or so very, very gently at around 30mph. Smooth as silk all the way home.
You never know the pump could be weak as has been said, float needles could be blocked, stuck float, float height set incorrectly, etc
It may be worth lagging the pipes in some kind of heatproof material too and seeing if that makes any odds.
A thought on fuel starvation: modern petrol contains more components that boil at low temperature than petrol did when this car was made: it is possible that hot weather and modern petrol are the twin cause of the problem. There is an interesting BP document here:
https://www.bp.com/content/dam/bp-country/en_au/me...
I know that doesn't really provide any kind of answer, but might provide food for thought.
I don't suppose you could -- if only temporarily -- add an electric fan on a simple on/off switch to increase air flow?
https://www.bp.com/content/dam/bp-country/en_au/me...
I know that doesn't really provide any kind of answer, but might provide food for thought.
I don't suppose you could -- if only temporarily -- add an electric fan on a simple on/off switch to increase air flow?
Certainly sound like lack of fuel. Also worth checking the ignition is in good order, points, plugs and advance/retard mechanism.
Check the flow from the fuel pump, and no blocked jets in the carbs or split diaphragm. Also check the carbs are balanced to a reasonable degree.
If no good, then cam timing, valve clearances, compression etc.
All good fun...
Check the flow from the fuel pump, and no blocked jets in the carbs or split diaphragm. Also check the carbs are balanced to a reasonable degree.
If no good, then cam timing, valve clearances, compression etc.
All good fun...
Fuel vapourisation/vapour lock is as others have pointed out a likely culprit. Many vintage car owners have suffered this in recent years with cars that have not previously mis-behaved.
Modern fuel systems are pressurised so can tolerate the more volatile fuels that don't agree with older cars.
A heat shield between the exhaust and float chambers may help.
Looking at the cooper fuel pipe, it's routed quite close to the back of the head (12/60 has the alloy coolant transfer port here which can't help), but perhaps more significantly the loop in the pipe is quite close to the exhaust manifold. Looks like an obvious possibility to promote vapour lock.
Given your limited time for experiment, I'd be tempted to replace the copper pipe from the pump to the t piece between the carbs with a bit of modern flexi fuel pipe routed safely away from the back of the head and the exhaust - take it lower down to the crankcase across the bulkhead, and feed up somewhere around the steering box.
May be ugly, but if it works for the weekend trip you can do something prettier later.
Modern fuel systems are pressurised so can tolerate the more volatile fuels that don't agree with older cars.
A heat shield between the exhaust and float chambers may help.
Looking at the cooper fuel pipe, it's routed quite close to the back of the head (12/60 has the alloy coolant transfer port here which can't help), but perhaps more significantly the loop in the pipe is quite close to the exhaust manifold. Looks like an obvious possibility to promote vapour lock.
Given your limited time for experiment, I'd be tempted to replace the copper pipe from the pump to the t piece between the carbs with a bit of modern flexi fuel pipe routed safely away from the back of the head and the exhaust - take it lower down to the crankcase across the bulkhead, and feed up somewhere around the steering box.
May be ugly, but if it works for the weekend trip you can do something prettier later.
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