Lurching under acceleration and dying when approaching junctions

[ddoherty958]

So Percy has had his first illness! Classic cars, to be expected.

It all began when I tried to adjust the mixture in the carb as it was running a little rich. After a while, the lurching started, only really when throttle is applied, almost like the car is gasping. It's happened at 45mph and 25mph, so anot dependent on speed. You can feel it vibrate through the wheel before it starts. I took it to a garage near me who did their best with it, one of the guys has had 3 MMs in times gone by. After examining they said I had a bent jet (which was why I had trouble changing the mix), but also changed one of my spark plugs (4 new ones and a jet en route from ESM).

Before I took ownership of the car, the previous owner had fitted accuspark electronic ignition for me for ease of mind. This came with triple ground plugs. The garage thought I might have an oil leak through the valves, and that the oil was fouling the spark plugs before being burnt off, leading to the lurching. Obviously very worrying. The dying out at jucntions is a recent thing (ie: after it came back from the garage), no spluttering or coughing, just smoothly goes from running to not while I slow down (Starts right up again though). Worrying indeed, however, while searching this forum I found this post from 2014 which seem sto have the exact same problem as I do, the issue then being the distributor. My question is, could my distributor be the real issue? The car ran fine for a few days after I got it, even going on a day trip to the country without issue. The problems started when I fiddled with the mixture (I know ). WIth the distribuitor being less than 2 weeks in the car, could it be the issue, or is my naivety showing?

viewtopic.php?t=55886 <= the post giving me hope I don't need to remove my head...

[Monty-4]

Hello!

It reads like it's running lean to me. This could be due to your mixture adjustment, a vacuum leak, dirt in the float bowl or jet, or perhaps the fuel pump not keeping up (if not "electronic" it has points inside that can be cleaned). A bent jet/needle would also effect the fuel metering.

I'd start by whipping out a couple of spark plugs and having a look at the colour (remember 1 & 4 will be leaner than 2 & 3, an A-Series quirk). See if the carb piston is moving freely (bent bits may hinder this) and that you have oil in the dash pot.

Welcome to old car ownership. Try not to panic and assume the worst - try the simple stuff first and get in contact with your local branch.

[svenedin]

Not sure about a bent jet but a bent needle is possible although I cannot see how that would just happen to a car that was running well. You can check this by using the lifting pin on the carburettor. Lift the piston up and let go. The piston should fall with a metallic clunk. Try the same test with the choke fully out (which pushes the jet right down). The piston should fall with a clunk again. Both tests with the engine off. That same lifting pin on the carburettor can help determine whether the mixture is correct. With the engine running you can lift the piston a very small distance 1/32 inch (0.8mm) and listen to what happens to the engine speed. See this for details on setting the mixture: http://tr4a.weebly.com/uploads/2/1/9/8/ ... vicing.pdf

Don't worry too much. It is probably nothing that serious. Might be an idea to check the obvious electrical connections are secure. Battery terminals, fuel pump, coil etc.

Edit: one possibility springs to mind. Did you "adjust" the wrong nut on the carburettor? One nut is the mixture adjusting nut. The one above is the jet locking nut. If you have disturbed the jet locking nut then the jet may be no longer correctly centred causing the needle to bind. That might be the "bent jet" comment. Not bent but no longer properly centred.

[James k]

Welcome to old car ownership. Try not to panic and assume the worst - try the simple stuff first and get in contact with your local branch.

As above, don't assume the worst but also rest assured that with these cars, even 'the worst' is not that bad and pretty easy and cheap to sort.

I'd check there's oil in the dashpot first.

[svenedin]

I have just been setting the mixture on my car. Something I haven't done in a long time. A few tips: get a head torch. Absolutely invaluable for all manner of jobs and I keep mine in the glovebox. Vastly better than a torch. Once you have checked your jet is centred, take off the air filter and the plate it is mounted on (be aware there are spring washers under the heads of the 2 bolts securing the air filter mounting plate and a gasket between the plate and carburettor). Now you can see what you are doing. Use the lifting pin on the carburettor body and look at the piston rising. Note that the lift to test the mixture is very small indeed (0.8mm). If you lift the piston too far the car will stall whatever the mixture and it isn't a valid test of the mixture. Assuming the car was running well before and you have not adjusted the mixture that much you are not going to need to adjust far. Adjust by 1 flat of the nut at a time. Clockwise adjusts the jet down and makes the mixture richer; anticlockwise moves the jet up and weakens (leans) the mixture. It is advisable to disconnect the choke cable at the carb lever or at least make sure the fast idling screw is not touching the cam. You may find that nothing much seems to happen with each small adjustment. Give the engine a rev (you can do this under the bonnet using the throttle lever) and let it settle down again. With mixture adjustment you may have to adjust the idling screw on the carb if the engine starts to run too fast. The correct idle is when the charging light on the speedo is just going out. You must have a hot engine for mixture setting.

Finally, and I'll probably get flamed for this, there is an alternative method for mixture setting. You can use a Gunson Colortune (sic). This is a special spark plug with a glass window in it allowing you to see the flame colour inside the combustion chamber. Bunsen blue is the correct mixture, pale blue is too lean and candle flame yellow is too rich (just like a bunsen in the lab; the ratio of air to fuel). I have had my Morris since I was 17 and have always used a Colortune and I used it this morning. You may find it runs best on a slightly blue/yellow mixture with the Colortune rather than the perfect deep blue (this is because the A-series and many older engines do not idle that well at the "ideal" fuel/air ratio). If you rev the engine you will notice the flame colour becomes more yellow; this is normal. The test is to set the mixture and then go out for a good run in the car (not just a run about town). After the run take out a spark plug and examine the electrode. Sooty black is too rich; biscuit colour is ideal; very pale is too lean. Then make a small adjustment according to spark plug colour (1/2 flat for instance). I may be derided by the pro mechanics for using a Colortune but I am just an amateur and it works for me (in conjunction with the 1/32" piston lift test). I still have the same Colortune I had 33 years ago so I hardly think it was a waste of money!

Best of Luck!

Stephen

[myoldjalopy]

Welcome to old car ownership. Try not to panic and assume the worst - try the simple stuff first and get in contact with your local branch.

As above, don't assume the worst but also rest assured that with these cars, even 'the worst' is not that bad and pretty easy and cheap to sort.

I'd check there's oil in the dashpot first.

So would I, although you would think the garage would have checked this....... but you have to wonder, as the 'dying out' at junctions occurred after the garage had the car

[ddoherty958]

Thanks for all your replies and tips, greatly appreciated!

I'll get a chance to check the spark plugs once the new ones arrive. I did check the dashpot, there is oil in it. It's pretty thick (by my never-seen-dashpot-oil eye) with a loud squelch when I pull the plunger out. It's not all the way up to the top but it's ~3/4 the way.

I've avoided touching the mixture since the garage, just changing one thing at a time so I can track down the cause of the issue. (Side note, the Haynes manual directs you to lift the carb piston 8mm (!!!) instead of 0.8 when tuning... maybe why I had some issues!). I did also make sure to turn the lower nut on the jet when adjusting, although I didn't notice much change I admit.

In addition, I also checked my engine oil this morning while greasing hinges and found it was extremely low. The car does leak oil a little, but it was below the min line when I checked. Obviously I haven't driven the car at all, and will be keeping a close eye on the oil level. Something the garage should have checked maybe, althought I'm not going to make excuses for not checking myself too. It's now full up again.

As it stands, I'll wait till the new plugs and jet arrive.

THank you all again!

[svenedin]

So you are going to change the jet with the carburettor on the car? Having just done this I can tell you it is not an easy job. I really would do the lift the piston and listen for a clunk test before you launch into something that may be unnecessary. Changing the jet (you do mean jet and not needle?) requires the jet to be disconnected from the carburettor and the float chamber. If you are going to do that you may as well take the lid off the float chamber and with no jet pipe attached to it you can remove any rubbish in there at the same time. Wash the float bowl out with petrol or carburettor cleaner. Have a new gasket for the float bowl as if you re-use the gasket it may well leak. When you put the new jet back in the carburettor raise the piston first or you could bash the needle and bend it. See this video and when you take the jet pipe off the float chamber make sure the old washer comes out. https://www.youtube.com/watch?v=ZPzu2zzyGCQ Personally I can't see why there would be a problem with the jet or the needle for that matter.

[ddoherty958]

So you are going to change the jet with the carburettor on the car? Having just done this I can tell you it is not an easy job. I really would do the lift the piston and listen for a clunk test before you launch into something that may be unnecessary. Changing the jet (you do mean jet and not needle?) requires the jet to be disconnected from the carburettor and the float chamber. If you are going to do that you may as well take the lid off the float chamber and with no jet pipe attached to it you can remove any rubbish in there at the same time. Wash the float bowl out with petrol or carburettor cleaner. Have a new gasket for the float bowl as if you re-use the gasket it may well leak. When you put the new jet back in the carburettor raise the piston first or you could bash the needle and bend it. See this video and when you take the jet pipe off the float chamber make sure the old washer comes out. https://www.youtube.com/watch?v=ZPzu2zzyGCQ Personally I can't see why there would be a problem with the jet or the needle for that matter.

I took your advice and just nipped out to do the drop test. The garage did say jet, so I'm just going opn what they told me. The clunk test sounded the same to me on both, I took a video and uploaded it here to see if I missed anything:

Choke in: https://streamable.com/e6aro4

Choke out: https://streamable.com/qsift6

I don't know if it's just me, but I didn't see the jet move much in the video, could be my lack of experience.

[svenedin]

I watched your videos. Looks fine to me. You can use the lifting pin on the HS2 carburettor to do this. You don't need to use a screwdriver. This test shows that the jet is centred and the needle is not bent. You won't see the jet moving. What you are seeing in your video is the piston with the needle attached to it (the brass rod in the centre) being lifted up by you and then falling back down. The needle fits down the centre of the jet tube. The jet tube is the brass tube that comes out of the carburettor with the pipe attached to it going to the float chamber. I would not be replacing the jet in these circumstances. Is there anyone from your MMOC Branch who could give you a hand I wonder? I bet this is something simple.

Could you check that the vacuum advance pipe has not been inadvertently disconnected? This is a thin pipe that goes from the distributor to the carburettor. It comes out of a part of the distributor that looks like a flying saucer. If it is detached the vacuum advance won't work but there will also be a leak of air into the carburettor which will cause the car to run lean.

[James k]

Just reading through your original post again. From what I understand, the car was running fine, albeit rich, you adjusted the mixture and then it started the lurching behaviour. Assuming that's right, and that something else hasn't happened as a pure coincidence, there are only too options: you adjusted it too lean or, as suggested above, you undid the wrong nut and the jet is loose. I suspect it's probably just running lean. There's a 'basic' setting that I can't remember which is a certain number of flats from fully tightened up. You might want to 'reset' it like that.

The video looks okay to me although there may be something more going on that doesn't come across without seeing it in person. Assuming that it was running fine before the adjustment, though, I reckon it's just the mixture.

[svenedin]

Just reading through your original post again. From what I understand, the car was running fine, albeit rich, you adjusted the mixture and then it started the lurching behaviour. Assuming that's right, and that something else hasn't happened as a pure coincidence, there are only too options: you adjusted it too lean or, as suggested above, you undid the wrong nut and the jet is loose. I suspect it's probably just running lean. There's a 'basic' setting that I can't remember which is a certain number of flats from fully tightened up. You might want to 'reset' it like that.

The video looks okay to me although there may be something more going on that doesn't come across without seeing it in person. Assuming that it was running fine before the adjustment, though, I reckon it's just the mixture.

That basic setting is wind up the adjusting nut so that the jet is level with the carburettor bridge (or up as far as it will go if it won't go that far) and then wind the nut down 2 complete turns (12 flats).

[ddoherty958]

I watched your videos. Looks fine to me. You can use the lifting pin on the HS2 carburettor to do this. You don't need to use a screwdriver. This test shows that the jet is centred and the needle is not bent. You won't see the jet moving. What you are seeing in your video is the piston with the needle attached to it (the brass rod in the centre) being lifted up by you and then falling back down. The needle fits down the centre of the jet tube. The jet tube is the brass tube that comes out of the carburettor with the pipe attached to it going to the float chamber. I would not be replacing the jet in these circumstances. Is there anyone from your MMOC Branch who could give you a hand I wonder? I bet this is something simple.

Could you check that the vacuum advance pipe has not been inadvertently disconnected? This is a thin pipe that goes from the distributor to the carburettor. It comes out of a part of the distributor that looks like a flying saucer. If it is detached the vacuum advance won't work but there will also be a leak of air into the carburettor which will cause the car to run lean.

Solid advice as ever, thank you. I do wonder why the garage told me the jet was bent... The vaccum pipe is attached, but I kow it's a bit tight to the top of the rocker cover, so I'll check it again in the morning. I think you're right, I'll look to get into contact with a local owner and get their opinion. I'm probably overthinking it all now! I was wondering if I'd see a diference in jet height when I lifted the piston, but now I know I won't. Learning as we go

[Grumpy21]

You don’t say where you are, someone local may be able to help.

The piston lift and drop looks fine to me, as does the needle (hard to tell though)

However I would suggest a relatively simple course of action. Undo the mixture nut by hand (no spanner) one flat at a time keeping a note of how many flats in total it’s moved. It’s easy to do if you reach under the carb and feel for the nut which you should be able to turn even if it’s a bit still. If it’s no better after say 12 flats than the problem is elsewhere. You can the do the misstate nut back up to where it was before you started.

Also the three earth plugs are a bit fancy for this old engine. The good old BP6E/N9Y are as cheap as chips and work perfectly well.

[keith rogerson]

I was just reading through the post about the jet settings.
I've returned to being a proud moggy owner, [ first one in 1979, then five others: 3 saloons, conv, woody, and now a pick up.[ this is especially driven by London vehicle restrictions ULEZ etc] I happily have my pickup on my drive. Since recently buying it, [and it is a very nice example] i have systematically replaced coil, electronic ignition, stripped the carb, cleaned etc but didnt adjust the settings as IO had driven it home quite happily a few days earlier.
I've just the SU fuel pump to replace a dodgy looking unnamed one[ which I feel maybe the culprit re my moggy coughing and spluttering when trying to accelerate.] Whilst waiting for copper connection for the SU pump to tank fuel line, I checked all the plugs etc. All choked up black soot. So, I went back to the beginning and set the carb as lean as possible and worked backwards.
Seemed to have done the trick, we shall see.