lumpy start up

[maverick]

Hi

Ive owned my Stag for 20 YRS and slowly but surely worked my way through alot of problems and generally putting things right how they should be,the car has rewarded me with what is now a lovely drive.
One of the last things thats haunted me for 20 yrs I would like to sort out is that when I start the car from cold with full choke,it starts very akwardly on what sounds like 4-6 cylinders, well definately not 8.
The other pots fire up eventually.

The best clue I can give you is that if I just drive the car out of the garage to work on it then drive back in with choke still on,the next time I come to start it roars into life on all 8 and sounds lovely.
This tells me that there is some left over fuel in the bores/manifold and that this aids statrting .

I think my choke mechanism is set up correctly as I did it via the haynes manual,full choke revs at about 1,500 rpm.

I think the fact is starts well after a garage in and out session and not starting from cold after putting back in garage after a journey,tells me that that maybe there is some fuel starvation at starting from cold and this causes the aforementioned lumpy starting.

My question is does any one else suffer this,and can anyone recommend an answer,?

Thank you in advance

Mark

[DJT]

Mark. For the past 30 years I’ve used the same method for starting from cold.

1. Turn on ignition, listening for the fuel pump ticking.
2. Depress accelerator and pull choke out fully.
3. Release accelerator.
4. Turn key to start position. Engine usually starts first time.
5. As soon as the engine is running, push the choke in to achieve a fast tickover around 1000x1200 rpm.
6. Drive off and push the choke in fully as soon as you are happy it won’t stall. This will depend on ambient temperature and road/traffic conditions.

Carbs and choke are set up as per the ROM.

[barkerwilliams]

Mark,

I would expect fuel to impact more than one or two cylinders but check carbs for split diaphragms

I would then fit a new set of spark plugs, cheap and simple.

Start the car and get it running rough for a few seconds and then stop and remove all plugs and inspect for differences in deposits / colour / perhaps some plugs wet with fuel and cold (i.e not firing)

Whilst the plugs out a compression test to spot discrepancies. If all Ok fit new plugs and start again.

Then perhaps a set of spark plug testers such as https://www.ebay.co.uk/itm/Spark-Plu...oAAOSwp-RahI3l . Fit and start the car after dark with the bonnet open and a quick visual inspection whilst running rough. Also in the dark with the engine running a quick visual check of the coil, plug leads, distributor cap for an ignition spark tracking to ground.

So far a minimum spend that might point you towards whether the issue is ignition, cylinder or fuel related.

Alan

[maverick]

Mark. For the past 30 years I’ve used the same method for starting from cold.

1. Turn on ignition, listening for the fuel pump ticking.
2. Depress accelerator and pull choke out fully.
3. Release accelerator.
4. Turn key to start position. Engine usually starts first time.
5. As soon as the engine is running, push the choke in to achieve a fast tickover around 1000x1200 rpm.
6. Drive off and push the choke in fully as soon as you are happy it won’t stall. This will depend on ambient temperature and road/traffic conditions.

Carbs and choke are set up as per the ROM.

Thank you ,Dave,
Im afraid thats exactly how I start mine as a tip from you about 15yrs ago LOL

[maverick]

Mark,

I would expect fuel to impact more than one or two cylinders but check carbs for split diaphragms

I would then fit a new set of spark plugs, cheap and simple.

Start the car and get it running rough for a few seconds and then stop and remove all plugs and inspect for differences in deposits / colour / perhaps some plugs wet with fuel and cold (i.e not firing)

Whilst the plugs out a compression test to spot discrepancies. If all Ok fit new plugs and start again.

Then perhaps a set of spark plug testers such as https://www.ebay.co.uk/itm/Spark-Plu...oAAOSwp-RahI3l . Fit and start the car after dark with the bonnet open and a quick visual inspection whilst running rough. Also in the dark with the engine running a quick visual check of the coil, plug leads, distributor cap for an ignition spark tracking to ground.

So far a minimum spend that might point you towards whether the issue is ignition, cylinder or fuel related.

Alan

Alan
Thank you,
I have the spark plug testers and along the way had new leads,rotor cap etc,
The quandry is that once she is running all is good,what I cant understand is difference between start up after putting away after journey out,lumpy and start up after she has been just backed out of garage for working on (ie left on drive,choke still on,then start up to go out again choke still on and fires up beautifully as I say I think the good start has got something to do with fuel residue.
Thanks

[wilf]

I suspect that the mixture enrichment is "weak" on one carb, or at least weaker than the other one. Your description leads me to this, since each feeds 4 cylinders (2 on each bank)

No carb'd petrol engine runs perfectly from total cold on initial stat up with choke. So long as it starts up OK, maybe you could consider it a "characteristic" rather than fault?

[flying farmer]

My 1st Stag used to suffer from exactly the same problem and I found that one carb was set far too rich and the other far too weak. So much for the previous owner paying the garage to get it set up to go through its MOT test. Fundamental problem was the needles and jets were worn out and replacing them sorted the 4 cylinder start up

[maverick]

My 1st Stag used to suffer from exactly the same problem and I found that one carb was set far too rich and the other far too weak. So much for the previous owner paying the garage to get it set up to go through its MOT test. Fundamental problem was the needles and jets were worn out and replacing them sorted the 4 cylinder start up

Thank you Neil,
Sounds very logical,I fitted new needles about 6,000 miles ago,but not new jets.
This gives me a new source of exploration,
Much appreciated,I look forward to your posts
Regards Mark

[maverick]

I suspect that the mixture enrichment is "weak" on one carb, or at least weaker than the other one. Your description leads me to this, since each feeds 4 cylinders (2 on each bank)

No carb'd petrol engine runs perfectly from total cold on initial stat up with choke. So long as it starts up OK, maybe you could consider it a "characteristic" rather than fault?

Thank you wilf,both carbs are set to 1.5 turns from the bottom of the threa if I remember correctly.
Stange thing is runs lovely and smooth when past the initail lumpyness,I can even do the 50p test on airbox,would just like to sort this anomomly
cheers

[wilf]

Yes, but the actual cold start enrichment is independent of the jet setting.

[maverick]

Yes, but the actual cold start enrichment is independent of the jet setting.

Yes your right,but I cant quite remember without getting out my strom manual out how it actually works I know its basically air starvation to enrichen the mixture and is there a disc with holes in it that lets in more fuel.

Im leaning to the theory of one carb needs to be richer but without checking individual plugs and corralting them to each bank or using a colourtune its difficult to establish which carb needs enrichment.
I always thought ideally the needles should be set at the same height,just like the carbs are both airflow balanced to be the same

Cheers Wilf

[mallardstag]

Have you checked that both chokes are operating and there isn't a broken choke cable?

[maverick]

Have you checked that both chokes are operating and there isn't a broken choke cable?

Cheers Mallard ,yes both cables are ok,but not sure on individual choke adjustment for each carb or how to idenfify which carb needs more choke

[wilf]

Well, you can, with the choke knob fully out, check to see if there is any possible further movement at each carb.

And, of course, check that with it fully in, the choke mechanisms are back to their "off" stops.

BTW, there is no air restriction "flap" with these carbs for the choke - the choke action is 100% about extra fuel being allowed in via the discs that rotate.

The fact that your car runs fine when warm leads me to think that additional fiddling with the jets isn't necessary, or wise. If you pull the plugs from one bank, the two outers are fed from one carb, the two inner cylinders from the other.

[maverick]

Well, you can, with the choke knob fully out, check to see if there is any possible further movement at each carb.

And, of course, check that with it fully in, the choke mechanisms are back to their "off" stops.

BTW, there is no air restriction "flap" with these carbs for the choke - the choke action is 100% about extra fuel being allowed in via the discs that rotate.

The fact that your car runs fine when warm leads me to think that additional fiddling with the jets isn't necessary, or wise. If you pull the plugs from one bank, the two outers are fed from one carb, the two inner cylinders from the other.

Thanks Wilf,
I tried this before ,ie check the plug colours and have got a note of which cylinders are fed by which carbs,but could not come to a clear conclusion .

Your suggestion re choke cable travel is a good point,i will check
Much appreciated