Bad running with some throttle - ideas?

[JimK]

The trav isn't running right: It idles very nicely and drives perfectly well on a light throttle, but as soon as you put your foot down it goes wrong. There is a massive increase in exhaust noise accompanied by some stuttering and very poor/zero acceleration.
When cold, the engine dies completely when you put your foot right down and takes a few seconds to start firing again. The symptoms improve as the engine's heat spreads through the ancillaries, but never quite go away.

The points are new and set to 0.015" gap.
Dizzy cap and rotor are in good condition, no evidence of arcing.
Coil is new-ish.
Plugs have a pale beige/white deposit.
Dashpot has engine oil.
Air filter is clean. Problem is worse with filter off (it was suggested the problem was over-rich mixture on acceleration).
Dizzy vacuum advance works, though not very smoothly.

Spec is a 1275 with HS4 carb on alloy manifold, standard airbox and filter. Standard cast exhaust manifold and stainless steel pea-shooter.

Ideas? Mine so far:

I wondered if the on/off throttle contrast means the dynamic timing adjustment is not working as it should. Any ideas how I can check that the plate moves as it should? I've done the suck-the-vacuum-pipe thing and the diaphragm is intact, though the plate doesn't seem to move terribly smoothly. The exhaust noise reminds me of when the points gap narrowed, which makes me think that the dizzy is affecting the timing somehow.

Anything I could try?

[bmcecosse]

Sounds like much too weak mixture - try richening up the idle setting a bit -it's almost impossible to have 'too rich' on acceleration, it just wastes fuel, but runs fine. What needle is in the carb ? Is the carb piston free to lift easily with a finger - and does it fall back with a nice 'clunk' when you let it go (assumes damper removed for this test!). You could also try changing that oil to 3 in 1, I find it works best.
On ignition side - it could be badly retarded I suppose - try advancing it till it pinks - then back it off again very slightly.
Obviously that exhaust is going to be a bit of a restriction - but that should only affect top-end performance and have little effect on acceleration.

[PSL184]

Sounds like its running too weak to me also.....

[JimK]

Sounds like much too weak mixture - try richening up the idle setting a bit -it's almost impossible to have 'too rich' on acceleration, it just wastes fuel, but runs fine.

OK, I'll give that a go.

What needle is in the carb ?

No idea. It's got filing marks on it though, and when I say that I mean those big coarse grooves made by someone who doesn't know how to use a file...

Is the carb piston free to lift easily with a finger - and does it fall back with a nice 'clunk' when you let it go (assumes damper removed for this test!).

Forgot to say that I dismantled and cleaned the carb and float bowl, and the piston moves very smoothly.

You could also try changing that oil to 3 in 1, I find it works best.

I've had 3-in-1 before, didn't notice any real difference. I think opinion is sufficiently divided that it's hard to say which is best overall. However, I do have some, so...

On ignition side - it could be badly retarded I suppose - try advancing it till it pinks - then back it off again very slightly.

How much adjustment can I make with the vernier? If I can adjust with that the potential for a snafu is less than if I have to unclamp the dizzy.

Obviously that exhaust is going to be a bit of a restriction - but that should only affect top-end performance and have little effect on acceleration.

That's what I thought.

[bmcecosse]

Yes -well that needle sounds less than ideal. Take it out and let's hear what it is. On the oil , I agree - what works in some carbs is not best in others. Time of year (temperature) matters too. The little hand adjuster will give you a few degrees - depends if it happens to be in the middle of the range or not of course! But easy enough to slacken the clamp on the dizzy and turn it say 1/4" clockwise to advance it - then play about with the hand adjuster. If worried - make a mark on dizzy and bracket before you move anything - then you can easily return to original setting.

[bmcecosse]

I had a look this evening in the old HS4 carb I used for many years on a number of engines, and it has a needle marked G1 or maybe it's GI - it worked well on various capacity units although never on a 1275.

[kennatt]

what do you mean by filing marks,has someone being altering the size of the needle

[JimK]

BMC: I'll pull the needle out at the weekend and have a look, but saturday's priority is to reassemble my saloon

what do you mean by filing marks,has someone being altering the size of the needle

That's eactly what I mean. I don't think it's unheard of, just usually done better and with a finer file...

[bmcecosse]

Needles can be modified - and if for a single carb - it can just be a 'flat' on the side! But obviously not ideal. One point that came to mind - has this carb been in use before ? And - is there definitely a spring inside the carb - above the piston ? It does sound like it's going very weak on acceleration - missing (or weak) spring may give that effect.

[JimK]

One point that came to mind - has this carb been in use before?

Yes, and it has previously run perfectly well.

And - is there definitely a spring inside the carb - above the piston ? It does sound like it's going very weak on acceleration - missing (or weak) spring may give that effect.

Yes, there is - I definitely remembered to put it back when I dismantled teh carb
Can't remember what colour though.

I'm going to have a look at the weekend.

[bmcecosse]

As long as it's there -spring colour is unimportant at the moment.

[TopClassics]

If the car has been unused for a while it could be stale petrol. I have had this several times. Just pump it out at the carb, dry out the float chamber and refil with fresh fuel.

[mike.perry]

Your most suitable needle would probably be an AAQ which was fitted to the HS4 carb on the 1275 Marina. Part no. CUD 1014. For the spring the correct fitting was AUC 4387. Sorry I can't tell you the spring colour.
Available from Burlen Fuel Systems, Salisbury. Tel 01722 412500