Lumpy running on choke

[MorrisJohn]

Morning gents. My car starts fine in the cold with the choke pulled out. It’s also idling (about 650rpm) and running fine once warm.

However, while I’m using the choke when it’s cold although it starts up it very quickly runs very lumpy and wants to cut out. As a result I have to use the choke to start it, then press the choke back in and sit with my foot on the accelerator until it’s sufficiently warm. By doing this the engine runs fine.

I should also say once it’s warm when I operate the choke to test it the effects on engine revs are negligible, though lumpy running is very quickly apparent. It won’t start in the cold without some choke though.

I’m guessing there must be a problem with my choke cable causing an incorrect fuel/air mixture?

I’m sure someone will have encountered this before and I’m hoping they can advise how to fix it. All the stuff I can find on the forums about lumpy running seems to relate to lumpy idle, which my car doesn’t suffer from.

[jagnut66]

Mine was subject to similar problems, try cleaning and resetting the carb.
Also, as you say, make sure the choke cable operates freely and doesn't 'snag'.
The cable linkages can get covered in a sticky crud over time, try spraying them with Plusgas or WD40 and working it in with a toothbrush or another small soft brush, you can 'flush' them after with another spray of the same.
Best wishes,
Mike.

[ianmack]

I start mine from cold with the choke full out. Soon after starting the engine, as you say, will run rough as the extra fuel for starting is no longer needed. I push the knob halfway in which eases the rich mixture but still gives an increased tickover speed and keeps it running smoothly to pull away. After a minute or so push it right in.

[liammonty]

Without meaning to sound patronising, are you sure you’re using the ‘choke’ correctly before you start stripping things down? All cars are a little different and require a bit of ‘feel’ when it comes to using the choke. If it’s operating correctly, the first bit of movement on the cable should raise the idle speed, and how much it does this is adjustable. It sounds as though this needs tweaking on your car to give an increased fast idle speed Throughout the full range of the cable movement, it also pulls the jet down the needle, giving a progressively richer mixture the further you pull out the choke knob. All of my 8 or so Minors that I’ve owned over the years like full choke from cold, but then need the choke returning about half way almost immediately to prevent the rough running you describe, which is due to an overly rich mixture. With the choke open about half way, you get a slightly enrichments mixture to help with the cold running, and still retain the maximum increase in fast idle speed which prevents stalling. Choke can then be progressively pushed home over about half a mile to a mile or so of running. You’ll find you may need to tweak it a little to pull away from a junction etc, compared with running at a steady speed. It’s a bit of an art but fairly straightforward once you’ve got used to it. SU carbs are much more forgiving on choke than Webers and other fixed jet carbs, so if it’s all set up ok it should behave nicely.

As above, if you know all this then apologies if it sounds patronising!

[MorrisJohn]

Thanks for the replies. Yes, it’s full choke to start it but then if it’s pushed in half way, 3/4 way or whatever once it’s running it’s still really rough and wanting to cut out. And barely any difference in rpm whether it’s in or out.

The only “solution” at the moment is using the accelerator instead until it’s warmed up. It’s not drivable until it’s warmed up for this reason.

Something must be not quite set up correctly.

[oliver90owner]

Pulling the choke control should increase the idle speed. Is yours not doing this?

[Bowie69]

As above, first bit of the choke should raise the idle, possible the adjustment screw for this is missing or maladjusted, it turns a cam against the main throttle butterfly.

[MorrisJohn]

Pulling the choke control should increase the idle speed. Is yours not doing this?

That’s correct. It’s not really increasing the revs. The difference is very slight.

Will check that adjustment screw.