23 replies to this topic

[EasterBern]

I'm trying to set my Mini up so that it runs properly.

 

The problem I've currently got is an inconsistent idle! I can set it up fine, nice idle at around 900 RPM and nice mixture (set with a colourtune), at operating temperature. 

 

Then if I drive it the idle might go back to 900, or it might be at 1500 or somewhere in between.

 

I suspect that there might be air getting in somewhere as the mixture often feels weak when accelerating and I've had to fit quite a rich jet (AAA) to get it near correct (though still too weak when accelerating up hill).

 

I've fitted 3 different carbs (currently on HIF38), and changed the manifold gasket, but still get the same issue.

 

It's a 998 with an LCB and Minispares heated inlet (though it's currently unheated).

 

The timing appears to be spot-on.

 

A compression test has all cylinders around the same value.

 

Any ideas? I've had lots of cars with SUs (twin and single), and have always had them running correctly, I'm out of ideas here!

 

Just had a thought, is it possible that the manifold is leaking into the water heating pipes? I'll have to try sealing them up to see if that makes a difference.

 

 

[nicklouse]

sticky accelerator cable

 

an air leak would not be as variable as you say. just constant.

[Twincam]

To check in used to spray WD40 around the joints - if it is leaking you'll get an increased idle. I had a leaky gasket and a worn butterfly on an old escort of mine however the exhaust manifold was at the front.

[Twincam]

sticky accelerator cable

an air leak would not be as variable as you say. just constant.

OP, going on from what Nick has said, can you pull the pedal up when the idle begins to go high to see if it drops back to normal?

[DeadSquare]

When we were motor racing, the rules forbade water injection, but over a few pints one evening, we came up with the idea of a leaking water heated manifold.

 

it seemed a good wease, but we couldn't work out how to refill the header tank between corners.

[EasterBern]

sticky accelerator cable

 

an air leak would not be as variable as you say. just constant.

 

It's a new cable, and the pedal itself isn't sticking. Though that has reminded me that it feels like I can't get the idle slow enough without pushing the butterfly actuator (!?) down manually, even though there is slack in the cable. It's like the return spring isn't pulling it back far enough - this has been the case with all 3 carbs and 2 cables!

 

To check in used to spray WD40 around the joints - if it is leaking you'll get an increased idle. I had a leaky gasket and a worn butterfly on an old escort of mine however the exhaust manifold was at the front.

 

I'll give that a go and see what happens!

 

 

 

sticky accelerator cable

an air leak would not be as variable as you say. just constant.

OP, going on from what Nick has said, can you pull the pedal up when the idle begins to go high to see if it drops back to normal?

 

 

I have tried that, and makes no difference.

[EasterBern]

When we were motor racing, the rules forbade water injection, but over a few pints one evening, we came up with the idea of a leaking water heated manifold.

 

it seemed a good wease, but we couldn't work out how to refill the header tank between corners.

 

If it turns out to be the problem with mine I can sell it to you, though I can't help with keeping the tank filled!

[nicklouse]

like I said an air leak normally manifests itself as constant high revs.

 

yours sounds more like a mechanical issue as you have had it on three carbs I would disconnect all cables after having run the engine so it will start without choke. and then see how it idles.

 

final thought. what oil are you using in the dash pot?

[EasterBern]

like I said an air leak normally manifests itself as constant high revs.

 

yours sounds more like a mechanical issue as you have had it on three carbs I would disconnect all cables after having run the engine so it will start without choke. and then see how it idles.

 

final thought. what oil are you using in the dash pot?

 

That's a good idea, I'll try that.

 

I've used both ATF and 3 in 1 in the dash pot, currently using 3 in 1.

[Twincam]

Try putting 20w50 (engine oil) into it as the 3 in 1 might be too thin

[EasterBern]

Try putting 20w50 (engine oil) into it as the 3 in 1 might be too thin

 

Thanks, I'll try that too.

[cal844]

All my minis use 20/50 engine oil in the dashpot, they pull well with no issues, so try that.

The 998 has an HS4 carb fitted with an AAU needle, unknown spring (think it might have been blue?)

Other than the inconsistent idle does the engine run well? I'd be tempted to try an AAU needle with the 20/50 engine oil in the dashpot because the AAA needle might be too rich once using thicker oil

[EasterBern]

All my minis use 20/50 engine oil in the dashpot, they pull well with no issues, so try that.

The 998 has an HS4 carb fitted with an AAU needle, unknown spring (think it might have been blue?)

Other than the inconsistent idle does the engine run well? I'd be tempted to try an AAU needle with the 20/50 engine oil in the dashpot because the AAA needle might be too rich once using thicker oil

 

Yes, the engine runs well, apart from the occasional flat spot. But I guess that could be due to having oil that is too thin.

 

I do have an AAU needle, but switched to AAA. I'll try them both with the 20/50.

[whistler]

My suggestion is that you measure the thickness of the inlet and exhaust manifold flanges to make sure they are the same. If not then your air leak could be because the inlet manifold is not being tightened onto the head properly. In which case you either have to make stepped stud washers or thin one of the manifolds.
I had the same and ended up buying a new inlet manifold of the correct flange thickness.
This was despite making and fitting stepped washers.
No leaks now.

[andyapanel]

The symptoms sound like hunting, which might suggest a weak mixture.

It could be an air leak, but as has been suggested, this would tend to give you permanently high speed.

Assuming you have followed the Haynes instructions, I would try making the mixture a bit richer; connecting a Tacho while you investigate can help you identify the best mixture.

If I change the mixture (which I tend not to do often once it is set up), I use TWO Colourtunes and the Tacho. I find using one Colourtune can set the mixture a little thin. 

Good luck.