Page 2 of 3

Re: Mixture screws- making no difference!

Posted: Sun May 20, 2012 9:26 am
by Taggers
Thanks all. Seems I'm not going to get much further until I properly understand the carb and have stripped it down and cleaned everything through. I notice that FRCs rebuild kit includes both the mixture screws and the power valve, so for £20 I reckon that's worth a go.

Does anyone know if the big-headed mixture screws will fit the tamperproof carb, or (as I suspect) I will have to file down the housing either side to accommodate them?

Finally, there is a peculiarity that must be relevant. The Idle jet housing on the driver's side is 1/2 turn out. Screw it fully home and the engine stops, 1/4 turn out and it runs fine. This is a clue, but to what, I don't know.

Cheers all

Chris

EDIT: I see both sorts of screws are included- result!

Re: Mixture screws- making no difference!

Posted: Sun May 20, 2012 11:58 am
by Gillsfan
The idle jet holders should be screwed all the way in. The FRC kits don't come with power valves, the kit is about £24 including postage and the power valve about £20.

Re: Mixture screws- making no difference!

Posted: Sun May 20, 2012 4:56 pm
by AJL Electronics
That was the point I was about to make. Idle screw may be backed off, but is it closing the butterflies? Then the CORRECT start position for the mixture screws is THREE turns out, rather that the often published 1.5 turns out.

Re: Mixture screws- making no difference!

Posted: Sun May 20, 2012 8:11 pm
by ScimmyMike
Gillsfan wrote:If the throttle valves aren't covering the progression holes properly then fuel will get though no matter where the idle screws are. Most of the carbs I've done have had that problem to a larger or smaller degree.
I'd second that, was the reason why the coupe was a wee bit thirsty, swapped the carb for one properly setup and economy came back (relative of course it is a Scimitar) and the idle mix screws did something.

Re: Mixture screws- making no difference!

Posted: Sun May 20, 2012 8:19 pm
by Taggers
Interesting, is that something that can be adjusted, or a manufacturing issue?

Re: Mixture screws- making no difference!

Posted: Mon May 21, 2012 12:07 am
by tony.idle
If you look closely at a carb (that isn't attached to a car :)) you will see one or more tiny holes starting under the edge of the throttle plate & just & so on the atmospheric side of the plate. If the throttle(s) close correctly the holes will only be slightly exposed at idle. There's no adjustment as such but problems likely to cause them to feed before they should (other than physical damage) are throttle plates not centred correctly or not synchronised (both easily cured). If the idle speed and fast idle screws are backed off so that the throttle plates are fully closed you should not be able to see light through the carb.

Re: Mixture screws- making no difference!

Posted: Mon May 21, 2012 6:14 am
by Gillsfan
Idle screw and fast idle screws badly ajusted are the common cause I've found. As tony say back them off and have a look.

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 2:59 pm
by Taggers
Hey guys.

Carb is off, cleaned, rebuild kit and new jets fitted. Loads of goo game out.

Looking from underneath, when the throttle plates close there are four holes. one of the holes is for the mixture screw, and is lower, so we know about that. There is a hole higher up, perhaps 0.5mm across which is obscured (not fully covered) by the plate.

Above that there are two tiny holes, these, I think, are completely covered by the plates. either the plates block them up, or they are above the plates.

Does that sound about right?

Chris.

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 3:05 pm
by philhoward
I think thats the holes Steve (Gillsfan) keeps going on about that need to be covered up by the plates at idle (otherwise any fiddling with the mixture screws is moot)?

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 3:21 pm
by Taggers
I think so too.

The larger holes (0.5mmish) are not fully covered, and so if fuel was getting through there it would negate the mixture screws. BUT, I can't see that the plates would ever fully cover them, they leave about half showing. To me that accords with what Tony is saying, but I'm not entirely clear about the finer points. To say the least.

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 3:30 pm
by trubac
if you loosen the plates mechanism, they are not covering the holes ???

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 3:34 pm
by Taggers
Cheers Trubac, not sure I understand. There is the screw at the rear right of the carb, (Fast Idle screw) I've backed that right off (as the carb is in the kitchen) but I'm reluctant to mess with too much else. I cant see that it would be easy to alter the final position of the plates... not without real know-how.

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 4:45 pm
by trubac
there is the screws that syncronize the flaps, with the gear mechanism, if you unscrew them the flaps can move fully.

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 5:27 pm
by Gillsfan
Below is a diagram which I hope will explain why I keep going on about the progression holes :-)

Image

Left is at idle right is with light throttle

Adjusting the throttle values isn't anything to be worried about.

Back off this screw

Image

And the thd one that would be here if a had an auto choke carb to hand.

Image

Then loosern this screw
Image

The right hand throttle value will now move independent of the toothed segment. The left hand value should now cover the progression holes. Move the right hand one to do the same. Take out the backlash in the toothed segment and tighten the screw. Now tighten this screw

Image

Untill it comes in to contact, adjust thd other one as required.

Hopefully at least some of that makes sence.

Re: Mixture screws- making no difference!

Posted: Sat Jun 09, 2012 5:55 pm
by Taggers
It does Steve. Wish I hadn't just put it back on the car.

It lives, anyway. It runs like a pig. But a different sort of pig. A pig with an airleak, but at least the mixture screws do *something*. Let's see how brave I am tomorrow.