Mixture screws- making no difference!

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Re: Mixture screws- making no difference!

Post by Taggers » Sat Jun 09, 2012 7:47 pm

Ok it's running better, at about 4.75 turns, seems to be a miss I can't tune out, but it's constant. I'll see how it runs over the next few days and see where we get to.

I had to blank off the vacuum tube to the cold start gizmo on the air intake, until I get more pipe. On the basis that I'm pretty sure it doesn't do anything, I hope it doesn't upset things much.

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Re: Mixture screws- making no difference!

Post by Spartacus » Fri Sep 14, 2012 12:51 am

tony.idle wrote:With the idle screws fully in the engine should be unable to access any fuel. It would be useful to know how it is managing to do so?

From dim and distant memory, IIRC there is a function on the DGAS where if the motor is starving of fuel and about to stall, there is a small vacuum held port that will open as the engine almost dies and loses vacuum and this will introduce fuel to keep it running. Thye idle jets only really perform a function at idle, there is a series of holes that link the fresh air side and the suction side of the carburettor throttle plates that allow air to pass and mix with fuel pulled, sometimes, from the accelerator jets by the excess volume. there's all kinds of little tricks that this carb gets up to.#

Mostr importantly, TBH I would be looking at float height before searching anywhere else, if this is wrong, everything else carburation wise will be too.

And that reminds me.... Something I used to meet quite often was where a manual choke kit had been fitted. Unless correctly set up, the throttle stop screw for the choke, the "stepper" if you like, could act as the tickover setting screw, giving a situation where no matter what you did with the throttle plates, the chokes would never close completely. Symptoms were, IIRC, high tickover, lots of soot. By the same token, if the auto choke was still in use (and to be fair, a properly maintained one was more than capable of doing it's job) and someone had twiddled with this screw, the same issue would occur.

Now it's been a few years since I messed with this kind of issue, so recollection might be not perfect, but I do know the float height effects everything else.

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Re: Mixture screws- making no difference!

Post by tony.idle » Fri Sep 14, 2012 1:36 am

There is (on some versions) an anti-stall device as you say. Its action is very specific & very obvious: the engine is about to stall, the anti-stall operates, revs rise, repeat. The effect is cyclic & very noticeable. The series of holes (progression holes) should be covered & inoperative at idle (throttles almost closed). They certainly won't keep the engine going if the idle screws are fully home. It's possible to set the idle incorrectly via the fast idle screw on both manual and auto chokes.



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Mixture screws- making no difference!

Post by Toecutter » Fri Dec 08, 2017 11:24 pm

It may be worth checking your ignition system is in good health, if you haven't already. A weak spark or intermittent fault in this area can cause all sorts of mischief. Just out of curiosity, how does the car run from transition off idle circuit? When pulling away normally, does the car stutter, then pickup smoothly through pilot circuit when accelerating up until the main jet circuit comes into effect? Any pops from exhaust at all? A combination of faults may be the issue. As I'm sure you're aware, It's a case of eliminating possible ignition system issues, air leaks on the intake/carb mounting before any mechanical fault diagnosis can be attempted. Leaks at the manifold, damaged gaskets at both manifold and carb mounting etc, then the carb itself. As you've mentioned, it seems like a rich problem. But, has this been adjusted to an extreme to allieviate lean running conditions?

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Mixture screws- making no difference!

Post by rebel alliance » Fri Dec 08, 2017 11:47 pm

Think this ship may have sailed 😊

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Mixture screws- making no difference!

Post by scimjim » Fri Dec 08, 2017 11:50 pm

Chris (the OP) hasn't logged on for a couple of years. IIRC he had a small loom fire shortly before his (second?) son was born and we haven't heard from him since - hope all is well.

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Current: SE5 (8Ball), TI SS1 (snotty), 1600 SS1 (G97), 1600 SS1 (C686CCR), 2.5TD SE5a (diesel 5a), 6 x random other SS1s.
Previous: SE5, 3 x SE5a, 2 x SE6a, 3 x SE6b, GTC, 2.9i GTC, 3 x 1600 SS1, 1300 SS1, Mk1 Ti Sabre, Mk1.5 CVH Sabre
Chief mechanic for: 1400 K series SS1 (Megan3), 1400 CVH EFi SS1 (Grawpy), Sabre/MX5 auto (The Flying Broomstick),
1300 SS1 (Number One) & Sarah's coupe.

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