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General Category >> Rubber Side Down! >> air fuel mixture screw is all the way in, problem?
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Message started by scruickshank on 05/31/17 at 18:08:40

Title: air fuel mixture screw is all the way in, problem?
Post by scruickshank on 05/31/17 at 18:08:40

Tweaking my wife's 2007 S40 to minimize/eliminate the backfire. PO had drilled out the mixture screw plug and I performed the white spacer mod on the needle, going from stock spacer to two #4 washers.

When adjusting the mixture screw per Lancers guidelines in the tuning section the mixture screw yielded good results when fully screwed in. No dead spots, only minor popping, good acceleration, no backfires, etc. Turning it out 1 - 1.5 turns increased popping and backfires.

Will keeping the mixture screw all the way in cause problems? I don't know if PO changed the jets and forgot to note the size when doing the washer mod.

Anyone have helpful info?

Title: Re: air fuel mixture screw is all the way in, prob
Post by IslandRoad on 06/01/17 at 01:30:51

They sound like unusual results. The common wisdom is that turning the screw out makes the mix richer and reduces the backfiring and popping. You seem to be getting the opposite effect.

Generally speaking, the ideal pilot-jet/screw-setting will see the screw sitting at about 1.5 to two turns out. Screwed all the way in should make the engine cut out (or almost cut out) and turning to 3 turns out should make it run too rich (any further and you risk the screw vibrating its way out). The sweet spot is somewhere between fully closed to 3 turns out.

A lot of guys find that setting the idle speed to about 800 rpm makes it easier to check the idle mix because the needle jet is coming into play at higher rpm. Once they get the best adjustment they set the rpm back up to somewhere between 1100 and 1400 rpm.

Are you running the stock muffler? Did the PO drill holes in the back of the muffler?

I reckon you might need to check the jets so you know what you're working with. Otherwise you're trying to tune based on an unknown quantity.

Title: Re: air fuel mixture screw is all the way in, prob
Post by IslandRoad on 06/01/17 at 01:32:54

BTW, when the Savage/S40 is tuned well for on the road, it will often still 'backfire' and pop between gear changes. Tuning out the popping at closed throttle is essentially tuning it rich at everything else.

Title: Re: air fuel mixture screw is all the way in, prob
Post by justin_o_guy2 on 06/01/17 at 07:55:45

If I Ever tuned out all the rumbling and growling I'd be trying to fix it. I enjoy the sound of it as I work down through the gears..