CO setting

Technical advice Q&A

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tim wood
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#1 CO setting

Post by tim wood » Mon Apr 26, 2021 4:09 pm

So I thought that having purchased a Gunsons gastester i would check my carb settings.
I’ve always thought that the mixture was too rich with signs of the engine hunting. The carbs are in OK condition with new jets and needles.

So with the jets set at about 65 thou down the CO level is around 8% and the exhaust is splashy. Tick over isn’t particularly stable at about 750-800 rpm

If I adjust to get the CO level to about 4.5% the exhaust sounds much more regular and the tick over is even. The entire set up seems better. Having done this I rechecked the depth of the jets to find that this is now around 30 thou. This just seems too high for comfort and I’m concerned about weak mixture at normal running speeds.

Does anyone have any thoughts?

Tim
Series 1 FHC purchased 40 years ago. Courted my wife in it.
Series 1 2+2 when the kids were small now sold.
Series 1.5 OTS in opalescent maroon, Californian car. My retirement present.

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mgcjag
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#2 Re: CO setting

Post by mgcjag » Mon Apr 26, 2021 4:20 pm

Hi Tim.....your set up seems better at tickover.....how about when you take it for a run.....Steve
Steve
69 S2 2+2 (just sold) ..Realm C type replica, 1960 xk150fhc

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tim wood
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#3 Re: CO setting

Post by tim wood » Mon Apr 26, 2021 4:30 pm

Hi Steve,
Thanks for the reply - do you mean with the jets raised and CO level down to about 4.5%. ?

I’ll take it for a drive tomorrow

Tim
Series 1 FHC purchased 40 years ago. Courted my wife in it.
Series 1 2+2 when the kids were small now sold.
Series 1.5 OTS in opalescent maroon, Californian car. My retirement present.

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mgcjag
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#4 Re: CO setting

Post by mgcjag » Mon Apr 26, 2021 4:50 pm

Hi Tim.....yes.....its no good haveing it running nice at tickover with low Co then find it dosnt perform on the road...however the jet do sound like they are high.....what needles do you have.....not what was on the packet whats actually on the needles.....Steve
Steve
69 S2 2+2 (just sold) ..Realm C type replica, 1960 xk150fhc

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christopher storey
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#5 Re: CO setting

Post by christopher storey » Mon Apr 26, 2021 5:35 pm

Tim : there is an interrelationship between float levels and jet levels which is fairly critical . Assuming you are using UM ** needles 30 thou down does seem on the high side, but if the float level is too high by even a minimal amount ( and I would regard say 35 thou i.e a more typical 65 thou against your 30 thou - difference as minimal when it comes to float levels), you will get this effect

May I also , without wishing to teach my grandmother etc , suggest that it is essential when using a CO meter to purge the carbs every 2 minutes at most by running the engine at 2000 rpm for 15 seconds or so. During this purge you should see the CO level drop to close to 0%, and then once idle is re-established it will come back to the true idle setting. Personally, I go for 5% . Also, false readings can result if you do not electrically warm up the meter for several minutes before calibrating it using the calibration knob

** and in theory since the alternative needles UO and UE are all pretty close at the bottom end you should not get much difference in level between any of them

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tim wood
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#6 Re: CO setting

Post by tim wood » Mon Apr 26, 2021 6:34 pm

Thanks gents for your helpful comments.

I’m fairly confident of the needle types and settings since I did renew those a while ago. Float height had crossed my mind and I may well revisit that.
Series 1 FHC purchased 40 years ago. Courted my wife in it.
Series 1 2+2 when the kids were small now sold.
Series 1.5 OTS in opalescent maroon, Californian car. My retirement present.

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MarkRado
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#7 Re: CO setting

Post by MarkRado » Tue Apr 27, 2021 4:33 am

Tim,
I got good results with 42thou on the 3.8 ( both UM and UE needle-most have same diameters at idle), and 61thou on the 4.2.
You must not underestimate even slightly worn spindle bushes; you can fit neoprene seals from Burlen, but the discs wont close really tight (at least not all the time), making fine adjustments impossible.
I rebuilt SUs from a Healey BJ7 and my 3.8 with new bushes recently; the result was amazing. Although my XK engine has seen better days, it runs super sweetly now. Just make sure that valve clearence and distributor as well as HT cables, caps and plugs are top. I guess you wont see a CO much lower than 5%. Dont mind, just go for Lambda around 0,87.
Mark
1963 OTS 880436

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#8 Re: CO setting

Post by tim wood » Wed Apr 28, 2021 5:45 pm

Just a brief update on this.
I removed the float chamber lids to check the distance twixt the lid and the fork on the needle valve lever.
On 1 carb this was around 9/16 vs the 7/16 specified. I reset this to 7/16 and checked the others which were only mildly out.
On reconnecting the fuel line and activating the pump the jet orifice copiously flooded. I rapidly turned off as I wasn’t sure whether the pump would eventually stop.

So clearly something wrong.
Haven’t progressed further due to lack of time but will update later.
Series 1 FHC purchased 40 years ago. Courted my wife in it.
Series 1 2+2 when the kids were small now sold.
Series 1.5 OTS in opalescent maroon, Californian car. My retirement present.

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tim wood
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#9 Re: CO setting

Post by tim wood » Fri Apr 30, 2021 6:39 pm

Further update.
The flooding on the front carb was clearly due to either a faulty float or the valve. I removed all of the floats and could find some barely detectable differences in size and weight. Although no sign of fuel inside. The valves looked fine but you can’t really tell.

As a belt and braces approach I’ve changed all valves and fitted the “stay up” floats from Burlen at not inconsiderable cost.
Checked the levels again and reassembled- no more flooding thankfully. I have a pressure regulator fitted so adjusted that to give a pressure of around 2.5 psi.

So tomorrow I will get back to setting the CO.

Tim
Series 1 FHC purchased 40 years ago. Courted my wife in it.
Series 1 2+2 when the kids were small now sold.
Series 1.5 OTS in opalescent maroon, Californian car. My retirement present.

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mgcjag
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#10 Re: CO setting

Post by mgcjag » Fri Apr 30, 2021 7:29 pm

Good news Tim.....at least your moveing forwards......Steve
Steve
69 S2 2+2 (just sold) ..Realm C type replica, 1960 xk150fhc

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#11 Re: CO setting

Post by abowie » Sat May 01, 2021 6:38 am

Sounds like you're getting there. Sorting out SUs does require a methodical, stepwise progression, and if you miss one thing they won't work.

Be aware that the quoted 65 thou is the equivalent of the 2 1/2 turns down setting in the manual.

But it's only a starting point for tuning. Depending on a number of factors you may end up significantly above or below that point. Having your idle CO around 5% is correct; the profile of the needle will do the rest.
Andrew.
881824, 1E21538. 889457. 1961 4.3l Mk2. 1975 XJS. 1962 MGB
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

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