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Unable to lean out mixture on one carb


daverclasper
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Hi everyone

 

Twin stromberg 150cd

 

Have been trying to lean out my mixture, as fuel consumption around town has been very heavy (about 15 mpg)

 

Haven't got a feel for the "lifting the valve technique", so using a gunson colortune on one plug hole for each carb and the revs at about 11000 as throttle spindles are worn.

 

Carb no 1 has leaned out with the top of the jet about 2mm below the bridge

 

Carb no 2 is not leaning out even with the mixture screw fully turned in and the top of the jet looks to be sitting about 1mm above the bridge.

 

The brass washer that sits on top of the spring, is 0.5mm on no 2 carb and 2mm on carb no 1, so maybe in the past someone has tried to get the jet higher.

 

It has new airfilters and I have reciepts for new jets and needles fitted about 8000 miles ago.

 

So, I don't understand why this no 2 carb won't lean out.

 

I have made sure the jets/needles have been centered when doing adjustements.

 

Any advice would be great.

 

Cheers, Dave 

 

 

 

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Hi and thanks.

 

The swapping sounds a good plan and maybe the jets as well?.

 

Have another set of carbs I've rebuilt with with new spindles, though intended using existing pistons/needles and jets.

 

Also forgot to mention, I have checked the throttle plates are balanced using a hose to ear and also the chokes do appear to be shutting off ok.

 

Cheers, Dave 

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is the float vent really clear, no gaskets covering any front ports ?    and on the air filters 

 

has  the jet tube got the same size bore  check with a small drill  0.9mm comes to mind 

but wrong spec jets will fit if someone fitted a mix and match odd ones .

 

as  base CD the needles are fixed,  have you tried dropping the needle lower ,  do both pistons drop to give the same clearance

to the bridge ,, use feelers to measure the air gap

 

 

Pete

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Hi

 

I put together my spare set on carbs with new throttle spindles.

 

I used my current pistons and needles (checked these were 6J's) and jets (the bores measured about 2.2mm).

 

Used the floats that were with the spare set, bit different but a floats a float I assume and set float height to 18mm (my current ones were around 21mm when I measured them a good while back, but car was running well then so left alone).

 

Made my own float chamber gaskets, but checked they were not blocking anything.

 

Used my current diaphrams, but checked there were no pin holes etc.

 

Screwed up jets until level with bridge's, then backed off 3 turns.

 

Checked choke was shutting off ok.

 

Primed carbs with lever on pump.

 

turned it over for ages and nothing. Checked a couple of plugs and one dry, the other a slight trace of petrol.

 

Didn't have the air filter on a this time and noticed that both the bridges had puddles of petrol and rear carb had fuel coming out of the float chamber breather/overflow thingy hole on the carb face, so flooded like mad.

 

Managed to get it going, though running very rough and black smoke out of exhaust.

 

Used the color tune on carbs, very yellow flame. Carb no 2 leaned out with about 1 1/2 turns in, though carb no  looked like it was only starting to lean out with the jet fully screwed up.

 

This was the opposite to how they were before.

 

Car running with low power and the exhaust note sort of farty sounding (couldn't think of another word).

 

Any ideas please.

 

Cheers, Dave 

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did the floats have two valve arms as this can get you to fit them upside down  and while  the height maybe right they dont float  correctly

 

the flat part of the float should be on top  the float sits at an angle in the fuel

 

Pete

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Hi Pete

 

Single valve arm, and flat of float was on the top.

 

Also forgot to mention that I used the existing float valves, though checked that they were freely moving up and down and I also removed them and blew through them to check they were sealing.

 

All cleaned behind the valves as were all internals of carbs.

 

Cheers, Dave 

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Hi

 

Have just swapped the pistons and needles over. Back to what it was (rear carb still rich with jet fully in and front carb leaning out quite close to bridge and still running really rough .

 

I wondered if it could be the timing (I swapped back to points after an electronic ignition appeared to fail recently and retarded the timing accordingly until pinking stopped and noticed dizzy was in same position as when original points were fitted, as I had marked it)

I advanced it with engine running and this made a huge difference and engine smoothed out and revved a lot freeer. The timing is now advanced to about where it was with electronic fitted.

Havent driven it yet, so don't know if it's pinking.

I'm surprised this tinkering has affected the timing so much, I guess the main difference would be the new spindes and the floats at factory height.

So still not able to lean out.

Pete did you mention lowering the needles. If so do know what the max could be without altering the plumb or the stability of them in the orofices, maybe a couple of mm?.

Not ideal I guess. Wouldn't mind buying new needles and jets if wear is causing this, but don't know if it is. 

 

Thanks again for both your help.

 

Cheers, Dave

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2,2 is 0.9" so thats correct,jet dia.

 

single arm eliminates float 

 

flat on top  correct

 

drop the needle is a get out of jail idea,   you can l drop until the screw wont grip .

 

the needle valves ...are these new or just as you say cleaned ,   the pump gives about 2.5psi  much more than you can blow so

thats not a deciding test.

 

make tea with two tea bags 

 

i still reckon there is a fuel height going wrong here.

 

advancing with engine running will allow too much advance , as you advance it will raise the idle a lot.  the old trick was hold it a t 2000  advance to get the highest speed then back it off,   if you fit a vac gauge you can advance to rise up to 23ins hg  but to run without pinking you draop it back to 18-20 in hg     has the same effect , be it ears , strobe or gauges   the result is the same ,, my guess she too advanced now and will pink

always use  super  99+ ron fuel   not **95    that was for mowers when these were built 

 

Pete

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Hi Pete and thanks

 

Have been mulling this over

 

When I fitted the second carbs with the jets 3 turns down from bridge and tried to start it for ages and it wouldn't, I also had the choke out as was quite cold day.

 

That was when I noticed the puddle of petrol on both the bridges and the the float chamber overflow/vent tube leaking on rear carb and also dry plugs.

 

Would this be the fuel not atomizing due to lack of throttle and way too much fuel coming out of the jets set very open (when it did finally start it was puffing a lot of black smoke).

 

After advancing the dizzy and it running tons better it did not appear to flood, though it did a bit out of float overflow on rear carb after switching off (rear carb is the one I can't lean out at all, using the colortune).

 

So would this likely be the float valve sticking/leaking on rear carb, as even if way too much fuel going through the jet to atomize fully, it would flood the bridge (or also come out of float chamber overflow/vent tube?).

 

Just trying to get a starting point on what to try.

 

Hope this makes sense.

 

Cheers, Dave 

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Hi Pete

 

Thought before buying new valves I would try the ones from original carbs as these hadn't flooded since I had car and been in use very recently.

 

Fitted these and rechecked float heights and also that no fuel leaking into floats and correct position.

 

Still flooding, now both carbs.

 

The measure of the original floats has always been 20mm/21mm which gives a lower fuel level I think. Thicker washers on valves, didn't look how much tabs had been altered.

 

Was wondering if the floats had been set different on purpose for some reason to do with flooding/over richness.

 

Thought, worth mentioning before spending, as cash slim at the mo.

 

Cheers, Dave

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If messing with the float needles is giving all these mixed results its telling you its the area for concern

 

they are generally quite reliable and you are having too much trouble, you cant effectively strip the needle

out of their base , try giving one a light tap witha small hammer to re seat it

 

can you devise a way to check the output pressure of your pump is it way above the max 2.5 psi

pete

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Cheers, Pete

 

Will give that a go. Girlfriend thinks I'm becoming obsessed and should take to a mechanic. Not done that for 30 years, so not good for my phsci  and properly not many about now who know these older ways.

 

I did wonder about the pump pressure output. It's an original pump. Can these over pump do you know.

 

Cheers, Dave

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Cheers Pete

 

Since tapping the valves with hammer, it's no longer flooding so far.

 

I'm managing to lean out the rear carb now, though with the jet adjustment screwed nearly all the way in. Just wondering if it's still leaking slightly and if this would be causing the lack of jet adjustment range.

 

Sorry to keep bothering you with this. Hopefully the last question, also many thanks.

 

cheers, Dave 

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