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mixture too lean

M

mbruskin

Donation Time
What would keep me from getting more fuel to my Strombergs? My plugs are too white, not tan or black, and my wide band oxygen sensor seems to agree with numbers from the mid 15's to sometimes 17 with the afr. As I turn the knob on the bottom of the carbs to increase the richness a little at a time they unscrew and fuel comes out. They seem to go about 3 and a half turns before that happens. After each small mixture adjustment I make a run of about 20 miles up the interstate and then pull the plugs. The color always seems to match the afr numbers. Can't seem to get it right.

Any ideas?

Murray
 
65beam

65beam

Donation Time
lean

why put an oxygen sensor on a car that everytime you take it for a drive everything changes? the carbs change as they digest the damper oil. the valve adjustment changes every mile and for what it's worth, these are old LBC's with old LBC engines. look at the problem you are having with carb adjustment by reading a sensor. do you expect a perfect tune of these engines ?????????
 
RootesRooter

RootesRooter

Donation Time
Do the sensor readings seem to follow along as you back the adjustment knob out and richen the mixture?

Strombergs are pretty 'dirty' carbs; I can't remember not having some soot on the plugs.
 
Ragg Mopp

Ragg Mopp

Donation Time
Are you sure the needles are the correct needles with the correct number stamped in them? I got a set 5 years ago that didn't have numbers but were supposed to be correct. They were significantly too lean and I had lots of problems until I ordered new needles and jets with the correct number on them. It made a huge difference in the way the car ran.
 
65beam

65beam

Donation Time
lean

another thought is that there are conventional strombergs and there are the emmision carbs that have no mixture adjustment. they are a self adjusting carb. parts from the two such as the needles and fuel bowl could have been mixed at some time or you may have the emmision carbs. the emission carbs run very lean. what do the screws at the bottom of the bowl look like? if they are flat with only a slot for a screw driver, they are probably the emmision carbs and there is no mixture adjustment to these carbs. probably three to four turns would allow the plugs to fall out. i checked the carbs that were originally on the wife's series 5 and it took 11 full turns to remove the mixture adjuster knob.
 
SIVAllan

SIVAllan

Gold Level Sponsor
65beam said:
another thought is that there are conventional strombergs and there are the emmision carbs that have no mixture adjustment. they are a self adjusting carb. parts from the two such as the needles and fuel bowl could have been mixed at some time or you may have the emmision carbs. the emission carbs run very lean. what do the screws at the bottom of the bowl look like? if they are flat with only a slot for a screw driver, they are probably the emmision carbs and there is no mixture adjustment to these carbs. probably three to four turns would allow the plugs to fall out. i checked the carbs that were originally on the wife's series 5 and it took 11 full turns to remove the mixture adjuster knob.
Click to expand...

I don't mean to hijack this thread, but this is the first I've heard of the emission Strombergs. Does anyone know if they do about the same "mileage wise" and "performance wise" as the conventional Strombergs?
 
puff4

puff4

Platinum Level Sponsor
Emission Zenith-Stromberg carburettors were never fitted to series Alpines, so I doubt that there's a parts mix-up, although I suppose anything is possible.

However, it is entirely possible that you have an incorrect needle or air valve spring. Needles are marked with stamped-in letters on the shank (i.e. you need to remove the needle to read them) and springs are colour coded, though very likely the colour is missing by now (and the correct spring happens to have *no* colour!), so you can simply compare it with a 'known good' spring to see if the compression rate is similar. Too loose a spring or too thick a needle will make for a lean mixture.

The proper needle is '5M'.
The proper spring has no paint color and is .032" in diameter.
 
SIVAllan

SIVAllan

Gold Level Sponsor
puff4 said:
Emission Zenith-Stromberg carburettors were never fitted to series Alpines, so I doubt that there's a parts mix-up, although I suppose anything is possible.
.
Click to expand...

What happened at the factory and what has happened since then, are two different things :)

If they fit I'd bet 'dollar to a dounut' that they have been installed here and there over the past several decades. For that matter, I'd not be surprised to learn I'm running a pair :)

Rgds,
 
todd reid

todd reid

Gold Level Sponsor
Another thing to check, if you haven't already, would be for air leaks that are leaning out the mixture.

The usual suspects: throttle bushings, PCV system, intake manifold & gasket, etc.

You may also want to verify your diaphrams are in good shape.

Good luck!
 
RootesRooter

RootesRooter

Donation Time
The emission Strombergs would be pretty easy to spot mounted on a Series car manifold. On the dashpot, the large letters say "CDSE" instead of "CD." Each carb has on its right side a 1-1/2" or so oblong capsule, mounted diagonally to the carb body, with a plastic cover. Inside the capsule, if I remember correctly, is a bi-metal bar which opens a valve when warm to let more oxygen in. And if you peer underneath, the lack of a mixture adjustment knob, replaced by a flat, one-setting-fits-all jet, should jump out at you.

One other minor difference is that the float bowls have a slightly different casting. CDSE float bowl gaskets will NOT work on CD's.

I've only seen one pair of CDSE's on a Series Alpine in 30 years, and those were on a junker.


SIVAllan said:
What happened at the factory and what has happened since then, are two different things :)

If they fit I'd bet 'dollar to a dounut' that they have been installed here and there over the past several decades. For that matter, I'd not be surprised to learn I'm running a pair :)

Rgds,
Click to expand...
 
65beam

65beam

Donation Time
lean

allan,
i know that you have seen my fastback. on the drive thru the north carolina mountains last year,the emmision carbs did a fantastic job of allowing me to pull that winding ,twisty, dog leg road that george had us on. i may have grabbed 1st gear a couple times. my car runs great with them and averages in the high twenties on mileage. tiger tom described them as self adjusting for the altitude. as dick said, they do add extra oxygen when warm. one thing that is different about them is that they require the intake and exhaust manifolds of the fastback to allow the carbs to work right since they draw heat off the exhaust. the intake bolts directly to the top of the exhaust manifold. there is a small chamber between the exhaust manifold and the intake manifold with a copper clad gasket between the two manifolds. this is entirely different from the setup of the series 5. my carbs have only a few hundred miles on them and perform great.
 
SIVAllan

SIVAllan

Gold Level Sponsor
65beam said:
allan,
i know that you have seen my fastback. on the drive thru the north carolina mountains last year,the emmision carbs did a fantastic job of allowing me to pull that winding ,twisty, dog leg road that george had us on. i may have grabbed 1st gear a couple times. my car runs great with them and averages in the high twenties on mileage. tiger tom described them as self adjusting for the altitude. as dick said, they do add extra oxygen when warm. one thing that is different about them is that they require the intake and exhaust manifolds of the fastback to allow the carbs to work right since they draw heat off the exhaust. the intake bolts directly to the top of the exhaust manifold. there is a small chamber between the exhaust manifold and the intake manifold with a copper clad gasket between the two manifolds. this is entirely different from the setup of the series 5. my carbs have only a few hundred miles on them and perform great.
Click to expand...

Yes I do remember how great they ran - it was amazing to see. Next time I see that car I certainly will tale a closer look at those carbs!
 
R

robertf

Donation Time
You ever fix this? I'm having a similar problem.

Idle speed 1000 rpm AFR around 13.5-14. Cruise its around 15-17 unless I pull the choke cable, then it'll go to 13.5-14.

Rebuilt carbs, new needles, innovate lc-1 with tailpipe clamp. Tried ATF, 15w30, now regular 30 in the dashpots.
 
R

RootesRacer

Donation Time
robertf said:
You ever fix this? I'm having a similar problem.

Idle speed 1000 rpm AFR around 13.5-14. Cruise its around 15-17 unless I pull the choke cable, then it'll go to 13.5-14.

Rebuilt carbs, new needles, innovate lc-1 with tailpipe clamp. Tried ATF, 15w30, now regular 30 in the dashpots.
Click to expand...


Are your throttle shafts worn?

On stroms when you cant get the main and idle mixture to work at the same main jet positions, its usually an air leak from the throttle shafts.

Your situation sounds odd though since usually adjusting the mains to get a good idle usually results in a "too rich" condition under load since the idle condition is weakened by the air leak.

Also I thought you had an SII, do you have a mix?
 
R

robertf

Donation Time
no, its a series 5 with wrong grill and a hood badge. maybe that threw you off. 1725 .020 over, KA camshaft (maybe KC, its the other one besides KB)
 
R

RootesRacer

Donation Time
robertf said:
no, its a series 5 with wrong grill and a hood badge. maybe that threw you off. 1725 .020 over, KA camshaft (maybe KC, its the other one besides KB)
Click to expand...

Well stroms are quite simple machines.

The taper of the needle sets the "mapping" of the fuel curve.

The diaphragm balances the force of the pistons gravity and the piston spring pressure and maintains a constant pressure drop across the piston.

The adjustable main jet allows the jets cross sectional area to be adjusted in relation to the needle for AFR adjustment.

If the diaphragms aren't torn, the pistons dont stick and the needles are correct AND barring vacuum leaks via the throttle shafts, they should work pretty well when adjusted.

One caveat though is that you have changed the camshaft and the taper on the needle is a single application kind of thing, and therefor probably not quite right for your camming.

I'm not sure what you think your AFR targets should be, but its not likely you will have a quality idle at idle AFRs leaner than 14/1.
Your running mixtures should probably be set to 13.5/1 under load.

Based on you description, it looks like your idle AFRs are OK, but your running mixture is a tad lean.
This can be cured in two ways.
1) Install a heavier piston spring.
2) Modify the taper of the needle for a smaller needle section in the "running" position.

#2 is a tricky business unless you can make your own needles out of hard stainless.
 
Ken Ellis

Ken Ellis

Donation Time
Regarding point 2 above -- the taper mentioned is not always a "straight line" point A to point B taper. Rather, they can be non-linear with "steps" along the way. I have an older StrombergCD-specific Haynes manual with several pages of specific dimensions of needles -- with measurements taken at several points along the length -- for a wide range of cars using the same carb(s).

Suffice it to say, I was never even close to the point where the specific needle taper made any difference when compared to the huge leaks around the throttle shafts. I was recently able to correct the issue, though... see sig. :D
 
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