3 lean 3 rich?

[Chicago]

70 TR6, ZS carbs, Pertr EI.
End of last year couldn't get past having 3 rear cylinders too lean and 3 front too rich.
Rebuilt carbs over the winter whole 9 yards.
Now although nice idle and seems to run fine still can't get past white dusty plugs in rear 3 and sooty 3 in front. Have adjusted both carbs to max still unable to fix mixture. Have checked for leaks can't find any in carbs. Checked and adjusted float levels. Logic would seem to indicate something perhaps beyond carbs. Checked vacuum lines they're ok. Carbon canister is not plugged. Need some ideas? : )

 

[Gary Pope]

If you are sure your carbs are good ...

I am guessing your intake manifold has an air leak on one side, probably at the gasket or the underside above the exhaust header. Any leak on the underside of the manifold is hard to detect, the compression collars on the exhaust bolts don't do a great job at sealing the manifold down.

If you take the manifold off you will see evidence of it leaking on the outside near the mating surface & the gasket.

 

[Chicago]

Upon further investigation found possible contributer. Despite pair new diaphrams found rear carb diaphram slightly different fit and tension. Just enough that it holds piston just a hair off definite bottom out and misses distinct click on drop. Perhaps part of problem, also contributes to slightly different pressure on piston lift. Thinking is this contributes to asymetrical power curve and possibly mixture difference. I wonder if anyone else has had this issue? Perhaps some quality control issue in diaphram manufacture?

 

[piman]

Hello Chicago,
the fact that the rear carb does not drop with a 'distinct click' means it will not function correctly but the cause is not the diaphragm but the main jet is not correctly centred. You need to adjust this before anything else. (This applies to any CD carburettor, Stromberg or SU)

Alec

 

[Chicago]

I'm all ears. Didn't think there was a centering
adjustment available? 70 carb has adjustable needle up and down how does one accomplish
lateral adjustment?

 

[piman]

Hello Chicago,
The usual method is to set the carb fully lean, in your case with the needle screwed down as far as possible. Slacken the jet holder and hold the piston down hard (with the dashpot installed) and then tighten the jet holder. This should centralise the jet relative to the needle. Reset your needle to normal running position and it should fall back with a click.
It is a long time since I ran Strombergs, but I seem to remember that the diaphragm has a small moulded tab on the underside, check that is lined up correctly to match the location point in the carb body.

Alec

 

[Rick O.]

Piman--The jet adjustment you recommend does not apply to the Strombergs (it works with SU's). The 175CD Strombergs have fixed (interference fit) jets that aren't accessible without removing the float bowl.

 

[piman]

Hello Rick O\Chicago,
Yes, that's correct, you need to remove the float chamber, and slacken the jet, then re tighten with the piston held hard down. This aligns the orifice bush.

Alec

 

[Chicago]

Think I'm gonna have to take a closer look.
Yes the tab was alligned. There was a noticiable
difference in the way the diaphrams set into carb.
The rear one having to be snugged in before capping seemingly compressing rubber & raising the piston slightly. I will be researching centering needle directions.

 

[MDCanaday]

My 2 centavos is watch the gross jets and the float height, if the front floods the rear can be starved. Is there adiquate fuel pressure?? 2.5 to 3lbs is needed less will drive you nuts.
MD(mad dog)

 

[Chicago]

thanks for all the good ideas.
so far while waiting for some parts swapped the
rear diaphram to the front piston. seems like the
diaphram issue may be the culprit after swapping
diaphrams needle clicked nicely home.

 

[Rick O.]

I've noticed a difference in the thickness of the diaphrams in the rebuild kits. While rebuilding a set of unmolested Strombergs, I noticed the original diaphrams were much thinner and more flexible than those supplied in the kits. Shouldn't matter as long as they're replaced in pairs.

Rick