View Full Version : Stromberg Carb Sync and adjust

08-09-2012, 09:49 AM
I am familiar with Datsun SUs and follow these steps to adjust and sync.
- Bottom out air/fuel needle, adjust out 2.5 turns for a baseline
- Adjust idle with idle screw on both carbs, verify equal flow
- Bring idle up to 2500, adjust linkage for equal flow
- Test drive, tweak air fuel equally
This process had served me well on a number of cars over the years

With this knowledge as my personal baseline, and not having the 72 TR6 in front of me until this weekend, I'm trying to understand the similar, simply stated steps for the Strombergs. I have read Buckeye write up 5X and I know the answers are probably right there, so here are my questions, if I were to try to follow a similar process.

- Is there a baseline # of turns for air/fuel? Where do you buy that special tool?
- idle is adjusted on the linkage? Seems like that would vary too much? Is there independent idle adjustments on the linkage for each carb?
- What does the bypass valve do?
- syncing flow at 1500 rpm as suggested is accomplished by adjusting a different point on the linkage?

Thank you in advance.

08-09-2012, 10:31 AM
Any LBC vendor will have the tool. For the "emissions" ZS carbs, my preference is to start full rich. The adjustment has very limited range, so full rich isn't very rich although you should wind up a turn or so leaner.

PS, don't go too lean, or the adjustment will come apart inside the piston and have to be re-engaged. It's only about 2 turns from full rich to full lean.

If every carb were identical, there would be no need for adjustments. Your process of always adjusting the mixture identically is assuming that the carbs are identical.

Yes, the idle stop screw is attached to the lever for the idle vent. There is a separate fixed stop for the vent lever, and it isn't attached to any other linkage, so it is plenty precise enough.

The bypass valve is an emissions device. When intake manifold vacuum is high during overrun (compression braking), the engine may not completely burn what little mixture does go into the cylinders, resulting in high HC emissions or even raw fuel in the exhaust. The bypass valve prevents this from happening, by bypassing the throttle and allowing more fuel/air into the cylinders. Some people chose to disable it, but you may get popping in the exhaust (caused by that unburned fuel I mentioned) without it.

Yes, there are separate adjustments for idle synchronization (as above) and linkage synchronization. There is deliberately some lost motion in the linkage, so that it doesn't interfere with idle adjustment, and the lost motion gets adjusted as well. Both lost motion and linkage synchronization is done with the engine stopped, by loosening the linkage clamp, inserting a drill rod in the lost motion stop, and tightening the clamp with the lever held against the drill rod. The 1500 rpm check is just a double-check that the previous adjustment was done properly.

08-09-2012, 11:48 AM
I might add this because on the surface it seems counterintuitive.
When adjusting the fuel air mixture, Clockwise turns with the adjustment tool raises the needle, enrichening the mixture.
To lean the mixture it takes Counterclockwise rotations..

And to repeat what Randall already said...From Full rich to as far lean as you dare go is just a hair over 2 Counterclock rotations. Any more and you'll unscrew the needle (carrier) from the adjuster. At that point, although the adjuster and the adjustment tool will still turn, the needle height will not change until your re-engage the threads.

If the ignition timing is correct and the engine basically stock, you should find a good mixture between 3/4 and 1-3/4 Counterclock turns from full rich (Clockwise)
Also, if you've never dealt with carbs that have an air valve diaphram..read up on it's proper orientation both on the air valve and situated in the carb.

08-09-2012, 12:59 PM
One of the guys in our local club changed his over to EFI using the cars as throttle bodies. He found someone in New England who specializes in that conversion, EFI (https://www.affordable-fuel-injection.com/browse-by-system/side-draft-tbi.html) this may be the site.

08-09-2012, 06:48 PM
Great info, thank you.

You helped put some of the pieces of the puzzle together for me. It looks like the Strombergs have a much finer air/fuel tuning range than the SUs and now that I know where the idle and the linkage adjustment is, I think I am set to straighten out some of the fuel delivery issues when I drag the car home.

08-09-2012, 11:52 PM
It looks like the Strombergs have a much finer air/fuel tuning range than the SUs
Pretty much all of this junk is because these are "emissions" carbs. The early, non-emissions ZS carbs were much simpler and more like the earlier SUs.

Emissions SUs got pretty weird too, with things like a thermostat built into the mixture jet. Some of them even had ball bearings for the piston to ride on.