TR6 Stromberg Carb Mixture Adjustment

[JP72TR6]

I know this issue has been written about in length, however I'm trying to reconcile several sources of information while trying to adjust the mixture on my two Strombergs ('72 TR6). Note, I've replaced most of the components of the carbs with the exception of the rebuilding the bypass valves and replacing the throttle rod bushings. Timing is set, valves adjusted and carb idles are synchronized. When using the screwdriver lifting the air valves on the carbs, the RPM continues to drop on both carbs. With the engine off, I'm using the air valve adjustment tool and turning all the way clockwise to stop but the RPM drop continues. My question is if I should be returning the damper rods prior to restarting the engine and retesting by the lift method or can the engine be run and tool adjusted without the damper rods in the carbs? And, if so, is this the reason why I haven't been able to get the lifting method to work properly?

 

[poolboy]

I'm not big on the 'lift the air valve' method of checking the mixture.
Best advice I can give is use the proper mixture adjusting tool and set the needles 1 and 1/4 CounterClockwise turns of the allen wrench after having turned the adjuster FULLY Clockwise using the allen wrench
Then take a few nice long drives, avoiding stop and go, and idling as much as possible and try to keep the rpms 2100 or higher, even if it means using lower gears....then pull your spark plugs ad see if they have developed a nice color indicating a satisfactory mixture.

 

[JP72TR6]

I'm not big on the 'lift the air valve' method of checking the mixture.
Best advice I can give is use the proper mixture adjusting tool and set the needles 1 and 1/4 CounterClockwise turns of the allen wrench after having turned the adjuster FULLY Clockwise using the allen wrench
Then take a few nice long drives, avoiding stop and go, and idling as much as possible and try to keep the rpms 2100 or higher, even if it means using lower gears....then pull your spark plugs ad see if they have developed a nice color indicating a satisfactory mixture.View attachment 85700

Thank you for the input. The reason I started messing with the carbs again was that in general around town driving the plugs looked like the picture above, however when I drove the car on a mostly highway 150 mile trip the plugs were black when inspected. I'll try the 1 1/4 turn and try another test run. Also, it seems like I'm topping up the dashpots more than I used to after replacing the o-rings (Moss) on the needles. Can this possible oil leak contribute to the blackening the plugs?

 

[poolboy]

I doubt it..the oil is there to slow down the rise of the air valve/needle thereby temporarily enrichening the mixture when you accelerate. The oil has nothing to do with the the mixture under any other conditions.
And the damper doesn't hold enough oil (3.5cc) to foul the plugs even if it dumped into the 3 cylinders that carb serves all at once.
BTW, the manual recommends using the same seasonal motor oil in the damper as you put in the engine...Also my recommendation
Usually the plugs do the opposite of what you describe if the mixture is close to right...dark in that stop and go town driving and fairly clean following a good long run on the highway.

 

[JP72TR6]

Thanks again, let's see what happens with the baseline 1 1/4 turn adjustment. I may also need to check the timing again. I know this has been written about ad nauseam, however I always followed my worn and oil-stained Haynes manual that specifies a dynamic mark of 4 degrees ATDC versus my newer British Leyland TR6 Repair Operation Manual that specifies a dynamic mark of 40 degrees ATDC. Is this the difference related to what some post as the erroneous Haynes publication?

 

[poolboy]

Must be....If you have a working VACUUM RETARD, the book says 4*ATDC at idle of about 850 rpms whereas if the Vacuum Retard is disabled, the book says something like 10* BTDC at idle....Maybe as much as 14*BTDC
But I'm kind of old school and just advance the timing until the rpms increase then retard just a tad and go for a drive.
Despite all those numbers, you'll find this in Haynes "ignition System" chapter.

 

[JP72TR6]

Strangely, when I looked at a couple of pages earlier in the British Leyland repair manual it stated the same as my Haynes Manual at 4 degrees ATDC dynamic at 850 RPM idle (vacuum retard). Not sure where the 40 degrees at idle came from - could be a misprint. I think you're right, old school is the best school for this given the variables of fuel type, engine wear, etc.
Thx