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TR6 Stromberg CD 175 Carb Tuning

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SherpaPilot

SherpaPilot

Jedi Hopeful
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Another Stromberg question again: OK, I know there has been a ton of material generated on these carburetors but I can't seem to find the answer to my question. I have recently downloaded the buckeye technical information and plan on going through each detail within the next few days. Although my TR6 runs very well, my question concerns the piston lift method of determining rich vs lean settings. With the engine properly warmed, I have attempted to determine if my carbs are running too lean or too rich by performing the piston lift method as described in my Bentley manual as well as other manuals and videos. According to this information as the piston is lifted approximately 1/4 inch, the rpm will either increase (indicating too rich) or decrease and shut-down (indicating too lean). In my case, these carbs always decrease and want to quit when the piston is lifted. I have adjusted each needle valve IAW the manual (turning it clockwise) to enrich the mixture. At full adjustment travel clockwise, there is no effect on the carbs and they continue to decrease RPM and will shut try to shut the engine down. Tonight I tested the car with both carburetors enriched as much as the adjustment would handle (to the full stop) and the car ran well as usual. I pulled the plugs and found normal conditions with a slight amount of carbon buildup at the base and within the threaded portion. I'm starting to think there is something such as an "o" ring that has failed in both carburetors causing this lean evidence. A part of me wants to leave well enough alone and continue to take plug readings but another voice is telling me there is something that is not correct and I had better fix it before I end up doing damage.

This is a freshly rebuilt cylinder head with new exhaust valves. All valves have been adjusted and checked. The ignition system is properly tuned and fresh including wires, plugs and timing. The fuel is fresh with the highest octane I could find in my area (91). The mechanical fuel pump is somewhat new and is working as advertised. The fuel filter is new. There is no problem with fuel flow. What point am I missing?
 
Yup, 1/4" is way too high. I gave up on the "lift the piston" method and instead went to reading the spark plugs which in itself was a learning experience.
 
Thanks for the help. I don't know why I couldn't find that link earlier but it certainly helps. I'll head back to drawing board later today and re-visit the adjustments.
 
There are several methods for assessing mixture - among them the lift-the-pin, examining the plugs & Colortune (viewing the spark). I use all three but in the end rely on looking at the plugs to see how I did.
 
If your carbs are too lean, the car will probably run hot. The best advice I can give you is to leave everything alone when the car is running well. Stromberg CD carbs have a limited amount of possible adjustment and they are very reliable once initially set up. If you just can't keep your hands in your pockets, give the carbs a little shot of cleaner spray every now and then.
 
I think if you put the needles in the middle of their adjustment range you'll be fine.
Put about 300 miles using fresh spark plugs on that setting, avoiding prolonged idling and and trying keep the engine in the 2500 rpm range as much as possible during that time, then remove the spark plugs for evaluating.
That's what I suggest to those who send me their carbs for rebuilding.
Since there is only 1/8" total adjustment range, setting the needles to the middle will make it necessary to deal with only 1/16" in one direction (either up or down) if further adjustment is even needed.
For all practical purposes, the middle is 1 and 1/4 CCW rotations of the needle adjustment screw from fully CW.
 
I'll add (because I just went through this) incorrect float height can make tuning difficult. Worth checking if the usual methods don't seem to get you where you need to be.
 
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