Carburetor Additional Notes

First Published Dec 2021, R. Kwas  [Comments added]

As the main SW-EM SU Carb page has gotten quite long, I have collected additional related topics here.


Mixture Optimization and the Gunson Colortune
    Wideband O2 Sensors
Carb Spacer/Thermal Isolator Notes and Observations

Reference Information
    Thermal Conductivity of Bakelite


Mixture Optimization and the Gunson Colortune:   

This cute tuning aid is essentially a "Spark Plug with a window", which allows monitoring the color of combustion, a visible parameter that correlates well with mixture ratio, so the comments here are applicable not only to SU equipped Volvos.  The major issue, and why the author does not like this tool so much is that such a tool has a serious limitation:  It must be watched during all tests and adjustment conditions...which is possible only when not rolling, like for the Idle...but Idle is a No-Load Condition!...what is maybe more important to me is the Throttle response and Loaded Operating Condition, which cannot be duplicated in the driveway while watching under the hood!  Indeed, a satisfactory idle adjustment, when stationary in the driveway, might easily give a unsatisfactory throttle response out on the road...!

A Rolling Road or Dynamometer, which loads the engine in a real-road situation allows the use of the Colortune tool while tester operator/adjuster is standing next to the car watching for the color, and able to make adjustments to optimize the results...but time on one of these rigs is typically expensive and so quite limited...but these instruments do give a printed out result as a record, so there is little subjective arguing about results! 

A Roadtest, with a hill section to monitor the Throttle response, including under continuous load, with the hill section taken at various speeds, steady, or accelerating, and monitoring the color of the Spark Plugs immediately afterwards, is the next best thing for us mere mortals on a budget. 

From Colortune's info: 

Orange - More Rich
Yellow - Less Rich
Blue - Optimum
White - Lean

Gunson Colortune in use, showing possible mixture conditions.

My responses to questions of mixture adjustment and mentions of this product on :

"Far be it for me to discourage a guy from buying another cool tool, BUT...Colortune shows lean or rich state of mixture, OK, so unless you have the car on a rolling road/dynamometer on which to check ALL ENGINE OPERATING CONDITIONS AND LOADS, usefulness of this device is very limited to adjusting Idle Mixture (no load!) only (and it seems to me to be an unnecessary expense, for setting only Idle, when there are any number of procedures in place for this)...and this goes for various carb(s). In the end, little beats a road test! Cheers" 

JD writes:  "Would giving it throttle apply load? While using Colortune?"

My response:  "Increasing Throttle loads the engine only very briefly against it's own rotating inertia, [that's why "lightening the Flywheel" is often employed by individuals looking to decrease Crankshaft inertia and for a faster Throttle response.] and this condition is/should be accommodated by Accel Pump/Enrichening function, (including very quick Throttle opening "Blipping the Throttle"), but shortly after that, equilibrium is reached at new Throttle opening, and it is a no-load condition once again...mixture wants to be checked at a continuous various loads analogous to pulling up a long hill at various speeds...that's what the Dyne can do for, with Colortune attached, you can be standing next to the Dyne watching it under these various load conditions...alternate to a Colortune and a Dyne, a wideband O2 Sensor gives you similar info while driving...!" 

JG writes:  "My experience is that as soon as you change the throttle position the colour will change for a moment. And to keep the engine revving at a specific number of revolutions by hand is virtually impossible. Therefore the colour tune product is only of use for setting mixture at idling speed. But you will not know anything about mixture under load. I'm not a fan."

My response:  "...precisely what I was saying above..."soon as you change the throttle position the color will change for a moment" ...that is transient response of setup to being momentarily loaded by engine inertia as RPMs increase...but as soon as RPMs settle at new point, load is minimal...Colortune has little value without a Dyne IMO...!" 

JDB writes: " I need a sensor installed with gauge. "

My response:  " Again, sure you can add a gauge, but it's not really necessary with a factory (or near factory) engine breathing and Metering Needle configuration, once set up...I'd start with that, adjust mixture at Idle, assure Dashpot reservoirs are filled with Damping Oil (20W) and finetune Jets after some roadtests and Spark Plug inspections...goal is satisfactory Throttle response, [and pulling/engine torque under load] and depending on where you are located and summer/winter temp variations, you may want to fine-adjust 2-3 flats richer for cooler, denser winter air. See also: "



Mike Fisher, motorcycle enthusiast down under, made his own see-through spark-plug, by gutting a retired spark plug and replacing the ceramic insulator with transparent epoxy.  That sounds like something I might like to try (it's on the list!): 

Apparently, for short durations of testing, the epoxy survives the high temperatures just fine. 

Mike Fisher picture used with his kind permission:

Home constructed transparent spark plug allows monitoring the combustion color during tuning operations!  

UPDATE (12/21):  I have begun the process of making my own "spark plug with a window" this space for further progress and pix.  


Wideband O2 Sensors: 

Maybe at some point, Gunson will develop a Smartphone camera interface, which will allow monitoring the combustion color from the driver's position during a Roadtest  This doesn't seem all that farfetched or expensive to implement with currently available technology [or maybe I'll simply rig the Go-Pro camera up to watch my own "spark-plug with a window", with toots on the horn acting as audible markers to identify the various operating conditions during playback! toot = steady state/low load/no Throttle, two toots = medium load/mid Throttle, three toots = hi-load/Wide-Open-Throttle, etc.]...until then, owners who have engine configurations other than what the factory spent lots of time, effort and expense developing, and must, or would just like to know(!), precisely how the mixture is doing at any one time, or just want another cool gauge on the Dashboard, do have the alternate option of installing a "Wideband O2 Sensor", and associated gauge, which monitors the state of exhaust gases and displays this as an indication of combustion mixture for the driver.

When searching "Wideband O2 Sensor", I came across several styles, including all digital, but it seems to me that an all digital mixture indication is about as useless as an all digital speed or RPM indication for such a dynamic situation (not to mention that this number is even flickering and changing during that interpretation!)...a bargraph where mixture indication and trend can be instantaneously noted are what is needed here!  Not some gauge which only displays an abstract number which must be's called System the time the interpretation is done, it has already changed to be something else, making the readout all but meaningless. 

I found this among the styles offered by Summit Racing (no relation!) with both digital and a bargraph of the reviewers on their site even noted:  "There are lots of times when the digital readout changes too fast to read, so the LEDs give you a darn good indicator [and with a quicker display response!] where you are at at all times. " 

Picture Source:

A digital indication is displayed on the 2 1/16" Instrument, along with a colorcoded, quick-responding, and more importantly quickly interpreted by a busy driver.  This instrument monitors the exhaust gases, so requires a Sensor in the Exhaust Gas stream.  A threaded bung must typically be welded onto the Exhaust Sys in order to mount that Sensor.  [Sticking such a sensor up the tailpipe won't do it as the sensor itself must be hot, and close to the hot combustion chamber.]

The Sensor and Gauge could be temporarily installed during Carb setup and adjustment, and could be removed after the procedure...the bung for the Sensor would obviously stay and could be plugged until and if it needed to be used again. 


Carb Spacer/Thermal Isolator Notes and Observations: 

These are an option, particularly useful in hot climates, which can be located between Carbs and Intake Manifold. Originally made of Bakelite, which has mechanical stability but poor thermal conductivity (compared to metal anyway, reference:  Thermal Conductivity of Bakelite), they decrease the rate of conducted heat from the manifold to the carbs after engine shutdown, thereby slowing/decreasing carb heatsoak and the resulting difficulty in Hot-starting.   


Thermal Spacers in-situ in Green on HIF44 carbs.

Excerpts from a Faceplant thread asking about the Carb Spacers... actually a subquestion in a thread about the fuel line between Fuel Pump and Carbs: 

Comment:  "...keeps the carbs cool."  My response:  "...but only after shut-off!...this must be stated, because without stating it, you are implying it does so during operation, and this is simply NOT the case [or necessary!]. See my explanation below!"

Comment:  "...they maintain/protect the air/fuel mixture settings from evaporation due to manifold heat to ensure correct combustion"  My response, and explanation:  "You have the evaporation part right, but not the mixture setting or combustion part, sorry! ...when engine is running, carbs are cooled by the massive amounts of outside air getting sucked through, not to mention the temp drop due to PV/T caused by Venturi (which can even cause carb-icing in certain conditions)...the spacers strictly serve to decrease heatsoak after engine shutoff, to help with helpful in hot climate and when making short stops..

Comment:  "Iíve had a couple instances of hot start issues, especially down in AZ."  My response:  "I would imagine a 5-20 minute "brief stop" at for instance the grocery store, in AZ on a 90Deg day to be about worst-case, to give you a difficult hotstart!  In that case, those carb spacers might just help a bit!...don't forget also to use the Hotstart Procedure!...paraphrased from the Owners Manual, is to floor it (no Choke obviously!) and keep it there while cranking, until she breathes enough fresh "cool" air (or at least outside, not superheated engine compartment air), to fire.  Hey, I might even suggest popping the hood to the first latch while going into the store, to let some of that superheated air escape (but hopefully NOT the Battery!!)..."

My general observation:  "Frankly, this whole discussion of hardline between FuPu and Carbs is moot, and almost entertaining to me...I'll continue to install a rubber line...which is incidentally a [much] poorer thermal conductor than a metal line, so less chance of fuel vaporization during heat-soak, so there is also little chance of me then complaining about vapor-lock!...and also BTW what the factory did for thousands of production cars! " 

Carburetor thermal Isolator/Spacer.





Reference Information:  

Thermal Conductivity of Bakelite compared to some other materials, measured in (W/m K):

Bakelite:  0.2 

Aluminum: 88 to 251  

Polystyrene (Styrofoam) 0.033 


External material sources are attributed.  Otherwise, this article is Copyright © 2021.  Ronald Kwas.   The terms Volvo, Colortune by Gunson AutoMeter and Summit Racing are used for reference only.  I have no affiliation with any of these companies other than to keep its products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the unique metaphor and probably (likely) wise-a** comment. 


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