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The Stromberg 175CDT, sometimes known as the W115 200, is the subject of discussion.


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Greetings, everyone.

I am in possession of a 1976 w115 200 model vehicle equipped with the Stromberg CD carburetor. I am now experiencing difficulty initiating the starting process, unless I manually elevate the needle/piston using a screwdriver. This behavior suggests a potential issue whereby the piston may possess excessive weight, hence impeding the starting mechanism.

This particular item has a red cap that is positioned slightly off center, with the damper located just below it. Based on the observation that the needle seems to have sprung, it is reasonable to posit that it is likely in satisfactory condition. The item does not seem to be worn.

I have used ATF (Automatic Transmission Fluid) as the dashpot oil. Is this compatible with a red cap? There have been occasional forum posts that discuss the use of various oils in correspondence to the color of bottle caps.

What is the recommended quantity of oil to be added? It is conceivable that an excessive amount of a certain factor may impede the appropriate upward movement of the piston.



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The aforementioned are carburetors that operate at a consistent velocity and function in a uniform manner. During the idle state, the piston should remain stationary, resulting in no upward movement. Consequently, the diaphragm does not exert any influence. The idling condition of an engine is influenced by the positioning of the piston and needle. Therefore, if the engine fails to idle properly, it is likely due to a misadjustment in either the piston and needle positioning or the enrichment device. The conventional throttle stops regulate the degree to which the butterfly valve is open at idle. However, there is also a separate stop for the piston and the ability to modify the height of the needle.

The dashpot functions as a damper that restricts the rate of piston ascent, leading to a transient increase in air velocity inside the venturi. Consequently, this induces a momentary enrichment in the air-fuel mixture or simulates the effect of an accelerator pump. The primary influence of this phenomenon is seen in the acceleration response.

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The diaphragm seems to be in satisfactory condition; nonetheless, I have opted to get a new in order to completely remove any potential issues.
I recently acquired a vintage Zenith 175 carburetor, originally sourced from a previous generation Range Rover, for a nominal sum of ten units of currency. My intention was to explore the potential utility of salvaging any functional components from this carburetor. Upon inspection, I observed that the piston within the Zenith 175 exhibits a noticeably greater degree of freedom of movement when manually lifted, in comparison to the piston in my own carburetor. Notably, when I remove the damper from my carburetor, its piston exhibits a similar level of freedom. In light of this, I am curious to ascertain whether any measures can be taken to address this disparity. Additionally, I currently have automatic transmission fluid (ATF) within my carburetor. I am uncertain as to whether this lubricant is excessively dense for optimal performance.
The phenomenon is really peculiar.

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It seems that you have thoroughly explored the available online resources pertaining to the adjustment of a Stromberg 175CD carburetor.

Based on the provided description, it is possible that the adjustment of the slow and/or quick idle screws may be misaligned. Does the vehicle possess an automated choke mechanism? Does the proposed solution demonstrate efficacy? Has it been appropriately adjusted?

This tutorial provides a concise overview of fundamental concepts.

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Prior to making any carburetor adjustments, it is imperative to first verify the float level, since an inaccurate level will significantly affect the fuel-air mixture, particularly at idling. Based on the information provided in the 190e Haynes manual, it is recommended that the float level be set between the range of 18-19mm. Subsequently, proceed to manipulate the idle speed screw, which is distinguished by its greater length, while acknowledging the presence of the alternative fast idle adjustment screw. The appropriate fluid for the damper should exceed the required viscosity, and its level should reach the bottom of the inspection hole. However, these factors are not relevant to the idle setting, nor is the condition of the diaphragm.

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I have extensively searched online resources for comprehensive instructions on the adjustment and tuning of Stromberg carburetors. Several of these sources have proven to be highly informative, enabling me to successfully locate a copy of the Haynes manual specifically dedicated to Zenith Stromberg carburetors. It is worth noting the remarkable diversity among Zenith, Stromberg, and Pierburg carburetors, as the multitude of variants is quite substantial. Consequently, I have encountered considerable difficulty in finding an online photograph that precisely matches the configuration of my own carburetor. The Ruddies website has proven to be a valuable resource in my research. Through its assistance, I have successfully identified my carburetor model as the Stromberg 175CD (T- with automated choke) "Build 3," which was installed in vehicles between July 1973 and July 1976. Based on my analysis, it seems that the distinguishing factor subsequent to 1973 was the incorporation of the pierburg fuel cutoff solenoid.

At this juncture, I will provide contextual information, perhaps aiding in the diagnosis or assistance of those with analogous difficulties.

The automobile was acquired in the middle of 2019 and had a tendency to be difficult to start. Once started, it required the application of throttle until it reached optimal operating temperature. This behavior led to the suspicion that the automated choke mechanism was malfunctioning, since online sources often mentioned issues related to choke units. The issue at hand was not of significant magnitude, and after the vehicle had reached an optimal temperature, starting its operation posed no difficulties. Consequently, I chose to postpone the resolution of this matter and instead derived satisfaction from using the vehicle for a few months.

In May 2020, subsequent to the passage of time, an acquaintance's paternal figure becomes aware of my difficulties in starting the automobile's engine and kindly extends an offer to inspect the matter. The individual has a considerable amount of mechanical expertise in relation to vintage automobiles, although excluding Mercedes models. The first step was examining the auto choke mechanism, during which it was seen that the coil functioned appropriately when subjected to heat. However, I am now contemplating whether this observation is indicative of the auto choke effectively operating, considering its reliance on a power source in addition to the provision of hot water from the thermostat.

The individual proceeded to inspect the dashpot and the diaphragm, both of which were found to be in satisfactory condition. Subsequently, attention was directed towards the examination of the distributor and the timing. At that moment, the observation was made that the points inside the distributor were noticeably dislodged, since the screw responsible for securing them had been stripped and was only producing a rattling sound. A new screw was installed, the points spacing was adjusted, and all components were reassembled. The vehicle successfully started, however, it is now experiencing a severe misfire.

Ultimately, the spark plugs were removed and discovered to be heavily coated with soot. However, despite the use of clean spark plugs, the misfire continued. We enlisted the services of an experienced technician specializing in carburetors, but lacking expertise specifically in Mercedes-Benz vehicles. The individual made adjustments to the timing and other relevant factors. However, it was discovered that in order to start the car's ignition, a direct connection of a 12-volt power supply to the fuel cutoff solenoid located underneath the carburetor was necessary. At this juncture, there were uncertainty among all individuals about the functionality of the fuel cutoff mechanism.

The engine required a direct current of 12 volts in order to initiate the starting process. While the engine was able to start and maintain an idle without any issues, it exhibited difficulty in achieving higher revolutions per minute.
In the event that the engine is started and the 12-volt power supply is disconnected, the engine will exhibit increased revolutions per minute (RPM) but will fail to maintain a stable idle speed.

Based on my extensive research and personal investigation, I have discovered that the pierburg fuel cutoff system, which was installed in vehicles starting from 1973, is designed to be fail-proof. Additionally, a delay relay connected to the no.4 fuse in the fusebox is installed in conjunction with this system. When the ignition is turned off, the cutoff solenoid receives power for a duration of approximately 5 to 6 seconds. This power supply effectively halts the fuel flow, thereby preventing the occurrence of engine run-on. It is worth noting that this setup appears to have been carried over to the W123s and subsequently to the W201/124 carbureted cars.
Upon inspection of my W115, I successfully located a little silver relay that was found to be disconnected. It is worth noting that this relay had not been tampered with by myself or anybody else over the whole duration of my ownership. I had seen occasional instances of excessive expansion during hot weather, among other factors, but these occurrences were not significant enough to warrant significant attention or concern.

However, it is unclear why it was necessary to supply power to the solenoid in order to initiate the car's operation. To investigate this further, I detached the solenoid assembly, which is integrated with the richness adjusting screw, and provided it with a 12-volt electrical supply to observe its behavior. Upon examination, the solenoid appeared to function properly, as the small spherical plastic float stopper ascended, seemingly obstructing the fuel flow, and descended once power was discontinued. One possible explanation is that the fuel, when introduced into the float chamber, may have had an unintended buoyancy effect. It seems that there is a spring assembly present, with the sphere positioned in the center. There is a possibility that the sphere may get dislodged from its position.

Subsequently, I contemplated the possibility of eliminating the solenoid entirely, which prompted me to acquire a Zenith 175 via a £10 transaction on the online marketplace, eBay. The vehicle in question is derived from a previous generation of the Range Rover model and lacks the solenoid component. The adjustment screw in question is a straightforward component. The design of the float chamber and jet assembly exhibits notable differences, particularly in the length of the adjusting screw. In the case of the Stromberg, this screw is relatively shorter and is threaded into the jet. Although it is just barely long enough to reach the bridge, it fails to effectively prevent petrol leakage. The W115 vehicle was started, however I refrained from operating it for an extended duration to allow the exhaust to reach high temperatures due to the presence of a fuel leak (despite my want to do so after a considerable period of time).
I began contemplating the inherent characteristics of the primary solenoid module, specifically with regards to the possibility of eliminating the float and spring components in order to transform it into a conventional adjusting screw mechanism. No, it would be unfeasible to extract them since the apertures are very narrow, and I am reluctant to do any harm to them. The procedure included inserting a slender nail into the upper part of the apparatus and afterwards applying light taps to displace the plastic float stopper downwards, so preventing it from floating upwards upon the introduction of fuel. This action ensures that the solenoid does not exert any upward force on the float stopper if it is not properly attached.

The current state of my progress is nearly aligned with my current position. I have reinserted the adjustment screw and tightened it fully just above the bridge. As a result, the car is able to start, albeit with the assistance of a screwdriver to lift the piston. Additionally, if the car is kept running until it reaches a warm temperature, it is able to maintain an idle. Consequently, I proceeded to follow the prescribed steps for tuning the car, as outlined in online resources and the Haynes manual. Starting from the idle position, I incrementally enriched the mixture by approximately 1/8th of a turn at a time. Simultaneously, I lifted the piston by approximately 1mm after each adjustment. This process was repeated until lifting the piston no longer had any discernible impact on the RPM. At that juncture, the vehicle was operating at an approximate engine speed of 1800 revolutions per minute (rpm). My vehicle is equipped with a solitary idle screw responsible for regulating the position of the butterfly valve. I adjusted this screw counterclockwise to achieve an engine speed of approximately 750 rpm, although I still perceive the idle to be somewhat elevated. It is noteworthy to mention that the vehicle consistently exhibited a high idle speed. This observation led me to speculate that the previous owner may have compensated for a malfunctioning automatic choke by intentionally setting the idle speed too high.

After making adjustments to the tuning, the vehicle exhibited smooth and consistent revving throughout its entire range without experiencing any instances of backfiring. Subsequently, I proceeded to take the vehicle for a brief drive along my lane, marking the first time in several months. Notably, the vehicle allowed me to turn it off and on multiple times without any complications. Following a short pause of approximately 30 seconds to tidy up my tools, I attempted to start the vehicle once more, and it successfully initiated without any signs of coughing or sputtering. Once again, I enlisted the assistance of another individual to rotate the object, afterwards using a screwdriver to elevate the piston, resulting in the immediate ignition of the mechanism. The situation is quite vexing.

If you have reached this point, I express my sincere gratitude for dedicating your time to attentively consider my predicament. I have developed a strong affinity for the stroke eight, but its perplexing nature has caused me several nights of restlessness spanning several months. Tomorrow, I will attempt to change the float level and make an effort to readjust the idle screw in order to assess its potential impact. Today, an experiment was conducted by applying a diluted solution of Fairy liquid and water as a fine mist to various surfaces in order to detect any potential vacuum leaks. The results of this investigation indicate that no such leaks were detected.

Thank you for your assistance so far. I am not prepared to abandon my pursuit of understanding the Stromberg, as it offers valuable educational opportunities. It is evident that the intricacies of the carburetor were once a perplexing and intimidating subject for me.

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The first step was examining the auto choke mechanism, which revealed that the coil functioned correctly when subjected to heat. However, I am now contemplating if this observation implies that the auto choke is really operational, considering its reliance on a power source in addition to hot water from the thermostat.

It is hypothesized that the power source is used for the purpose of heating the automated choke. If the structure of the choke in question has resemblance to the substantial SUs found in my previous XJ6, it is likely to include a coiled bi-metallic strip. In warmer temperatures, the system disengages and deactivates the choke, whereas in colder temperatures, the choke remains engaged. The power supply potentially originates from a regulated source that operates on a predetermined timing mechanism. This timing mechanism facilitates a progressive increase in temperature inside the strip, therefore leading to a controlled deactivation of the choke. In the event that the engine is at a low temperature and the ignition, perhaps the starting circuit, is activated, it is expected that a voltage of 12V will be present at the terminal in order to facilitate the heating of the bi-metallic strip.

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I am not acquainted with the precise type of carburetor you own, hence I can only discuss broad ideas pertaining to constant velocity (CV) carburetors. One notable characteristic of the Stromberg carburetor is the lack of a distinct idle circuit. The gasoline required for idling must be supplied by the jet in which the needle is positioned. The task at hand is somewhat challenging due to the little fuel use during idle as compared to regular operation, rendering it highly susceptible to needle positioning during idle. The process starts with the elevation of the piston, which subsequently raises the needle out of the jet, so allowing an increased flow of fuel. The issue pertains to the stationary orientation of the piston and needle. As previously discussed, it was noted that the modification of the mixture ultimately results in the elevation of the piston; nevertheless, the insufficiency of the mixture hinders its ability to sustain operation. This phenomenon may be attributed to the influence of the mixture modification on the whole spectrum of operation. There may exist several methods for modifying the needle's resting position with relation to the jet. Subsequent iterations of CD3 carburetors had a needle height adjustment mechanism that could be conveniently accessed from the top using a long Allen key. However, it seems that your particular carburetor does not possess this specific feature. Are you aware of the precise model identification of your carburetor?

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I had already conducted extensive research on the carburetor used in the 115 230 engine at a prior period.

Acquire a carburetor repair kit, proceed to detach the carburetor, disassemble it completely, meticulously cleanse its components, and afterwards rebuild it. Additionally, it is important to verify that there are no leaks in the flexible rubber gasket located between the carburetor and the intake manifold. The rubber's age sometimes resulted in the occurrence of tiny perforations, which gained notoriety.

The 115 engines were not equipped with inline fuel filters, leading to the accumulation of debris in the float chamber and subsequent obstruction of fuel supply via the needle. Install an inline fuel filter inside the engine compartment along the fuel supply line.

Additionally, it is recommended to acquire a carbon monoxide (CO) meter in order to accurately adjust the mixture. Once the engine is operational, it is advisable to set the CO level to around 1.5%. The adjustment of the jet is accomplished by manipulating the adjuster located underneath the carburetor, specifically positioned inside the central region of the float chamber.

The ATF level should be maintained at the bottom of the filler.

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I had a similar problem with my Stromberg 175CDTU carburetor, whereby a gasoline leak originated from the vicinity of the automated choke mechanism. The carburetor was disassembled, and the gaskets and diaphragms were replaced, with the exception of the one on the automated choke, due to a stripped screw.

What is the procedure for doing a vacuum test or inspection on a carburetor while it is detached from the engine? Additionally, is there a method to verify the functionality of the automated choke?

I conducted three rounds of verification on the floats, which were adjusted to a measurement of 16/17mm. Additionally, I replaced the float needle with a new one.

Do you have any suggestions or ideas to contribute? I have observed that the startek handbook seems to lack a certain PDF file, namely 072-136. If somebody had this, it would be advantageous.

The included image depicts the location of the leak.


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It is presumed that the issue has been resolved at this point.
Nevertheless, in the event that the engine fails to start until the air valve piston has been left open, it indicates that the fuel-to-air mixture is too concentrated.

Adjust the fuel screw towards its maximum limit.
Rotate the carburetor counterclockwise, as seen from below and above, by 420 degrees, which corresponds to a 1 1/4 turn.
Initiate the activation of the engine and afterwards exercise patience until it reaches an optimal operational temperature.
Utilize the screwdriver to elevate the air valve piston by about 1-2 millimeters. In the event of an increased engine speed, it is necessary to diminish the amount of gasoline by rotating the fuel screw in a clockwise direction, specifically by 1/8th of a complete revolution.
Please wait for a period of 30 seconds until the engine reaches a state of normal functioning, and then attempt the task once again. In the event that there is a high probability of the engine ceasing to operate, it is advisable to augment the fuel supply. Rotate the screw counterclockwise and do the action again.
The appropriate fuel-to-air ratio should be maintained until the engine experiences a little decrease in speed or remains unchanged, without stalling.

Wishing you the best of luck.

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