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How to Set-up Triple CD Stromberg Carbs |
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I am pretty disappointed with my first video I took about setting up a GTR XU-1, so I do apologise for not releasing it until now. I really don't think it is something I am totally proud of. My aim is to present something of substance that will be useful to anyone watching. A couple of things are not as they should be and hopefully I have edited in the necessary amendments. I took this footage over 20 yrs ago so I just have to work with what I have. It was one of my first video attempts and it was painfully obvious to me then that being a movie director/actor and cameraman is quite difficult, but I have learnt from this experience and hope I have improved on future videos. So I will leave the talk through below as is in its original form so it will fill the gaps I missed in the video. Thank-you. |
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The prelims to this video is as follows. The Torana rocks up and the owner wants me to set-up the carbies. He explains that he had a carby expert already set them up and it still runs un-even, surges while cruising, no power and blows black smoke on acceleration. Yeh right, I am used to this story. Here is how it goes. There is no such thing as setting up the carbs without first setting up the ignition system. As you read on you will see exactly why this is so. I explain that I will check over the distributor and timing advance specs before I even bother trying to set-up the carbs. He is quick to point out that the same mechanic just o/hauled the distributor as well. I am liking this story more and more. Did I surprise the owner when I revealed all the faults I found as I progressed through the job. He just spent over $400 getting all this fixed. At the end of this, I will give you my opinion of what I think about the work he paid for. The owner was with me the whole time and understood everything I was talking about when I explained how it all worked. I like it when the owner is around, it gives me someone to talk to and I like to explain what I am doing because most people who own older cars are car enthusiasts and like to know more about their cars. Secondly, some owners are a bit weary about getting ripped off. Having them watch re-assures them. I even did this when I owned MBS but back in those days I would charge an extra $5 an hour as doing this did take longer to get the job done. Anyway lets get into it. First-up a quick compression test TT1. All is good there. I connect the timing light to check the initial timing and centrifugal advance. There was a problem, the timing mark was moving several degrees and did not settle while it was idling and also the engine took a considerable amount of time to return to idle. The problem was that the centrifugal advance was starting too early. This engine when finished with tuning should idle about 850-900 rpm with initial timing of about 10 deg. When I checked the timing it was on 16 deg and moving about slowly. I stopped the engine as it was pointless trying to set the timing. I had to revert to checking the timing statically. It was a few degrees retarded in actual fact. So what I had to do was remove the distributor cap and turn the engine over until I could see the primary advance spring adjustment post. I grabbed the rotor and felt what resistance it had when I moved it in the advance direction. From experience I could tell straight away that it offered almost no resistance which meant the advance would start to early. All I needed was a small screwdriver to bent the primary advance adjustment post outwards. This will tension the spring and slow the initial advance curve. I have made a video on how this is done and can be found on my U-Tube channel. For this engine I want the centrifugal advance to start at 1050 rpm, about 150 rpm above idle. This will ensure that the timing will stay steady while the engine is at idle. I checked the secondary advance spring tension and once again from experience I determined it to be reasonable. I will check the adjustments I made once I re-start the engine. I did a bit of other inspections while I was there and I will cover these in another Tech Talk later. With everything back together I restart the engine and immediately a difference could be noticed. The engine returned to idle very quickly from a crisp stab of the throttle. I now was able to time the engine with a timing light. Once this was out of the way I was now ready to check the carbs. While the engine was still running I used my carb flow meter to measure the flow of air through each carb. The flow for each carb was as follows measured in Kg/hr. #1-10, #2-11 & #3-10. I guess this was not to bad but I hope to get them all exact by the time I finish. With the engine shut down it's time to start dismantling. Click on the link below to see the video.
Since I did not rebuild these carbs and I was not the one who set them up initially, I had to partially strip them down to check that all has been done correctly.
I was not going to take the fuel bowls off to check the float level but there is a little trick to check them. Place your mouth over the jet as close as you can, then give a quick sharp blow down the jet. Quickly back away and observe if fuel comes out of the jet. You will probably see it bob up inside the jet orifice. Now repeat this on the other two carbs and compare how high the fuel bobbed up each jet. What you want ideally is that each carb fuel bob is the same. If they aren't, then the fuel levels may not be identical. This test does not tell you that the level is within specs but rather that they are equal. The picture above is the front carb, notice the gap between the jet retainer and the carb bore indicated at the two positions. This could be normal and does not indicate that the jet is off centre. If the manufacturers machined the tolerances to be perfect then centralising the jet would not be necessary. Unfortunately, mass production means that not every hole machined in the carb will be exact, so this made it necessary to allow the jet retainer the ability to be moved into the central position in relation to the needle. In summary, how many things were wrong with this set-up. (1) Initial timing retarded (2) centrifugal advance was too early and too much (3) Carb sync at idle not perfect (4) mixture jets were not even (5) diaphragm in one carb was swollen (6) one jet was not centralised (7) part throttle sync was bad (8) no oil in dampers initially So what do you reckon, was $400 well spent to get his carbs and distributor done by whoever this mechanic was? Below is a video I made in conjunction with the re-kit I filmed on one carby. That video is in the Carburettor section. This is basically a carry on from there but with added bits and pieces that relate to setting them up as triples. The video covers how to syncronize them on the bench.
I have a needle chart that may be useful to you for the XU-1 set-ups, download it in PDF here, in Word here. Ah yes the following question has been sent to me many times, so I will answer it here. The question is "why is oil used in the damper, some manuals say to not use oil." Firstly why the damper is there. The purpose is to restrict the sudden upward movement of the piston or air valve when the throttle is suddenly opened. Why?? The damper in a way provides the acceleration circuit that is needed when suddenly opening the throttle, otherwise a dead spot may occur in engine response when the throttle is stabbed suddenly.
No matter what type of carburettor is utilised, when the throttle is suddenly opened to gain acceleration, a richer mixture is momentarily required. This is because fuel is heavier by weight and will lag behind in response to air flow. This enrichment is accomplished by the damper assembly as follows. As soon as the piston begins to rise, oil is restricted from flowing through the damper by a partial hydraulic lock causing the movement of the piston to be restricted also. This will cause a temporary increase in the depression just above the jet at point S, therefore resulting in a temporary increase in fuel enrichment. That's it, simple as that. Now you know why oil is required in the damper, but there is always deviations. The viscosity of the oil is variable to certain conditions in engine state of tune. Thin oil allows the damper to raise a little quicker, thicker oil obviously restricts the movement significantly. In speedway and certain racing engines utilising the CD or SU carb I rarely use oil, but in a street mild application I will use oil and adjust the viscosity to suit engine response. You may be able to get away with no oil in some engines, it really is suck it and see, but from my experience, a light oil generally works best. To test the damper is working correctly is a simple matter of filling the piston tube with some oil when the piston is in the raised position. Use your finger to do this. Allow the piston to then fall. Place on the damper and secure it. With your finger raise the piston at various rates. The quicker you try to raise the piston, the more resistance you will get. Raising the piston slowly generally goes up easily dependent on the oil thickness. If the piston raises with no resistance felt, then buy a new damper. I hope this little addition to my article has answered the question.
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