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Trying to understand turbo fueling process

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  • #16
    Well I've started looking again at what might be wrong with my installation, so that the car will be working correctly for the show season. First thing I did was to add a clip to the pipe connection onto the anti-run on valve and also on the valve exit pipe as neither of these looked particularly secure. Then I noticed that the valve connection pipe was very loose onto the carburettor and looking rather age worn, so that is now up for replacement and clipping tight. I'm hoping that these two issues will go a long way to explaining, if not actually fixing, the fuel pressure drop-off on boost. Ive also looked at the balance pipe. That seems to be very tight and sound, but possibly age-hardened, so might swap that one as well just to be on the safe side.
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    1990 MG Maestro Turbo #436 (To keep me occupied in retirement)
    2023 Hyundai Tucson Ultimate Hybrid(Daily run-around)
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    • #17
      Yes it is just a length of normal fuel line and is a VERY common cause of problems so swap it for some new hose.
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      • #18
        Dan, just removed the balance pipe. Sealing one end and blowing through the other, getting no indication of it being air porous. It's just quite hard and inflexible. Also required warming to allow it to be pulled of the two outlets. Just wondering if there will be any benefit in changing it.
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        1990 MG Maestro Turbo #436 (To keep me occupied in retirement)
        2023 Hyundai Tucson Ultimate Hybrid(Daily run-around)
        1974 Austin Allegro Semi-works Rally Car


        You know you are getting old when your knees buckle and your belt doesn't

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        • #19
          A length of fuel line will be less than a quid, why take a chance on an old one, especially if it has gone hard.

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          • #20
            Having replaced the carb balance pipe, I have now fitted a fuel pressure gauge betwen the fuel line and the carb connection. At idle and up to 2000 rpm it seems to be a healthy 4-4.5psi pressure. Taking the revs up (with a warm engine) to 3000, the fuel pressure falls away to 3 psi. Made me wonder if the vapour seperator could have some crud in it that is blocking the circuit, so just removed it to check. Fuel removed appeared to be clear, so now I'm stuck as to why the pressure is dropping off and/or the mixture is going lean. For the record:
            • Pressure regulator is new
            • Carb overhauled
            • Tank cleaned before fitting
            • New carb balance pipe fitted
            • New pipe from anti run on valve to carb fitted
            • All pipe clips checked to ensure full tightness

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            1990 MG Maestro Turbo #436 (To keep me occupied in retirement)
            2023 Hyundai Tucson Ultimate Hybrid(Daily run-around)
            1974 Austin Allegro Semi-works Rally Car


            You know you are getting old when your knees buckle and your belt doesn't

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            • #21
              If it isn't a blocked pipe or filter, I'd say the fuel pump..

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              • #22
                Originally posted by Doctordiesel77 View Post
                If it isn't a blocked pipe or filter, I'd say the fuel pump..
                I was thinking that as well, later on yesterday. I know the standard pump is still available, but are there better alternative pumps on the market?
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                1990 MG Maestro Turbo #436 (To keep me occupied in retirement)
                2023 Hyundai Tucson Ultimate Hybrid(Daily run-around)
                1974 Austin Allegro Semi-works Rally Car


                You know you are getting old when your knees buckle and your belt doesn't

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                • #23
                  Pete, this all sounds similar to a problem Al has with #496 which we've been chasing off and on for a couple of years! It drives OK up to about 3000 / 3500 rpm in 1st 2nd and 3rd but when you change into 4th and hoof it, it just dies away with no oomf. Like you we've swapped pretty well everything, including a new race tank, external swirl pot and aftermarket Bosch-type pump. One of the 2 carbs we've tried is an as-new rebuilt one, done by a MMM member quite a few years ago now (and embarrassingly can't remember by whom). Can't offer any fresh ideas but will be v interested to hear what the eventual outcome is. We've also had it on the rolling road twice but the operator couldn't tell us the problem, I don't think he was very good at using the equipment for problem diagnosis.
                  ETA - when we get a chance (the car's stored offsite at the moment) we will check the gasket and the plug gap.
                  midget1380@btinternet.com

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                  • #24
                    Originally posted by Sussex Pete View Post
                    Having replaced the carb balance pipe, I have now fitted a fuel pressure gauge betwen the fuel line and the carb connection. At idle and up to 2000 rpm it seems to be a healthy 4-4.5psi pressure. Taking the revs up (with a warm engine) to 3000, the fuel pressure falls away to 3 psi. Made me wonder if the vapour seperator could have some crud in it that is blocking the circuit, so just removed it to check. Fuel removed appeared to be clear, so now I'm stuck as to why the pressure is dropping off and/or the mixture is going lean. For the record:
                    • Pressure regulator is new
                    • Carb overhauled
                    • Tank cleaned before fitting
                    • New carb balance pipe fitted
                    • New pipe from anti run on valve to carb fitted
                    • All pipe clips checked to ensure full tightness
                    Hi Pete
                    I would be inclined to check that the boost sensing pipe can hold 10 psi or whatever boost pressure you are hoping to eventually run. This is the pipe from the plenum chamber that runs to the fuel pressure regulator and the wastegate actuator, numbers 8,9 &11 on your diagram . Being a collection of rubber hoses, a metal pipe that has a tee junction, pressure regulator and the wastegate diaphragms there are quite a few areas that can leak. The steel pipe in particular is age vulnerable to corrosion. The ideal tool is a mittivac pump, but any tyre pump with a gauge and a few adapters would do the job. Pull the rubber hose from the plenum chamber and pump in the required pressure and make sure the pressure holds. Any air leaks in this area will cause the mixture to run lean under boost conditions because the fuel pressure will not be increased when needed. If this draws a blank then you can move forward to checking the fuel pressure and boost pressure under load on the road.

                    Sorry to ask this but I have not followed the whole history of the rebuild, other than the bottom end balancing what non standard performance enhancing engine and breathing system modifications if any have you undertaken?

                    Cheers, Gary

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                    • #25
                      Originally posted by G Force View Post

                      ...... Sorry to ask this but I have not followed the whole history of the rebuild, other than the bottom end balancing what non standard performance enhancing engine and breathing system modifications if any have you undertaken?

                      Cheers, Gary
                      Hi Gary, Other than a larger diameter clutch and the balanced botom end, the engine is totally stock. Regarding your suggestions, as I have currently got the underbonnet fuel system dismantled, I'll take the opportunity to test the pipe that runs to the pressure regulator.

                      Reason for the dismantling was to have the pressure regulator checked when running with the 044 type pump. Upshot of that is that it would not work at less than 0.8bar, rather than the standard 0.3bar, even with a 1mm restrictor on the fuel inlet to the vapour seperator. This is tending to indicate that the 044 pump is too much for a stock engine.

                      I have a possibility of another pump of similar spec to the standard, and on the basis that I have made a number of changes to the fuel system without being able to prove if they have made any difference, I'm going to work with a standard setup for the time being.

                      What I have done recently is:
                      • Changed the pipe from the carb to the vent valve; add clips to both ends
                      • Changed clip on fuel inlet pipe on carb to get better clamping
                      • Confirmed vent valve is working electrically and seals with current through the switch
                      • Fitted new carb balance pipe
                      • Fitted restrictor to the oneway valve on the inlet manifold for the crankcase vent system

                      I really need to test the car again, to see if these have made any difference, maybe also including a new standard fuel pump giving a consistent pressure flow.
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                      1990 MG Maestro Turbo #436 (To keep me occupied in retirement)
                      2023 Hyundai Tucson Ultimate Hybrid(Daily run-around)
                      1974 Austin Allegro Semi-works Rally Car


                      You know you are getting old when your knees buckle and your belt doesn't

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                      • #26
                        Yet again, its a 2 forward, one back process! These are my fuel pump choices:

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                        On the left is the original pump - no markings, so totally unknown specification or manufacturer. All I can say is that it worked and gave about 4psi line pressure. In the middle is the current replacement (once I can get the correct fuel output connector). Its a Bosch 0580464044, as used on some Citroens and Renaults; and to the right is the original replacement 044 pump, that had such a high fuel flow and pressure that we couldn't get the pressure regulator lower than 0.8bar, so way OTT! I hope with the new pump, I will be getting somewhere....
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                        1990 MG Maestro Turbo #436 (To keep me occupied in retirement)
                        2023 Hyundai Tucson Ultimate Hybrid(Daily run-around)
                        1974 Austin Allegro Semi-works Rally Car


                        You know you are getting old when your knees buckle and your belt doesn't

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                        • #27
                          At least it looks about right....the original one may have been a replacement in the past? I'm sure all the factory ones were either Lucas or Bosch and they alwayß seemed pretty keen to label their products....

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                          • #28
                            Without wishing to jinx anything, I am very tempted to say IT'S FIXED!!!!! Possibly some of the things I attended to earlier may have had a positive impact, but the final thing was to replace the (new) Champion spark plugs with NGK ones and at the same time to tighten the spark plug connection in the end of the plug lead. Don't know if it was either or both in combination, but with a slight leaning of the mixture (from the 5% it was showing), back off the idle speed to 750 and the engine is running so sweetly now, with 70 in 5th coming up remarkably quickly on the test run. Now I can properly atend to the remaining snagging items to finish it once and for all!
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                            1990 MG Maestro Turbo #436 (To keep me occupied in retirement)
                            2023 Hyundai Tucson Ultimate Hybrid(Daily run-around)
                            1974 Austin Allegro Semi-works Rally Car


                            You know you are getting old when your knees buckle and your belt doesn't

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                            • #29
                              Well done Pete, Champion spark plugs are not what they used to be, cheaply made now and not a quality product.
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