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The OTHER one - Gemini #2

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    #16
    Bbbbbbbbbbbbbbbbbbbbbbbbbbbboooooooooooooooooooooo oooooooooorrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrtttttttt tttttttttttttttttttttttt!!!!!!!!!!!!!!!!!!!!!!!!


    Love it.
    Originally posted by Marv
    Only be concerned if that Dunning-Kruger Motorsports bloke is there and goes all Captain Damphands McGigglefingers on you after a couple of lemon squashes....


    I'M NOT A HOOKER BUT 20 BUCKS IS 20 BUCKS...

    robslothyoung - follow me on insta.

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      #17
      The noise is so fucking addictive its not funny, cant wait to actually be able to rev it out properly. Seems to go better than the relatively basic sum of its parts suggest it should.

      Carbs are going to take a fair bit of fine tuning - its an absolute bitch to start cold or hot which it shouldnt be given that it has a relatively healthy amount of timing advance [10btdc] and the vac advance connected too. That said, it actually drives without any hysterics cold without a problem after youve given it 30sec or so on throttle to settle into an idle on first start. Another issue ive got is i still think its a bit rich as the fuel is washing out but theres no black marks on the rear bumper from excessive fuelling so that contradicts itself.

      The main issue at the moment with the carbs is i think its suffering from a bit of spit-back, happens mainly around 3000rpm - say cruising on the highway, back off throttle and then go to pick up the throttle again. Its not a huge backfire out the inlet ala too much timing advance, more like a small cough out of the carbs. So far seen the cause as being described as cam related [possibly, but its not a huge cam so doubtful], lean mixture particularly on progression [again, doubtful as its pretty rich], and carb syncing [they are synced properly, so not that]. Anyone have any suggestions?

      Im also concerned that the main venturi is a bit on the small side - currently has 28mm chokes and it drives ok [caveat being fresh engine so havent revved it hard yet], but all the choke sizing graphs ive looked at for IDFs seem to suggest its way under-sized. Looking at the graph for approx 1700cc and 7000rpm i should need a choke around the 34mm-36mm area, not 28. Could airflow being too restricted be causing the spit back? ie choke too small restricting airflow and leaving fuel mixture still in runner rather than being drawn into cylinder?

      1978 Gemini Sedan - Powered by i change my mind every week
      1980 Gemini Sedan
      1992 Land Rover Discovery 200TDI

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        #18
        induction system is titz
        www.holditflat.com

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          #19
          Best thing about my old rally gem was the noise on twin idfs

          Sent from my G8141 using Tapatalk

          1998 MX5 - Ohlins DFV coilovers, Roll bar,15 x7.5 Konig Litespeeds, Mania Intake
          2014 Colorado LT - Oversized Whitegoods. Kid/bike/track hack/horse hauler.

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            #20
            yep, chokes too small. the prob is, changing those means rejetting everything. id go 36mms and see what it does. have you got a wideband on it?
            Originally posted by Marv
            Only be concerned if that Dunning-Kruger Motorsports bloke is there and goes all Captain Damphands McGigglefingers on you after a couple of lemon squashes....


            I'M NOT A HOOKER BUT 20 BUCKS IS 20 BUCKS...

            robslothyoung - follow me on insta.

            Comment


              #21
              Originally posted by F3ARED View Post
              The noise is so fucking addictive its not funny, cant wait to actually be able to rev it out properly. Seems to go better than the relatively basic sum of its parts suggest it should.

              Carbs are going to take a fair bit of fine tuning - its an absolute bitch to start cold or hot which it shouldnt be given that it has a relatively healthy amount of timing advance [10btdc] and the vac advance connected too. That said, it actually drives without any hysterics cold without a problem after youve given it 30sec or so on throttle to settle into an idle on first start. Another issue ive got is i still think its a bit rich as the fuel is washing out but theres no black marks on the rear bumper from excessive fuelling so that contradicts itself.

              The main issue at the moment with the carbs is i think its suffering from a bit of spit-back, happens mainly around 3000rpm - say cruising on the highway, back off throttle and then go to pick up the throttle again. Its not a huge backfire out the inlet ala too much timing advance, more like a small cough out of the carbs. So far seen the cause as being described as cam related [possibly, but its not a huge cam so doubtful], lean mixture particularly on progression [again, doubtful as its pretty rich], and carb syncing [they are synced properly, so not that]. Anyone have any suggestions?

              Im also concerned that the main venturi is a bit on the small side - currently has 28mm chokes and it drives ok [caveat being fresh engine so havent revved it hard yet], but all the choke sizing graphs ive looked at for IDFs seem to suggest its way under-sized. Looking at the graph for approx 1700cc and 7000rpm i should need a choke around the 34mm-36mm area, not 28. Could airflow being too restricted be causing the spit back? ie choke too small restricting airflow and leaving fuel mixture still in runner rather than being drawn into cylinder?

              Spit back is progression - the progression drillings are possibly not ideal. May also be accelerator pump, but that tends to manifest when you give it a bootfull rather than just picking back up off trailing throttle.
              I'm running 35mm chokes, 1800cc and ~7100rpm, and a few other mods to the auxilliary venturies to improve flow.

              Get bigger chokes into it (34s), and post up the rest of your jetting.
              "Where can we get hold of a Vincent Black Shadow?" "Whats that?" "A fantastic bike," I said. "The new model is something like two thousand cubic inches, developing two hundred brake-horsepower at four thousand revolutions per minute on a magnesium frame with two styrofoam seats and a total curb weight of exactly two hundred pounds."

              Comment


                #22
                Accel pump being set incorrectly should give dumps of black smoke when revving even when stationary, no? And how does one establish if the progression circuit is the problem? Not seeing excess fuel in mirror at night but that doesnt mean shit without getting hard figures from an o2 really - its definitely rich at some point because the oil is washing out. Havent had the o2 on it yet, Roddy is meant to be making an adaptor so we can put it in the tail pipe and check it hopefully this weekend, otherwise ill throw it on a dyno somewhere during the week.

                Max venturi size off the shelf for a 40IDF is 34, so according to the graph still technically undersize but probably ideal to go a little bit smaller given itll live on the street. Jetting sizes are as follows:

                Fuel Press [Recommended] - 2.5psi to 3.5psi
                Float height: 11mm w/ Gasket
                Needle and seat: 200
                High Speed Enrichment: N/A
                Acc pump nozzle: 50
                Acc pump bleed: 55
                Choke/Main venturi: 28
                Idle jet: 55
                Emulsion tube: F11
                Main jet: 125
                Air corrector jet: 185

                If you follow the formulas that they have in print/online everywhere for recommended sizing:

                Venturi size rule of thumb = inlet valve size minus 6mm, so 42mm inlet valve should have 36mm venturi [way, way off]. 34mm is biggest off the shelf available for 40IDF so being 2mm short of ideal has surely got to be a better compromise than being 8mm smaller.

                Idle jet = 1.6 x Venturi size. Current idle jet is way too big for 28mm chokes [1.6 x 28 = 44.8] but bang on for 34mm chokes [1.6 x 34 = 54.4].

                Main jet rule of thumb = 4.1 x Venturi size. Current [4.1 x 28 = 115] is two sizes smaller on the main jet vs recommended, ideal with 34 chokes [4.1 x 34 = 140] is too small so ill either have to go up here or wind the accel pump up?

                Anything else im missing/thoughts?
                1978 Gemini Sedan - Powered by i change my mind every week
                1980 Gemini Sedan
                1992 Land Rover Discovery 200TDI

                Comment


                  #23
                  Originally posted by F3ARED View Post
                  Accel pump being set incorrectly should give dumps of black smoke when revving even when stationary, no? And how does one establish if the progression circuit is the problem? Not seeing excess fuel in mirror at night but that doesnt mean shit without getting hard figures from an o2 really - its definitely rich at some point because the oil is washing out. Havent had the o2 on it yet, Roddy is meant to be making an adaptor so we can put it in the tail pipe and check it hopefully this weekend, otherwise ill throw it on a dyno somewhere during the week.

                  Max venturi size off the shelf for a 40IDF is 34, so according to the graph still technically undersize but probably ideal to go a little bit smaller given itll live on the street. Jetting sizes are as follows:

                  Fuel Press [Recommended] - 2.5psi to 3.5psi
                  Float height: 11mm w/ Gasket
                  Needle and seat: 200
                  High Speed Enrichment: N/A
                  Acc pump nozzle: 50
                  Acc pump bleed: 55
                  Choke/Main venturi: 28
                  Idle jet: 55
                  Emulsion tube: F11
                  Main jet: 125
                  Air corrector jet: 185

                  If you follow the formulas that they have in print/online everywhere for recommended sizing:

                  Venturi size rule of thumb = inlet valve size minus 6mm, so 42mm inlet valve should have 36mm venturi [way, way off]. 34mm is biggest off the shelf available for 40IDF so being 2mm short of ideal has surely got to be a better compromise than being 8mm smaller.

                  Idle jet = 1.6 x Venturi size. Current idle jet is way too big for 28mm chokes [1.6 x 28 = 44.8] but bang on for 34mm chokes [1.6 x 34 = 54.4].

                  Main jet rule of thumb = 4.1 x Venturi size. Current [4.1 x 28 = 115] is two sizes smaller on the main jet vs recommended, ideal with 34 chokes [4.1 x 34 = 140] is too small so ill either have to go up here or wind the accel pump up?

                  Anything else im missing/thoughts?
                  Ok, accelerator pump setup is really done by feel/o2 sensor, you can still be too rich without black smoke.
                  Venturi size "rule of thumb" is junk. Ignore.
                  Does it respond to idle mixture adjustment? How many turns out is best (smoothest) idle?
                  Main Jet size needs to be right, as does air corrector - accel pump is a momentary thing, you can't use it as a band aid for the MJ/MAJ being wrong.
                  "Where can we get hold of a Vincent Black Shadow?" "Whats that?" "A fantastic bike," I said. "The new model is something like two thousand cubic inches, developing two hundred brake-horsepower at four thousand revolutions per minute on a magnesium frame with two styrofoam seats and a total curb weight of exactly two hundred pounds."

                  Comment


                    #24
                    ive been thinking about this all day, and the more i think the more t sounds like a cam timing issue. does this thing have a vernier pulley?
                    Originally posted by Marv
                    Only be concerned if that Dunning-Kruger Motorsports bloke is there and goes all Captain Damphands McGigglefingers on you after a couple of lemon squashes....


                    I'M NOT A HOOKER BUT 20 BUCKS IS 20 BUCKS...

                    robslothyoung - follow me on insta.

                    Comment


                      #25
                      Originally posted by Gammaboy View Post
                      Ok, accelerator pump setup is really done by feel/o2 sensor, you can still be too rich without black smoke.
                      Venturi size "rule of thumb" is junk. Ignore.
                      Does it respond to idle mixture adjustment? How many turns out is best (smoothest) idle?
                      Main Jet size needs to be right, as does air corrector - accel pump is a momentary thing, you can't use it as a band aid for the MJ/MAJ being wrong.
                      It does respond to idle mixture but the "window" where it gets better is pretty small. Cant remember exactly how many turns out, will need to wind one in then wind it out again to check but i *think* it was somewhere around 2.5 turns. Really need to get the wideband onto it but my feeling based on driving is idle mixture ok/mixture and main jets ok [based on nothing more than not overly fuelly out the pipe and responsive on throttle] but something wrong to cause it to spit back. The worst part about the spit back is its right in the rev range [3k] where you sit on 100-110 in 5th.

                      With the venturis, do you mean ignore the formula or the graph? 28 still sounds small to me based off what other engines seem to be running relative to their engine size.

                      Originally posted by Sloth View Post
                      ive been thinking about this all day, and the more i think the more t sounds like a cam timing issue. does this thing have a vernier pulley?
                      Standard cam gear so non adjustable, cam zero'd on standard timing marks. I take it your thinking the cam may have been ground a few degrees out?
                      1978 Gemini Sedan - Powered by i change my mind every week
                      1980 Gemini Sedan
                      1992 Land Rover Discovery 200TDI

                      Comment


                        #26
                        Originally posted by F3ARED View Post
                        With the venturis, do you mean ignore the formula or the graph? 28 still sounds small to me based off what other engines seem to be running relative to their engine size.
                        Ignore the "Intake valve minus 6mm" bullshit. The graph is ok. Throw the 28s in the bin (or a lathe - that's how I wound up with 35s with an improved venturi profile) and go to 34s. Alfa 1600s turning to 6,000rpm came with 32s for fucks sake. Not much point trying to set them up for 28s when you're going to change to 34s. 28s are probably ok on a stock beetle motor.
                        Fatten up the idle a 1/4 turn and see if it helps with the progression spitback - highly unlikely to be cam timing if it's only a narrow window of rpm it happens in. You can also raise the float height 1-1.5mm to richen it up in the progression (Only just remembered that trick) - it will make it slightly fatter everywhere else as a trade off though.


                        Feel isn't a good indicator of air fuel unless you're so far off one end it's not funny - I need to lean off the topend (air correctors) on mine because it's so rich you can smell it from the following car - but it doesn't feel doughy or anything (6-7000rpm range).
                        "Where can we get hold of a Vincent Black Shadow?" "Whats that?" "A fantastic bike," I said. "The new model is something like two thousand cubic inches, developing two hundred brake-horsepower at four thousand revolutions per minute on a magnesium frame with two styrofoam seats and a total curb weight of exactly two hundred pounds."

                        Comment


                          #27
                          Had a good read of the pdf you sent me this morning [thanks again]. I *think* ive got a slightly better understanding of whats going on. Going to have to order a few main jets and air correctors as well as the venturis i think. So, the spit back issue around 3k rpm - if we are to assume for a moment its caused by lean/weak mixture around the progression phase [based on my interpretation of the problem] the possible causes/fixes are:

                          -Too lean on idle jet. Up idle jet or turn up idle mixture screw [like you suggested earlier by 1/4 turn]. Idle jet being a 55 i think thats more than big enough given that the book recommends a 45 or a 50 for similar engines, so may need more turns.

                          -Too lean on air correction [185]. Smaller holes is richer mixture so that would mean the air correction must come DOWN a size [to, say 175?]

                          -Not enough static advance. Currently set on 10BTDC, may need more? I think this ones unlikely as it pulls across its rev range [keeping in mind im not turning it over 4500 yet] cleanly from 1500ish rpm under load without going stupid

                          Now, something else struck me as interesting in the pdf which got me thinking - what are the chances of one cylinder being slightly leaner than the others causing the spit back only on that pot? I have a sneaking suspicion if that is a possibility that it is cyl #2 thats the problem because the idle mixture screw on that barrel is a lot stiffer to turn than the rest. Might give the mixture screws a quarter turn and see if it irons out.

                          Im thinking 34mm venturis, leave the idle jet size alone [provided the o2 says idle mixture is ok], up the main jet size to 135/140 from 125 and leave the air corrector alone. If it still acts like a shitcunt and running it richer helps, then drop the air corrector size from 185 to 175
                          1978 Gemini Sedan - Powered by i change my mind every week
                          1980 Gemini Sedan
                          1992 Land Rover Discovery 200TDI

                          Comment


                            #28
                            Your dissy problem sounds like you are trying to run an electronic dissy off a ballast resistor.

                            10deg BTC is not heaps you could bump it up to 16ish provided the total timing is not too much.

                            Comment


                              #29
                              I wouldn't worry about what the o2 sensor says for idle mixtures - you're adjusting them for smoothest running (and adjusting them independently to get there) at idle - that will probably be a little fat according to the WBO2

                              Not sure the Air corrector has much input at those sorts of flow speeds.
                              You *may* need to up the idle jet size when you go to 34mm venturis.
                              "Where can we get hold of a Vincent Black Shadow?" "Whats that?" "A fantastic bike," I said. "The new model is something like two thousand cubic inches, developing two hundred brake-horsepower at four thousand revolutions per minute on a magnesium frame with two styrofoam seats and a total curb weight of exactly two hundred pounds."

                              Comment


                                #30
                                Originally posted by mopar lover View Post
                                Your dissy problem sounds like you are trying to run an electronic dissy off a ballast resistor.

                                10deg BTC is not heaps you could bump it up to 16ish provided the total timing is not too much.
                                I was thinking this too. When i had my first Gemini and fitted a facet electric pump, i couldn't figure out why it kept dying. Turns out because i took the power for it after the ballast, Moved to the other side, problem solved.

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