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  • Need some help tuning Strombergs

    I first got my car running about 5 years ago with PoolBoy's help and haven't touched the carbs since. They've been progressively leaking more and more, and having some idle issues so I decided this would be a good time for another "mini-rebuild" - basically just new gaskets, and seals around the throttle shafts. I had just replaced the diaphragms a little over a year ago with a pair from Joe Curto after a failure in both that I installed about 5 years ago.

    So my son and I did this over the weekend and reinstalled, and I did a quick and dirty tune on them just to get them out and driving around a little bit. Not too bad but needed some tweaking. Brought the car in today to do a full-on sync and tune following Buckeye's page. I had this issue yesterday when driving around but actually since tuning the carbs further it's gotten quite a bit worse. The issue I seem to be having is a not-great idle (which I'll live with, 65,000 miles on the original motor except for thrust washers, crank bearings and ignition parts). But what I have is, when I blip the throttle, the engine will settle back at about 1300 rpm, way too high. It will then sit there at 1300 for at least a full minute or two, and then as if someone had flipped a switch, drop way down to like 400-600, at which point I can't even keep it running. So needless to say trying to tune these carbs is an exercise in frustration.

    I have checked for vacuum leaks over and over and can't find any.

    I'll try and walk through my process going back and forth between the BUckeye page for reference:

    1. Warm up engine with idle set high. I have verified the engine is up to temp with an IR thermometer, 170-175F

    2. Loosen nut on spring clip so carbs can rotate independently

    3. Opened top of carb up to verify diaphragms and pistons are installed in the correct orientation.

    4. Connect vac gauge to manifold.

    5. Run car and try to sync. I have a snail horn, rather than a uni-syn, and I use a stethoscope too. I am confident I'm doing OK getting them to sync except for the fact that they are wandering all over when the idle drops suddenly. When the idle does drop, it doesn't seem to be consistent which carb ends up having lower airflow.

    6. Bypass valve: I thought this was my eureka moment but no. I futzed with the carbs for an hour or two, thinking I wouldn't even mess with the bypass valves. When I finally did I found both of them floating wide open. Disconnecting the vac feed to the distributor (coming off the bottom of the front carb) had no change. I worked on the bypass valves and got them set to the point that with the distributor feed plugged, they are shut plus 1/2 turn. Although, there was no change in idle when I pulled the screwdriver out of the vac retard hose, there was no change in idle, nor was there a change when I reconnected the hose.

    7. Repeat steps 1-5 after messing with bypass valves. FYI this entire time my idle trim screws have been fully shut, in fact I've never opened them.

    8. Mixture on both carbs is set such that when I raise the piston slightly the RPMs increase then fall right off. I also have a Gunson Colortune in but for some reason on this car it always, always indicates rich. I had good luck using it on my Volvo 1800S but not so much here, so it's mostly just there for gee-whiz factor.

    9. At this point I'm back to trying to get an idle speed set. Unfortunately it's still doing that dropoff thing. I can set it for about 1300-1500 rpm, in which case it will stay running when it drops off to about 4-600 rpm after a few minutes, or I can set it for 900-1000 rpm, in which case it just dies when the "switch is flipped".

    Getting really frustrated here. Anyone have any idea what might be going on?

    The throttle shafts were replaced when I did the initial rebuild 5 years ago, this time around I changed the shaft seals, and verified no play or wear on the shafts in their bores.

    Nothing is hanging up. Even when it's running at 1500 rpm I can go around and push on everything and try and get it to "seat" and nothing moves or changes, until a minute or two later....bam, stumble stumble stumble. The throttle cable (I have a Ratco cable) is disconnected for all of this.

    Help!

  • #2
    Do you have manifold vacuum hooked up to the brake booster? If so, disconnect and plug the line to the carbs or cap the nipple and see if it still happens. Any other vacuum leaks? Use carb cleaner or a propane torch torch around the manifold and carbs to see if the engine rpms increase when the gas or cleaner is sprayed near suspected leaks.

    The other thing to check is that you have a good fuel feed at 2.5 to 3 psi and good volume coming out of the hose disconnected at the front carb turning the engine with the starter - disconnect the coil wire for this so it doesn't start. What kind of float valves did you put in during he rebuild?
    72 Pimento w/overdrive

    Comment


    • #3
      Hey again Darel...and thanks again for lend of the Colortune back then.
      What I'd do is try to eliminate some of the carb related variables, that sometimes under certain conditions have a mind of their own, so to speak.
      Those 2 being the Throttle Bypass Valve (TBV) and the Temperature Compensators (TC).
      Use the TBV spring tension adjustment screw and turn it CCW as far as you can. That'll put maximum spring tension on the TBV diaphragms which may be enough to temporarily disable it's function.
      On the TC's just turn that tiny nyloc clockwise in order to keep the TC from opening and more importantly keep it from 'floating' which like the TBV can create very frustrating conditions when trying to set the idle.
      And once all that's done (and I suggest doing it simultaneously) set the mixture needles in the middle or 1& 1/4 CCW from fully CW...
      If the idle stabilizes, I'd leave the TC and the TBV disabled and concentrate on fine tuning mixture AND ignition timing.
      If none of that helps, I'd still leave the TC and TBV disabled....I've been doing that more and more on rebuilds...following the example of the 2 BIG BOYS who are doing carb rebuilds.
      Driving a 1973 TR6
      Doing ZS carb repairs
      email kencorsaw"at"aol.com

      Comment


      • #4
        Thanks guys. I'm about done for today but I'll jump back into it tomorrow. I hadn't touch the TCs because I set those up pretty well "way back when" and I don't like to touch stuff when I don't really feel the need to, but I'll go ahead and disable both the TCs and TBVs. I've been kicking that around for a while now, but I always fall back on the "smarter people than me at the factory thought they needed them" argument. It will make things simpler though.

        I do have the booster hooked up to manifold vac at the big banjo fitting. I hadn't removed it but I will tomorrow.

        I have never, ever had any success with the starting fluid / propane torch method for finding vac leaks, even when I knew there was one - once I even tried loosening 2 or 3 manifold bolts all the way and soaking it, with no change (this may have been on the TR6 but also on several other old cars). Tried the starting fluid earlier today just one more time and again no dice. Either I'm good on vac leaks or the method is suspect. I did retighten all the bolts everywhere. I'll cap off the booster line tomorrow.

        Will update.

        Comment


        • #5
          If it'll ease your mind about the TC and the TBV suggestions...both those were 'concessions' to meeting US emissions and really not performance enhancements neither were they detrimental or create a loss of power ...but they can affect idle stability, that's about it. The carbs will still be good carbs.
          Disabling those 2 functions may not be the solution, but at least you'll know one way or the other.
          And if you decide to enable them again, just keep track of what it took to disable them so you can get back where you started.
          Last edited by poolboy; 09-05-2018, 02:23 PM.
          Driving a 1973 TR6
          Doing ZS carb repairs
          email kencorsaw"at"aol.com

          Comment


          • #6
            OK, I think I got it - pretty sure the rear TBV is bad. I did disassemble them and check the springs and diaphragms and they were all good. I blocked off the brake servo, no change. Tightened down the TCs, no change. When I turned the TBVs in the problem disappeared, and I did notice the front TBV bottomed out at full CCW, but the rear never bottomed out, just kept turning. In any case I have it settled in to a nice, much more stable 800 rpm idle, took it for a quick drive, no issues. No more idle hanging up. Balanced out nicely. I'll come back to this thread if the problem recurs, but if it doesn't, thanks again guys!

            D

            Comment


            • #7
              Ken - as you predicted in a previous thread of mine where i inquired about vac nipples that could be disabled, i am starting to have idle stability issues on my '74 carbs (still have not driven car, just running in driveway, but have about 2 hours of run time at varying RPMs), so i am making the adjustments you recommend: tightened down Temp compensator nyloc to prevent it floating, adjust float transition valve plunger via the adjustment screw set with approx .040 sticking out the bottom, TBV spring/plunger adjustment screw completely turned out CCW, capped vac nipple on top of TBV, and made a blanking gasket for the TBV between the TBV and carb body. i made the gasket out of a spare (new) front engine plate gasket, which is a tad thinner than the gasket I removed (i had installed it last year during rebuild with the kit from Joe Curto). Do you think the thinner gasket could become an issue and I should make one out of thicker material. the difference is subtle, but definitely there - maybe .005 less. Attached pic is of gasket i just made sitting on the TBV and the one i just pulled off sitting next to it.

              Thanks - DaveR

              Comment


              • #8
                The gasket has to be strong enough material to withstand the vacuum that the bottom hole is exposed to, Dave.
                Driving a 1973 TR6
                Doing ZS carb repairs
                email kencorsaw"at"aol.com

                Comment


                • #9
                  thank you, that makes sense. what is your preferred gasket material in this application?

                  Comment


                  • #10
                    Originally posted by robertsdw6501 View Post
                    Ken - as you predicted in a previous thread of mine where i inquired about vac nipples that could be disabled, i am starting to have idle stability issues on my '74 carbs (still have not driven car, just running in driveway, but have about 2 hours of run time at varying RPMs), so i am making the adjustments you recommend: tightened down Temp compensator nyloc to prevent it floating, adjust float transition valve plunger via the adjustment screw set with approx .040 sticking out the bottom, TBV spring/plunger adjustment screw completely turned out CCW, capped vac nipple on top of TBV, and made a blanking gasket for the TBV between the TBV and carb body. i made the gasket out of a spare (new) front engine plate gasket, which is a tad thinner than the gasket I removed (i had installed it last year during rebuild with the kit from Joe Curto). Do you think the thinner gasket could become an issue and I should make one out of thicker material. the difference is subtle, but definitely there - maybe .005 less. Attached pic is of gasket i just made sitting on the TBV and the one i just pulled off sitting next to it.

                    Thanks - DaveR
                    If I recall correctly, one of my TBV's would not bottom out by itself (message to Ken, this was before you rebuilt the carbs). I found the captive "nut" inside had been pushed out of the recess where it is supposed to reside, so no amount of turning would accomplish anything. Disassembled it and reset the nut so it would work. Just a FYI.
                    1976 TR-6 BRG - CF57239U
                    Carbs by Poolboy
                    Rear Camber Kit, Rear Hubs by Goodparts
                    Gear Reduction Starter by TSI
                    Distributor by British Vacuum

                    Comment


                    • #11
                      Thanks for the tip. My adjustment screws seemed to bottom out ok. Question for all: What is best material to use for making a blanking gasket to go between the TBV and the carb body, since it must be able to withstand the vaccuum (where there was a hole) and also be compatible with fuel/air vapor, and not flutter?

                      Comment


                      • #12
                        Soda can....

                        I'd wait for the Doctor

                        cheers
                        Tush
                        81 Alfa Romeo Spider Veloce, 81 Triumph TR8
                        73 Triumph TR6 CF4874UO, 68 Triumph TR250 CD5228LO
                        62 Triumph TR4 CT6716LO, 60 Triumph TR3A TS69891LO
                        60 Triumph TR3A TS64870LO, 59 Triumph TR3A TS44836LO

                        Comment


                        • #13


                          ......I can't give away all my secrets, but yeah, what Tush said will work as long as it's thick enough
                          Driving a 1973 TR6
                          Doing ZS carb repairs
                          email kencorsaw"at"aol.com

                          Comment


                          • #14
                            Again, late to the party. A soda can is fine.
                            I have been doing the same thing on Japanese carbs too. Imagine having FOUR of these valves! The ones for my CB1100F are only $30 each (the beer can was only $1.81).
                            My buddy has been bringing all sorts of 12-14 yo dirt bikes by my house for "a tune-up." Each time I drop a floatbowl to find green goo, black tar, or dirt dobber dust plugging float bowl vents, fuel tank vents, dirt-conaminated fuel tanks and ruined TBV diaphragms. He laughed when I pulled out a beer can and fashioned the same shim. Non-adjustable needles get shimmed up with a #2 brass washer at roughly .005" thick. Fortunately he has just about finished putting "new" dirt bikes under all the boys fannies in his household!
                            If you look on BobbyD's website, find his throttle body conversion to see an example.
                            Last edited by TR6_Jim; 10-10-2018, 09:59 PM.

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                            Need some help tuning Strombergs

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