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'82 Seca 750 Refresh Project: Now Scooby's Bike

Discussion in 'XJ Technical Chat' started by Nuch, Aug 24, 2019.

  1. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    I'd be concerned about a torn diaphragm. If it tore while the slide was up, the slide would fall quickly and possibly get stuck. Worth looking into.
     
  2. Nuch

    Nuch Well-Known Member

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    I can confirm that all diaphragms are good... held each up to the light and everything...

    Given the time to think things over, I’ve decided that I was over zealous (obsessed?) with lubrication and anti seize...

    As I said, diaphragms are in great shape, but perhaps I should not have massaged them with silicone grease. During this last disassembly, I wiped off excess grease that I now realize just didn’t need to be there... maybe between the unnecessary anti seize (as discovered earlier in this thread) and the grease, I inadvertently created more problems...

    Guess I’ll see when I get the carbs back in... looking forward to the weekend already...
     
  3. kerriskandiesinc

    kerriskandiesinc Active Member

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    Well, hopefully you'll get her running soon.....be interesting to see a high speed 'plug chop'.....and, also find out what the 'backfiring' through the carbs might be??

    Where in Suffolk County are you located??, I'm probably close by, if you need a second opinion/set of ears/eyes, lol........though I'm nowhere near as good a (XJ) mechanic as I used to be, lol !!
     
  4. Polock

    Polock Well-Known Member

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    i put silicone grease on the diaphragms but just a wee little dab
     
  5. Nuch

    Nuch Well-Known Member

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    Ok, so essentially, no change.

    Benched sync done again. Carbs back on the bike. Another brand new set of plugs put in just in case. Choke on, turn the key, hit the button and she started right up. Raced up to 6-7k so I rolled the idle knob back to a reasonable level (lets say 3-4k) while the choke was still on and she warmed up. I let it go for a few minutes and after some time went by, I began gradually removing choke.

    Sync gauge showing 1, 2 & 4 relatively the same height with #3 very low.

    However, as she continues to warm and I attempt to back the choke all the way off over time and adjust the idle knob to keep the idle at a reasonable level, she goes into a rhythmic up and down cycle of the RPMs... and then it happens... The Sync gauge tubes begin to pop from the intake nipples!

    Whoa!

    I also saw a blue flame come from the #2 nipple after the gauge tube popped off.

    New plugs just as black as the last set.
     
  6. XJ550H

    XJ550H Well-Known Member Premium Member

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    you want your idle to be 1050 rpm (spec) 1100 rpm
    when choke sends rpm up back off choke. see where you can drop it to and still run no less than 1100 rpm the choke makes motor run at higher than idle rpm.

    carb 3 adjustment is the idle knob use the knob to bring up #3 vac then tune down the other carbs to 3.
     
  7. kerriskandiesinc

    kerriskandiesinc Active Member

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    Are your valves in spec/closing properly, I wonder....?
    (also, valve timing good?)
     
  8. XJ550H

    XJ550H Well-Known Member Premium Member

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    not to say he has more than 1 issue going on. the leak down test is used for valve leaks.
    psi going up with oil is oil sealing the gap in rings to cylinder walls.
     
  9. kerriskandiesinc

    kerriskandiesinc Active Member

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    I missed the bit where he'd already tested the valves/rings to bore....

    I suppose that only leaves the carbs then, if the valves are in spec, and the rings are sealing......

    I had poor WARM starting,( but not cold) and spitting back ( massively) thought the carbs when my 900R ate its valve guides/inlets.......I was convinced it was the carbs, turned out the valves/guides were sinking into the head.....not a known XJ problem,I know !
     
  10. Nuch

    Nuch Well-Known Member

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    Today she started as usual... Took a few tries, but she gets there.... displaying the usual popping and rough stuff...

    It's clear to me that #4 is not firing. Or at least not consistently or strong enough for that matter. I hooked up one of those spark indicator things and it appears to flash as if the coil is doing it's job. On previous attempts (other days) all 4 were getting hot... though, once I checked it initially, I never checked later on for risk of burn... I just assumed once she got hot, she'd stay there.

    Throughout the trial today (several minutes), I can leave my hand on the exhaust. If I did that on the others, I'd definitely leave my skin behind.

    Question #1. Can one side of a coil go bad?

    Question #2. I'm getting a reading of 4.5 -ish on the coil thin wire resistance check... The book says it should be 2.5 + or - Isn't this high? Honestly, The cap to cap test does not seem to give me any display at all. I'm doing something wrong I think...

    Question #3. Can I swap the 1 & 4 leads to the plugs to see if the cylinders swap the "no fire" condition?


    Also noteworthy... I clipped the #4 lead back, cleaned the screw shaft inside and replaced the cap. I cleaned the carbon from the #4 plug and put it back in. Bowl levels look good (I thought maybe I was back to a starvation problem but clearly it is not) and after this last attempt, the plug came back out squeaky clean, but wet with fuel.
     
  11. XJ550H

    XJ550H Well-Known Member Premium Member

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    get a temp gun 15$ aim and test save the burnt fingers
    the plug wire can go bad and the connection where the wire goes into coil.
    primary wires (thin ones) meter should be set to 200 ohms
    secondary wires (sparkplug wires) should be tested at 20k setting with out the caps on the wires.
    go to next highest with caps installed
    you can swap leads to see if problem follows wire and cap. you can also swap plug into known firing cylinder to test plug

    regap plug?
     
  12. Polock

    Polock Well-Known Member

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    you have to get the sync right first. try turning the idle knob till you get some kind of reading on #3, then make the rest match 3.
    that says #3 was shut off, the idle was too low
     
  13. Nuch

    Nuch Well-Known Member

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    Ahhh, This makes sense... however now I need to get #4 firing first...
     
  14. Polock

    Polock Well-Known Member

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    maybe that would work but the better way is to get the sync closer first. even if you need to disconnect the tci to keep it from starting and running away, and just get it closer with the starter.
    with the sync that far off #4 may not fire when 123 do and vise versa.
    there is another way, pull the carbs and get a good bench sync. didn't want to hear that, did ya :)
     
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  15. Nuch

    Nuch Well-Known Member

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    I’m not opposed. It would be the 6th time I’ve had them off the bike. The last time, (only a few days ago) I decided to pull them again and re bench them. At bench sync, the vacuum looks like that. #3 is low and 1,2&4 are high and relatively similar. As it stands now, I’ve not touched the sync screws, only the idle screw...

    I just assumed that I NEEDED all firing to do the vacuum sync.
     
  16. Polock

    Polock Well-Known Member

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    don't get me wrong here, it will need to run around 1k to 1.7k to get the sync close and tweek the mixture screws. it seems, as it is now it runs on 1+2 and maybe 4 hits now and then. that could be the
    that's#4 coming and going .
    you need to turn the idle up (that opens #3) now get on the 1-2 to 3-4 screw and while the starter cranks try to bring 1-2 down closer to 3
    stop, take a brake, let the starter rest
    get on the 3 to 4 screw, hit the starter and get #4 down closer.
    somewhere in all this, it's going to start at a reasonable rpm, then you can tweek and tune till your happy
     
  17. Nuch

    Nuch Well-Known Member

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    Seems like you have a strong suspicion that it isn't a coil/electrical issue.

    My "to attempt" list is growing.

    I'll give it another go this week end. That's the next chance I'll get to go out there and try...
     
  18. XJ550H

    XJ550H Well-Known Member Premium Member

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    better to rule out things before spending money that you do not have to.

    I do agree with Polock that your carbs are way out of sync or at least thats the place to look.
    swapping plugs and wires is a good teat yu also maybe able to swap ignition coils around too
     
  19. Polock

    Polock Well-Known Member

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    the plug caps on these coils are connected to the same wire, it goes up the plug wire, around and around many many times inside the coil and goes out the other plug wire.
    so if something happens to that wire, both plugs quit. same with the 12v side, it's all or nothing
     
  20. Nuch

    Nuch Well-Known Member

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    OK. Small success. I set everything back up and gave it another try. Given the idea that I'd try to sync without worrying about #4 NOT firing, I set out on my task.

    Got her running and I kept the RPMs up with choke on while I tackled the sync. As I said before, 1, 2 & 4 read significantly higher than 3... 1 & 2 were nearly the same with 4 being even higher than 1 & 2.

    I didn’t pay attention to how smooth it was running, my goal was to bring the 4 carbs to the same level. Over time I was able to back the choke completely off. I got real close with the sync… low and behold, when I reached down to feel #4, it was screaming hot! All 4 were firing.

    However, there was still a fair amount of popping. So much popping in fact that my sync tubes for 1 & 2 would not stay on. I think that once warmed up, the sync gauge tubes get so soft that they can’t hold on to the intakes anymore under the popping pressure... Also worthy to note is that 3 & 4 do not have this issue.

    I decided at that point the sync was close enough... I couldn't keep the gauge attached to the engine anymore anyway. I put the caps back on the intakes (the caps won’t blow off while popping!) and I played with the enrichment screws a bit (1/4 turn either way as @Polock suggested earlier in this thread). While it was hard to discern a rise or fall in the idle while turning the screws, I was able to affect the popping.

    The popping became minimal and the cyclical, deranged up and down of the idle disappeared.

    I’m happy to say that I put her back together and was able to ride around the neighborhood a bit. It is no where near where it needs to be, but I feel like I got her far enough at this point to not feel totally defeated by it.

    At throttle blip, the idle takes too long to come down, but of course more tuning is necessary. I’ll tinker as time permits. I will register and insure her in the spring as she will be making the trip back to the carb clinic in June... Next to one of her sisters on the trailer of course. I’ve no doubt she needs only a few minutes with the master (@hogfiddles) to dial it in.

    There's still a long road ahead. She still needs a lot of love but there's a ton of life hiding in there. I know it.
     
    Last edited: Oct 27, 2019
    kosel, Rooster53 and Polock like this.

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