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My bike died today during a ride and I'm not finding the cause

Discussion in 'XJ Technical Chat' started by SQLGuy, Apr 4, 2021.

  1. SQLGuy

    SQLGuy Well-Known Member

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    Hi folks,

    This is my '82 SECA 750 that I have converted to fuel injection. I've made a lot of changes since the original conversion, and put about 13000 miles on it since converting it. Current setup is sequential injection with a single throttle body and a custom manifold and fuel rail. The manifold attached directly to the head (no carb holders) and the injectors are in bungs very close to where the manifold flanges bolt to the head. Converting to sequential was the last change. I did that a couple thousand miles ago.

    I was out for a ride with a friend today. After about 140 miles I was just getting back into his town... I was waiting at a light to make a left turn, and when I had a gap, I gunned it and it just died in the middle of that turn. Up 'til then it had been running really well. Not a hint of a problem.

    So far I've verified spark on all four plugs, fuel pressure, and I can smell that there's fuel. Still only get the slightest occasional cough when cranking, with the battery on a charger/booster. I also tried starting with propane running into the air box, but that didn't make any difference either. And, actually, come to think of it, when I checked AFR on my mobile app, I was seeing 9.9:1 or so, so there's definitely fuel. I also checked timing, and it's correct. Just wanted to make sure nothing had broken with the crank wheel or the pickup, but the timing light shows that spark is where it should be.

    Compression doesn't look great but I just realized I had forgotten to open the throttle when testing compression. It's quite even across the four cylinders, though. I can't think of anything that would cause a loss of compression on all cylinders, to the point of not running, without being something that resulted in crunching noises or locking up of the engine. Any other ideas here?

    So, I have spark, I seem to have fuel, and I don't have any completely failed cylinders. I had previously owned a GPz 750 that had a bent valve in one cylinder when I bought it. Compression only measured 30 PSI for that cylinder, and it still ran... just badly.

    The only other thing I can think of is that something broke in the sequential injection and I lost two of the four... but I would have thought that the bike would still run off of propane at that point.

    So, any ideas?

    Cheers,
    Paul
     
  2. Franz

    Franz Well-Known Member

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    Are there any electronics on the throttle body that could have failed? I had a car with a throttle cable and a potentiometer which was faulty. Should not shut the engine down though.
     
  3. cds1984

    cds1984 Active Member

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    SQLguy send me a link to your fuel injection upgrade thread.
    Would love to have a read through.

    No help to your issue of course, just purely selfish :) and maybe, helpful for diagnosis.
     
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  4. SQLGuy

    SQLGuy Well-Known Member

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    There are electronics in the throttle body. They have not failed. The tuning app is showing correct readings from the throttle position sensor.
     
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  5. jayrodoh

    jayrodoh YimYam Premium Member

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    The carb'd bikes will run (poorly) on two cylinders so I suspect yours would as well. I can't think of anything that would cause it to die suddenly that you haven't checked. I don't know if it is even possible for the cam chain to slip/jump but did you pull the valve cover and check the cam timing with the marks on the crank wheel? About the only thing I can think of that would cause the whole thing to stop running and cough slightly when attempting to start.
     
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  6. SQLGuy

    SQLGuy Well-Known Member

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    I checked compression again. 7o PSI +/- on all cylinders, throttle wide open and plugs out. Checked on my Virago for comparison, and to make sure the gauge is accurate... that showed 130PSI. :-(

    In the next couple of days I guess I'll be pulling the valve cover, and maybe the head to see what happened.
     
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  7. Timbox

    Timbox Well-Known Member

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    So you are thinking that at that the bike is just tired in the PSI and need a little help? I would start that if it only has 70 PSI across all 4. I good honing and maybe a set of new rings and your bike will be better than ever. Good luck to you and keep us posted. I will also have to take a look at the throttle bodies you used and see what your project is. Sound great if you no longer have to clean carbs and you have a nice running XJ.
     
  8. SQLGuy

    SQLGuy Well-Known Member

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    No. I'm thinking that something must have happened internally in the engine. It was running great until it died. That's not how tired would happen. My current theory is that something went wrong with the timing chain tensioner and allowed the cams to get out of time... hopefully not enough to cause valve/piston contact, but maybe...
     
  9. jayrodoh

    jayrodoh YimYam Premium Member

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    That is where my head is at as well, you would think it'd be a noisy event though. Keep us posted.
     
  10. Jetfixer

    Jetfixer Well-Known Member

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    A couple of items to check ...check battery voltage , and the TCI could be going south , recently I had my Johnny Cash bike idling away and it quit. I took cover off and was corroded sprayed with contact cleaner and it started up ran for 10 minutes and died again . Lucky me had another , check your coils if they are stock look for cracks in housing ...even if you have spark it may possibly be weak . Just a few thoughts to check before tearing into the engine.
     
  11. SQLGuy

    SQLGuy Well-Known Member

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    OK. Finally had some time to dig in to it. The cam sprockets and chain look OK, but the intake cam is one tooth off (retarded) from where it should be. The exhaust cam is spot on.

    I checked clearance on a couple of exhaust valves, though, and they both measured a bit too much: .202mm fits, .228 mm does not fit. I checked/adjusted valves pretty recently - maybe 1000 miles ago.

    Any thoughts on what caused the cam to jump? Tensioner failure?

    I'm thinking I'd want to reset the cams and then check compression again to see whether/where valves might be bent, but I'd want to first make sure that whatever caused the loss of timing wouldn't recur.
     
  12. Polock

    Polock Well-Known Member

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    look in the cam cover for marks from the chain. i saw them in mine, guess it could jump a tooth
     
  13. SQLGuy

    SQLGuy Well-Known Member

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    Did see a bit of a mark towards the front edge of the middle chain guide inside the cover.

    Other things from today's investigation:

    Removed the tensioner, reset the intake cam to its correct position, and reinstalled the tensioner. Then checked compression again. Numbers were 95 to 105. For reference, my XV750, which runs great, reads 110. Both were cold. Same gauge. I'm at about 6500 feet elevation here, so the air is a bit thin.

    Then I checked the valve clearances. They're kind of all over the place.

    #1 - zero clearance on intake, a little too much on exhaust
    #2 - good on both
    #3 - too tight on intake, good on exhaust
    #4 - too tight on intake, too loose on exhaust

    I could see valve damage causing valves to be too loose, but I don't understand what would have caused them to become too tight. I adjusted the valves relatively recently... within the last 2000 miles for sure. When I did them last time, the ones that were out of spec were barely out, and moved to the other end of the range by installing the next numbered shim.

    Not sure whether I should pull the head, or first set the clearances and re-check compression. Thoughts?
     
  14. Timbox

    Timbox Well-Known Member

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    If you have the shims I would re-set the clearances and see what you have.
     
  15. jayrodoh

    jayrodoh YimYam Premium Member

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    I'd start here for sure. Avoid overthinking it and easier/cheaper than pulling head.
     
  16. SQLGuy

    SQLGuy Well-Known Member

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    Did some more checking yesterday, and also tried a different (Lisle, so, maybe, better) set of gauges. Actually, all clearances were in spec except #1 intake, which was way too tight (about .06mm). Will try swapping out that shim.

    That's the lobe nearest to the tooth that I added for cam position sensing. I'm pretty sure I checked clearance after adding the tooth, but maybe I'm wrong about that. If that's the case, maybe welding that tooth on was enough heat to warp the cam by .05mm... At least it would be an explanation of why that clearance closed so suddenly.
     
  17. Timbox

    Timbox Well-Known Member

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    SQLGuy, do you have a link to this build. After reading the above I would like to see all the different mods you made for the FI to work. Adding a tooth and so on. Thanks.
     
  18. SQLGuy

    SQLGuy Well-Known Member

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    I do not.
     
  19. SQLGuy

    SQLGuy Well-Known Member

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    But, briefly (specifically to the FI work):

    Custom 36-1 wheel to replace stock ignition reluctor.
    Remove one pickup, since only one is needed.
    Custom VR conditioner circuit based on NCV1124, since stock Microsquirt had problems above 5K RPM. They may have fixed this in newer Microsquirts.
    Modify air box to replace front with flat plate and single outlet.
    Block temp sensor screwed into back of block (originally from a Porsche 914).
    Air inlet temp sensor (GM) added to air box.
    Custom fuel rail using Hyundai Accent fuel pressure regulator.
    Originally used Hyundai Accent injectors. Later changed to 14# Ford four-hole injectors.
    Golan engineering fuel filter.
    Add fuel return tap to fuel tank in place of stock petcock.
    Add fuel outlet tap to fuel tank on right side.
    Add in-tank fuel pump (Bosch pump theoretically used in some Harley and Saab applications).
    Custom intake manifold which bolts directly to head (no carb holders).
    Ford Escort throttle body with built-in throttle position sensor and idle air control valve.
    Custom throttle cable to the throttle body and with retainer welded to the frame.
    Remove choke cable. :)
    Add tooth to intake cam for cam position sensing.
    Add cam position sensor to valve cover.
    Weld oxygen sensor bung to exhaust collector and mount O2 sensor.
    (Not strictly FI related, but added catalytic converter before muffler)
    Replace coils with Saturn SC coils and modify harness to drive these from Microsquirt (Microsquirt bolts in place of original TCI module).
    Switch to resistor plugs since SC coils are designed for higher resistance in the secondary than stock coils (still using 5K caps, too)
    Two MAP sensors: one for manifold pressure and a second one for ambient pressure (for full time altitude compensation).
    Modify Microsquirt to add second pair of injector drivers for sequential injection.
    Modify Microsquirt to add Bluetooth connectivity to allow easier data logging and tuning from a smartphone app.
    All the wiring, relays, and fuses, needed for Microsquirt, fuel pump, injectors, sensors, IAV, etc.
     
  20. Polock

    Polock Well-Known Member

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    WOW !
     

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