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  1. #1
    CB750 Member Steezentheknees's Avatar
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    Lost Spark on Cyl 2-3

    I have been slowly rebuilding a 1980 CB750F Super Sport for the better part of 5 years. It's been a saga blah blah blah. I finally got it running well enough to ride for longer than a few minutes. Took it on a nice long ride to see what the plugs were doing. Pulled them, rich, re-jetted, started back up, idled as normal, died, lost cyl 2-3. Heard a bit of a noise coming form the coils, not quite a 'buzzing as much as a ringing" it's intermittent. Went back to original jetting, nothing. Cleaned carbs, nothing. Tested coil resistance, swapped them to see if it would follow one or the other, nothing they're fine. Tested plug boots and wires they are fine. I have solid blue spark on all 4 plugs. Plug gaps are fine. Replaced pulse generator. No change. Swapped spark units, no change. Wiring harness is brand new. Clip from harness to coils has been replaced. I have good compression, the motor is freshly rebuilt. The carb set up is a 2 mikuni vm34 into 4 manifold. The manifold is 2 pieces, 1 for Cyl 1-2 and 1 for cyl 3-4 so it would make no sense that that would cause me to lose fire in 2/3.

    TLDR: Lost fire in cyl 2-3, have checked/replaced everything. I'm kind of at a loss. I would be grateful for any suggestions. Thanks in advance.

  2. #2
    CB750 Guru amc49's Avatar
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    Compression test and the exact numbers you got on all 4. Anybody ever set the valves?

    Your carb setup there is one that makes problems where there were none before.

    If rich enough to foul the plugs they will NOT fire under load to run. May well not even start.

  3. #3
    CB750 Member Steezentheknees's Avatar
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    Thank you for getting back to me so fast. Compression is 150 +- across the board on my cheap compression tester. I have not checked the valves since before my first "ride". Will do that right now. I'm aware of the drawbacks of running a 2-4 system but I won't pull and rebuild another set of CV carbs, air box and boots, ever. The one thing I can absolutely confirm is that I am running very, very rich. Even though it ran for hours a few weeks ago. I was convinced this was an ignition/electrical issue but am now leaning more towards "washout". Plugs 1-4 are sooty black, plugs 2-3 are wet with gas. Virtually no sign of combustion.

  4. #4
    CB750 Guru amc49's Avatar
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    Were the carbs pre-jetted for that particular engine setup? If not you are in carb hell and much worse than with the CVs, which at least were nominally jetted. Two big carb bores never pull across the board nearly so equal as 4 smaller ones at part throttle; the big bores will kill you and it would show up as your issue. Fuel falling out of suspension to then wet the plugs when it does not vaporize properly due to low velocity at part throttles. If your manifold has the fuel hitting a flat before turning sideways even worse. 9000 rpm engines will not do that like 3500 rpm lawn mowers do. The difference in manifold lengths to each carb will mess things up too.

    The ignition is not a strong one and wets out pretty easy but not much available to replace it, the Dyna tends to burn up in the hot oil, massive trouble with those. The coils alone may help.

  5. #5
    CB750 Member Steezentheknees's Avatar
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    The carbs were pre-jetted for DOHC's. I hear what you are saying. I agree with you. You can't run from physics. But I will say this bike has run at every jetting combination ive tried for months. And it was running very decent until I lost spark in 2/3 recently, albeit rich. Nothing ive tried fuel delivery related has improved or worsened the symptom as of today. Leaned it out and nothing changed. 1-4 plugs are dark brown, 2-3 are just gas with no sign of combustion. I've had the dyna ignition in my cart many times but cant seem to justify $400 on something not guaranteed to fix my problem. I'm tired of throwing parts at it. My question is would valves cause a set of cylinders to simply not fire? Could I have skipped timing? I'm not convinced its fuel delivery TBH. I'm leaning towards electrical IMHO. Will need to check timing and valves tomorrow. But i'm still not sure how that would correlate with a whole pair of cylinders.

    UPDATE: I was plugging and unplugging plug wires as the motor was "running". 1 and 4 were obviously running strong and shocking me as I pulled and replaced them. 2 and 3 were seemingly intermittent but low and behold when held just the right way the bike would run completely normal if only for a few seconds. Ive re-crimped and replaced that wire but cannot recreate it. Let me know your thoughts. Thanks.
    Last edited by Steezentheknees; 11-13-2020 at 06:49 PM.

  6. #6
    CB750 Guru amc49's Avatar
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    If you truly have 150 psi then it is not the valves.

  7. #7
    CB750 Member Steezentheknees's Avatar
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    I picked up a spare wiring harness today and am going to see if that changes anything. Otherwise, once again I am completely at a loss.

  8. #8
    CB750 Guru amc49's Avatar
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    Who supplied the carbs/intakes?

    No trouble likely with harness, if you have 12 volts at the coils + (black/white) you are as good as you will get. The kill switches themselves can make for low volt issues.

    Go to non-resistance plug wires if not already and same on plug caps and plugs, R in plug number is a resistor plug, dump it. Those moves will allow you to open the plug gap up a wee bit more to do something useful. Assuming you are using the NGK D8EA plug.

  9. #9
    CB750 Member Steezentheknees's Avatar
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    Cycle X for the Carbs, Speedmoto for the intakes. I have 12 volts for sure. Could a kill switch have anything to do with what i've got going on? Running NGK D8EA non-res plugs but the plug caps have a 5Ω symbol. Any chance you could link non-resistance plug caps?

  10. #10
    CB750 Guru amc49's Avatar
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    The kill switch will lower the volt to under 12 if a problem.

    If you carefully unscrew the end up inside the plug cap you can replace the resistor above it (like a fuse) with a correct length of cut bolt to then have a zero resistance cap. You must make sure the parts all touch while still having enough thread left to tighten the part back up. Some won't come apart though, the ones that do have a screwdriver slot in them.

    Sure you didn't get the carbs metered for SOHC instead of DOHC? CycleX seem to have dropped most of the DOHC parts they used to carry and the carb kit page says the carbs do not work with DOHC.
    Last edited by amc49; 11-14-2020 at 11:54 PM.

  11. #11
    CB750 Member Steezentheknees's Avatar
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    I purchased the carbs when they still made the dohc kits. I spoke with him and he said he took them off the website due to fitment issues on the stock frame. I purchased another manifold from speedmoto as it fit better and the fuel routing was much more direct, if not close to equal on all four. Again I am quite convinced I have an electrical demon somewhere as opposed to fuel delivery. I can get the bike to idle very strong for a few seconds if the #2 cable is wiggled just right. #3 cyl is intermittent and the headers are warm. 1 and 4 are hot.

  12. #12
    CB750 Guru amc49's Avatar
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    If you do not have brand new plugs in then all that literally means nothing. Fouled plugs have warped the thinking of thousands of people; they literally trick you with all the ways they can act up.

    Thinking myself that setup not metered nearly so well as claimed, a proper prejet does not put the plugs black in that short a time.

    Look at the firing order. Or 1-2-4-3. The first number of each pair (1 and 4) has a clean intake event because proceeded by dead time with nothing happening. The 2 following the 1 though, what if the intake manifold volume there is such that when the intake pulse for 1 then hits the closing cylinder 1 intake valves it reverses back to go through the intake backwards, and common physics. It hits the carb and pulls more fuel out because the carb feeds fuel either way there is a pressure change, a known phenomenon. The cylinder 2 opens valves and pulls even more fuel to make that pulse super rich, it passed the carb fuel exit 3 times. Same thing with 4 and 3. Reversion like that has messed up way more than one setup when things added up wrong to create problems. And there is no way to jet for that with only one carb bore, the single bigger bore even aggravates the issue.

    Just thinking out loud but why 4 separate carbs get used on 4 cylinder bikes mostly, it stops all that. 2 will work but all the volumes and pipe lengths have to be tested for that to make sure it doesn't happen. Who guaranteed that on that setup? Nobody, they simply assumed it would work. Mostly it does, but sometimes.............look at how many fours use twin carbs when the makers could save great bundles of money doing it..........you don't see any to speak of.
    Last edited by amc49; 11-15-2020 at 09:02 PM.

  13. #13
    CB750 Member Steezentheknees's Avatar
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    That makes perfect sense. The simple physics of going from 2 big bore carbs to 2-double manifolds makes perfect sense. And would make complete sense why this motorcycle would never run right. But it did. For months. Albeit rich. And i'm simply trying to track down what changed that and why I can get it to run perfectly for a few seconds by wiggling some wires and then I lose cyl 2. 4 Carbs for 4 banger bikes. Yes. I get that. But this is what im working with right now.

  14. #14
    CB750 Guru amc49's Avatar
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    It makes perfect sense but it is only a theoretical, meaning it does not have to happen.

    If you have not changed plugs yet that can do every bit of your complaint as it is NOW. Plugs running rich do it in various amounts, some can take months to finally overwhelm to go into that state.

    'I finally got it running well enough to ride for longer than a few minutes. Took it on a nice long ride...'

    That does not say months. Not trying to be an -ss but to bring it up to maybe hit the full truth. As it often figures into any cure. Running rich is an accumulative thing if not very bad and I myself have been to where slightly rich on these ended up a couple months later in the blind staggers but using the CV carbs instead. Leaned carbs way out and suddenly I picked up 2000 rpm on the top end. More, actually.

  15. #15
    CB750 Guru amc49's Avatar
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    There is a ignition mod using GM car parts that some have done..........I had some success with it back in the day. You CANNOT use the GM coils longterm with Honda spark units, they will fail. The setup will blow like Accel motorcycle coils away.

    http://gpzweb.s3-website-us-east-1.a...odCavCoil.html

  16. #16
    CB750 Member Steezentheknees's Avatar
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    'I finally got it running well enough to ride for longer than a few minutes. Took it on a nice long ride...' Essentially means I was riding it around the neighborhood many times leaning it out and then 1 long ride over an hour, a few down the street in between. All running strong. But still rich. And i'm changing plugs every time I try something new to eliminate that variable. You're not being an -ss. I know you are just trying to get all of the information and at the end of the day we are working toward the same goal. So where it stands: Had a buddy come come over today to take a look at it, only runs on all cylinders past 3/4 throttle. So maybe its the pilot circuit. I'm running K & N Filters, 25 pilot, 220 main, 2.5 slide, no air jet. idle screws out 2 1/2 turn, air/fuel screw out 1 1/2. Going to throw bigger pilot jets in tomorrow and see if it will at least idle. I'm honestly more confused than I was before.

  17. #17
    CB750 Guru amc49's Avatar
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    What indicates you need to go up on pilot if rich? You are at 2 1/2 turns now on air screws, that says pretty rich already if it's not dying. You are adding a lot of air, normal being 1 1/2-2 turns. Assuming VM type air screws BEHIND the slide, those being pure air with no fuel in it. You should be able to kill the motor toward lean if the pilots are anywhere near right. Screw them open each separately and look for the die point. You got richer pilots simply by screwing them shut more at that far open. A truly correct pilot will die going both rich and lean, lean it will die faster, it may just begin to run like crap rich, but both ways will affect smooth idle. Ignore fast or slow but STILL SMOOTH idle, you readjust those using speed screw.

    What is the needle setting slot used?

    Wondering if that main is too big too.

    Pilots only control up to maybe 1/4 throttle, going to 3/4 before clearing up says way more than pilot there, it points at needle which covers 1/4-3/4 throttle.

  18. #18
    CB750 Member Steezentheknees's Avatar
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    TBH i'm still not convinced my main issue is fuel delivery because it has run all of these jetting settings fine previously. However i'm sure it's part of the problem. I'm running a stock 6DH8 needle at middle clip. I have access to a 6DH4 which I believe is a bit of a leaner needle taper. I'm happy to try rejetting again but i'm wondering if I need to start at the beginning with a valve check and a timing light. Leak down test, etc. Also can you link me to resistorless spark plug boots? Thanks.

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