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  1. #1
    CB750 Member Fatherof2's Avatar
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    I have a dead cylinder and low compression.

    Hi Gang,

    I have a 93 NH750. The number two cylinder is not working, but the compression is 85. I completely cleaned the carbs. Full tear down, carbs separated and cleaned with new jets installed. Brand new plugs. Swapped plug wires. Nothing worked. Compression in 1,3 & 4 is 60. I am pretty bummed here.

    Any thoughts on the dead cylinder? Also, with compression down on 1,3 & 4, what needs to be done? Is this a big job?

    I appreciate any input!

    Thanks All.

    Guys............Im an idiot! I did not warm up the bike, so my numbers are off. I will have to wait until tomorrow to try again. Can I expect a big jump in compression? Still does not explain dead #2 cylinder. Thoughts on that?

  2. #2
    CB750 Guru amc49's Avatar
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    Use another compression gauge to back your numbers up, compression can be notoriously finicky about getting reliable numbers and more testing adds to the reliability and some gauges are garbage.

    Because you MUST have well over 100 psi to make good power, more like 120+, the bike new will be over 150 easy. No sense messing with anything until the compression comes up, nothing will work. Mileage? It gives even more backup to that.

    When compression is low the plugs will not burn hot enough to stay clean and then they don't work (foul) and in variable ways. Dead cylinder is irrelevant to all that until the compression is known good on all 4.

    I worked out the 100 psi as a bad cylinder over 40 years ago while in garage work, it fits all types of engines except for very small 2 strokes. I've proved the number out a thousand times. If you don't get above that by at least 20 psi you are wasting your time, the engine will never run well. Unless blown or supercharged, that can throw a wrinkle in the rule.

    Hot vs. cold engine can be worth another 5-10 psi. The throttles all must be held wide open or the number is false. All plugs need to be out and I suggest using car battery to keep all numbers and cranking close to the same speed, the bike battery will drop in amp too quick. You can use car battery while it is in the car but do NOT start the car up!

    Numbers still low you go after valves first (head comes off) and then rings if the mileage is high enough. It can be helpful to add a few drops of oil to each plug hole to lube old dry rings but don't get carried away with it, if you hydrolock the cylinder with too much oil you will bend a rod and engine pretty much junk then.

  3. #3
    CB750 Guru amc49's Avatar
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    Those later engines are sometimes hydraulic liftered, if so and head has been off there will be no compression until the lifters are bled down, they hold valves open at reassembly.

  4. #4
    CB750 Member Fatherof2's Avatar
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    I am happy to report that compression is decent.

    #1 = 132

    #2 = 127 (The dead cylinder.)

    #3 = 136

    #4 = 136

    The bike has 22K on it.

    Any thoughts on the dead cylinder? What can I check? What might be the cause?

  5. #5
    CB750 Guru amc49's Avatar
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    Carb issues or ignition...............

  6. #6
    CB750 Member Fatherof2's Avatar
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    AMC,

    Thanks for your input.

    I am leaning towards ignition issues. Can you give me some tips on what I should check? Can I have a bad coil if the other cylinder is fine?

    Thanks again!

  7. #7
    CB750 Guru amc49's Avatar
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    'Can I have a bad coil if the other cylinder is fine?'

    Commonly no but in freaky circumstances yes. The waste spark twin coil thing relies on BOTH plugs to work as the plugs form the ground circuit of the secondary. One plug fires backwards, or from the outer wire to the center electrode to do that. Because the 'fire' pulse is a/c and not d/c. Meaning either plug having something wrong can mess the other plug up. The engine should go from 4 cylinders to only 2 working.

    But not always. I've seen car coils only drop one cylinder and using the exact same setup as some do now. It has to do with how the fault occurs I think.

    Anyway, plugs cannot be wet and being clean may be wet if you have allowed enough time for fuel to dry up off one that is fouled. And, once a plug has jumped down the side to short out the gap it may not want to jump at the gap any longer meaning new plug. Something about the electricity jumping down the side reinforces the ability to repeat that over and over, it becomes a path of least resistance and electrical always likes that.

    I try not to use resistor plugs, or wires either or even in the plug caps. The wire must ohm check out at less than 5000 ohms per foot of length and the insulation must be good. I have seen a plug cap go bad to drop a cylinder out.

    You can swap the coil wiring from one coil to the other to see if the bad cylinder switches position. Same with wires.

  8. #8
    CB750 Member Fatherof2's Avatar
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    Thanks for all your input AMC!

    Here is where I am at.

    I was hoping it was a freak thing with the coil, but I don't think so. I was able to pull the #2 wire over onto the #3 plug. No change. (I previously did a wire swap with not luck.)

    I swapped the #1 plug with the #2 plug. No change.

    I can see gas on the #2 plug after running the bike.

    My conclusions are that its not the plug, wire or coil. Can I rule out spark and electrical issues?

    As I mentioned, I just completed a full rebuild of the carbs. Full teardown, ultrasonic cleaner, brand new pilots. I did a bench sync but obviously not a full gauge sync yet. I played with the mixture knob with no success. Could it be a fuel issue? Bike actually runs OK considering. There is defiantly fuel on the plug. Could it be too rich? I'm using stock size Pilots.

    I just don't even know what to check next. The compression is a bit less on the #2. Would that stop it from firing? After riding it for a bit, the #2 pipe is hot, but not anywhere close to the other 3. Might just be residual heat.

    I should mention that the bike wont run at all without the choke on. Even after it warms up, the bike will die without choke.

    I would love any suggestions!!

    I have searched the internet for ideas, and I can find threads about similar issues. The problem is that the OP never comes back to say what happened. Its that or it was something simple like a bag plug etc.

  9. #9
    CB750 Guru amc49's Avatar
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    'I should mention that the bike wont run at all without the choke on. Even after it warms up, the bike will die without choke.'

    That says something seriously still wrong, engine will still run without one cylinder, at least partly as the carbs have like 5 systems and they are all 4 totally messed up up and down the range???. Meaning carb work suspect now. I suspect you getting 85 psi on the bad cylinder then it going up to 125ish too. Crap gauge or other issue. Plug getting wet says it is getting at least some fuel.

    You're beginning to get to where you really need somebody there to weed out the issues one by one as there could be more than one. Part of the problem of working on them over the net is never being able to grasp a full picture of both the bike AND the person working on it. Not saying anything bad of course but it IS a problem. That model as well has pollution control all over it and can have a vacuum leak in more than one place to mess up all your work to nothing. The carbs are complicated enough now that most of the time people think they are clean to run when they are not. I personally do not regard ultrasonic cleaning as having any worth at all. A full teardown as well means nothing if every single passage is not known to be 100% open and most aren't.

    You must have compression enough to run, fuel and air mixed properly and properly timed spark to run.

    If you put oil in the weak cylinder to bump up to 125 you faked yourself out. The number likely not real.

    The mixture screws must be cracked open say 2 turns. You need to try swapping the COILS with each other, plugs won't cut it there past a point.

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  11. #11
    CB750 Member Fatherof2's Avatar
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    Hi Gang,

    I just wanted to leave an update on what my issue was. (So many threads don't leave a final update which was frustrating as I was trying to solve my issue.)

    First off, big thanks to AMC for all the input!

    I stopped being lazy and pulled the carbs back out. After a brief inspection, I discovered that the rubber diaphragm on one of my vacuum pistons had become un-seated! Huge air leak! I corrected that and put the carbs back in. Viola! She runs like a champ! I did the needle shim thing too. Nice Improvement! (More on shimming the needle here if you are interested. https://www.nighthawk750.com/)

    The #2 cylinder is running fine. Pipe gets crazy hot just like the other 3. I will do a compression check to see if that improved. (AMC, do you think the diaphragm issue could have contributed to a weaker compression result?)

    By the way, if you are looking for instructions on how to rebuild your carbs, please check out this guy:https://cody-preston-richards.mykajabi.com/
    The videos were super helpful and very detailed.

    Thanks all!

  12. #12
    CB750 Guru amc49's Avatar
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    You found it and great job!

    No, it will not affect the compression reading but if engine not run in a while that will often. The rings sometimes gum up and have to run a bit to let engine realize it is back to doing work again.

    Persistence and the ability to go back to critique your own work saved you like it will a lot if you do a lot of things.

    Luck.........and thanks for the comeback, like you said, so many don't.

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