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  1. #1
    CB750 Member fg1972's Avatar
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    Engine not running right

    Hi all,
    I've got an 82 CB750 that I picked up over a year ago not running and have been slowly rebuilding and converting to a cafe racer. The project is around 85% complete and have had a few attempts at getting the engine to run properly with not a lot of success.

    I'll start with a bit of history on what has been done. (I've done all the work myself except for the engine machining).
    Engine, new bearings, rings, seals, gaskets, cylinders honed. Head machined, valves lapped, valve shim gaps set between 0.10 Ė 0.15 mm.
    Carburettors, fully dismantled (except throttle shafts/butterflies), cleaned, new jets, needles, float valves, seals. Bench synced each carb to #2 carb. Needles 2 Ĺ turns out.
    New carb to intake boots, new coils, spark plugs and wires. (Stock airbox)

    First start up, fired up straight away, idle was all over the place and slow to decelerate and running rough. Spraying carb cleaner around various spots to find a leak revealed vacuum leaks around the throttle shafts. Ordered a set of throttle shaft felt seals and off with the carbs again to replace them. Again, bench sync and paid special attention to ensuring no binding and crisp spring back to idle position. Also took the spark advancer off for a good clean and lube which I hadnít yet done which now seems to move a lot smoother. Checked compression, carbs fully open, spark plugs out using a car battery to turn the engine over. Compression was around 140psi on all cylinders a bit low probably due to my untrustworthy gauge and or new rings still to bed in however they were consistent which I think is the main thing.

    Next start up, seemed better but not great, idle reasonably consistent. Loosened off and retightened the timing chain adjusters (one at a time) to tension the chains. Noticeable reduction chain in noise after doing the rear tensioner (2 acorn nuts). Confirmed spark and combustion in all cylinders with a Gunson colour tune plug which seemed to indicate mixtures were in the ballpark. Checked timing on cylinder #1 which was ok.
    Still runs a bit rough and seems to not rev that happily, like there is a bit of hesitation and an odd pop sound here and there (not as loud as a backfire) whilst revving.
    When at idle, itís pretty stable now if left alone but sometimes blipping the throttle makes it not return to back to the same idle. Iím yet to vacuum balance the carbs but donít believe this can cause it to run like crap but I could also be wrong. So running out of ideas on what else to check and was hoping for some assistance please.

    Check it out on instagram @fiorellogalluzzo if you're curious.
    Cheers
    FG

  2. #2
    CB750 Guru amc49's Avatar
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    '...sometimes blipping the throttle makes it not return to back to the same idle. Iím yet to vacuum balance the carbs but donít believe this can cause it to run like crap but I could also be wrong.'

    You're wrong. Hanging idle can easily be caused by vacuum leak or the carbs not synced up. One high then induces the slide to drift up as they are CV type.

    You're off OEM compression by 30 psi, they start running like crap at 120 and lower. Rings not seated might be 10 of that.

    Did you break all 4 carbs apart? The only way you can get to the aircuts which can leak vacuum badly.

    The off pop sound is misfiring plugs.

    Every valve better have gone back in its' original hole or a horror story.

  3. #3
    CB750 Member fg1972's Avatar
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    Thanks amc.
    I'll do the carb balance and report back.
    The carbs were taken completely apart and the aircuts were also bypassed.
    What can cause the plugs to misfire?
    Each valve, spring, bucket was certainly kept in separate labelled compartments and I made sure to reassemble them in the same order.
    The lower compression numbers hopefully are just my cheap gauge.

  4. #4
    CB750 Guru amc49's Avatar
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    You hope. You MUST have the throttles way open too or you get a drop like that. Why you retest using another gauge, they are a constant issue as is all compression testing. I typically run 3-4 tests on each cylinder doing them to be sure of the numbers.

    Plugs misfire due to constantly changing fuel/air ratios, almost always toward rich when they wet up to short down the glass instead of through the plug gap. Usually a fuel supply issue (erratic at some point involving too much), or can be weak ignition or maybe not even plug, can be light backfire out the back of carbs too,l signalling valve leaks or slow timing (cam or ignition). Usually plugs though.

  5. #5
    CB750 Member fg1972's Avatar
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    Yes throttles wide open, have done several runs, same results on a cold and warm engine. I'll definitely have to borrow a known good gauge at some point to verify the compression numbers, however right now I'm not too concerned about the number since as all four read the same. Valves leaking is also a possibility which I certainly hope not but what are the chances of each cylinder losing the same amount via the valves unless I've cocked up the cam timing some how? I clearly remember getting the timing right during assembly as I had to assemble twice because I forgot to install the cam chain guide the 1st time (yeah I know).
    Not sure how relevant or if it proves anything but I have done a crude leak down test on each cylinder with a modified compression tester hose / air fittings using compressed air to pressurise the cylinder, rotating the crank by hand, hearing the air escape out the intake, then exhaust, then silence, then could feel the crank wanting to rotate under pressure.

    Thanks for the info on causes of plugs misfiring. Mine are a bit black so definitely something going on here. Just an observation I made today with the timing light connected to #1, the flashes seemed to be missing a beat, ie; flash flash flash flash, nothing, flash flash flash flash. Tried #2, #3 which behaved the same however #4 was not as bad. I thought this was really strange unless timing guns aren't that sensitive in detecting the pulse every single time or am I likely to have an ignition problem in the mix?
    In the mean time, I should take a good look/check resistance of the ground connections for the coil and the CDI units, check the leads and caps. The caps are the type with 4.5 or 5 kohm resistors in them.

  6. #6
    CB750 Guru amc49's Avatar
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    '....I'm not too concerned about the number since as all four read the same.'

    What are the odds of that? The same as all 4 leaking valves the same amount. Pretty rare and screams gauge is trash. They should vary slightly from each other and will with good gauge.

    '...unless timing guns aren't that sensitive in detecting the pulse every single time...'

    Hit the nail on the head you just did. Likely.

    I for one remove all resistance in the spark path, zero on plugs, wires or caps. Bump the plug gap open a fraction more to use resistance where it can do something.

    You should know the tricks those carbs play. If the dead idle gets stopped up then lean of course but as soon as you even think about opening throttle off dead idle the carb goes super rich as the idle circuit serves as part of the airbleed to the primary jet, making it go rich as you barely open the throttle. It drives people nuts.

    If you did not change timing chains then the marks will not line up and off by often 1/2 tooth, you assemble with them retarded, where they moved once they stretched. Those chains stretch a LOT.

  7. #7
    CB750 Member fg1972's Avatar
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    Engine not running right

    Ok, pulled the coils, checked primary and secondary winding resistance comparing to the old set, checked leads. re terminated the coil ends as I wasnít happy with the fit into the coils. Double checked earthís for the coils and spark units.
    Plugs cleaned and gapped slightly bigger, 0.75mm. Carbs vacuum synced, they were a bit out.
    Idle seems better now no hanging however pretty much the same when increasing the revs, ie; doesnít rev freely, few pops here and there.
    Observations, cylinder 4 not getting hot during idle but does get hot if revs are up. Adjusting the pilot screws seems to have no effect at all on any cylinder.
    Plugs look wet afterwards.
    From what youíve said AMC, could it be running lean at idle (maybe super lean #4) then super rich when increasing revs?
    When I did rebuild the carbs, the idle jets were completely blocked but I managed to get them out of the carbs and cleaned out with a fine wire. Iím pretty sure I had sprayed out the passages with carb cleaner. The main and secondary jets were replaced with the stock sizes that came in the kit. Any ideas or advice is much appreciated.
    Cheers
    FG


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    Last edited by fg1972; 10-10-2020 at 05:21 AM.

  8. #8
    CB750 Guru amc49's Avatar
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    double post

  9. #9
    CB750 Guru amc49's Avatar
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    The 'fine wire' can be no thicker than 'E' string on a guitar or thinnest one and .013" in diameter. The mixture screws should each affect the idle, you should be able to kill that cylinder going clockwise to a light seat. #4 is obviously not hitting all the time and the plug wets instantly like that. If the idle speed has the carb butterflies exposing the first hole that they should not expose then the engine goes rich too. There is one hole in carb bore way forward of butterfly, idle feed comes out of that one. The a series of holes right behind butterfly that each open one after another as throttle is opened up, if the first one is exposed then incorrect and problems. One of which is mixture screw not killing cylinder, the fuel leak from the hole makes up for it. Hole is either supposed to be barely covered up or just slightly beginning to expose and no more. Any more says throttles are open too far and an engine condition issue. You leave idle speed where it was and yank carb bank to check where the butterfly feed holes are. If you flip carbs upside down there is a silver plug behind the idle mixture post that has the mixing chamber for those holes above it. If stopped up it can be hell to open back up and all kinds of off-idle issues. Most people find themselves cleaning the carbs 3-4 times before they are right.

    http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf

    The slides are matched to the caps and you never ever sand on the slide OD or cap ID.

    As I've said the part throttle above idle gives people utter fits.

  10. #10
    CB750 Member fg1972's Avatar
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    Thanks AMC, much appreciated.
    The wire used to clean out the idle jets was around .3mm roughly converts to .013". Leaving the idle speed screw where it was, I pulled the carbs out last night, and yes the hole that is supposed to be nearly covered was clearly exposed so the idle circuits are obviously not working. I'll have another good look inside and re read the manual I was following (same as what you linked) particular in cleaning the idle circuit passages.
    I'm not sure what you mean by "The slides are matched to the caps and you never ever sand on the slide OD or cap ID", is this relating to the slides and caps on top of the carbs? If so, they were all labelled and reassembled in their original order and all move freely and smoothly.

  11. #11
    CB750 Guru amc49's Avatar
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    Yes, the actual CV slide and top cover on carb.

    That design is the least efficient CV type as it leaks unlike rubber diaphragm ones but the leak is calibrated by the clearance between the big OD at top of slide and the cap it slides up and down in. The center also has a damper hole in the cap center bore that stops fluttering. Some people if slide gets damage on the big OD sand it off and death doing it, the slide clearance opens up and slide quits lifting AS QUICK, which is super hard to measure as vs. lift just not opening at all. The slide opening slower goes rich when the same amount of area gets fed a higher vacuum when slide did not lift accurately and the cause of a lot of trouble. If the engine falls off in efficiency the slide slows down too as well as with more jetting to go rich, it kills slide lift to go even richer and a big hole. Why pods instead of OEM airbox mess the carbs up so bad.

    If the caps are properly centered on slide then slide and bore in cap should show literally pristine as the air cushion (the leak back and forth there) between them keeps them from ever touching. The slide BOTTOM can touch in the carb bore though and often does at high vacuum of decel, you will see wear there and on the lower slide part. That wear if sticking the slide on the lower small OD part can be cleaned up smooth with #600 sandpaper but DO NOT EVER touch the bigger top OD with it.

  12. #12
    CB750 Member fg1972's Avatar
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    Thanks AMC.
    Checked idle jets were clean, sprayed the crap out of the passages as per the manual blocking the holes and watching it come out the pilot screw holes. Did this with carb cleaner and compressed air in both directions until happy.
    Refitted the carbs and noticed an instant improvement and a big step forward. Sounds like a motorcycle now rather than a tractor. All exhausts getting hot even on idle, throttle response is great and revs quite happily. Re synced the carbs but still having some trouble getting a steady idle. Ie; adjusting the idle screw until it idles nicely but often revs won't come back to idle after blipping the throttle unless pressing on either #1,3,4 sync tabs. Checked that the carbs do spring back naturally and definitely not binding. It seems that you need to force one of the carbs to drop its vacuum a bit for the others to come back to idle. Could this be something going in with the slides perhaps?

  13. #13
    CB750 Guru amc49's Avatar
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    Possibly but possibly indirect. The pressing on the sync tabs says you have enough flow past butterflies to coax the slides into trying to float up. They are open too much at least on one if that happens.

    You can make sure all slides drop the same not fast but positively all the way top to bottom. Any drag on one says break out sandpaper to sand the carb bore and slide on the SMALLER OD ONLY. You can play with the cap centering too, sometimes slightly to one side a slide sticks.

    The aircut covers must be sealed too any leak there is an issue.

    Any time you do any work there you should always be looking to readjust idle SPEED lower if it will do it with no problems. 1000-1100 rpm.

  14. #14
    CB750 Member fg1972's Avatar
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    Before removing the carbs again, I'll double check the aircut covers for leaks.
    I'll check the slides and observe the drop rate and travel. If it does need sanding, you're referring to the part of the slide that has the notch down the side that inserts into the carb bore and not the thicker bit that inserts into the top cap? I do remember seeing some minor scratch marks on the thinner section but didn't pay too much attention as it didn't seem to bind when moving by finger but I wasn't watching for travel or comparing to the others.
    The idle speed has come down a fair bit from where it was whilst running like crap however I'll hopefully get it lower.
    Thanks for the speedy reply.

  15. #15
    CB750 Guru amc49's Avatar
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    'you're referring to the part of the slide that has the notch down the side that inserts into the carb bore and not the thicker bit that inserts into the top cap?'

    Yes. Don't touch the bigger part. The lower gouges because you are aluminum on aluminum there, they tend to seize. #600 sandpaper and smooth that out, it just has to be smoother, not disappear. They will always drag there as they lock at decel and high vacuum.

  16. #16
    CB750 Member fg1972's Avatar
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    Engine not running right

    Thanks for the info. Carbs off again, slides seem to be moving smoothly and not binding despite the minor scratches on the small diameter section. Anyhow I lightly sanded smooth with 800, 1200, 2000 and they glide silky smooth. Refitted and start up, touched up the carb sync. Idles well but even slightly touching the throttle and the revs donít come down unless pressing on the sync tabs like before. Interesting observation, turning the idle screw out where it wonít idle but using the throttle to keep the engine idling, blipping the throttle works great with no hanging. Turning the idle screw in to get it idling by itself then blipping the throttle brings us back to the hanging issue. Iím not sure what is the difference between using the idle screw or throttle position to hold the idle but itís almost like it needs to be able to idle lower via the idle adjustment for this to work. Iím getting close but this has me scratching my head.
    Cheers
    FG


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  17. #17
    CB750 Member fg1972's Avatar
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    After a good nights sleep and clear mind this morning, I checked the choke cable to ensure it wasn't engaging the choke when choke was not required which was ok but I loosened it off just a touch in case there is a slight tension on the cable when things heat up.
    Pulled the spark plugs, they were dry but very black which looks like it's telling me its too rich. Connected up my Gunson colortune to each cylinder which confirmed my rich theory so I adjusted the pilot screws to lean it up as per the colortune. Doing this allowed the engine to idle lower so i could unwind the idle screw out a bit and still hold a steady idle. Now I'm pleased to say that it runs nicely with no hanging idle !!!.
    Big thank you AMC for your assistance.
    FG

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