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  1. #1
    CB750 New Member umacs4's Avatar
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    1981 CB750C custom carbs

    Hello everyone. I have bought an 1981 CB750c four custom Honda and have had the carbs gone through with kit for the bottom side (jets,orings etc) then turned around and replace air chut off valves and throttle tube orings on the set. Replaced the boots on both sides and I still cant get a vacuum check on number 4 carb when trying to sync them. Cylinders 1-4 running hotter than 2-3. Backfiring from cylinder 4. Reshimmed all the valves to speck and that didnt help either. Three diferent techs doing all this in the last 6 months and we are at a lost. Any help or suggestions are welcomed.

  2. #2
    CB750 Guru amc49's Avatar
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    Get a solid series of compression readings to make sure you are not wasting your time. If #4 is backfiring due to plug there being fouled then you will NOT get any vacuum reading of meaning at all, the cylinder is not working enough to. Plug black or wet or both is fouled.

  3. #3
    CB750 New Member Madfox22's Avatar
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    Correct amc49. Compression test is next. You don't want to pay much attention to what the books and most people say about what the compression is supposed to be. The true compression reading is dependent upon many things. Such as, amount of charge in the battery (cranking speed), throttle position, miles on bike, and other factors. In this case you are trouble shooting a problem, so the compression check needs to be looked at as a comparison. All 4 cylinders need to be fairly close to each other in the amount that the gauge reads. If one cylinder is a lot lower then the others, that is a problem. Valve not seating properly, broken ring, gaulded piston, etc. If all cylinders measure within, i'd say. 10-15 lbs of each other, look elsewhere for the problem. Main thing is to have all spark plugs out when measuring the compression to avoid having the pressure cross from one cylinder to the other in the chance of a blown head gasket between the cylinders. If the compression on all cylinders is a little lower than book specs, not to worry, if all cylinders are a lot lower then book specs, worry! All this is assuming that whoever rebuilt the carbs knew what he was doing, they are tricky.

  4. #4
    CB750 New Member umacs4's Avatar
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    Thanks guys i'll check with the tech to see what the compression has been. I know he has done that and a leak down test and it never repeats the same numbers or results. One time holds the next time it doesn't. On the compression two cylinders are the same and the same with the other two but 10 lbs or so diference and two are running hotter than the other two by 20 degrees or better. This is my memory ill check numbers for sure and thanks for answering the post. It helps to know your heard when sometimes it doesn't feel like it.

  5. #5
    CB750 Guru amc49's Avatar
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    I can tell you that book spec optimum compression is around 170 psi and 100 psi is a junk motor that never will run right ever. 130 or higher is fine. The problem with compression testing is repeat reliability and why I always run like 3-4 times on each cylinder as backup and using nothing but a screw-in type gauge only, the push to seal ones simply drive you nuts. I use a car battery as well and all plugs yanked so motor spins the same on all that spinning. A car battery IN THE CAR works fine but NEVER start the car up doing it, that can easily fry the alt or wiring in bike.

  6. #6
    CB750 New Member umacs4's Avatar
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    Well guys its been awhile .I talked to the tech and #4 compression checked 128, 1-3 148. cant get a vacuum in #4 but the others are synced. checked timing and it seems to be good.with the engine running and pull plug wires on 2&3 and it continues to run but pull either #1 or#4 and the engine dies. While feeling the exhaust pressure out of mufllers the right side is hard while the left is more of a puff. It seems 1 and 4 are the only ones firing. There is good spark down to plug but there's seems to be no fire from cylinders. I dont know if its an electrical or a fuel problem.I'm getting different readings on spark plug wires at idle on my gauge. should read 1400 rpm at idle speed and 2-3 were reading 2800 rpm and it wasn't doing that but that is what it read and still backfires out of right exhaust pipe. Stumped

  7. #7
    CB750 Guru amc49's Avatar
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    Run the compression check again, the numbers make no sense. Think about the odds of ALL 4 cylinder readings ending at an even 8 for a number. Impossible. Suspect the gauge too, common for most gauges used to be garbage.

    Explain further what 'can't get a vacuum' means in greater detail, can be more than one thing.

    'I'm getting different readings on spark plug wires at idle on my gauge. should read 1400 rpm at idle speed and 2-3 were reading 2800 rpm and it wasn't doing that...'

    What does that mean too? I didn't understand it. Standard idle speed is around 1000-1200.

    Could easily be dirty carbs too, the bikes are known for it and often they need to be cleaned more than once to work right.

  8. #8
    CB750 New Member umacs4's Avatar
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    Syncing the carbs the vacuum on 1 thru 3 on the dial was 30 on #4 it was 0..no movement. tried to adjust the jet in (all the way) or out and it didnt change nothing on the running bike.we had the idle speed set high then we were checking rpm around the plug wires with some sort of gauge that looked like what you put around a wire to see if there is current and could not get it to be solid. 2-3 were bouncing between 1400-2800. #4 was 123 #3 150 #2 150 #1 149. for some reason cylinders 2 and 3 are not running right. we're trying to find out why. thats why were checking spark and fuel problems to see which is the culprit. if theres spark and no fire does that mean no gas leaning towards fuel but cant rule out electrical. (coils,spark plug wires and boots etc)

  9. #9
    CB750 Guru amc49's Avatar
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    Take a working vac gauge hose like #3 and trade it with #4 and see if anything changes. Then another to be sure. Same with coils, swap the two coilpacks out and see if anything changes there. Set all mixture screws at in lightly to hit and back out 2 turns and leave them there.

    Checking current in plug wires is witchcraft. Check the wires each for resistance at 5K ohms max per foot of length or less and then begin to suspect coils, the plugs are easy enough to weed out.

    Get engine up and running to as warm as it will go and then kill it to then shoot a SHORT burst of starting fluid into back of #4 carb and then quickly trap it in place with the choke closed and then crank bike up while dropping choke back off to see if #4 suddenly hits for a bit, that will give you some clue as ro if a fuel issue.

    You probably need to know if not already that the pairs of plugs ground each other, meaning one not grounded right then messes up spark on the other. One plug of each pair fires BACKWARDS if you look close enough, from the outer bent wire to the center electrode.

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