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  1. #41
    CB750 Enthusiast Old Okie's Avatar
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    Been away for a while, lots of company this summer, problems with the weather, hotter than...
    Got back to the bike today and began with a compression check, very disappointing.
    1, 2 & 4 at 120 psi, 3 @ 90/100. What does that mean and what are some of the causes??
    I can do a lot but I am not comfortable rebuilding this motor. Will the bike run at those numbers???
    Going to check valve clearances, may be a problem there. When I started the bike it hissed at me
    through a carb, not sure which one yet, that could be a sticky valve or bad adjustment on #3.
    So much for cleaning this thing up and riding it.

  2. #42
    CB750 Addict Dave's Avatar
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    120 is fine for cold dry cylinders.

    Set the valve clearances on the loose side.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  3. #43
    Moderator dirtdigger's Avatar
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    120 is on the ragged edge for running, 90/100 is a no go. Do check valve clearance make sure not holding valve open but I doubt it on a sohc. Use standard valve clearance on sohc motor, go too wide and you will beat up the valve stem tips.

  4. #44
    CB750 Enthusiast Old Okie's Avatar
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    I will get back on this project on Monday. The bike has electronic ignition, a round black plastic module.
    So there is no points or any of that stuff so what do I use for determining TDC?? I have not looked at the
    stator side of the motor, anything there that I can use?? I need to download another manual, my computer lost the one I had.

  5. #45
    Moderator dirtdigger's Avatar
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    Even with electronic ignition there still has to be a way to check ignition timing.....timing marks which are on the ignition advancer assembly. post some pictures of what you have sometime so we know what you are working with

  6. #46
    CB750 Addict Dave's Avatar
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    Hook a timing light to #1 wire and shine it in the hole. For points you repeat on wire #2 but I can't remember if the solid state units can be separately adjusted.

    Click image for larger version. 

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    @DD I always go 'on the loose side' (but within specs) on OHV Hondas with medium or high mileage (car and bike).
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  7. #47
    Moderator dirtdigger's Avatar
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    @DD I always go 'on the loose side' (but within specs) on OHV Hondas with medium or high mileage (car and bike).[/QUOTE]

    The sohc is not like the dohc with the issues of valve clearances tightening. The sohc like to get looser as time goes on because of all the extra parts associated with the rocker arms. The valves seldom sink in the head so the clearance hardly ever gets tighter unless you get a fluke problem with a soft seat or ingest a lot of dirt and wear the valve face. Honestly cant say I have ever had a valve clearance go tight on the sohc, the rocker pivots seem to wear the most especially on the later models of the sohc where they dont allow the rocker pivot shaft to rotate. The valve stems do tulip and pit from excessive clearance on these...

  8. #48
    Moderator dirtdigger's Avatar
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    There has to be marks somewhere, somehow you still have to do an initial adjustment for the electronic ignition. Even on electronic you still have to be able to adjust them separately as 1,4 still functions as a separate system from 2,3. They still have to do the job independent, still two pickup units and two coils. Hiding somewhere there are timing marks.

  9. #49
    CB750 Enthusiast Old Okie's Avatar
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    Finally back to this bike. How did I get so busy??
    The bike has a Martek 440 ignition, I found instructions but
    the print is very rough and mostly unreadable.
    A good copy of the instructions for this Martek 440 would get me going again.
    From what I can read on the instructions there is a timing mark to set the timing
    and no individual adjustment for 1-4, 2-3??? Still trying to understand this system.

    Edit: Some progress today, starts on choke and then idles fair but stumbles coming
    off idle. Accelerator pump does not work, next thing to fix. Checked for air leaks in
    the intake area, none found. I am testing the bike in the shop with the air box off,
    does that make a difference??? Also added a photo of the ign module, there is a small
    window underneath the upper right screw. I will remove the module, again,
    to see what I can see. If the accel pump works that may help the stumble coming off idle???

    Click image for larger version. 

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    Last edited by Old Okie; 08-22-2018 at 09:36 AM. Reason: Added info.

  10. #50
    CB750 Addict Dave's Avatar
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    I see that small window in your pic is in exactly the same place as the stock one was, just fire it up and shine a timing light in there.

    Having the airbox off makes a lot of difference. Look at the multitude of posts about "airbox vs pods" you'll find here.

    I'm really surprised by your question on the accelerator pump. I'm pretty sure you already know the answer is "hell yes" as that is it's purpose, to keep the engine running when you suddenly open the slides.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  11. #51
    Moderator dirtdigger's Avatar
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    On these carbs as well as the early round top the airbox will make very little difference especially at idle, I run them all the time without so I can see if each accel pump jet is working. These arent the dohc cv carbs, these are old school carbs and can be made to run pods pretty easily.

    The pump diaphrams go bad, there are two check balls that stick all the time, you need the restrictors in the accel pump crossover tubes for the system to work correct. The accel pump is very important on these carbs as they came very lean from the factory.

    Yes your timing mark will be in the window, guessing the stock advancer setup is underneath, make sure it moves free. If it has the stock advancer then you will have the stock tdc mark and fire mark and advance mark.

  12. #52
    CB750 Enthusiast Old Okie's Avatar
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    Yes I understand the accelerator pump, I guess I was thinking out loud with that question.
    I will be able to set the timing, I think. Not too worried about the timing or carb sync, the bike
    was running when parked. My main question is, how do I clean the idle circuit, is that part of the
    slow jet?? Can I clean the idle circuit with the carbs on the bike?? I still have brakes and tires to do
    but I really want this bike to start and run correctly before I start on the easy stuff.

  13. #53
    CB750 Enthusiast Old Okie's Avatar
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    Yes, I am still working on this bike and getting no where.

    Frustration is setting in, she is still not running. So far I have checked the compression, 120 psi, low but should run. New spark plugs. checked valve clearance. removed carbs from the engine(left the rubber on the engine) and cleaned every thing I could clean. Slow jets were plugged, was able to push Marvel Mystery Oil through the accel ports, MMO because my eyesight is very poor and I can see the red oil, new accel pump kit, set floar heights, did not need adjusting, started her uo, same story, starts OK on choke, runs on 1/2 half choke dies on no choke. No fuel through the accel ports.
    Throttle cables hang up with wheel turned right, OK other wise, I can fix that. I can see that these carbs have been worked on, probably several times. I am beginning to think some of the problem is in the carb linkage, the cam not pushing the accel pump, slides seem to work OK but the running on choke has me stumped. Also the float valv on #4 leaks a full stream, has been leakie before. So here I am looking for advice and assistance. may have mentioned it before, I thought the starter was noisy. The problem is that tin can starter cover is bolted to the starter and makes a good resonator. Without the cover the starter is quiet. Any how, that's my story, got any suggestions.??

  14. #54
    CB750 Member pidjones's Avatar
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    Replacement float valves are notoriously bad. This is a part that is best sourced from Honda or even used from eBay if genuine.
    "Love 'em all.... Let GOD sort 'em out!"

  15. #55
    CB750 Enthusiast Old Okie's Avatar
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    These are the original float valves but #4 has caused trouble earlier.
    I may have a replacement for #4. What is the proc edure for setting float heights??
    Some say 12.5 mm and others say 14.5 mm?? Where does the tiny hole in front of the slides go???
    Last edited by Old Okie; 09-12-2018 at 06:11 PM.

  16. #56
    CB750 Enthusiast mrtelec's Avatar
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    my 78k was 14.5 for the floats, with them just seating they should be level with the slow idle jets. I shot carb cleaner through my slow idle jets to make sure it was getting through when I had them out. did you check the accelerator jets in each carb to see if you are pumping fuel through them? when I had the carbs apart I shot carb cleaner through the accelerator pump rail that goes to the little brass tubes in front of each carb. In my case I had one that was plugged so I carefully pulled it out and ran a e string and carb cleaner through to clean it out.

  17. #57
    CB750 Enthusiast Old Okie's Avatar
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    How did you remove the accel jets?? I probably need to do that,
    I have been advised not to remove them but they need cleaning.
    Did you remove the hoses in the rail to the accel jets??

    Some progress, accel jets are working, still no idle, runs good above 2500 rpm.
    I will try to post some pics. Pics did not work, float level by the clear tubing method,
    #1 is about 1/4 inch below the joint between the carb bowl and carb body, #4 is slightly
    higher. Fuel levels may be below the tip of the slow jets.???
    Last edited by Old Okie; 09-13-2018 at 10:45 AM.

  18. #58
    CB750 Enthusiast mrtelec's Avatar
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    I would take them apart and soak each carb first. Make sure you don't lose the restrictors inside the hoses. While you have them apart you can block off 1 side of rail and shoot cleaner in other side to see if they are clear. I took the one out as a last resort as it was blocked after eveything else I did. I used channel lock pliers to press it out and in, somewhere there is a thread or video on it, you have to be very careful cause I'm sure it can go bad quickly if you don't do it right. In my case just gentle and straight force did the trick. On your floats have you tried the clear tube method to verify your float levels? It shows you where your levels actually are set at.

  19. #59
    CB750 Enthusiast Old Okie's Avatar
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    This where I got to today...

    Some progress, accel jets are working, still no idle, runs good above 2500 rpm.
    I will try to post some pics. Pics did not work, float level by the clear tubing method,
    #1 is about 1/4 inch below the top of the bowl, #4 is slightly
    higher. Fuel levels may be below the tip of the slow jets.???

  20. #60
    CB750 Enthusiast mrtelec's Avatar
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    I believe you want about 2 mm below top of bowls so you should be pretty darn close without seeing a picture. do you have the air box and filter on the bike? if so once your running spray a bit of carb cleaner on the boots to see if it revs up, if it does you have an air leak around the intake boots. do you have the boots clamped tight?

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