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  1. #1

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    Default Help/Advice sought

    Hi all, to start I've been lurking a bit here and finally signed up. So, hello! Now that we have that out of the way, I'm having some issues with a CB750 I purchased. Honestly, I feel like I was sold a bill of goods. Anyhwo, I've been having a whole host of issues, but my main one now is that I cannot get the bike to stay above 50mph, even at WOT. The bike is set up with the Cycle X dual Mikuni carbs. I've torn them apart and cleaned them, I've reset the valve tappet gaps. Still not sure what the issue is. I can run it up to 60-70 in 3rd and 4th, but as soon as I hit 5th, she starts to slow. I'm at a total loss here. Outside of the walking down on speed issue, the bike seems to run and shift fine, so I haven't touched the timing or points yet. Is there anything that I'm missing? No brake drag, tires are inflated... I'm lost. I bought the bike under the guise of "the only issue Honda's have, is that they run". Well, I have spent more time wrenching than riding.

  2. #2
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    What kind of breather, What kind of exhaust??? Try advancing the timing some and check the points or both.......

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    How's the compression .. ??

    I had a 76 CB750A .... Did the 836cc Kit to it ... (Boring only)

  4. #4

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    Has a 2 2-1 side exit exhaust. Maker unknown to me. Breather looks like just a run of the mill mini air cleaner style.

  5. #5
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    Guessin' you got the dual VM 34's with the kit:

    http://www.cyclexchange.net/Honda%20...rbs%20Page.htm

    Check the points and timing - when the cover is off, observe for sparking across the points - if so, an indication of bad condensors.

    Got adequate fuel flow to the carbs? Could be float bowls going dry after sustained high speed operation.

    Sounds like the main jets in the carbs needs adjustment - try your WOT with the enrichener (choke) on to see if it improves response.

    Check for air leaks too - at fittings and see if there are any little caps missing on tuning nipples.

    Have you done any plug chops to read what mixture is at WOT?


    Here is some basic carb tuning info which may be helpful:

    "Motorcycle carbs have a few circuits that effect different throttle positions.
    Idle Circuit - for idle and off idle
    Pilot Circuit - small effect on idle. Higher effect at lower RPM's with decreasing effect to full throttle
    Needle jet/jet needle - Mid range circuit
    Main Jet - Wide open. Fuel is metered through the main by the jet needle at different throttle positions.
    Choke circuit - Initiated by a valve or butterfly that increases the vacuum and opens the circuit up. Cold starting and warm up.

    Symptoms of Lean and Rich

    Lean

    Reduced Power - Sluggish at certain RPM's. Wide open throttle yields no power.
    Difficulty Starting
    Spark plugs are clean - No residue. Insulator may be white.
    Runs better at higher altitudes
    Backfires - Popping on deceleration for a lean idle circuit.
    Runs on choke.
    Idles poorly - Fluctuations in idle RPM, stalling
    Engine runs hot
    Hanging idle
    Sharp Odor - The exhaust smell may be sharp and burn your nose.

    Rich

    Reduced Power - It has less power but seems to be OK and runs.
    Reduced Fuel Mileage
    Rough Idle
    Spark Plugs Black
    Exhaust Exit is Black and Sooty
    Odor of unburned fuel
    Runs better with more air - If you remove your air filter and the engine begins to run a bit better
    Runs worse at altitude
    Black Smoke - If you have black exhaust when you rev it, it's rich.

    Rich conditions can be created by people too but the condition is usually caused by:

    Installing a jet that is too large
    Setting a float level that is too high.
    A Choke that is stuck or not turned off.
    A stuck float in the float bowl.

    https://mechanics.stackexchange.com/...n-a-motorcycle
    Last edited by TriNortchopz; 09-04-2018 at 12:10 AM. Reason: too rich

  6. #6
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    TriNortChopz - just... post... the... link.
    Thank you.

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    Quote Originally Posted by 72Savior View Post
    Has a 2 2-1 side exit exhaust. Maker unknown to me. Breather looks like just a run of the mill mini air cleaner style.

    Baffles or straight through exhaust??

  8. #8

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    Quote Originally Posted by Tattooo View Post
    Baffles or straight through exhaust??
    Straight through, baffleless. So, I ran it up to 70 in 3rd on the way home. 4th will slightly lower the speed, and 5th brings it all the way back down to 50ish. Again, I haven't checked the points yet, nor have I done a plug chop. There is some decent soot in the exhaust, so maybe it's rich. I've had cars run rich but never had a speed issue. Really banging my head on this one. This is also my first bike so this is very frustrating.

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    You have a multiple of problems going on...

    No back pressure, air filter to small....

    But before I go on has the bike ever run correctly????? Ever??? Is this something new??

  10. #10

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    Quote Originally Posted by Tattooo View Post
    You have a multiple of problems going on...

    No back pressure, air filter to small....

    But before I go on has the bike ever run correctly????? Ever??? Is this something new??
    That's a good question. I purchased it, under the guise that it did run fine, just needed some carb tweaking. Well, it seems it needs a lot more than that
    Last edited by 72Savior; 09-04-2018 at 3:26 AM.

  11. #11
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    Quote Originally Posted by 72Savior View Post
    That's a good question. I purchased it, under the guise that it did run fine, just needed some carb tweaking. Well, it seems it needs a lot more than that
    Yep just as I thought.... I'll bet it's never run right in that present configuration......

    I've had several of those bikes with that style motor through the years....

    Those motors don't like to much air through the breather and no back pressure through the exhaust....

    Yes I know it can be done but it takes a lot of tweaking...

  12. #12

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    Quote Originally Posted by Tattooo View Post
    Yep just as I thought.... I'll bet it's never run right in that present configuration......

    I've had several of those bikes with that style motor through the years....

    Those motors don't like to much air through the breather and no back pressure through the exhaust....

    Yes I know it can be done but it takes a lot of tweaking...
    What kind of tweaking am I looking at here?

  13. #13
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    Before you do anymore work you need to go back to the beginning.
    Check the compression. Plugs out. Battery fully charged.
    Cam chain slack taken up as the book tells you.

    Adjust the valves the way the BOOK tells you. Intakes .002 and exhaust .004 (IF it has a stock cam!!!)

    Let us know how that goes.

  14. #14

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    Quote Originally Posted by Luky View Post
    Before you do anymore work you need to go back to the beginning.
    Check the compression. Plugs out. Battery fully charged.
    Cam chain slack taken up as the book tells you.

    Adjust the valves the way the BOOK tells you. Intakes .002 and exhaust .004 (IF it has a stock cam!!!)

    Let us know how that goes.
    I thought the exhaust side was .003?

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    Quote Originally Posted by 72Savior View Post
    I thought the exhaust side was .003?
    What does your manual show for valve clearance?
    Tried a plug chop yet?
    Tried with choke on yet?
    Tried with air filter off yet?
    Know what size main jets in the carbs?
    Looked at points and timing yet?
    What's the compression readings?

    Once that's all done, let us know what you find...

  16. #16
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    Plugged fuel filter/petcock? I had a T500 did this, it was a plugged petcock filter. Lossen the gas cap (air not getting in tank).

  17. #17
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    I'm fine with TNC posting content AND the link, because links have a way of disappearing over time.

    More than just thread participants read these threads. Fifty years from now when most of us are dead it will likely still exist via the Wayback Machine and other archives. Post for the ages!

    I agree with checking the petcock. Lots of customer bikes past and present come in with ghastly shit in the bottom of the tank and fouling the petcock. Tiny fuel filters can restrict flow too.

  18. #18

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    I did check the pet cock and it was clear. Oddly mine doesn't have a screen on it, so I have an inline filter. Lines are 1/4" and so is the filter inlet. Could the fue lline size be a factor? Maybe it should be a tad larger? I guess what really confused me is that I can run it up to 70-80 but when in 5th it walks down.

  19. #19
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    Quote Originally Posted by 72Savior View Post
    I did check the pet cock and it was clear. Oddly mine doesn't have a screen on it, so I have an inline filter. Lines are 1/4" and so is the filter inlet. Could the fue lline size be a factor?

    Yes sir, and those inline filters are notorious for causing your kind of problem.....

  20. #20
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    Here is what we heard so far:
    "I cannot get the bike to stay above 50mph, even at WOT...I can run it up to 60-70 in 3rd and 4th, but as soon as I hit 5th, she starts to slow."
    and
    "So, I ran it up to 70 in 3rd on the way home. 4th will slightly lower the speed, and 5th brings it all the way back down to 50ish."
    and
    "I guess what really confused me is that I can run it up to 70-80 but when in 5th it walks down."
    and
    "my first bike so this is very frustrating."


    What we never heard is: what do you do, or what happens after this, in 5th it walks down...

    Does the engine stop? does it backfire? do you have to pull over? pull in the clutch and play with the throttle to keep it runnin'? shift down to 4th or 3rd and ride it at half throttle, or ride it in third at WOT to go 70+? This would be helpful information...with details if you can.
    Is it the same each time? How many times have you repeated this sequence?

    Gotta ask - have you tried anything different - you know what they say about doin' the same thing over and expecting a different result.

    When it does it thing in 5th at WOT, try, just try, to engage the enrichener on each carb, by pushing the levers down - all that will do is allow addition fuel to be fed into the carb - if there is any left in the bowls. Let us know if there is any difference. The carbs operate independently from each other with that manifold setup (not actual cycle-x setup as their photos came up as invalid - go to the link in post #5 to see 'em) :
    Click image for larger version. 

Name:	Mikuni-VM-carburetors-for-triumph-T140-Bonnieville.jpg 
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    Also wondering if the air filters might be the wrong ones for those carbs.

    As noted, it could be lack of fuel; by the time you get it to 5th, there is not enough fuel left in the carbs to allow full throttle operation...Maybe only one carb is not getting enough fuel...
    but we can't be sure, because we are not getting all the details.

    To get it up to 70 in 3rd and 4th, are you at full throttle?

    We are also assuming it has the correct carbs - could be the guy you bought it from just got the 2-1 manifolds from cycle-x and stuck a couple of Mikuni carbs on there from his Polaris snowmachine or watercraft. Or had some new ones that are not the right size for the 750 - can you confirm if they are the VM34s?

    When you had them apart, are you sure you assembled them properly? Floats not in upside down?
    Do you have a Mikuni Manual?
    https://www.mikunioz.com/tuning-tips...ual-downloads/
    and
    http://www.mikuni.com/pdf/vmmanual.pdf


    Can you post a couple pics of your bike here?
    What type of gas tank and petcock do you have?
    Pull both of the fuel lines off the carbs and stick 'em in a bucket, turn on petcock to check for even, consistent and adequate fuel flow.

    Details man.
    Last edited by TriNortchopz; 09-06-2018 at 12:27 AM. Reason: clarified that photo of dual Mikunis is not actual cycle-x

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