Question about Honda CB 750 Motorcycles

7 Answers

I have a 1982 CB750 Dohc Frame RC041023283 eng RC013201156 My Sons bought it for me as at 75 I cannot Kick Start my Triumphs which I gave to my sons. The prev owner sold it as he had problems with it . It never ran properly for me it would start missing after a few miles and replaceing the wet plugs it would run ok for a while. I carried out a compression test and 4 cyls were 125 to 150 psi.I replaced the Ign coils and several sets of Plugs.all no good Itried to check how the eng was running using a colortune and found no4 rich which I adjusted.I could not shift any of the other pilot screws and found on removeing the carbs that they were seized and 2 carb bodies damaged beyond repair where pilot jet is fitted.The Carbs on the bike are VB52A With Prim jet 68 and Secondry 75 I have been given a set of VB42A Carbs with Prim jet 68 and Sec 102 and fitted with an acc pump (not on 52A and they Supposedly came off a running bike) I fitted them as is and the bike ran very erratic spitting and backfireing. I would like to Know (1) Can I rejet the VB42A with the Jets from the 52A P68 Sec 75. Or can I split the 42A and use the bodies to rebuild the 52A . Please help as I am missing out on some good runs

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  • 6 more comments 
  • pateli Apr 29, 2010

    This reply refered to a 1975 bike with mine is a 1982 vwith electronic ignition

  • pateli Apr 29, 2010

    this reply is not covering my bike

  • pateli Apr 29, 2010

    According to Honda Ireland This bike was for the *** market as a CB750CB. It is badged as a CB750 Custom Exclusive.Can I use the 42A carbs or should I loook fo a 52A set Probably not available in Ireland

  • pateli Apr 30, 2010

    I have a set of manuals incl. a factory one. Not very helpful with my problem

  • pateli Apr 30, 2010

    I have plenty of manuals dont cover my problems

  • pateli Apr 30, 2010

    Bike Doc gave the best help and wanted bike details which I have passed on.Bike is CB750CB for Japanese Mkt.imported into US then into Ireland it is Badged as a CB750 custom exclusive. I rejetted the 42A carbs as 52C carbs 68/75 bike is running but spitting back and wont idle below 1700RPM I will check Ign timing next

  • pateli May 01, 2010

    Hi Bike Doc Thank you for your helpful info I gather from your last message that the 42A carbs are suitable for my bike as I said the 42A jetted Prim 68 sec 75bike is running but spitting and backfireing occasionallyand will not idle below 1700 rpm.I tried it with 102 sec and it would not run properly very rough and backfireing loudly.Should I go up to next size Sec in hope that it sorts prob. Sorry if I appear time wasteing but I am new to Japs and Multi carbs. I will be accepting your info.

  • pateli May 04, 2010

    Thanks For your suggestions The Back fire is through Exhaust not Carbs. I will take on Board your suggestions I still dont Know if I have correct carbs for this engine

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7 Answers

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The problem here is that VB42A carbs have a 30mm diameter venturi, whilst the VB52A carbs are 34mm diameter so bodies cannot be mixed. Also the frame no. doesnt match the engine no Can you please let me know what model yours is, SC nighthawk as the engine suggests, or F Supersport as the frame no. says It looks like the engine has been changed Ride safe Bike-Doc Please mark this as very helpful (if it has been!!)

Posted on Apr 29, 2010

  • 2 more comments 
  • steve Apr 29, 2010

    Hi
    I must leave for 1/2 hour but will be available for assistance / livechat when I return

    Ride safe





    Bike-Doc



    Please mark this as very helpful (if it has been!!)






  • steve Apr 30, 2010

    Hi again
    I obviously didnt explain myself fully last time
    The CB750C models had type VB42A or VB42C carbs as original, these have a smaller main hole through them (venturi size) than the type VB52A
    UK model CB750F supersports (F-B, F2-C & F-D) have VB52As which are a different venturi size & so let more mixture in, and that is why your plugs are fouling
    I dont know the jets for the Custom models but most CB750s with VB42 carbs use #68 primary & #98/100/102 secondary jets

    So you've got the right carbs now & it sounds like the AIR passageways may need a clean
    Also check the condition of the air cut-off diaphragm where fitted

    I hope this solution is acceptable
    Ride safe





    Bike-Doc



    If you need more info on Custom models I can get it (eventually)


  • steve Apr 30, 2010

    Hi
    Here is a better explanation !
    The
    CB750C models had type VB42A or VB42C carbs as original, these have a
    smaller main hole through them (venturi size) than the type VB52A
    UK
    model CB750F Supersports (F-B, F2-C & F-D) have VB52As which are a
    different venturi size & so let more mixture in, and that is why
    your plugs are fouling
    I dont know the jets for the Custom models but
    most CB750s with VB42 carbs use #68 primary & #98/100/102 secondary
    jets

    So you've got the right carbs now & it sounds like the
    AIR passageways may need a clean
    Also check the condition of the air
    cut-off diaphragm where fitted

    I hope this solution is acceptable



    If you need more info on Custom models I can get it (eventually)

  • steve May 03, 2010

    Hi
    If the bike is 'spitting & backfiring' back through the carbs, then either its firing at the wrong time or most likely an inlet valve is not closing properly, either because the valve gap is too small or because its bent/broken/burnt away.
    For it to backfire out of the carbs then the flame is coming back when the valve should be shut
    The cylinder at 125psi (when cold) is most suspect as its a bit low & when hot the valve gap could easily disappear holding the valve open
    Check & adjust the valve clearances, fit new plugs then synchronise the carbs

    Ride safe





    Bike-Doc


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I would try rejetting and check float and needle height as well

Posted on May 01, 2010

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  • Honda Master
  • 1,851 Answers

Sounds like you have cam timing and or valve lash out of adjustment.

Posted on May 01, 2010

  • 1 more comment 
  • Gary Rice
    Gary Rice May 05, 2010

    Perhaps a call to a local bike shop is in order?



  • Gary Rice
    Gary Rice May 05, 2010

    PS can I get a YES for trying here - ?

  • Gary Rice
    Gary Rice May 07, 2010

    Hey,
    Please vote me up for effort - cost you nothing extra and really makes my day.

    Thanks,
    Gary


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Would love to help another biker like myself who has fallen on this problem of getting old and unable to bounce around on kick starts ,now as for re-jetting these carbs with the jets from the old ones i would go for it and get some newspaper and put them on the kitchen table and have a good play ,but use the secondary jets in the carbs with the accelerator pumps because the pumps push neat fuel in so the larger jets will only make it run rich on acceleration unless you are after having a burn up.And if you are like me then 80kms is my limit as my boy racer days are long over .

Posted on Apr 30, 2010

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I can tell you right now why your bike is spitting a sputtering. The secondary jet is not close enough to the stock carbs it is more then likely getting too much fuel under acceleration. and to answer your other question the jets from your old carb will screw in to your new set for sure. Otherwise if your lucky you can find a jet kit and carb kit online for this bike. I found one for my fathers 82 Honda. You said your jets are frozen seized in the old carb. Soak that carb in mineral spirits or lacquer thinner over night and those jets should come right out. Remember if your carb also has a bad seal and it leaks air it will loose vacuum and also cause a miss. Hope this can help point you in the correct direction.

Posted on Apr 30, 2010

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Maybe a service manual can be of some help
A lot of manuals can be found here including carburetor tips on motorcycles

http://ezinearticles.com/?Find-the-Free-Auto-Repair-Manual-You-Need-Online&id=92717

Posted on Apr 30, 2010

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You can split and make 2 become one as long as the parts would fit in the carb. Also a different carb or setting would mean retuning air intake, idle speed and needle( if it has one). It is a time consuming process but if you are patient with it then you will succeed. Now one spark you mentioned was wet and that may be the case of too much petrol going into the cylinder too. But since you mentioned it is a 1975 model it would have a manual thing in the magneto,which can be adjusted with a screw driver, I cant remember the word for it, but I think you know what I am talking of here. For backfiring etc on these engines it is a possible fault of the mentioned part, o I remembered now, it is called points. Have a look at this tutorial before you spend time with the carb. Remember old engine like this will run badly when points are dirty or not set up properly.I assume coils etc are perftect here.Good luck.

Posted on Apr 29, 2010

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I have a 1982 CB750 Dohc Frame RC041023283 eng RC013201156 My Sons bought it for me as at 75 I cannot Kick Start my Triumphs which I gave to my sons. The prev owner sold it as he had problems with it . It...


Sorry I thought the bike was a 1975 when in fact it is a 1982. I would suggest you check the points and make sure it is clean and adjusted properly. Regarding the carb, you can make 2 become 1 but if the parts will fit and bear in mind that air mixture will require careful tuning since it will not be the same as manufactured default. Back firing is often caused by faulty spark, plugs, coil or even bad ground to engine.Also before you consider doing any work on the carb, try to swap the points to a new one.(I assume it has a points engine due to the year of manufacture)

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