CB750 Carbs

[pateli]

 Since I retired my sons bought me a 1982 CB750 as I can no longer kickstart my Triumphs.Since getting it I have been having trouble in that after a few miles it breaks down and changeing plugs (Wet) it runs Ok for a while The Carbs on it  were VB52C jetted P68 S 75. I tried to tune them and found the pilot screw houseing on two damaged beyond repair covered in fillers.I was given a set of VB42A carbs which I jetted as the 52c was P68 S75 the bike runs erratically and wont idle below 1700RPM and is spitting with odd backfire I understand that the 42A has a slightly smaller venturi than 52C. Since haveing the Bike I have replaced the Ign Coils and fitted new leads.Fitted a few sets of Plugs D8ea.A compression check shows all 4 cyls 130 to 150 psi. Valve clearances have been done.Any suggestions what further steps I should take.The Bike is a CB750CB made for Japanese Market imported into US then into Ireland it is Badged as a CB750 Custom Exclusive, Frame RC041023283 Eng RC01E3201156. I have been told that this is not the original Engine if so are the VB52C carbs the right ones. Please help as I am missing out on some lovely runs and at my age(75) there are not many left

[mikey440]

Hello, does the cb have a stock airbox on it or does it have single pods on every carb, makes a big differenc, those carbs are designed to run on vacuum and the only way to get the proper vacuum is to have the stock airbox on it, also the carbs have to be very very clean inside or you will have running issues, suggest taking carbs completely apart and soaking the big parts in pinesol for a few days and blowing out all jets, my jets are primary 68 and main is 102, that is stock for 1982 cb750k model,
 if your plugs are comin out wet then your not hittin on all four, next time you fire it up feel the header pipes 1 at a time till you find one thats not hot, if you just replaced the coils then perhaps theres a bad one or a wire is ajar perhaps, hope this helps, mikey440

[pateli]

Thanks for the info There is a standard air box on the bike with a new filter its not just any one cyl fails it could be any one of them . The bike was the same on the original coils I have checked all the Ign wireing ang replaced some connectors.If I put in 4 clean plugs it runs on all Cylinders but will eventually fail. I have tried 42A carb with P68 and Sec102 it was spitting and backfireing I then replaced the Sec with a75 as was in the original carb and it runs better still spitting with odd backfire.I have been told that the Eng in the bike is not the original so I am wondering what is the correct Carb.The Eng No.is RC013201156My manuals dont help.

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Date: Fri, 30 Apr 2010 19:07:19 -0700
From: [hidden email]
To: [hidden email]
Subject: Re: CB750 Carbs

Hello, does the cb have a stock airbox on it or does it have single pods on every carb, makes a big differenc, those carbs are designed to run on vacuum and the only way to get the proper vacuum is to have the stock airbox on it, also the carbs have to be very very clean inside or you will have running issues, suggest taking carbs completely apart and soaking the big parts in pinesol for a few days and blowing out all jets, my jets are primary 68 and main is 102, that is stock for 1982 cb750k model,
 if your plugs are comin out wet then your not hittin on all four, next time you fire it up feel the header pipes 1 at a time till you find one thats not hot, if you just replaced the coils then perhaps theres a bad one or a wire is ajar perhaps, hope this helps, mikey440

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[davew]

if you keep the vb42 carbs and jet themas vb42 carbs you idle should be about 1100 + -
you will not have any problems just remember to keep the original air box

[pateli]

 I am not with you I have the 42 carbs on the bike jetted P68 S75
and the bike runs but is spitting and some backfireing If i fit as I got them P68 S102 the bike runs like a pig Spitting with loud Back fireing. What Jetting should I use THe original Airbox with new filter fitted. Thank you for suggestion can you elaborate. 

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Date: Mon, 3 May 2010 06:32:11 -0700
From: [hidden email]
To: [hidden email]
Subject: Re: CB750 Carbs

if you keep the vb42 carbs and jet themas vb42 carbs you idle should be about 1100 + -
you will not have any problems just remember to keep the original air box

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[davew]

if you have the vb42's jetted as they shuold be p=68 & s=102 & float set at 15.5
& pilot screw set at about 1 3/4 out  it should be running ok
but if it still backfires i would take a serios look at the valve timing and clearences
you have  ignition in the cylinder and the  intake valve is  not closed ---therefore backfires
through the carb
start from the begining--get an aftermarket manual and follow it to the letter
i say aftermarket because the honda manual has lots of pictures but few words
but "Haynes or Clymer " manuals have lots of words but naver enough pictures

[woodsrider250]

2nd that.  the 68/102 is stock jetting....You have something else wrong like cam timing (since you have pretty much rules out everything else).

[Pat Lackey]

  Thanks for info my plans now are to go back to the begining check everything again inc valve and ign timing fingers crossed I will soon be on the road again. I have Haynes and Clymer manuals
Pat

[davew]

keep us filled in

[Danny Boy]

Pat - just seen your letter in CMM. Was intrigued by your comments about the valve timings. Am trying to to help a mate with an 81 750 DOHC. When I checked his sprocket marks against the timing gear, it was out slightly, too. Wondered if that turned out to be the problem.
Cheers.

[Pat Lackey]

 I am still trying to find the easiest way to reset the timing. Anyone got any suggestions on doing it without rloosening thr Cam shafts.

[mikey440]

 you could have carb float sticking causing a flooding issue, will cause it too run like shit, backfire what not, try tappin on the float bowls with the handle of screwdriver,,mykee440

[mikey440]

 fuel mixture screws 3 turns out ??

[mikey440]

sounds like you have a flooding issue thats fouling plugs to me,
 its definetly a fuel issue i believe, check floats and fuel mix screws...

[pateli]

Thanks fo info Carbs are going for Cleaning I have to abandon due to family illness should get back next week. Does 3 turns not sound too much or did it work for you 
 

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Date: Mon, 17 May 2010 14:20:27 -0700
From: [hidden email]
To: [hidden email]
Subject: Re: CB750 Carbs

 fuel mixture screws 3 turns out ??

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