750 Carbs (Again)

[RCOBill]

Hello all, new to the site and first post..  (1981-82 CB 750 all stock)
Another VB42A carb problem (I Think) to resolve. I can turn a wrench and have rebuilt a few carbs in the past but the VB's are a challenge for sure. Buddy's bike and this started as a favor to rebuilt carbs that is now on its 4th month and still on SQ#1.  Bike sat for a year with fuel in the tank and the fuel valve open, the result was green junk and a mess. Went through the rebuild process three times, new O-rings, new jets, maim and idle, new diagrams and gaskets. Checked the rubber boots between block and carbs, returned the setting back to where they were before stating, used fresh fuel and still will not run at idle, mid-range or high RPM. Starts on full choke and dies, pump the throttle, revs and dies. It will keep running with short sprays of starting spray. Running 150 to 152 psi on each cly. and good suction if you crank it and cover the intake on each carb with your hand. The spark is just a little weak in that it’s not that bright blue flash you like but it does start and run, so it is there. Same feeling with the timing, it can’t be that far off that it will not run, I’ll set that with a light once this problem is resolved and it will idle. I have the carb section of the manual and did checks the bench adjustments as directed. I even used a shop vac and tested the carbs on the bench, full choke plenty of gas, no choke and the pistons pull up to open the mains, plenty of gas. Not a real safety minded check but it worked.  So, with all that said, what is common to all four carbs that I have missed. Additional questions, are the overflow tubes in each bowel also the vents to allow air in as fuel is sucked out? This has the plastic floats, how are they adjust, I don’t see a way to bend anything to change the setting. Last, checked to see what the points looked like, left side eng. small cover removed, found the spark advance in there, no points, I also found oil, not much, but I don’t think there should be any, is that correct and is the electronic ignition?  Also, the throttle pump balls on #2 carb, are working as it pumps fine, visible in the all four carbs and when pumped it revs. I’m lost as this point so please help.. Thanks

[Lucky 1]

These are CLASSIC signs of clogged up idle jets.  

The hole in a stock idle jet is only .0013 thousandths!!

You will need to remove the idle jets and clean them with a #10
electric guitar string and carb cleaner with compressed air.

NO...A paper clip is way too large, and will damage the soft brass.

Hold them up to daylight and make sure you can see a hole in them.

The engine will idle with mixture screw and idle jet.
USe the stock mixture screw setting.

You need to get a workshop manual or down load the pages on carbs for your motorcycle.
A MUST HAVE.

[seestheday]

Yep, your idle jet sounds plugged.

If you don't have the MacGregor carb manual, get it and follow it. Search for the thread on this forum called dohc carb cleaning for many more details.

Floats aren't adjustable on these carbs.

Oil behind that cover is normal.

[Hoosier Daddy]

Both have good valid suggestions above, I'll also add about the SOS (Spawn of Satan). If this auto fuel valve is not working properly, you will not get good fuel flow to get the carbs. Without the constant or restriced fuel supply and it would try to start, may even briefly run but die, especially on either just as you described.

[RCOBill]

Sorry, for the edit in a hurry here at work.

OK, checked idle jets, not clogged all four carbs. rechecked all the jets and found nothing clogged. Reset and checked everything per the book. Used clean fuel from clean gas can and not the tank and still no joy. It's not getting fuel for some reason, plenty of fuel in the bowls, floats are not the adjustable type. It will rev and sounds great with spray, stop the spray and it falls off sounds like fuel starve.  Pump the gas and it comes back up but dies again and will not hold the RPMs. I can see the pistons lift with the spray as it revs up and it still looks like a carb problem. I have checked the boots, yes they need to be replaced, old and hard but no visible cracks and if there was a vacuum leak it should still run just badly. Must be useable spark as it runs on spray so, what did I miss here.  Ready to move on to a different set of Carbs and give up on the VB42 A set.. What can I replace them with ??  (1980/ 81 bike)

Will trade carbs my VB42 A with good partsm rebuild kit and old CB 650 carbs. will also toss in a set of Mikuni fuel injection bike carbs in excellent shape for something that will work on a CB 750, must be ready to bolt on, I’m done cleaning..

Anybody interested ???

[Beekeeper]

Will the motor do much more than rev and die or pop without the airbox choking things down?  I've never tried running mine without it, so I don't know.  Some of these guys will be able to tell you.

[motogrady]

Well, I'll tell ya what worked for me after 2 years of the choke and high idle war.
Getting the stuff out of the small passages that
take the fuel from the jet to the carb opening.
There are 2 or 3 really small openings, one of them on mine
actually sits under the edge of the choke butterfly.
You hardly see them.

I used an e string off one of the guitars, the thin one, with starting fluid.
Squirt, probe with the wire, then repeat each carb 3 or 4 times.

Then 100 psi of compressed air, get a rubber nozzle that will fit right against the throttle body,
right over the hole.  Go to the jet side, blow it out from that side also.

There's prob other ways, think about it, you have a real small passage maybe 1 inch long,
with turns in it, that is full of grease like varnish at best.  The stuff might have gotten hard,
heaven forbid, but that's what you are up against.


I still can't believe it when I start mine up now, or come to a stop, and it idles at 1000rpm.

If you go with something else, let us know how it works out.  Good luck guy.