Okay this is getting annoying

[Seacat]

To start I have a 1981 CB750C.

The first problem was number four cylinder stopped firing. I checked things out and pulled the carbs. I cleaned the carbs and put them back on. (Oh did the carbs ever need cleaning.)

Now number four cylinder was firing, but number three wasn't. (As noted by checking the exhaust pipe temps.)

The wires and boots were bad on the plugs, so I replaced them.

Number three still wasn't firing. I'm getting spark on number three and there is gaas in the bowl.

I pulled the carbs again. Maybe I screwed up putting the carbs back together. I pulled number three apart again and cleaned it once more. (I can't truly clean the low speed jet as it's a press in. I did dump a bunch of carb cleaner through it though.)

Tomorrow morning I'll put the carbs back on and see if the problem is fixed. I'm hoping so.

The only other check is to swap the wires. Attach the wire from Coil Lead one to plug three and coil lead two to plug one and see if that changes things. Maybe it's the coil or the coil attachement. I just don't know at this point.

Any other checks or options? I'm open to suggestions.

Cat

[Seacat]

Well. I put the carbs back on with the same results. I swapped the wires from cylinders two and three and two ran just fine so now I know it isn't the coil or the connection.

Okay so here is what has been done so far. Oh and the bike is a 1981 CB750C.

Compression Test:
Cyl.#1 170psi. Cyl.#2 170psi. Cyl.#3 185psi. Cyl.#4 180psi.

Spark Plugs and wires have all been replaced. Coils checked.

Carbs pulled and cleaned twice.

Boot checked for leaks by spraying with WD-40. No change.

Carb. is getting gas. (There is gas in the float bowl and I can smell gas on the Plug when I pull it even though it doesn't look wet.)

Today when I was running the engine I heard what sounded like a couple of backfires through the carb. A short sharp hissing accompanied by a misting of the WD-40 I had previously sprayed on the boot.

To say I'm now confused would be an understatement. I have no idea what to check next. Any ideas? Or should I just pull the carbs again and drag them down to the nearest shop and have them professionaly cleaned?

Cat

[ScottE-2]

HI, It sounds like you have clogged jets to me.what will work for you is a set of welding torch tip cleaners they come in assorted sizes,that is what I used when my jets got clogged,run them through the jet to clean it,they are small in diamater Good Luck Scott.

[cbrydr]

carbs are a pain- i had mine off several times . small idle jets can be pulled out with a pair of regular pliers by twisting back and forth while pulling up - clean and then tap back in with a small hammer.these jets have a small hole that goes all the way through the only way i"ve found to clean them is to take a small piece of an electric guitar string clamped in a pair of vice grips and work it through the hole.i use about a 2 or 3 inch piece of the smallest string on an electric guitar. hope you have good results . god bless you.

[cbrydr]

IDLE JETS CAN BE PULLED OUT WITH A PAIR OF PLIERS -TWISTING BACK AND FORTH WHILE PULLING UP --AFTER CLEANING TAP BACK IN PLACE WITH A SMALL HAMMER .I USE A 2 OR 3 INCH PIECE OF THE SMALLEST STRING ON AN ELECTRIC GUITAR CLAMPED IN A PAIR OF VICE GRIPS. WORK IT BACK AND FORTH UNTIL IT GOES ALL THE WAY THROUGH THE JET --SHOULD TAKE CARE OF YOUR PROBLEM---GOD BLESS YOU.

[sasquatch-2]

Problem could be in the plug wires.Try pulling the plug boots off of the wires,trim about a 1/4 inch off the wire and reassemble........................

[gonebiking-2]

Why not just replace the plug leads with new ones insted of working with old electrical wiring that probably has become brittle over the years and may have cracks in the cables  - thats they way I would go

[Seacat]

Cylinder three is firing.

Replaced the wires and caps. Rebuilt that carb. Still didn't fire.

Kept the bike runnin and started turning in the Pilot Screw. Finally got it firing. Played with it a bit and got the Cyl. firing normaly.

Now I have to play some more and synch. the carbs. While I'm doing this I'll play with the pilot screw and lean out the cylinder.

At least it's firing now.

As close as I can figure, te previous owner synched the carbs without cleaning them and this one got out of wack. We'll see.

Cat

[gonebiking-2]

its the little things with these carbs that tend to annoy all of us in this forum. It seems to be the major topic since the forum first started. When they are new they work like a dream, but when the get neglected and dont deliver, thats when they become tempremental and drive us to the funny farm ( Lunatic asylum ) out of sheer frustration. GLad you go it sorted out. Get it properly synchronised, and hope fully it will run like a dream

[cbRider]

>IDLE JETS CAN BE PULLED OUT WITH A PAIR OF PLIERS<

Glad you've got it sorted out. I am still struggling with my carbs. Compression is good but I can not get #3 to idle. No reaction of the idle air screw. No vacuum leaks on the boots. Throttle or air flow is balanced.
The idle jet on my carb sits deep in a well and you can't get to it with pliers. We must have different carbs. Mine are VB4B's.
After the third removal, cleaning etc I feel a stick of dynamite would do the trick.
I just got this bike and it has been nothing but a pain.

Karl

[gonebiking-2]

I guess I spoke to soon. Now we run into a problem in that you have the VB4B carbs and my bike has the VB52A carbs, and the only problem I have had has been dirt in the float chamber causing it to leak. I usually drain all the fuel out of all four carbs and collect the fuel in a glass jar. Amazing to see what residue comes out of them . Since installing a fuel filter no more problems, although the carbs were rebuilt over a year ago. Depending on how much you want to spend and what you want to do with the bike, I would buy a new carb of the same make. You can buy them individually, or a complete set of four ready to just slip on and replace the air box etc. Have a look at www.hdlparts.com , type in the model Honda and year and then go to the carb section and see what they have. Been a while since I`ve been in the website, but am seriously looking at spending the $1000 + for the entire set of four. They will outlast me thats one thing I know. I wish I could be of more help, but it seems that the idle jet is the culprit and has lodged itself deep in the housing. Maybe worth your while in drilling the jet out without damaging the wall where the outside of the jet fits into the housing. How do the other idle jets sit in relation to number three carb ????. Maybe heating the carb and tapping it may loosen the idle jet sufficiently to be able to pull it out with a pair of thin long nosed pliers

[cbRider]

My mistake, sorry.
The carbs are >VB42B<, I left out the 2 in my post.

Karl

[woodsrider250-2]

http://cgi.ebay.com/ebaymotors/HONDA-CB750-CB-750-CARBS-CUSTOM-cb750c_W0QQcmdZViewItemQQcategoryZ10064QQihZ013QQitemZ230312267517QQrdZ1QQsspagenameZWDVW

http://cgi.ebay.com/ebaymotors/HONDA-CB550-CB650-CB700-CB750-F-K-C-REBUILT-CARBS-w-EXC_W0QQcmdZViewItemQQcategoryZ35597QQihZ023QQitemZ360113515153QQrdZ1QQsspagenameZWDVW

[gonebiking-2]

Hi Karl. "Woodsiderider " has submited a post. Geeez mate, if I were you ,instead of mucking around and giving your self a heart attack over the problems with your carbs , get a set of fully rebuilt carbs from the guy on E-Bay !! Thats what I would be doing in tripple quick time.
My workshop manual lists the make or serial number of your carbs but also have a VB42A and B  series, but it states they are for the CM model. Darned if I know what the CM model is as in Australia we have the VB52A carbs. But seriously I would exchange your carbs for the ones that are up for grabs on E-Bay. Pity they were not on the Australian Ebay website otherwise I might have got a set as well. Let me know how you go with the carbs. I think that will solve all your problems

[sasquatch-2]

gonebiking wrote

 am seriously looking at spending the $1000 + for the entire set of four.

If your willing to spend that kind of money for carbs, go to Cyclex and get these
Or Ken also has  " Honda cb750 Keihin CR Special carburetors.
We are offering these carburetors in 29mm and 31mm for you Cafe and stock Honda framed enthusiasts.
We recommend phone calls for exact requirements.
$ 695.00           # KCR-21     " ..............................................

[sasquatch-2]

If you want to use rebuilt carbs,go to Hondachopper.com ,and get ahold of houndog750 .He does some amazing things with these carbs and doesn't charge a whole lot to rebuild them for you.Of course when you get them back,you my be torn between running them,or framing them and hanging them on the wall as purty as he makes them..........................

[woodsrider250-2]

The SOHC carbs are different, but I would imagine eh could rebuild them just the same!