Carb issues I think! (1981 CB750K)

[Cleric]

Hello all, I'm new to the site and I've enjoyed reading ya'lls post.

Last year I purchased a 81' 750K, it ran but it would "bog" down every time you got gave it any throttle.  It doesn't do it as bad when it's in full choke but still it doesn't have a smooth acceleration of RPM's.  I bought a carb rebuild kit last week and pulled the carbs and replaced all the jets, gaskets and needles, cleaned up the bowls and floats and then re-assembled and yet there is still a problem.  I checked the #1 air-cut off valve and it seems to need to be replaced but I don't think this can be caused by air-cut off, but I'm not that great with carbs so...  Anyway, rode it down the road again today and it still has bogging issues.  It seems to happen more when under a load.  I can manipulate the throttle a little and eventually she'll get up to speed, but it takes a while and it's not fun in traffic :-)  So that's my issue, any ideas???

[TOOLS1]

Welcome to the forum. i am not very familiar with the DOHC 750s, but it sounds like a lean condition. Does your carbs have an accelerator pump?
TOOLS

[Cleric]

Yeah tools, It has a accelerator pump on the #2 carb.  I'll have to pull the carbs again and check it's condition.  When I pulled them and cleaned them last I didn't mess with it because I wasn't sure what it was and I didn't have any replacement parts in my rebuild kit for it.  I've seen some rebuild kits for the pump itself around the internet for $20 or so.

[TOOLS1]

That's about right, for the price. the problem is the two little balls. They get corroded, and gunked up. One is in the little pump cover, and the other is in the float bowel. You have to drill a very tiny hole in the plastic plugs (use a pin vise like the ones that you use to clean torch tips.) Then insert a No# 2 sheet metal screw, and work the plug out. Dig the ball out, and roll it on sand paper until shiny. Clean out the port, and seat. Then reassemble. Easy as that
TOOLS

[Cleric]

Is that procedure in addition to a rebuild kit, or in replace of buying a rebuild kit?  Sorry if I'm not understanding...I don't know what balls you you're referring to.

[TOOLS1]

If the rubber diaphragm is still in good shape, you might not need a kit, but either way I'll guarantee you will need to free up the little balls. One is located in the little cover that goes over the pump diaphragm. When you remove it, you will see a little plastic plug in the very center of it on the inside, The little ball is under it. You can check it by blowing air through the hole that is between the two screw holes. Air should go through when you blow, but not when you suck through it. The other is in the float bowel. It is on the rear, and there is a little o-ring where it seats against the carb body. you will see the little plastic plug. You can blow on the hole in the float bowel where the Acc pump attaches. Air should go through when you blow, and not when you suck.
TOOLS
P.S. Yea I know I said"suck"

[Cleric]

Thanks again tools, I found the points you described, but instead of plastic plugs, mine are brass.  I tried messing with them a little and but I didn't want to mar it up.  I figured I might be able to use the procedure you described, but I'd have to drill a slightly bigger hole in it with a drill and then patch the hole or get another plug.  Regardless of all that, I sprayed a little carb cleaner into the holes and they seemed to be blocked at first but I guess the cleaner and the pressure freed it up.  I ran a little more cleaner through it until it was free flowing then I ran some compressed air through it.  The flow did work only one way so it seems to be working now, I'm guessing the pump is mechanical by the look of it, so I'm gonna put some gas in the bowls tomorrow and test to see if it's spraying fuel when the pump is activated via the throttle.  If that fails, i'll attempt to clean the balls.

cleric

BTW....Yes, I did say I was gonna clean balls!

[TOOLS1]

ROFLMAO
TOOLS

[Cleric]

Well, It's running much better, but I still have a little bit of hesitation.  I think it might be related to the Pod filters that were put on it, I have the stock air-box, so I might put it back on and see if that makes a difference.  I also have a couple of exhaust leaks, but I don't know if it's enough to make a difference.  Anyway, it's rideable now.

cleric

[TOOLS1]

Try raising the needle up one notch.
TOOLS

[Cleric]

Apparently my needles are not adjustable!  I guess that's why I see a lot of guys saying that Pod filters are not great on DOHC's.  I opened up my fuel/air pilot screws some and it helped out a lot but it still isn't right.  I guess maybe I should try to re-install the original air box, or I need to re-jet it.  Decision's Decisions

cleric

[TOOLS1]

If you are brave. You could get a set of numbered drill bits, and a pin vise, for them, and start hogging them out yourself. that's how we used to do it years ago.
TOOLS

[Cleric]

Well, I put my stock air box back on and I took it down a little side road next to my house, and it took off like a scalded dog!  It was perfect.  So I grabbed my helmet and gear and decided to go drive it a little, and as soon as I pulled out of the drive way, it started hesitating again.  I thought one of the rubber sleeves may had come off the carb or the air box, but that wasn't it.  So I'm back to square one.  It seems like it has to be something in the carburetion, I guess it's just beyond my limited knowledge.

cleric

[Cleric]

I rebuilt my carbs from the bottom up.  New air shut-off valves for all 4, new o-rings for the tubes connecting the carbs, cleaned all the jet that I just replaced for good measure and changed the gaskets and needle valves.  So...it seems to be running much better and I'm not having bogging I was having before, it runs smooth sitting in the garage.  The problems come when I'm riding it.  It seems great until I get to 4th and 5th gear, and it's like I have no power...I can't get over 60 unless I down shift to 3rd and run the RPM's up and then shift, but it still eventually drops down to about 60 mph.  I'm about ready to just buy me some Keihin CR Special carbs, but I"m not entirely sure it's a carb problem anymore.  Any suggestions or ideas?

[TOOLS1]

What kind of ignition system do you have?
TOOLS

[Peterwh]

Have you balanced the carbs?  I bought a 78 SOHC a while ago and it had hesitation issues.  I checked the carbs and the balance was way off.  Once balanced it was fine.

[Bill B]

I have an 81 750 and I THOUGHT I was having carb issues until I found out that my rotor was bad on the electrical system.  Believe it or not, it had a massive impact on the operation of the bike.  Mine would work great up to about 1/2 throttle and then it would start to bog down.  After the replacement, the bike ran fine up and down the rpm range.  I still have some minor carb issues and I have found out that carb cleaners can really screw up the elastomers in the carbs that you can't get out.  I have heard that the accelerator pump is definitely hurt by them.

[TOOLS1]

It's the ETHANOL in the gas that screws up the rubber parts on these older vehicles, for no longer then carb cleaner is in the system it does not have time to hurt anything.
TOOLS

[kramit]

I have a 1981 CB750C. and was wondering if you got your bike to run any better. My bike has the exact carbs Keihin 42A . On top of these carbs there is a automatic petcock valve looks like a octopus, which only allows gas to enter carbs when starting. This part has been known to starve carbs for gas. If you have this on your carbs throw it out, and run the gas line, and put in fuel filter directly to the carb gas inlet tube. They put this on so you would not have to turn your gas petcock off manually evertime you park the bike. There is a small black hose that will need to plug that comes off the number 2 carb, I cut the tube about 1 and 1/2 inch plug with bolt or screws. This may not solve your problem, but this part
on all web sites will eliminate this to narrow down carb issues. These carbs I had to take apart a number of times, and they can look clean, but there are a lot of very small passages, and they need to be cleaned.
The pilot screws have tabs to keep from adjustment just cut small tab off with drimel cutting wheel, I have mine set at 2 and a 1/2 turns out I run stock pipes with pod filters so this can be set diferent if running header exhaust. It sound like 1 or more float bowls are running out of gas, runs great then bike needs more gas. other causes can be your accelerator pump as others have mentioned. These carbs were the sync-screws ever removed becase this would throw off carb being in sync. Also check plug wires,and put in a new set of plugs they are not expensive. If you have rubber hoses on bottom of carb, get a container, and make sure each bowl has gas comming through open 1 at a time, and watch flow.

Good Luck
Kramit

[kramit]

Sorry but the pilot screws are set at 1 and 1/2 turn out, not the 2 and 1/2  mentioned above.

Kramit