Carb adjustments? 1979 DOHC

[lvin4jc]

I have searched the internet to find a nice diagram of where the adjustment screws are on these carbs and I haven't found anything great. Also, I was born in '85 and while I like carbureted engines, carbs are just before my time and no one has really shown me how to adjust them properly.

My '79 came with pod filters. They are taped down to almost nothing so that it will run with them on. However, I pulled the plugs today and they show as being a bit carbon fouled. It's my understanding that this makes sense and indicated a lean condition. Also, when decelerating in gear the bike backfires. That all sounds consistent with what i've seen researching the problems with pod filters.

The bike runs well, don't get me wrong, it will go where you need to go. Starts right up every time and sounds good too. But I know it's not the way it should be. I will probably look into finding a stock airbox and going back that route.

In the meantime, can I make it run a little richer to make up for the filters? Should I tape them up more so the filters take in less air?

Also, the bike didn't want to idle last week and I got to messing with the adjustment where the throttle cable goes into the carb bank and the idle air mixture knob (the only adjustment knob/screw I have located on the carb bank[Black plastic knob underneath the carbs in the middle]). I did manage to get it to idle better but i'd like to know how it's supposed to be adjusted. Am I supposed to turn the idle air screw until the rpm's peak and then adjust the throttle linkage back down to 900-1100 rpm's or do it another way?

Thanks guys!

 

[amc49]

Lost, oh so lost....................The black plastic knob is idle SPEED. Once set the cable where it goes into carb bank never gets touched. All it will do anyway is mess up the black knob, they affect same thing. Idle mixture screws are underneath each carb toward the front and easily broken having limiter caps on them makes it even easier as you cannot turn them more than maybe 3/4 turn without hitting the cap, an EPA thing for emissions.

This.............

'...a bit carbon fouled. It's my understanding that this makes sense and indicated a lean condition.'

Dunno where you got that but absolutely WRONG, carbon fouling is RICH.

The backfiring is either that and wetted out plugs or at least one of the air cut valves has gone bad. Look at left carb sitting on bike, the two bolt circular cover on outside facing side has a diaphragm under it that richens when you are on decel to again lower emissions, with them failed the bike will backfire when you roll off the gas to decelerate.

Service manual here, join for free and download it and tells you how to adjust the carbs..............

http://www.cb750c.com/modules.php?name=Your_Account&redirect=viewtopic&t=301

Manual is in the technical documents section. Use the CB750K one.

 

[lvin4jc]

Lost, oh so lost....................The black plastic knob is idle SPEED. Once set the cable where it goes into carb bank never gets touched. All it will do anyway is mess up the black knob, they affect same thing. Idle mixture screws are underneath each carb toward the front and easily broken having limiter caps on them makes it even easier as you cannot turn them more than maybe 3/4 turn without hitting the cap, an EPA thing for emissions.

This.............

'...a bit carbon fouled. It's my understanding that this makes sense and indicated a lean condition.'

Dunno where you got that but absolutely WRONG, carbon fouling is RICH.

The backfiring is either that and wetted out plugs or at least one of the air cut valves has gone bad. Look at left carb sitting on bike, the two bolt circular cover on outside facing side has a diaphragm under it that richens when you are on decel to again lower emissions, with them failed the bike will backfire when you roll off the gas to decelerate.

Service manual here, join for free and download it and tells you how to adjust the carbs..............

http://www.cb750c.com/modules.php?name=Your_Account&redirect=viewtopic&t=301

Manual is in the technical documents section. Use the CB750K one.

Well if I wasn't lost then i'd be the guy answering the questions, lol.

You are right about the plugs, maybe I didn't get enough coffee? They are dry fouled but that may be too many short trips around the neighborhood and not enough longer trips too, i'm still practicing riding, this is my first bike.

Actually that's great news about the black knob because the idle speed was my only concern at the moment. Last time I rode I set that once the bike was up to operating temperature so I think that'll be good.

Once I get out on the road i'll get a proper plug chop and go from there. If I can get the backfiring fixed (i'll check that diaphragm) then I don't have any other complaints with the carburetors, especially since they are running pods. I would like to mess with the carburetors as little as possible.

 

[Dave]

PS That black knob is large and easy to get to for a reason, so you can use it often. You'll find it's easily reachable while sitting on the bike.

It's normal to turn up the idle when the bike is cold (on cold days) to keep it from stalling until it warms up, then adjusting it back down later while sitting at a stop sign.

 

[lvin4jc]

PS That black knob is large and easy to get to for a reason, so you can use it often. You'll find it's easily reachable while sitting on the bike.

It's normal to turn up the idle when the bike is cold (on cold days) to keep it from stalling until it warms up, then adjusting it back down later while sitting at a stop sign.

That makes sense. It was a little warmer when I got it two weeks ago. Then it cooled a bit (and got used instead of sitting for the winter) and it just wouldn't idle for me. Otherwise it hasn't given me any issues other than the aforementioned backfiring. Well I appreciate your comment because I won't think it's a problem when it happens in the future.

 

[amc49]

Nah, set the idle with that hot only. The choke if working right has a fast idle mechanism built into it that bumps cold idle speed up. If that is not enough then a cruise control makes short work of that. Cruise control used for warmup then flip it off to have normal hot idle. Then you don't have to do the double back stretch down burned fingers shuffle.