Rear brake dragging, & smoke from one exhaust pipe

[NoodleBeard]

Hello my 81 CB750F seems to be dragging its rear brake a little bit is there an adjustment for it that I am not seeing, the rear brake pedal was frozen I had to redo the bearing and such and that works now but the rear brake seems to always be dragging(brake light is not on, so i don't think its an issue with the brake pedal) not sure if there is an adjustment or if its from it sitting and the caliper needs to come off and be cleaned.

Also finally got the bike running, little cold blooded but it runs OK after it has warmed up, I have noticed as its warming up its blowing smoke out of the muffler on the right side, I assume cylinder 3 and 4. Smoke is grayish black, not a ton but its noticeable, the other pipe does not have smoke coming out of it. The pipe that is blowing smoke you can also hear some popping, or misses? hard to tell. Been through the carbs, timing seems to be on. I had messed with the fuel mixture screws but have not had a chance to get back to getting them properly set, just curious if you guys have any ideas for the weekend coming up

[Re-run]

The pedal actuates the light and caliper separately. What is likely happening is 2 things. The master needs cleaned out, there is a return hole that clogs and won't allow pressure to release fully.

So, clean the master fully and run a bit of wire into the holes to break up the crud.

the second issue is with the seal in the caliper. There is a rubber seal that the piston moves through. What happens is crud builds up between the seal and the caliper housing. You need to pull out the piston and seal and clean the groove. Use a stiff nylon of brass bristle brush that will fit into the groove, a dremel wheel works well. Clean ever millimeter including the corners of this groove. You may need a new seal.
When cleaning, a white powder will come out. This is the corrosion caused by the dot3 fluid.

Once both units are cleaned, bleed everything while making sure there are no high spots in the line.

Once done, as long as all seals are good, you brake should work great.

[shinyribs]

Yup. I was thinking crud around the caliper seal,too. Pretty common issue actually.

[Re-run]

Noticed we didn't address the smoke issue. Easiest thing would be to pull the plug and see what it looks like. If it is pretty dark, then that cylinder is running rich or has weak spark. Find which plug is rich and swap the plug leads, 1&4 or 2&3. See if the problem stays or moves. If it moves, spark issue. If it stays, carb issue.
Clean the plug before testing, or replace it.

[NoodleBeard]

Thanks for the ideas with the brakes, lol its a pain to push it around the shop, Ill give the plug idea a go. I had replaced all the plugs because of weak spark on one of them and I had figured if im going to replace one ill do all of them and just never bother to look at them again. Ill give them a check and let ya guys know whats up

[NoodleBeard]

I did some work on the bike last night, fixed the brake issue. Rear brake working good need to get some fluid to do a proper bleed on it. I pulled all the plugs, looked like some minor carbon but did not really look bad no oil or gas smell. I am not entirely sure now if my timing is set right trying to find a decent guide to set the timing statically. I found a good timing write up but its for an older cb750 not sure if its the same as I have 81 cb750f. It also looks like the smoking switched sides on me. I did not swap the plugs or the wires put everything back the way it is and some how on its own the smoking and popping moved to the other exhaust, It was also doing some back fires if I would rev it aggressively and let of the gas it would backfire, either at the peak of the revs or on the down rev cant remember pretty tired by the time i got to that point.

[LukeM]

If you haven't done so already, we should have a copy of the Factory Service manual for your bike on the home page of our happy little forum here.  There should be a good procedure on how to set the static timing.  The DOHC bikes use a reluctor and Hall-effect sensor in place of the points of a conventional ignition.  It then goes to a couple of spark boxes that fire the coils and from there, the plugs.

Hope this helps.
Luke M

[NoodleBeard]

  Yeah I had already been looking through it, how accurate will static timing get me, or assuming I do it right should that be good enough, bike still seems to be running rich even after adjusting the fuel mixture screws or pilot screws whatever they go by. I tried adjusting following the manual, but did not see a noticeable difference so i then i just set them all back to 1.5 turns out to factory specs with no improvement. What gets me is how the issue jumped from one muffler to the other.
  I have a hard timing believing its anything but timing or a carb issue but just cant seem to nail the issue. The bike runs ok aside from it smoking a bit from one muffler and the popping and occasional back fire. around 4000-5000 rpms the there is some hesitation then what almost feels like a jump in rpms to get into that upper rpm range.
  It leads me to believe that its a timing issue, perhaps its not advancing correctly or the carbs are still out of tune. The things i know for certain is that its running rich, smoking a little does not* smell like oil and it does not linger, and the popping sound from the muffler, the plugs look clean no build up on them and all have good spark. To help narrow it down im not sure if its a timing issue, or a carb issue or dreadfully both. Bike is so close to running as it should witch makes me think its not a major mechanical issue.

[TOOLS1]

What color is the smoke?
TOOLS

[NoodleBeard]

grayish black, does not smell like oil or anything and the bike seems to hold its oil pretty well. Just mostly smells rich

[shinyribs]

Since it's popping on the same side it's smoking from it's likely they are related.

Popping can be cause by valves out of spec. Sometimes you can get a pop on decel if you are a tad lean. The CV carbs on these bikes have a feature called the air cut-off. If that messes up you can get a pop from that as well.

But a op accompanied by smoke doesn't sound like a lean condition. Pops can also be from an overly rich condition and the pop is unburnt fuel exploding in the exhaust system. Generally caused by dirty/ out of tune carb or valves out of spec (trying to fire with the exhaust valve open).

If the bike has been sitting a good once over is really in order. Pull the carbs and clean/synch and/or rebuild them, check valve specs and adjust the cam chains.

[NoodleBeard]

Hey was able to get out to my shop and do some work, have not been able to make much improvement on how the bike is running. I had already gone through my carbs and cleaned them. twice I have gone threw them. So on to what I found out today.

1. did a compression check all cylinders  are between 140-145, all of them are basically the sam

2. cylinders 1 and 3 spark plugs on the center bit look nice and tan, where as the others look a bit darker and almost black, faint smell of gasoline

3. The exhaust that is smoking (maybe even steam, grayish black) on the right side, if you hold your hand on it is coming out much harder/faster than the left side a huge noticeable difference and is warmer, where the exhaust on the left side comes out slower and is not really warm at all. The exhaust on the right side when it pops/backfires you can feel an increase in pressure on the exhaust if you are holding your hand over it differently pushes hard when its popping.

4.To try and narrow down the smoking bit from one side one at a time with the bike running I pulled the spark wire off of cylinders 3,4 and it seemed to make no difference in the amount of smoking coming off witch I thought was odd.

5. Thinking it was cylinders 3,4 I fiddled with the fuel mixture screws, I tried adjusting slowly to listen for the increase in RPMS but could not hear any, I then just turned them all the way in and the bike did not really seem to run any different, the same is when I just went nuts and turned it way out.

I did manage to change the oil, and fix rear brake so lol some progress I have made. I  have also noticed with the throttle if slow increase it around 4500-5k it hesitates and gets a little doggy but you can eventually throttle threw it and the RPMS will jump. I have also noticed even at around 1200RPM the bike is still idling rough. I really don't see as it sits now idling at 900RPM Pretty sure it will just die  

[shinyribs]

I've never been able to get one to idle at 900 rpms. SOHC or DOHC. 1100-1300 rpms seems to be what they like IMO.
On the smoking:

You have really good compression numbers, so I doubt it's coming from worn rings.

Pulling one plug at a time was a good idea. Since that had no affect on the issue it would lead you to think it is not related to the combustion process (oil past the rings or fuel smoke). Maybe it's oil getting past the valve guides. An oil leak there would remain constant whether the bike is firing or not. That would turn the corresponding plugs black too, like you described.

We had one bike at my buddy's shop awhile back that smoked terribly but everything checked out perfect on it. After much head scratching I noticed that the slip-on muffler didn't appear new, but also didn't appear to have been on that particular bike for very long. I called the owner and he said he had just bought that slip-on off Cragslist from a guy parting out a bike that had recently blew an engine. Basically, the slip-on was full of oil residue from the previous bike it was on and would start smoking after it got hot. That was tough to figure out since oil smoke was obviously present, but the engine was healthy. Just throwing that out there for what it's worth.