79 CB 750K...Idling Problem...Overheating as well.

[dagettem]

Hey!  About a month ago I bought a 79 CB750K with 9000 miles on it for $800 and have been getting quite the education.  The other day I took it for a ride "around the block."  I live in a farmland area.  I put about 15 miles on it, and it ran like a dream.  I still has some foul gas in it which I am going to drain here pretty soon, but I noticed when I got back home and pulled it into the garage, the bike was running a little too hot, so in turn I figure it is running alittle rich.  Personally, I feel there is an intake leak at the carb intake at the carb holders.  So I am going to buy a new set of carb holders and see where that takes me.  Another little hint is the fact that it doesn't like to idle when its warm.  It idles at about 3000 RPM, when its cold it idles at 1000 RPM, where it is supposed to be.  I saw some other posts on here of people with the same problem.  

Is there any good way to check that this is the problem before I put down $75 on eBay?

I was also going to have a mechanic do some carb work for me, sync them up, do a rebuild, but after that little ride I feel the main problem is the carb holders and the carbs can be left alone for the season.  The carbs were spotless when I took them apart.  Unless you are supposed to get them resynced after an issue such as this.  Like I said, the bike ran like a million dollars while riding it.  Never missed or anything.

Thanks,
Dave

[blackhawkrider]

Its not being rich that causes excess heat so if you feel it is too hot its not because its too rich. That would actually cause it to run colder. More than likely your idle is just too high. Leaking carb intake boots would cause the idle to be lower or it would idle very irregularly. I would check to see if lowering your idle to 1200 rpm when the bike is fully warmed up helps things. When its cold is it runing at the 1000 rpm like you stated with or without the choke? Also your idle mix screws could be out of spec but I would start with what I said before. What makes you think its too hot? Most air cooled bikes you really can;t tell unless its excessive heat like you can't even get near it or when riding you have a sudden loss in power. Other than that anything else is pretty much normal temp not much to go wrong.

[dagettem]

Thanks for replying!  Well, what was giving me a hint that it was running hot was it was actually during the exhaust pipe blue coming out of the motor.  The motor idles at 1000 RPM with the choke off.  Also, once the bike is back together I'll check the idle adjustment.

The problem may have just reared it's ugly head.  I was riding it yesterday, noticing it didn't have any power past 5000 RPM, it would just bog, but if you stayed consistent with it in the throttle it was fine.  On my way return ride home (about a mile from my home) it started losing power.  I started backing down through the gears, and it lost power all together.  I turned the key off to give it a quick go through before trying anything again, and I turned the key back on and hit the button, it just clinked.  The lights were bright as can be so the battery had enough butt (I think), so I was pushing the rest of the way home.  My Dad realized I was gone for too long and helped me push up the final hill.  

He coasted it down the hill and popped the clutch and it started, but it still was not right.  It sounded as if it was only running on two cylinders and the cams were chattering.  It had no power.  It wasn't even putting hardly any charge to the battery, so badly that the headlight was hardly lit and the tail light wasn't lit at all.  My Dad was worried about the cams making that much racket, so we started off by pulling the plugs, they all looked good.  We checked that there was blue spark to all four cylinders, we had spark.  We pulled the cam cover off and everything was good and tight and where it needed to be.  Even the timing chain was adjusted to where it needed to be.  The top end was spotless, no dings or scratches or anything.  All the lobes were on the cams.

Alittle more detail about the bike though.  I bought it, and it wasn't running right, so I put half a can of seafoam in the gas.  Changed the plugs (the Autolite equivalent to NGK), put a newer set of CDI igniter units on it with the hard backing.  That helped it out to atleast run and keep running, although it still had small pops alittle out of the exhaust when running and after about 5 minutes the idle would go high.  So yesterday I finally did drain the tank, there was about 2 gallons of old fuel still in the bike and put 5 gallons Cam2 (aka Turbo Blue, leaded fuel) hoping that would help it out.  Obviously not, if finally gave up.  I want to learn how to do this stuff myself, but I know when I get it back together I am just going to take it to a mechanic.  A mechanic wanted $150 to clean and sync the carbs.  This is getting way too expensive and frustrating.  

Sorry for the lengthliness.  

[blackhawkrider]

well it sounds like you overheated the bike. Im sorry to hear that hopefully you didn't damage rings or warp anything. Did you change the oil when you got the bike? Blue pipes are usually an indicator you are way to lean. This might actually be why it overheated. A carb sinc will not fix this issue, save the $150 for after you get it running without overheating then finish your project off with the sync. Unless you want the mechanic to fix the actual problem then I would tell them what it is doing. Expect it to cost a fair amount more though. I would suggest getting back into the carbs and making sure everything is stock and is in the right place. good luck

[dagettem]

The gentleman that I bought the bike off of had just changed the oil right before I bought it.  Could there be any indicator to damaged rings? Oil out the exhaust?  Also, just out of curiousity, when it was getting hot, shouldn't I have lost power even then?  When my Dad noticed the small spot of blue (probably the size of a quarter or half dollar) on the pipes, the bike was still running strong from that ride.  He just stopped at a friends house on it, and they were checking it out when he noticed the spot.  Heck, maybe the spot was there from the previous owner.  Hmmm....this is rather depressing if I did hurt that motor...

Thanks!

[blackhawkrider]

if you did any damage a compression test would indicate it. Whether a warped head or damage pistons. Since you didn't seize the motor you have a good chance its ok. So only one pipe has blueing? I would start by looking at that carb to make sure it is all in order, so check that the jets in that carb match the jets in the other carbs and continue from there if need be. If its only one carb it very likely could be causing your issue. Three cylinders can still pull one badly running one with out you noticing too much especially if its firing but is just lean as hell. It would not be a bad idea to check compression on that cylinder as well, as the heat could still have done some damage. Better to know for sure than to fix the carb problem still have issue and not know why. Don't be depressed anything can be fixed and when these things run right they rarely have issues and are fun to ride.

[gonebiking-2]

if you still have the four into four pipes on the bike, check that you are not leaving a small trail of blue smoke out of one of the pipes. If there is no smoke when you are riding, and no smoke on start up then the rings are about 99% good and the valeve stem oil seals are still OK. if its over heating then check that the main jets are the correct. In my Workshop manual the main jet is a 68 and the secondary is a 102. Anything below those numbers means that the main and secondary jets are too lean. Also , replace the inlet rubbers from the carbs to the inlet manifold. They develop cracks as the rubber tubes age, and they start to suck air through them which makes the engine run leaner.

[blackhawkrider]

actually I think you meant main is 102 and secondary is 68. I have a feeling the two are swiched up in one or more of the carbs.

[liljohnnie69-2]

Hi Dave,
 I just bought an '81 cb750k and am on  the same learning curve as you.After replacing a burnt valve and then lapping in all the rest of the valves i noticed it would only idle at about 3000 rpm and was worse when it was hot.The fix that i suggest is checking camshaft to cam bucket clearance with a feeler guage.
This will require you remove the valve cover but it is pretty easy if you dont have a fairing.Rotate the engine until the cam lobe is facing away from the bucket you are checking and then insert or try to insert your feeler guage.The clearance should be between .002" and .005".If you do any business at all with your local honda dealer hopefully they will trade you one for one for the shims because they are about $8.00 a piece and can run up the repair bill in a hurry.
   

    Hope this helps and good luck
          liljohnnie69

[gonebiking-2]

Nope, Just checked with Honda Australia and in my workshop manual and the main jet is 68 and the secondary jet is 102. Something else comes to mind. Has anyone ever bothered to check the rubber diaphragm , O ring and spring on the side of the carb body. This is the Air Cutoff Valve, and if the diaphragm is worn and persihed, this can cause air to leak in and many problems will occur once the diaphragm is past its use by date.  Replaced mine last week and its running even better then ever. Was able to source the diaphragm , O ring and spring locally ( Australia ) without having to buy from overseas.

[Samuelouie]

Was this issue ever resolved? I have a '81 Honda CB750 and it's symptoms seem identical to what you've described. Any help would be great!

[OnyxWolf]

My 79 did this when I got her. I did a full rebuild on the carbs, resync, and rest the idle and air fuel mix to fsm specs. I also replaced the carb boots. This fixed my issues with idle and temp. Just make sure you get those jets clean.
Good luck.