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1979 DOHC CB750Kz Cafe Build

Cqyqte

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Started in November when my eldest son delivered a tired CB750K for me to Cafe for him over the winter. I have been retired now for 8 years and have taken up custom building of bikes in the garage. Been doing this for more than ten years as a hobby but now it's more than a hobby. I started a Facebook page OlCoyoteCycles if you are interested, where you can watch the build progress daily. https://www.facebook.com/OlCoyoteCycles1/
 
I have noticed the hit count on my Facebook page has increased so I hope some of you have visited as a result of this thread. Thanks in advance, things are coming along quite well. I haven't run into too many struggles finding parts... yay!
 
I have noticed the hit count on my Facebook page has increased so I hope some of you have visited as a result of this thread. Thanks in advance, things are coming along quite well. I haven't run into too many struggles finding parts... yay!
For those that haven't a Facebook account here are a few up dated pics
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Just a few, there are many more on my Facebook page... it's worth it.
 
The weather has been much better the past few days so I rolled the '79 CB750 outside to get some air, while we wait for the seatpad and get ready for a tuning session.

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If motor has good compression then 75-115 or so can work pretty good. Resist the urge to go higher, the CV slides aren't as active when the bike is slightly rich, they then don't open as fast and the rich on top of rich then drives owner batty. CVs open max when the engine is well sealed up and slightly lean.
 
I was surprised she started right up and idled quite well after her long sleep. PO said he road her weekly before my son bought her, however the centre two cylinders are struggling. Using a IR tempurature gun the centre two cylinders are not getting gas at idle but do when throttled up, so the pilot/idle jets are probably clogged. I know for sure they are getting spark, (good spark too) and I can hear them cut in as the ilde rises.


The clanks are things dropping on the table due to vibration, LOL.
 
The pilots clog easier than spit and then right above them at off idle the primary runs rich as crap due to the pilots being clogged. The one-two lean-rich instant plug fouler thing. The primary system is hooked to the pure idle and the idle serves as air bleed to primary when you switch from one to the other. So, clogged idle makes just off idle go super rich. A DOHC pull your hair out oddity.

If that early a model then pressed in pilots and there are a couple of cross passages above the pilot feed hole itself, you can't get to them unless jet is out. There is a mixing chamber under the off idle transfer fuel holes too that can clog to give off idle issues. Flip carb upside down and you can see the aluminum plug at the fuel bowl flat, the plug covers the bottom of that chamber. I clean the #35 pilot feeds using a .013" metal wire, the 'E' string (smallest one) of a guitar is perfect for it.

Is that bike synced at carbs? The ka-chunk, ka-chunk, ka-chunk at idle says primary chain rattle from lopsided due to either no sync or cylinders not firing like you say.
 
After the video and some further evaluation tests I determined I needed to open the carbs to see for myself their condition and the jets that I am dealing with. Stock 102 - 68 jets and pressedin pilots, I used the E string as I too find it the most useful cleansing device. As I mentioned earlier in a PM to you I have acquired an assortment of jets and I replaced the stock for 115-75 as you had mentioned might be a possible bandaid to running pods. After reassembly fired her up and found a way more responsive throttle, so I took it for a short roll up and down the road, up here we are still coated with winter debri (sand) on the roads making riding very dangerous. Here are the two links which will give you some idea of my next find. She pulls reasonably well but I found at 5000 rpm the telltale crackle of a lean situation shows up and the revs are higher on my return, she feels leanish when fully warmed up. Did I hear you say that lifting needles in these carbs are a waste of time? Because my next plan would be to lift them if that was possible, any knowledge you could lend will be much appreciated.



Thanksfor helping...
 
I personally never had to lift needles. Or drill slide lift holes.

There are people who have lifted them but not really necessary on these because bumping up primary jet does pretty much the exact same thing. The needles MUST still be loose when done so that they can self-center back in the jet needle (why they are loose to begin with).

Be aware that the valves not sealing show up first in engine refusing to go all the way up in rpm, many do end up thinking it is a fuel issue when not and why most of the carbs are off motor when you buy the used bike. Carb work absolutely will NOT solve a valve leaking issue. Compression test says it all, OEM spec is 170 psi and they run pretty good until they go below 135 or so. 120 says the motor needs work. 100 is dead cylinder not putting out any useable power.

Could be lean like you think though. You are up in colder territory, here in Texas that 115 secondary main would pull out to 10,000 rpm with ease.

Anything .003" or less is same as zero valve clearance to me and why they run like crap. The engines will definitely burn valves set at .002" because the numbers you get there are NOT REAL, you set to a .005" minimum to guarantee you got good compression and screw what the service manual says.
 
The aircuts in carbs can give trouble but are easy to do away with. If you separate carbs individually do yourself a favor and use a dremel with a small rock to remove the staked end of the choke plate screws, the screws then come out easily to and later go right back in with a bit of removeable loctite to hold them in place.

Leaking aircuts will foul idle quality in a second.
 
Thanks for all the help and inherent knowledge it paid off well. I installed the 115-75 jets clean every nook and cranny I could. Set the float levels as per the 15 degree rule and flushed the petcock clean, installed a see through fuel filter and adjusted the PMS screws to 1 3/4 turns out, oh sync'd the carbs. My son came by yesterday and we took it for a ride. The grin from ear to ear was my reward. Listening to it wail at 5000 and up was a treat, also my nose told me the fueling was as good as she gets for the pod setup on these carbs. Again thanks for the words of wisdom, much appreciated.

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My son has ridden it for a week or so now and has said he can feel a slight stumble at the upper end of the throttle range. Question, I saw someone on this forum indicate they too were running 115 - 75 jet set up but they also indicated they have their mixture screws turn to 3.5 turns out. I have my sons at 1 3/4 turns out what's your experience in this aspect. I realize that the pilot setting is specifically an idle circuit will still provide that fuel level throughout the entire throttle range. Therefore a slight increase in fuel in the mix will be a slight increase at full throttle which could remove the stumble. What setting does everyone with this jet selection use.
 
The answer is yes, my son road down yesterday with a number of minor modifications he wanted done. The first was to resync the carbs and set the mixture screws. So I turned the screws out to a total of 2.5 turns out and then resync'd the carbs. I also tightened the swivel ball on one mirror, and removed the slider bolts on the front brake, checked the tightness of the points cover. My son reports that the bike has way more balls and the stumble is gone. :)
 
Sure about that???

I can show you solid engineering and more importantly physics reasons for why not so.

The fuel circuits do supply some slight amount up higher but think about it enough and you will realize what I'm talking about. On the DOHCs there is ample reason actually there for opening them up more LEANING it out. I'll give you a minute to chew on that one..............
 
I read the MacGregor Carb Cleaning Services document on cleaning DOHC carbs and was confused by the different numbers they provided for setting the pilot screws 3.0 turns out as compared to the OEM service manual stating 1.5 turns out. I emailed the author to ask why the difference, and he said that the factory set the DOHC carbs to run lean to meet EPA standards. He said that if the bike was running well with the screws turned out 1.75 turns as I had set them for my son, he said try turning them out another 3/4 turn and see if the motor lights even more. Following his suggestion I did and the bike responded extremely well (improving performance) and that stutter at 4500 all but disappeared which to my son made his day. Now I will bow to your experience, but I will say the performance change of that slight adjustment was amazing, albeit Rich or Lean. Your recommendation of 115 and 75 jets and Sean Grier's suggestion of 2.5 to 3 turns out for the pilot screws is working for my son... means he's happy I am happy.
 
Yes, the idles are lean because of the EPA. Open the screws more and a better idle and as well it makes up for the aircuts too. The aircuts are needed because idle is so lean that shutting off throttle at decel then backfires through pipes due to slow lean burn afterburning. When you open screws more the aircuts then are not even needed and can be blocked off to prevent their own special problems.

When you open mixture screws you are opening with throttle shut and high vacuum at the mixture outlet, the curb idle port. At full load and higher rpm the slides are open and that same port has much less vacuum at it and then the mixture screw effect is much less. As well, the primary main system and idle system are linked together, when the primary comes on line then the idle mixture output drops off and the curb idle port and idle air bleeds become extra bleeds that lean things out since again the curb idle port is then at a lower vacuum number, meaning the pressure has come UP in it. The primary outlet though will be then out in the active airstream and more pull on it at that time. The so-called 'seesaw' effect these do between the idle and primary systems, as one feeds less the other feeds more. Why if the idle fuel feed gets stopped up you are lean there at dead idle but as soon as you go toward the primary starting up then the mixture goes super rich, because the idle feed being plugged has the air bleed effect plugged too. Super lean then super rich dead back to back and why so many have fits figuring out just what the hell is wrong with the carbs.

Actual correct on the idle pilot jets is when you can open OR close the mixture screws and make the engine run worse either way, if it gets steadily higher rpm in opening them with no trying to go lower then you need a slightly bigger pilot, some go from the OEM #35 to #38 to get that. You consider the mixture screw setting the 'main jet' for pure dead idle only, then the pilot is a bit bigger to cover off idle until the primary starts up.

Yes, the lower load fuel systems feed at higher loads and rpm but depending on where the bleeds and discharge points are they can full flow at higher rpm or drop way off, every carb design is different in that. These the idle feed drops since the port is in the side of passage where at higher rpm there is boundary layer. The mains though discharge in the center of the airstream where the air is much faster. Idle flow drops as the engine gets faster and more loaded, in fact it drops hugely as soon as the butterfly edge opens up past the discharge point, at that point the pulling vacuum takes quite a dump.
 
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