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Hard Starting , Popping, Missing Etc / 80 CB750F

Oh , Holy crap , I'm a dumb ass, yea totally different picture now! Why did I not pay more attention in math class !!!!!!!!!
Thank you !!!
 
OK , finally have all the valves back in spec,,,,,,, and may I say , what a PITA!!! Anyway, ran another compression test and #2 came up from 120 to 150, perhaps then once I get it sorted out, some hard running will produce even better results to match the other cylinders 160 cold.
 
You might get lucky....................now you know why commonly even the Honda techs did not run them for real as much as they charged for. They'd have to charge enough for you simply to sign the bike over to them.
 
Well,
this took some time for the start up, due to a leaking Petcock and the time consuming search for a replacement. None of the Chinese ones on E-bay are suitable although they all say they are for 80 CB750F,,,,, either too small a feed tube, or wrong size fuel nipple. One even said fits 3/16 or 1/4 inch fuel line, how does that work? anyway found a used correct one on e-bay that does not leak and bought a rebuild kit (must drill out the rivets) for the original one now my spare.
Filled with fuel and ,,, same old thing,, hard starting, popping out of one cylinder etc. Very frustrating. Wondered about the igniters or coils???? then went back over my trouble shooting notes and one item was to check the pulse generators gap. Off came the cover and wow, one was .027" the other .032". Re-gapped them to .014" and rolled it outside for a test fire. Started up much easier, still popped some for a few minutes, then it cleared up, revs clean, sounds pretty good. Only one troublesome symptom , does not want to settle down to idle. If I lower the idle speed knob, it takes a bit for it to react, then the idle is to low, try to bring it up and similar not much change then too fast and after a rev does not want to settle back to idle.
The carbs have not been balanced ( I've not purchased a kit or taught myself yet) So these symptoms sound familiar?, and possibly and easy fix, or as I have found on these DOHC bikes usually not!!!
Thanks for all the help!
 
When you fouled out plugs you made them easy to stay that way, may need new plug(s), at least in your errant cylinder. Not idling may be a symptom of that. It won't idle right with carbs not close to each other either but you should be able to simply bench sync to get around 90% of that. Not settling back down to the previous set idle speed after a rev is a sync issue or vacuum leak.
 
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Desperately trying to like this Motorcycle but it has been a struggle. #2 cylinder not firing consistently so I pulled the carbs and sent them to a friend who has work with these CV carbs before (he has restored two CBX 1100's). Sure enough he finds some crap in the jets in the #2 carb, cleans and bench syncs. I reinstall the carbs starts and runs fine, I now get to actually ride for the first time since I bought this unmolested 3,900 mile bike back in March. Runs great, smooth, fast, good brakes,,,, all is well with the exception of on occasion the idle would not return to low idle,,,, not always, but just sometimes, I would drag the clutch a bit and it would settle down to low idle. So I check for vacuum leaks, good there, check the advancer unit, good there, reset timing, new plugs and then do a carb sync. They sync up nicely and sounds good, and Idles consistent. Now for the test drive, now it's developed a new trick, dead stumble or hesitation off idle till about to 2000 RPM then hold on to the bars. Pulled one of the new plugs, rich fouled. Son of a !!!!!!! So for one week it ran great. Wow the thought of having to pull these carbs back off is maddening , what a PITA. Soon becoming my least favorite bike.
 
Thats why I always say ditch the stock carbs and spend the money up front and get some aftermarket carbs. Keihin cr's or Mikuni's. You will be money ahead and power ahead in the long run.
 
While it's nice to have a set of $1000 aftermarket carbs in hand I have no trouble with the OEM CVs whatsoever. Yeah, they can fudge up when they sit but easy peasy getting back right. I've rebuilt many to have them go to running fine longterm once issues are worked out.

If bike has had ethanol fuel run through it to then sit the tank will be rusting and the finer red rust powder will go past any OEM filter there (they are not nearly fine enough) to repeat cause issues with flooding, clogging, the like. These carbs are quite open (vented) to the air and it wrecks havoc with ethanol in the fuel. One week sitting often sticks the needles.

The only thing that you can write the carbs off for is worn butterflies in bore or slide to body worn. Both those not a problem until like 40K+ miles. Of course, if ethanol left in it to corrode critical places/parts, that can do it too. I've run some unreal corroded carbs to have them run fine before as long as it doesn't hit a critical spot.

If idling FOR SURE on all 4 consistently then look at accel pump, the check valve is the next thing to mess up and can do that. The fouled plugs can make pull-away from idle wonky too. If plugs new enough not to be black yet you can rinse them in acetone and if needed re-scratch the electrodes up to make clean jump off points for spark and throw them back in to re-use. Assuming you are using a series -8 NGK plug.

If idle mix screws too far open the plugs can foul.

Hard aircut rubber diaphragms will drive you mad if they've never been changed..............they can be neutered to stop problems.

Ignition needs to be supreme, even the new ones had crap ignition, it looks hot as hell, definitely isn't. Weak, weak. Make sure the pulser airgap is tight, it strengthens spark. Thinking .016" the number, I have gone as low as .012". They MUST NOT HIT. Bumping the timing up a line or two advanced will help with fouling plugs.

You can check across kill switch for resistance, some have had issues with that lowering spark. There's a mod for adding a relay and bigger, shorter wire to make the coil power supply more substantial and the main power no longer goes through the kill switch, it only works the relay then. Some swear by that one.
 
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So, some more info,
Only non ethanol in this bike since I got it. Yes, NGK-8 series plugs. Idle mixture screws at 1 1/2 turns. Pulser air gap set at .014 .
So my thinking leads me to it being something I did? Before chasing the inconsistent low idle, it ran great. Set timing, changed plugs (old ones were not horrible, a little darker than I liked but not bad) and synced the carbs, then all of sudden it runs like a turd. Seems like too much of a coincidence, runs great one day, I wrench on it and now runs bad.
Sorry I touched it now, should have been happy with occasionally dragging the clutch when it went to high idle. The sync gauges were new and needles were equal before start up, they had knobs that allowed you to control flutter, and made it easy to get a good read on getting all equal.
I'll get a look at the accel pump and check valve to see if they are functioning.
Frustrating.
 
Don't be sorry, can't learn if scared to mess with stuff. Question your own work, a healthy skill to have, most can't go there.
 
Well,
Of the three actions I took, new plugs (certainly not the issue) reset timing (actually advanced some so don't see this as the culprit either). So my thinking takes me back to the carb sync. So will go back and triple check this. This time I will test each gauge individually on #2 to assure that the readings are the same before proceeding. Who knows? Probably should have bought the Motion Pro unit where you calibrate on before each use.
May not be the sync but I highly suspect it. I'm a bit confused how this effects going dead rich, as the plugs are now black, not the case before the sync?
 
Why I like the automatic setting of all 4 carbs drawing on the same working fluid, you get away from the potential of 4 gauges not being exactly the same readings.

Pull the carbs off and first thing take careful note of the butterfly positions as relates to the very first hole on bottom of bore that exposes when the throttles roll open. Ignore the curb idle hole about a half inch-ish in front of the butterfly. Looking at carb bores from the front. If engine in good shape so that the throttle does not have to be artificially open too far to get correct idle speed, then the butterflies will be dead equal and all ever so slightly just not exposing that first hole that begins to show when the throttles open up. That hole showing entirely or even more than that (3-4 holes there IIRC) and the engine will be running super rich at idle to load your plugs up. That is regardless of where you have the mixture screws. Correct is just barely closed and so much as the slightest movement then begins to expose the first hole. Maybe .005" showing. Half the hole is too much.

If you have no idea of where the aircuts are as to condition, well, that can screw you around too. If leaking there your sync is worthless, indeed, it will be tilted to make the carbs OUT of sync rather than getting better as the sync will include the occasional aircut leaker into the idle air flow for sync speed. Thus throwing off getting them evened up.

You should know that on these pressed in pilot carbs the idle pilot jet that's pressed in has cross passages drilled sideways above the idle fuel feed restriction for air to bleed in and no real way to know if they are open since you can't pull that jet. One reason why people can have fits with these, the seeming plugged airbleed then makes that cylinder idle too rich. Or, the cross linking of the primary main jet to the idle pilot jet then makes the primary off idle fuel too rich because the pilot is stopped up to be too lean. The pilot serves as a partial airjet for the primary once the primary airflow takes over from pure pilot idle only flow. Yeah, it drives people nuts. The primary and pilot have a careful seesaw action, when one begins to fall off the other is pulling into action and both are hardlinked to each other. Why a clogged idle feed makes the engine lean as heck at dead idle then instantly it goes super rich on the other side of that or just off idle.

Are you able to get a .013" wire up through the idle pilot jets to confirm they are at least open on the fuel hole? Each individual idle mixture screw should be able to drop the idle enough to try to kill motor when cut all the way off clockwise, if not then something wrong.
 
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Well, after testing my carb sync gauges each on #2, two were off by one inch of vacuum. So took plenty of time to re-sync to where I thought they were perfect. In test driving though it just was a turd, hesitating , bucking etc, so set about to see if fuel starvation could have been the issue, rerouted the fuel line and changed to a different filter to assure that fuel was always present. Same result. Always seemed ok on the centerstand, but ride it and put load on it a total turd, not sure it would make it around the block. I'm now at a such a frustration level I can't see straight. So I decided to change tacks, and see if ignition could be the factor, I had already regapped the pickups, and checked the advancer unit. Decided that before I set fire to this bike to pay it back, I would try two more items, new coils, and a pair of new Ignitors. Replaced both today, and it was the IGNITORs!!!! As soon as I hit the starter button, I could tell the difference in how it started and sounded. The test drive was awesome, smooth, fast, and idles perfect. So although it did have some carb issues that I had to sort thru, and the symptoms seemed always to point at fuel,,,,,, the final piece of the puzzle was ignition.
So glad I did not give up on this thing although my frustration level was at Devcon 5!
Thanks for all that chimed in with info and help,,,,, appreciate it!
 
Hmmm, must be getting old... (oh wait, I really AM getting old - 65 this year!), but I'm having similar problems with my '80 750F Supersport and need someone to clarify what mrmach5 means when he says his problem was "ignitors." I see he mentions coils (yep, still know what those are), but somewhat confused by his terminology WRT ignitors - is he perhaps referring to the high tension ignition wires? Looking for some enlightenment please...
 
Ignitors = spark units, the little boxes under the seat above battery. Long shot but possibly your issue if one is not cutting ground on time to mess up timing. Pretty rare but nothing is impossible on electronics.
 
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