Topic: 75' cb750f carburetor troubles, or is it valves? insight needed

[proudsnacks]

to start off i have already read through every thread i could find and checked each issue, the bike is a 1975 cb750f, purchased barely running 2 years ago, its completely stock except for an EMGO slip on. i cleaned it up from new jets/ needles floats and gaskets to new breaker plate assembly, carb/insulator rubbers, new oem coils/ wires and caps/plugs. still had issues with popping gas out of the #4 carb at low rpm. i ended up taking it to a local shop whom i have dealt with before and does good work. he did a compression test,120PSI on all cylinders,  found that the #4 valve was way to tight and adjusted, did a leak down test and the valves all held "strong'. the #4 carb pilot jet was actually cracked and broke when taken out, i put the original one back in but the problem persisted, i was also waiting for a better muffler, i brought it back for a proper vacuum sync and it was much better, no more gas popping back through. i drove it a bit but its still not quite right, so i checked the plugs and the #4 plug was fouled from the threads to the tip and the others hard some carbon build up around the threads but they were whitish at the tip, i put new plugs in, and check to see if the popping of gas had returned, it didn't but i noticed that its still puffing air and its hard to get it up to 6k when riding. also, float heights were within spec.

what im trying to figure out after reading about bent valves and clogged pilot circuits, at this point which could it be? there is very little noise from the engine besides a couple small exhaust leaks(and noisy clutch) but it runs nice. would i hear a bent valve? i checked tappet clearance again and its right on, from the pilot jet breaking, i can assume that the kit i put in is garbage,
would non OEM needles/jets make it run slightly rich?  from what i have read they dont like aftermarket jets/needles. i sold my KZ in hopes id being daily driving it by now  worst case it goes back to the shop for a valve job i suppose? id do it myself but i have never done valves and i work in a kitchen, so all day. any thoughts and advice is greatly appreciated!

[robvangulik]

A bent valve would show up clearly in a compression test.
It doesn't so it isn't!

[proudsnacks]

thanks for the reply, thats what i was thinking but not 100% sure, i think im going to order all new OEM jets/needles and see where that takes me. any thoughts on aftermarket carb kits, this was the first time i used one.

[proudsnacks]

also, im trying to identify which carbs i have, but cant seem to find this number

[Jerry Rxman Griffin aka MuthaF'er]

064A. Original stock 75 CB750F carbs just like mine. 105 mains, needle in middle slot, 40 slows

[robvangulik]

It is in your own picture....

[proudsnacks]

I realize that, but when trying to search for
OEM needles I couldn't find the matching number to determine which needles I'd need to run,  I feel like the current ones are to rich, and moving the clip is to big of a step, but I'm not an expert

[Jerry Rxman Griffin aka MuthaF'er]

The needles/needle jets sets are not to be found. NLA. Look for the -392 as the middle number. It is specific. That will give you the correct sets. Your best bet is to ensure all the tune up steps have been done then mess with the carbs. PITA to remove them, split the carbs and take of the top retainer ring but that's what you gotta do. New aftermarket carbs such as Keihin CR's or Mikuni RS are currently NLA and when they return the price is PROBABLY going from ~ $700 - $800 to $1200.

Do yo have access to online parts fiches? The part number you need is 16012-392-004 Needle set, jet. Others will have a different needle profile.

[proudsnacks]

that's what the Honda dealer told me today, not much out there. only in kit forms, cant remember where i got these ones from. its been raining for almost a week so its a good time to muck with it. also going to  try and get my tank flushed/cleaned. i have the original needles but some are discolored, however i didn't 'inspect" them. I will go look at them and see if its worth trying them out instead.

[Jerry Rxman Griffin aka MuthaF'er]

That part number IS for the needle and needle jet KIT. And, yes, they are no longer available through Honda for quite a few years. I once used Keyster kits but for only a short time. I replaced most of it with my original parts even if they were old. May have kept the bowl gaskets. Not likely you will find 4 NOS kits. Make what you have work.

[proudsnacks]

Ahh k, after inspection, the needles that were in it look pretty good, so I'm going to try them out and see if it helps,. Putting all the OEM bits back on

[ofreen]

That part number IS for the needle and needle jet KIT. And, yes, they are no longer available through Honda for quite a few years. I once used Keyster kits but for only a short time. I replaced most of it with my original parts even if they were old. May have kept the bowl gaskets. Not likely you will find 4 NOS kits. Make what you have work.

They were briefly available again from Honda last year, which was the second time I had seen them listed since they originally went NLA.  The last set I bought from Honda were a little different than previous sets.  I always ran them with with the clip in the center groove on the needle, but I had to move it to the second groove from the top with the newest set.  Works perfectly that way.

Other F0/F1 parts have popped up from time to time, such as the rear caliper indicator cover that had been NLA for years.  I don't see any listed anywhere now though.  I wish now I had bought a couple more when they were available.  Maybe Honda commissions a run of a batch of parts occasionally, but I'm not privy.

I see David Silver Spares is listing a "repro" kit in stock.  I don't know anything about them or who makes them.  I tried the Keyster kit many years ago and found them completely unsatisfactory.  The jet was OK, but the needle taper was all wrong.  I don't know if they have improved. 

[Bankerdanny]

Did you save your old needles and other brass?

[proudsnacks]

I did, I spent last night putting it all back in, don't have vacuum gauges but I bench synched as best I could, it fired up nice and sounds like it has a happier idle, I was getting some popping before which I can't hear now, gonna test it out next day it's not raining. I'm hoping the crappy rebuild kit has been the culprit all along

[proudsnacks]

I was able take take it out for a test ride, and I'm pretty sure that was the issue, I can tell it still needs the vacuum sync to even it out but it revs out a lot easier, the only thing I noticed is the idle changing as it warms up, only 1 and 4 chokes close all the way, 2 and 3 don't fully close. I tried cleaning up the mechanism but did help much.

[proudsnacks]

But I see it's not just me, I'm used to my 1980 kz choke/ idle set up...

[proudsnacks]

back again after some time away from the bike, had the gas tank cleaned and coated, no more rusty carbs 
i  had also gotten the carbs re synced after re installing the original brass and it works a lot better.
However, i cant seem to get rid of the popping on the 4th carb. i know its clean, i checked the plug and its pretty fouled compared to the rest, it looks like its rich but i dont understand why if its puffing part of the fuel back into the air-box, if the combustion ratio is off, would unburnt fuel collect and foul?
im thinking another compression test is in order.
what are the possible causes of gas puffing back through? its kinda hard to hear when riding, plus a couple small exhaust leaks but i can feel its through the insulator. it idles fine, no popping or puffing its only under load at about 2500-3500. peeking into the cylinder with a flashlight, the piston looks pretty cruddy, and the intake side of the engine with the carbs off looks pretty gunky going down to the intake valves.

[HondaMan]

back again after some time away from the bike, had the gas tank cleaned and coated, no more rusty carbs 
i  had also gotten the carbs re synced after re installing the original brass and it works a lot better.
However, i cant seem to get rid of the popping on the 4th carb. i know its clean, i checked the plug and its pretty fouled compared to the rest, it looks like its rich but i dont understand why if its puffing part of the fuel back into the air-box, if the combustion ratio is off, would unburnt fuel collect and foul?
im thinking another compression test is in order.
what are the possible causes of gas puffing back through? its kinda hard to hear when riding, plus a couple small exhaust leaks but i can feel its through the insulator. it idles fine, no popping or puffing its only under load at about 2500-3500. peeking into the cylinder with a flashlight, the piston looks pretty cruddy, and the intake side of the engine with the carbs off looks pretty gunky going down to the intake valves.

This can be caused by a slight vacuum leak on the #4 or the #3 carb's rubber hose (to the head). It can also be caused by using 'pod' type air filters, or by ignition timing being too advanced for the 2-3 set of points. If it pops out of the #4 while you are coasting down a hill, then that carb is set slightly less open (in vacuum-sync terms) than the #3 in the set. A vacuum-sync job is the best fix, or you can try adjusting the slide up in 1/8 turn steps (turn the 17mm nut downward after loosening the little 10mm locknut) and test-riding it, the hard way...

[proudsnacks]

it sort of sounds like the carb sync wasn't done quite right then. i just installed all new OEM clamps and the rubbers were all new last year, cant seem to find a leak to the head. plus a new air filter in the stick airbox. ill check what the timing looks like, shop guy said it was pretty close and adjusted it, but people make mistakes. i watched him do the sync the second time around, had a little trouble getting the needles all balanced. looked decent enough, all within a few cm's. i think i might invest in a set of gauges for myself at this rate 

[proudsnacks]

i checked the timing and the point gaps, 2/3 was a little tight and the timing looks good but 1/4 timing is slightly retarded compared to 2/3. but the plate is already turned as much as it can go, it looks likes it about 1-2 cm away from lining up with the marking. what am i missing here? would an electronic ignition be a better set up?

[1976cb750f836]

You said earlier, all is well except ex leaks, exhaust leaks can cause some strange things!
Loss of power, possibly your spitting thing, they can lean the mixture and cause poking in exhaust.
Also be sure your points, or Dana is in sync.  Both sides timed correctly, and the same dwell!
Keep us posted.

[HondaMan]

i checked the timing and the point gaps, 2/3 was a little tight and the timing looks good but 1/4 timing is slightly retarded compared to 2/3. but the plate is already turned as much as it can go, it looks likes it about 1-2 cm away from lining up with the marking. what am i missing here? would an electronic ignition be a better set up?

This sounds like the points are Daiichi points. Their characteristics put the timing plate (and marks) just as you describe.
And no, electronic ignition is not the answer: all of the electronic units out their use too much power for these bikes to support, unless you ride like a roadracer at 5000+ RPM most of the time. Sort out this points issue first, and then if you'd like electronics added to reduce (as in, stop) the points wear, consider one of my Transistor Ignitions, which runs from those points and stops their wear.

All this said: the thing to do at this point is to try to adjust the L points (1-4) first to get them onto the timing marks. If they are indeed Daiichi, one of the simplest ways I know of to make this happen is to bend the ground contact arm slightly outward, then re-bend the outer part of it upward again, then set the gap and timing. The general idea here is to move the stationary contact down (away) from the moving contact about 0.002" in distance, which is the geometric error in the Daiichi points that cause all this trouble. Then, you can usually set the gap at 0.014"-0.016" and get the timing correct. Then repeat for the 2-3 side.

Where my Transistor Ignition can help with the Daiichi points is: if you just simply bend the stationary arm downward a little bit and then set the timing/gap there, the points contacts are no longer parallel to each other, which will cause rapid wear from arcing and pitting. This is where my Ignition shines: it removes all but 100mA of load (and 100% of arcing) from the contacts, which also stops the wear. T prove this out, when I [re]created these units in 2006 I also put some Daiichi points on my own 750 and adjusted them just like this, with angled-contact faces, and rode it 6 years with no ill effects as my 'test plan'. At the end of that time nothing had changed and I had no need of any readjustments, but I was rebuilding the engine (2013) so I bought some Honda/TEC points plates anyway to start fresh. The old Daiichi points plate is my carry-along spare set now.

[proudsnacks]

going off of what the shop guy said it was pretty good..from what i can tell its not as good as it should be. i mucked around with it to see if i could get it closer but my test light died in the process, i switched to a multi meter and tried it that way, it seems like its now set correctly. when i started it up and to check strobe timing my #1 float bowl started over flowing, as if the gasket wasn't seated properly. not sure what happened over night but its just not sealing, also my petcock it now leaking, so thats really annoying. i have a new petcock on order, i think i have other gaskets i can use. i had some sea foam in the tank but i didnt think that would have any affect?

[proudsnacks]

Well the gas leak is fixed, and I think I was able to set the of ition timing a bit better, not entirely sure what I did, bit I was able fiddle around with the points gap set to .012, slightly tight but it brought things more aligned with the marking, now they are about the same, 1/4 when I check with a strobe light are kinda "jump" as in the F mark will jump around a little bit  when Im trying to get a reading. Still showing the same misting of gas, doesn't seem.as bad with easy throttle but mists hard with a semi quick twist of the throttle. Hoping another carb sync will clear it up.

[proudsnacks]

tried syncing the carbs again myself after setting the timing as best as i could, i noticed that as i apply the throttle, the 4th carb isn't building vacuum like the others do. does this mean there is a vacuum leak that i haven't found yet?