Topic: Potential 1970 CB750 K0/K1 Hybrid

[denward17]

Good troubleshooting MikeyG, keep at it, it will be great when you get there.

[Mikey G.]

Managed to get the 750 out for a couple of test runs around the block, and things are very promising.  First things first, I did end up making a purchase that I probably should've made a long time ago:




I've been getting by with vacuum gauges until now, but this bike has been touchier than any of my others, and I'm tired of THINKING I have my gauges calibrated correctly, but never 100% sure.  Well, this thing paid for itself almost immediately.  Easier to read, more responsive to changes, no farting around with dampening valves.  Excellent.

Second, I managed a longer ride today, and things went well.  Bike seemed responsive and didn't hesitate or whatnot.  The only problem was I had the idle set WAY too low, so when I got back I ran each idle set screw in a single line each.  The good news is that this got the idle revs to a good place.  The bad news is because I only did it by the adjuster, not with a proper sync, is now the idle sync is a little off.  So eventually tank off, Carbtune on for hopefully five seconds to lock the idle down, and we're good.

I pulled the plugs just for fun.  I'm not super happy with 4- it's always seemed a bit rough, and I noticed I was getting a HECK of a lot of decel pop during my ride.  On the one hand, all my other bikes have air cutoff valves, so a bit of popping might be normal.  But what I was getting seemed a bit excessive.




Plugs are in order, 1-4, left to right.  Overall, they don't look bad.  The inner electrode on 4 is quite a bit lighter than the other three, so I may adjust the mixture screw on that one a little.  I'm not super happy about the deposits on the outer ring of 2, but everything else seemed fine.

Still, it's nice to know I'm close.

[Mikey G.]

I feel like I'm inching up on the proper settings for this bike.  I've been gradually enrichening the pilot screw over the last couple rides, and every time it get just a hair better.  My last ride was pretty much issue free, even though I only did about 25 minutes, so the engine didn't get HOT-hot.  Still, it's much better than it was.

I had to drill the vent out on my gas cap, which took care of most of my "inconsistent" issues.  Which was odd, because it was a brand new Yamiya unit.  Regardless, the difference is night and day.

That said, and speaking of new Yamiya units, we can't have this much good without a little bad, can we?  I saw this the other day, and was just.... annoyed.  I mean, I can fix it.  But why should I have to?

Ah, well.  Such is life.

[BenelliSEI]

Well done. Perseverance is the key! I’m still using my ancient Motion Pro liquid filled “sticks”. How do you like the Carbtune?

[Mikey G.]

My only regret is not buying one sooner.  I did vacuum gauges for many years up until this point, and they work fine.  But the Carbtune eliminates the two biggest question marks while using the needle gauges-

--Are they calibrated?
--Are the valves to dampen needle flutter closed or open enough?

I'd get to a point on the K0 where the valves would be closed as much as they needed to be to do the idle, but then closed too far for the sync at 2500 rpm.  So I was endlessly messing with those, while trying to stay on top of the carb adjustments themselves.  It was just a hassle.

And it's a minor thing, but it's much easier to compare readings off a face 4 inches wide, vs. a bank of dials that's nearly a foot across.

Honestly, I think the needle gauges- are fine for 90% of bikes.  The K0 just happens to be goofy and touchy enough to require that extra bit of fidelity.

If you have the mercury sticks you're probably fine.  The only upside to the Carbtune would be because of the spring-loaded rods, there's no risk of sucking up mercury if you rev the motor.

[Mikey G.]

This bike.... confuses me.  I finally got a little more road time on it, and while there's a lot of good, I'm still not there yet.  First up, the good:

-I thought I had a bit of an oil leak out the head or top end someplace, but that's seemed to have taken care of itself. 
-When the bike is on the mains, she runs EXCELLENT.  Responsive, happy, pulls well.  Runs like a proper bike should.
-Idle is very nice.  Rock steady, and quite happy.

And now, for the bad.

-I'm STILL getting real bad popping on deceleration out of at least 1, maybe 2 pipes.  It's tough to tell which ones while riding the bike.
-Off idle, or while low-speed cruising still hits me with a bog, hesitation, or burble right when I give it a bit more throttle.  It almost seems like the plugs, or a couple of them at least, load up (or starve, I can't tell) for fuel right when that throttle is cracked.  This, by far, it what's stopping me from a perfectly running bike.
-Plugs on 1 and 2 seem pretty good, but 3 and 4 still seem a bit lean.  Which is annoying, since the idle air screws on 3 and 4 are only about half a turn out at this point.

One last oddity- at the end of my last ride she was idling really good, but I noticed a bit of light popping out of 2, of all carbs.  That's still showing deposits on that plug, so I may be burning a bit of oil, so I'm okay with that one so far.  But still odd.

I"m real close to pulling the carbs and checking the idle passages on 3 and 4.  It's just odd she's still running lean, and I can't figure out any other easy option to check. 

[newday777]

Did you use a High E guitar string to run through the brass intake ports like HondaMan says and shows in his book, withlots of spray carb cleaner and compressed air?

[Mikey G.]

Not yet.  The carbs got the carb cleaner + compressed air treatment first thing after I bought them, and got ran through an ultrasonic cleaner as recent as a couple months ago.  That said, they did have quite a few hard white deposits when I first bought them, which may not have busted loose during those cleanings.

I really wish I knew the symptoms of slightly lean vs slightly rich better.  Idle mix screws are the bane of my existence, and always have been.

[newday777]

Clean them properly to get rid of your problems.

[BenelliSEI]

Next time you put on the synchronizer try this (assuming the carbs are clean):

1. Make sure all four throttle cables have a little bit of slack

2. Set the idle using the knurled wheels (when HOT) to the idle you like. Ensure all four are running even on the gauges.

3. Starting with #1, slowly back out the throttle cable base adjuster until the gauge moves fractionally. Now set it back and lock it down so the guage is back even with the other three.

4. Do the same to the other three carbs.

All this should ensure that when you initially lift the carbs off the idle pins they are still well sync’d. Should cure the hesitation you describe.

[BenelliSEI]

P.S. Just set the idle mixture screws to the base setting for your bike. Leave them alone.

[scottly]

I really wish I knew the symptoms of slightly lean vs slightly rich better.

A lean miss will produce a sharper hesitation than a rich mix, which is more like a "burble".
Don't worry about popping in the exhaust on deceleration with the throttle closed.   

[BenelliSEI]

I really wish I knew the symptoms of slightly lean vs slightly rich better.

A lean miss will produce a sharper hesitation than a rich mix, which is more like a "burble".
Don't worry about popping in the exhaust on deceleration with the throttle closed.

+1 to that. My cb750 K1 burbles a bit on shut down too. No worries.

[newday777]

Not yet.  The carbs got the carb cleaner + compressed air treatment first thing after I bought them, and got ran through an ultrasonic cleaner as recent as a couple months ago.  That said, they did have quite a few hard white deposits when I first bought them, which may not have busted loose during those cleanings.

I really wish I knew the symptoms of slightly lean vs slightly rich better.  Idle mix screws are the bane of my existence, and always have been.

The internal carburator ports will have hardened MTBE deposits in them that have to be manually cleaned out after ultrasonic cleaning using a high E (.010) guitar string, lots of spray carb cleaner in each port, run the E string multiple times, spray and blow out with compressed air is the only way to properly clean the deposits out that is blocking the proper mixture of air and fuel into the venturi mixing. Hondaman clearly spells it out in his book. You will continue to chase your tail on this running problems until you do it.
Edit: The running problems in post #248 of popping out the exhaust at idle is the signs of blocked internal ports of your carbs with MTBE powder, from old gas deposits exposed to water over the years and left in there to dry out.it has to be cleared out to get rid of the idle pops.
"I realized that cylinders 3 and 4 were popping out the exhaust quite a bit."

[Mikey G.]

Thanks for all the advice.  If I can't get things figured out this year, I'll probably pull the carbs again this winter and go poking around with some wire.  I'm generally not a fan of that invasive of a cleaning when it comes to carbs (too easy to mess something up), but if it's what I've got to do, it's what I got to do.

For reference, current idle mix screws are about 3/4 turn out on carbs 1 and 2, and maybe 5/8 of a turn out on 3 and 4 (which seems quite far in, to me).

I managed to get another decent ride in the other day.  I had to keep the choke at least partially on for the first 10 minutes or so to keep everything nice and smooth, but I was able to get it get it to behave.  When it got up to operating temperature, I still had a bit of the persistent off-idle miss after deceleration, but I noticed something strange, and (more importantly) repeatable. 

If I stayed "on" the throttle as I slowed down, ie never closing it all the way, but keeping it at least cracked slightly open all the way down, the engine would hesitate for a second when I got back into the throttle.  HOWEVER- if I closed the throttle completely, even momentarily, then throttled back up, there was no hesitation at all.  Like, I could blip it closed, then immediately open it up again, and it would run fine.  I have no idea what this means, but the fact it's consistent and repeatable made it worth mentioning.

As far as idle goes, after the ride idle was great.  1, 3 and 4 were very happy.  I have a small miss in 2, but I think I have a bit of oil getting in that cylinder as well that may be messing with things, so I'm not sweating it.

[newday777]

That is the same results I had this summer until I pulled the cleaned carbs off and poked out the MTBE in the little mix air internal ports with the high E wire. It's the only way to get it running right.

[seanbarney41]

Stu, you keep talking of this MTBE?  I don't believe that additive was ever mandated in Michigan or the vast majority of other states.  I remember reading about it in Hemmings Motor News back in the 1990s and all the problems it was causing and car clubs joining together to help lobby it's banning.  I think it was only selected east coast states, a few out west, and California that mandated it's use.  If fuel with it was ever sold here?  I have no idea.  Nor do I know the full maintenance history of MikeyG's bike

[seanbarney41]

Also, the carb sets I have encountered with the worst damage typical o what you attribute to MTBE ended up running fine with normal cleaning.  I agree you have to get those passages CLEAN, but the proper chemicals and compressed air have always got the job done for me.

Also MikeyG, gasoline itself is one of the best carb cleaners there is.  Hell, I can remember my grandfather, who was a WWII aircraft mechanic sitting out front of his garage with a bucket of gasoline and a toothbrush scrubbing the Buick's Carter AFB.  So, try to ride around the carb problem as much as possible, and after a while re-check the tune-up and sometimes these problems will go away.

[Mikey G.]

I've still been tinkering, and things are getting better.  Setting the screws on carbs 3 &4 to 3/4 out improved things quite a bit.  While I still got that little bit of hesitation here and there, when I got back home I realized carb 2 was still set at 1/2 a turn out.  I set that to 3/4 like the rest, but haven't had a chance to test ride it before a cold snap hit.  I've got a good feeling about it, though.

Eventually I'm going to have to look at my gauge needles as well.  They start off working just fine, but after about 15-20 minutes they start wiggling quite badly.  Both speedo and tach cables are new and lubed, so there's that.

I've also ordered new seals and fork tubes, so hopefully soon I can get the correct forks and front fender installed.

[Mikey G.]

Alright, I'm going to have to re-do these cables from scratch.  I did another ride, ended up getting the mixture screws to stock, and pretty well figure I'm going to leave them there.  I'm still getting rough running out of 2, and as a result of getting the mix needles back to stock, now my idle is a touch too low.

But the kicker was when I got back.  I still got that annoying miss out of two, but on a whim I decided to rev it up.  I did, and ended up with a pretty heinous miss out of cylinder 1, of all places while running on the mains!  So something isn't set right someplace.  On the plus side, carb 2 works just FINE on the mains, so at least I've only got one problem (that swaps places).

I think I'm going to pull the carbs, double check some of the smaller passages with a wire, then re-set the slides per Benelli's suggestion.  I hate to go backwards, but you gotta do what you gotta do.

[BenelliSEI]

Congrats!

[newday777]

Stu, you keep talking of this MTBE?  I don't believe that additive was ever mandated in Michigan or the vast majority of other states.  I remember reading about it in Hemmings Motor News back in the 1990s and all the problems it was causing and car clubs joining together to help lobby it's banning.  I think it was only selected east coast states, a few out west, and California that mandated it's use.  If fuel with it was ever sold here?  I have no idea.  Nor do I know the full maintenance history of MikeyG's bike

MTBE was used in any location where air pollution was high to curb air pollution levels. The gas stations didn't have to label that they had it in gasoline. It was done at state level of government to meet EPA regulations.
The reason it was phased out was it got into ground water and wells. It is a lifetime pollutant, meaning it doesn't go away. I have a friend in NH that has a water filtration business and tests wells for MTBE as the state has a program to pay for filtration with any well containing it(it is even in municipal water supplies!)
This label is what I noticed the other day on my trip through AZ.
And some information on MTBE pollution.

There are new lowered acceptable levels that are being changed to that haven't been released to the general public yet, most people don't have any idea the damage of MTBE.
Hondaman mentions having to remove the dried MTBE powder out of the mixture ports in our carbs to get rid of poor running conditions of this thread. That is why I bring up the need to manually clean out the internal ports that most people fail to do and wonder why they are continuing to have running problems.
You can do what you want to, I have had to do it on my own carbs because of the same popping.

https://archive.epa.gov/mtbe/web/html/water.html

[Mikey G.]

Couple small updates.  Carbs got pulled again, and ran through with some brass wire.  I didn't see any deposits get pulled out, but at least I have peace of mind.  The idle stop screws were a bit off on carbs 1 and 2, so I set all 4 identically to at least give me a good baseline.

Unfortunately, the weather turned quite cold, so carb tuning will have to wait.  In the meantime, I decided to swap my forks to my correct K0 set.  Taking them apart was easy enough- it's a surprise to see the only thing keeping the forks from coming apart is the single circlip in the lower tube.  Also, all the bushings and snap rings on the upper tube was interesting.  My old downtubes were trash, so I got a new aftermarket set.  I also ordered new caps and seals, and new upper clips as the first was broken, and I'm not taking chances with the other.

Once it warms up a bit I'll strip and polish the lowers, then get this front end swapped out.

[BenelliSEI]

Looking better every day!