Topic: Kaze's CB750K6 Running (Video)

[Kaze]

Disclaimer: This video may not be available forever. If the video doesn't run here, you might need to click the "Youtube" icon on the video and watch it from there.

My bike's most recent health checkup data will be below this post.

My bike runs better than it did last year, so that part is very good. But as you can hear, it's not running as well as it could. I'm not a daily rider anymore, and haven't been touring on this bike at all. In fact, most of my time with this bike has been spent with a wrench in hand. It's on the road now, but it's still running a bit rough. (EDIT: lots of farting on the downshift still when coming off the freeway)

At the beginning of summer, before I was riding, compression was 125-120-120-125. I didn't remeasure it at the end of summer. My sense is that at least some of this poor compression must be due to the leaking head. I have to admit that I've gotten so used to seeing a leaking head on a Honda that I sort of glazed over it until watching my video. That's sad, but it's a reality of old, crappy barn-rescues.

[Kaze]

GENERAL
- Mileage 11,483 on odometer (actual history of bike prior to 2014 is unknown)
- K6 engine, carbs, pipes, front end, sprockets and chain.
- K7 frame, rear-end, as well as wire-harness, ignition switch, and indicator lights.

2023 Health Check Results (July)
- Compression (psi): #1 = 125, #2 = 120, #3 = 120, #4 = 125
- Valve clearances: Set to K6 spec. .05MM intake, .08MM exhaust.
- Oil & filter change: 237 miles since last change. Non-synthetic 10W40.
- Gas: Fresh. No old gas from last season.
- Fuel filter: 100 miles.
- Air filter: Standard airbox with 200 miles on filter. No dust, clear in front, dry and dark in the back.
- Exhaust setup: 4-into-4, OEM K6 pipes (inside condition unknown)
- Throttle cables: 2023, summer. Stock K6 length.
- Handlebars are stock K6 size.

FUEL
- Carburetor: Keihin 086A (dated 1976)
- Jet size unknown. Have not replaced them (brass).
- Carbs cleaned Aug. 18: Carb bodies soaked in Berryman's Chem-Dip, all ports cleared with carb-cleaner & compressed air. All jets clear and clean, all holes appear free of obstructions under magnifying glass. New idle screw springs. New O-rings. New rubber lines between carbs. Rubber top seals and float bowl seals are supple and re-sealed with silicone grease.
- Vacuum synced to 200mm (20cm) Spec. is between 16-20cmHg)
- Idle screws out 1 turn across the board.
- Float level at 26mm. (Measured with carbs at angle, so floats not depressing needles)
- Gas cap gasket replaced and inside cleaned out in summer 2023. Air vent cleared out but still seem to have some "gas starvation" issues at 1/2 tank. Opening the gas cap solves issue.
- Stock airbox with new rubber boots.
- No apparent vaccum leak or cracks on original intake boots (from engine to carbs)
- FUEL TREATMENT: "Sea Foam Motor Treatment " used according to prescribed amount, 1 tank ago.

ELECTRICAL
- Points and condensers have 450 miles on them. Contact surface clean.
- Ignition timing: Spark advance “F” is aligned with timing mark.
- Point gap: .35mm (Spec: .3 – .4mm)
- Ignition advance angle: Unknown. (I don't own a dwell meter) (Spec. 35°)
- Point dwell? Unknown. (I don't own a dwell meter)
- Spark-plugs: NGK, D8EA. New in August, but after 100 miles already sooty on #2 and #3.
Plug gap: Spec. .6 to .7mm Unchanged from factory, measure @ .65mm. (Prior to carb clean, plugs had 200 miles on them, #2 and #3 were sooty, #1 and #4 were much cleaner)
- Plug caps: Brand new NGK caps (as of Sept. 2023)
- Ignition Coils: Stock TEC brand FL703
- High tension lead for cyl. #2 missing a bit of the hard plastic insulation at tip.
   - Trimmed leads back 1-2mm.
   - Primary Winding, cyl. 1&4:  5.8 k ohms
   (Initial reading 6.2 ohms, minus .4 internal resistance)
   - Secondary Winding, cyl. 1&4: 14.6-14.8 k ohms
   - Primary Winding, cyl. 2&3:  5.6 k ohms
   (Initial reading 6.0 k minus .4 internal resistance)
   - Secondary Winding, cyl. 2&3: 14.15 k ohms

[robvangulik]

Your valveclearance should be .05 and .08 MILLIMETER in and ex! Not .005 and .008, never mind if you used mm or inch, it's not right.
That is about  .002 and .003 inch....and that can make a real difference in the way it runs, don't you think?

[Kaze]

It would make a real difference indeed if I had written in millimeters... I was thinking centimeters.
.005cm=.05mm. Exhaust is .008cm=.08mm. Have edited the original post to read in millimeters.

[Tracksnblades1]

Nice looking bike…was it a police bike..?

It looks like it has 750 F1 instrumentation….what year is it 77+…? * edit* I see it’s a 77 frame..
I
Have you sync the carbs…?  Have you checked your tach for accuracy..? It sounds on iPad speakers to be idle slower than your tach shows…

My F1 will idle real slow and sound like that....but it’s tach will be showing an unstable 700rpm reading..

900+ it really smooths out and no chunka chunka chunka chunka….clutch and primary chain noises…

[seanbarney41]

sounds really healthy except for that lumpy idle, much better than most new builds seen around here.  I don't put much stock in idle quality as my bikes spend little time at idle speeds.  Agree that your idle speed is set a little low plus a carb sync

[Kaze]

Tachometer wrong? To me it sounds so low that it's ready to die, but the tacho said 1400 so I just blindly followed the needle. It IS low isn't it. So my tachometer could be off.

Paint. The original paint was trashed, front fender was scratched up, and I had to buy new side-covers. So I decided to be free and paint it a solid color. It's not a CB750P, but I like them.

Ignition switch/indicator lights are K7, yup. K7 wire bundle has a larger weather-pack connector for the ignition, with wires set up for the directional turn signal indicators. It seemed like a good idea to just stick with what the bundle was set up for. And, I like the steering lock from the K7.

Carb-sync with the 4 gauge set-up was done in August and that made a big improvement. But after you mentioned it, I looked at some other posts and it seems we're meant to sync. at 1krpm and again at 3krpm. I can't recall if I did it twice, so I'd better do it again. This waning memory b.s. is not cool. I'm only 54, not 94... wtf.

"Chunka-Chunka" is the exact way I would describe it. My ears say that the timing is off, but hard for me to be sure. I used the static timing light method. Some fellas on here suggest using a dwell meter, which I haven't bought yet. I will, once "birthday season" is over.

Any thoughts on that oily evidence of a head gasket leak?

I stopped by the local motorcycle dealership this evening. The service gents gently reminded me that they don't take in vintage bikes, but two of the mechanics were nice enough to come out to look, listen, and sniff anyway. I'm paraphrasing here, but the message I received was that when trying to tune my bike, that I'm fighting the low compression, so my adjustments are going to be "off" somehow. Anybody have a comment about this?

[Kaze]

Can an iPad test the tachometer?!
I see some no-touch tachometers on Amazon for cheap, but Amazon has a bad rep for selling crap.
Any suggestions?

[PeWe]

That idle reminds me about  before I cut the advancer springs. Cut the loop that attach spring around advancer tower. Fold up a new loop with pliers.  Both springs.

Make sure advancer sit in its most counterclockward position (if it has a play) before tighten the small M6 nut.
Ignition adjusted with timing lamp spot on marking at an idle of 1000-1200 rpm.
(I prefer 1200)

Full advance happen at around 2700rpm.

Sloppy springs will advance ignition so idle will suffer.

Both my CB750 have that mod. My modified K6 with +107whp at last dynos a year ago and my almost stock K2 with K7 type pistons and cam.

Idle is more stable too.
I can ride my K6 in 50kmh in 5th gear rather smooth and sound just over 2000rpm. Megacycle 125-75 cam can behave well as a street bike need.

[PeWe]

About oil leak.
It can leak under cam towers/holders around the rubber coins and eventually via cam studs threads.
Remove spark plug and feel with a finger inside. If the cavity roof has oil, leak via rubber coins.
They need sealer that help them to seal before placing them and mount cam holders etc.

Important to tighten the head nuts again 24 hours after the first time.
Stock studs can be tightened 20ft lbs.

I tightened my K6 with stock studs 28Nm due to 836 and modifications that encouraged heavier use of the throttle. It later got stiffer mess flexing hd  studs.
Now MLS gasket and 29.5Nm.

[Kaze]

Pewe: Does it matter that I have points & condensers? I thought your bikes had electronic ignition?

[PeWe]

I did the spring fix on my K2 when it had points. I had to do that to get ignition correct at idle. It was too advanced, ca 1mm.
Point plate turned  max clockwise. Spring fix made it to match exactly. I tested Yamiya's "race springs". Cut stock worked better.


My K6 when it had Pamco ignition, later Dyna-S, after that points,  and finally Dyna-s again.
That advancer advanced way too early, before 2000rpm. Advancer cam could move several degrees without springs held it back. It work better if springs apply force on cam direct, holding it back. More stable idle.


Some set the ignition at full advance. Skip a nice idle and slow riding on low rpms.
Dyna-S installation doc say that.
I should not like that kind of bike.
A race bike another thing.

[Deltarider]

Doesn't run that bad. Also my first impression was that tachometer is showing a too high rpm for what I hear....
If it was my bike, I would doublecheck the tachometer with a known good one and set the gaps with a dwellmeter. Then recheck/readjust ignition timing and do a dynamic sync once more.
BTW, you have a tendency to aim at the 'middle', cf:

- Vacuum synced to 200mm (20cm) Spec. is between 16-20cmHg)

This is overthinking it. The mms are hardly relevant, concentrate on equality across the four.
You're almost there.

[PeWe]

When syncing, slowly  inrease the throttle so all will follow in sync to at least 3000rpm.
Slowly decrease to see they all are in sync.
When good, take offs and city riding will be enjoyable.

Make sure that all carbs are jetted OK,  air/fuel screws equal and valves adjusted before sync.

A cooling fan in front of engine a cool thing....

[seanbarney41]

Honda never even claimed that the tacho is accurate at low speeds.  Every manual I have seen expressly tells you NOT to rely on the on board tachometer for tune up settings.

[Kaze]

Thank you guys, all of you. I'm sort of speechless at the moment, just going over what you all wrote. Appreciated!

[Kaze]

Just in case someone asks... these plugs are only 150-200 miles old. (In order, 1-2-3-4)

[Kaze]

Well... the timing plate for cylinders 2-3 was off a little. Maybe 1mm, or less? Enough that the bike is already sounding better. Bad that I missed doing it last time, but a better running bike makes me happy, so I'll just skip flogging myself this time.

Next I'll redo my carb-sync.

[Deltarider]

Just in case someone asks... these plugs are only 150-200 miles old. (In order, 1-2-3-4)

Next time, position your plugs so that the experts here can have a look inside at their ceramic noses.

[Kaze]

Noses?

[Deltarider]

The white ceramic insulators.

[newday777]

Like this

[Kaze]

Gotcha

[Kaze]

Well... I think I fixed my fuel starvation problem. My fuel lines were too fat to go through the carb body so I had routed them under the T's. Holy misnomers Batman! That's not a T junction, it's an upside down T!

The week I put the Seafoam into the gas, and replaced my gas-cap gasket, my bike almost died on the freeway with 1/2 tank of petrol. I THOUGHT that opening the gas cap at the gas station let air in... and that's why it came back to life. Up until that day, I was working on the bike and never rode more than 40 miles at a time. I'd come back, fix something else, and refilled the gas. That's why I never noticed the problem.

Now I'm thinking that the "fix" was keeping the tank full. The lines were too low and gas was fighting gravity. So now I need to go out and ride out the rest of this tank of gas and see if it was my crappy plumbing that caused the problem.